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The Bad Taste of Medicines: Overview of Basic Research on Bitter Taste

      Abstract

      Background

      Many active pharmaceutical ingredients taste bitter and thus are aversive to children as well as many adults. Encapsulation of the medicine in pill or tablet form, an effective method for adults to avoid the unpleasant taste, is problematic for children. Many children cannot or will not swallow solid dose forms.

      Objective

      This review highlights basic principles of gustatory function, with a special focus on the science of bitter taste, derived from studies of animal models and human psychophysics. We focus on the set of genes that encode the proteins that function as bitter receptors as well as the cascade of events that leads to multidimensional aspects of taste function, highlighting the role that animal models played in these discoveries. We also summarize psychophysical approaches to studying bitter taste in adult and pediatric populations, highlighting evidence of the similarities and differences in bitter taste perception and acceptance between adults and children and drawing on useful strategies from animal models.

      Results

      Medicine often tastes bitter, and because children are more bitter-sensitive than are adults, this creates problems with compliance. Bitter arises from stimulating receptors in taste receptor cells, with signals processed in the taste bud and relayed to the brain. However, there are many gaps in our understanding of how best to measure bitterness and how to ameliorate it, including whether it is more efficiently addressed at the level of receptor and sensory signaling, at the level of central processing, or by masking techniques. All methods of measuring responsiveness to bitter ligands—in animal models through human psychophysics or with “electronic tongues”—have limitations.

      Conclusions

      Better-tasting medications may enhance pediatric adherence to drug therapy. Sugars, acids, salt, and other substances reduce perceived bitterness of several pharmaceuticals, and although pleasant flavorings may help children consume some medicines, they often are not effective in suppressing bitter tastes. Further development of psychophysical tools for children will help us better understand their sensory worlds. Multiple testing strategies will help us refine methods to assess acceptance and compliance by various pediatric populations. Research involving animal models, in which the gustatory system can be more invasively manipulated, can elucidate mechanisms, ultimately providing potential targets. These approaches, combined with new technologies and guided by findings from clinical studies, will potentially lead to effective ways to enhance drug acceptance and compliance in pediatric populations.

      Key words

      Introduction

      Most children at some point in their lives are prescribed medicine. Some refuse to take it, and they and their parents suffer the consequences. Although children are subject to many of the same ailments and diseases as adults and are treated with the same drugs, most drugs (nearly 75%) available in the United States lack US Food and Drug Administration (FDA)–approved pediatric formulations and therefore do not have labeling information about pediatric safety and efficacy.
      • Roberts R.
      • Rodriguez W.
      • Murphy D.
      • Crescenzi T.
      Pediatric drug labeling: improving the safety and efficacy of pediatric therapies.
      The lack of “child-friendly” formulations leaves an estimated 40% of the world’s children at increased risk of avoidable adverse events, such as suboptimal dosing, lack of adherence to medication regimens, and reduced access to new medicines.
      • Milne C.P.
      • Bruss J.B.
      The economics of pediatric formulation development for off-patent drugs.
      Although recent legislation in the United States and European Union has created incentives for testing drugs in this special population,
      • Giacoia G.P.
      • Taylor-Zapata P.
      • Mattison D.
      Eunice Kennedy Shriver National Institute of Child Health and Human Development Pediatric Formulation Initiative: selected reports from working groups.
      this process is confounded by the requirement that the formulation be suitable for the pediatric patient population—actually a continuum of many smaller populations, such as preterm infants, term infants, infants and toddlers, preschoolers, school-age children, and adolescents.
      • Milne C.P.
      • Bruss J.B.
      The economics of pediatric formulation development for off-patent drugs.

      The Problem: A Matter of Taste

      A central challenge of administering medicine to children is a “matter of taste”—drugs, by their very nature, often taste unpleasant, with bitter taste a primary culprit. More than 90% of pediatricians reported that a drug’s taste and palatability were the greatest barriers to completing treatment.
      • Milne C.P.
      • Bruss J.B.
      The economics of pediatric formulation development for off-patent drugs.
      Most drugs work by interfering with physiological processes within cells, so medicines have the potential to be toxic when ingested in sufficient quantity. Bitter taste is thought to have evolved as a deterrent against ingesting toxic substances,
      • Glendinning J.I.
      Is the bitter rejection response always adaptive?.
      which may explain why many drugs taste bitter. The basic biology of the child, as reviewed here, explains why children (and adults) reject bitter-tasting drugs. In fact, bitter compounds are effective agents in deterring pediatric poisonings when used in conjunction with other preventive measures, such as child-resistant closures.
      • Rodgers Jr, G.C.
      • Tenenbein M.
      The role of aversive bittering agents in the prevention of pediatric poisonings.
      Although many solid oral dosage forms (eg, pills, tablets) have the advantage of masking or encapsulating bitter tastes, such methods are ineffective for many children because they often cannot or will not swallow pills or tablets. The cutoff for needing liquid formulations typically is between 6 and 8 years of age,
      • Schirm E.
      • Tobi H.
      • de Vries TW
      • et al.
      Lack of appropriate formulations of medicines for children in the community.
      but older children (and teenagers and adults) vary greatly in their ability to swallow tablets and capsules.
      • Ruark J.L.
      • McCullough G.H.
      • Peters R.L.
      • Moore C.A.
      Bolus consistency and swallowing in children and adults.
      • Sadrieh N.
      • Brower J.
      • Yu L.
      • et al.
      Stability, dose uniformity, and palatability of three counterterrorism drugs-human subject and electronic tongue studies.
      In addition, fixed doses are impractical because the dose often varies according to the size of the child. The Physician Drug and Diagnosis Audit revealed that 6 year olds were 4 times as likely as 16 year olds to not take their medications as oral solids.
      • Milne C.P.
      • Bruss J.B.
      The economics of pediatric formulation development for off-patent drugs.
      Drugs usually are administered not alone but rather as part of formulations that are in either liquid or solid form. Liquid formulations are complex mixtures containing many other components besides the active ingredients; excipients include, but are not limited to, bulk materials, flavorings, sweeteners, buffers, preservatives, and coloring agents.
      • Pawar S.
      • Kumar A.
      Issues in the formulation of drugs for oral use in children: role of excipients.
      Because masking the bitter taste of medications is a major challenge in formulating liquid medications, drugs are often combined with more pleasant-tasting compounds, for example, sucrose, high-intensity sweeteners, and flavors popular with children, such as bubble gum. Adding both sugars and acids to medication formulations reduces, but does not completely eliminate, the bitterness of drugs.
      • Ishizaka T.
      • Okada S.
      • Tokuyama E.
      • et al.
      Suppression of bitterness and improvement of palatability of commercial prednisolone powder.
      However, frequent use of sucrose-sweetened medicines has been linked to dental caries in children.
      • Roberts I.F.
      • Roberts G.J.
      Relation between medicines sweetened with sucrose and dental disease.
      • Hobson P.
      The treatment of medically handicapped children.
      • Feigal R.J.
      • Gleeson M.C.
      • Beckman T.M.
      • Greenwood M.E.
      Dental caries related to liquid medication intake in young cardiac patients.
      • Greenwood M.
      • Feigal R.
      • Messer H.
      Cariogenic potential of liquid medications in rats.
      • Manley M.C.
      • Calnan M.
      • Sheiham A.
      A spoonful of sugar helps the medicine go down? Perspectives on the use of sugar in children's medicines.
      This concern is responsible, in part, for the general decrease in sugar content in prescription medications in recent decades.
      • Maguire A.
      • Rugg-Gunn A.J.
      Changes in the prescribing of liquid oral medicines (LOMs) in the northern region of England between 1987 and 1992 with special regard to sugar content and long-term use in children.
      • Baqir W.
      • Maguire A.
      Consumption of prescribed and over-the-counter medicines with prolonged oral clearance used by the elderly in the northern region of England, with special regard to generic prescribing, dose form and sugars content.
      In contrast, acids remain in frequent use in medicine formulations to improve flavor and to maintain chemical stability.
      • Maguire A.
      • Baqir W.
      • Nunn J.H.
      Are sugars-free medicines more erosive than sugars-containing medicines? An in vitro study of paediatric medicines with prolonged oral clearance used regularly and long-term by children.
      Children like more intense sourness than do adults,
      • Liem D.G.
      • Mennella J.A.
      Heightened sour preferences during childhood.
      so lowering the pH increases the palatability for children, probably more so than for adults, and can contribute to bitter taste masking. Using buffering agents to adjust pH into the acidic range also increases the stability of medications otherwise prone to hydrolysis in liquid formulations.
      • Allen Jr, L.V.
      Dosage form design and development.
      However, adding acids to medications has the potential to cause dental erosion (at pH <5.5).
      • Maguire A.
      • Baqir W.
      • Nunn J.H.
      Are sugars-free medicines more erosive than sugars-containing medicines? An in vitro study of paediatric medicines with prolonged oral clearance used regularly and long-term by children.
      About half of 97 pediatric medication formulations used regularly and in the long term by children have an endogenous pH <5.5 and thus are capable of damaging tooth enamel.
      • Maguire A.
      • Baqir W.
      • Nunn J.H.
      Are sugars-free medicines more erosive than sugars-containing medicines? An in vitro study of paediatric medicines with prolonged oral clearance used regularly and long-term by children.
      Citric acid was the most frequently used acid, which raises concerns because citric acid has been linked to tooth erosion due to its ability to dissolve the hydroxyapatite of tooth enamel and dentin.
      • Grenby T.H.
      • Phillips A.
      • Desai T.
      • Mistry M.
      Laboratory studies of the dental properties of soft drinks.
      • Leung VW-H
      • Darvell B.W.
      Artificial salivas for in vitro studies of dental materials.
      The need for liquids for some children may be bypassed by newer modalities for delivering medications. However, solid oral minitablets were refused, spat out, or chewed by half of children younger than 5 years of age,
      • Thomson S.A.
      • Tuleu C.
      • Wong I.C.
      • et al.
      Minitablets: new modality to deliver medicines to preschool-aged children.
      and chewables and melting tablets trigger bitter taste responses, limiting the compounds amenable to such formulations. In addition, new research suggests that the ingestion of bitter compounds may also act in the gut to elicit nausea.
      • Peyrot des Gachons C.
      • Beauchamp G.K.
      • Stern R.M.
      • et al.
      Bitter taste induces nausea.
      Whether children’s encounters with a relatively novel-tasting medication followed by nausea can result in a long-lasting learned aversion to the flavor
      • Bernstein I.L.
      Learned taste aversions in children receiving chemotherapy.
      is an important area for further research.
      Although liquid formulations are often the preferred form of oral delivery for infants and young children,
      • Milne C.P.
      • Bruss J.B.
      The economics of pediatric formulation development for off-patent drugs.
      • Allen Jr, L.V.
      Dosage form design and development.
      • Nahata M.C.
      • Allen Jr, L.V.
      Extemporaneous drug formulations.
      the vast majority of drugs are not commercially available in this form.
      • Nahata M.C.
      • Allen Jr, L.V.
      Extemporaneous drug formulations.
      The need for liquid formulations is expected to increase; most newly approved drugs are not yet labeled for use in pediatric patients, and an appropriate formulation usually is not available unless the drug is approved for that population.
      • Nahata M.C.
      • Allen Jr, L.V.
      Extemporaneous drug formulations.
      In this article, we draw knowledge from the chemical senses literature, with emphasis on bitter taste research involving animal models and on recent developments in the psychophysical assessment of taste responsiveness in children (see Mennella and Beauchamp
      • Mennella J.A.
      • Beauchamp G.K.
      Optimizing oral medications for children.
      for an earlier review), to better understand the nature of bitterness and suggest further ways to make medications more acceptable to the pediatric population. We focus on bitter taste but acknowledge that other sensory attributes (eg, texture, sourness, bad odors) also play a role in compliance. We review the set of genes that encode the known proteins that function as bitter receptors, as well as the cascade of events that leads to multidimensional aspects of taste function, highlighting the role that animal models played in these discoveries. We also summarize psychophysical approaches to study bitter taste in both animal models and pediatric populations, and we compare and contrast bitter taste perception in adults and children and identify gaps in knowledge. When appropriate, we reference review articles to direct the reader to the wider literature.

      Overview of Bitter Taste

      Taste is one of the senses through which humans and other animals perceive their environment. One of the primary taste qualities is bitter, a sensation that arises when specific chemicals are detected by specialized receptors in the tongue, as well as other parts of the oral cavity (eg, throat). In developing effective strategies for reducing the bitter taste of medications, it is important to consider the basic functional architecture of the gustatory system (illustrated schematically in Figure 1). We summarize the state of knowledge about bitter taste, from peripheral receptors to the brain, and link this system with perception.
      Figure thumbnail gr1
      Figure 1How bitter works: the process of bitter perception. The generation of bitter taste starts when a bitter compound enters the oral cavity, where the ligand binds to a T2R G protein–coupled receptor (TAS2R) expressed in the apical membrane of receptor cells found in taste buds, triggering a cascade of signaling events, leading to the release of neurotransmitter that activates an afferent nerve fiber that transmits the signal via the cranial nerve to the brain. Taste buds are distributed in distinct fields in the oral, pharyngeal, and laryngeal epithelia, with each field innervated by a different cranial nerve branch. Only the taste buds on the tongue are depicted in the figure. The taste buds of the laryngeal epithelium are thought to be involved more with protection of the airways. Taste receptors have also been identified in a variety of nongustatory tissues, such as the gut, where they have been proposed to play a role in nutrient and toxin sensing. The taste signals course through the brain and provide input to circuits that subserve various functions, such as oromotor and physiological reflexes, discriminative perception, and affective processing. The figure illustrates the complexity of the mechanisms intervening between the application of the bitter stimulus and the generation of the behavioral response, providing a variety of potential targets for strategies to modulate the bitterness of medications. DAG = Diacylglycerol; Gαgus = G-protein subunit α-gusducin; Gβγ = G-protein subunits β and γ; IP3R = inositoltriphophate receptor; PIP2 = phosphatidylinositol 4,5-biphosphate; PLCβ2 = phospholipase C β2; TrpM5 = transient receptor potential ion channel subfamily M member 5; VPMPC = ventral posterior medial nucleus, parvicellular subdivision. *The insula/operculum is actually lateral to the sagittal plane of section shown.

      Neurobiology of Bitter Taste

      Peripheral and Central Anatomy of the Gustatory System

      The principal sensory organ of gustation is the taste bud, a collection of ∼50–100 specialized epithelial cells, some of which serve the role of receptors. Receptor proteins are expressed on the apical membranes of microvilli, which protrude into a pore in the epithelium, where they have access to the oral environment (see area labeled “Taste Bud” in Figure 1). Thus, stimuli must be in solution to adequately reach and stimulate the receptor cells.
      The taste buds are bathed in excretions from the sublingual, submaxillary, and parotid salivary glands, as well as from numerous minor salivary glands throughout the oral epithelium. Although there is sufficient evidence that saliva plays a significant role in taste receptor activation by orally applied chemical compounds, its contribution has not been extensively studied. Proline-rich proteins found in saliva can bind with bitter-tasting tannins found in some foods, increasing their acceptability.
      • Glendinning J.I.
      Effect of salivary proline-rich proteins on ingestive responses to tannic acid in mice.
      Proline-rich proteins arise from gene clusters that are interleafed with bitter receptor genes, hinting at a common regulatory mechanism and function.
      • Cabras T.
      • Melis M.
      • Castagnola M.
      • et al.
      Responsiveness to 6-n-propylthiouracil (PROP) is associated with salivary levels of two specific basic proline-rich proteins in humans.
      A better understanding of the function of saliva in taste receptor processes may help us curtail the bitterness of medicines.
      Taste buds are distributed in distinct fields in the oral cavity (see “Oral” area in Figure 1).
      • Miller Jr, I.J.
      Gustatory receptors of the palate.
      • Miller Jr, I.J.
      Anatomy of the peripheral gustatory system.
      In the anterior tongue, taste buds are housed in specialized protrusions called fungiform papillae. In the posterior tongue, the taste buds are found in a series of trenchlike structures in the lateral margins, referred to as the foliate papillae, and in moatlike structures in the dorsal surface, referred to as the circumvallate papillae. Extralingual taste buds are also found on the soft palate and in the laryngeal epithelium. Each field is selectively innervated by a specific branch of the seventh, ninth, or tenth cranial nerve, all of which project to the rostral nucleus of the solitary tract (NTS) in the medulla, where they terminate in a roughly overlapping orotopic fashion.
      • Hamilton R.B.
      • Norgren R.
      Central projections of gustatory nerves in the rat.
      • May O.L.
      • Hill D.L.
      Gustatory terminal field organization and developmental plasticity in the nucleus of the solitary tract revealed through triple-fluorescence labeling.
      • Corson J.
      • Aldridge A.
      • Wilmoth K.
      • Erisir A.
      A survey of oral cavity afferents to the rat nucleus tractus solitarii.
      Interestingly, the pathway of gustatory signals through the brain varies somewhat across the mammalian species examined.
      • Lundy R.F.
      • Norgren R.
      Gustatory system.
      For example, in rodents and lagomorphs (eg, rabbits and pikas), taste-responsive neurons in the NTS, in addition to contributing to local hindbrain circuits involved with oromotor and autonomic function,
      • Zaidi F.N.
      • Todd K.
      • Enquist L.
      • Whitehead M.C.
      Types of taste circuits synaptically linked to a few geniculate ganglion neurons.
      • Travers S.P.
      • Travers J.B.
      Reflex topography in the nucleus of the solitary tract.
      • Norgren R.
      Taste and the autonomic nervous system.
      project to the parabrachial nucleus (PBN). The projections from PBN neurons bifurcate, with 1 set terminating in ventral forebrain structures associated with homeostatic functions and affective processes and the other in the parvicellular subdivision of the ventral posterior medial nucleus of the thalamus, from which neurons send their axons to terminate in the insular cortex’s gustatory zone. In primates, the projections of the taste neurons of the NTS bypass the PBN and terminate in the thalamus, whose cells project directly to taste cortex (see Figure 1, “Central Nervous System”).
      • Pritchard T.C.
      • Norgren R.
      Gustatory system.
      Thus, the ventral forebrain in primates receives its taste input from cortical structures. Many of these pathways are reciprocal, setting the stage for significant feedback to modulate the signals.
      Regardless of the different anatomic paths of taste signals through brain in various mammalian orders, the significance of which remains to be understood, in all cases, taste signals can be modulated not only in the periphery but also anywhere along the central gustatory pathway. For example, the adage “a spoonful of sugar helps the medicine go down” receives some support from evidence that sucrose can indeed decrease the perceived intensity of quinine, a phenomenon referred to as mixture suppression.
      • Lawless H.T.
      Evidence for neural inhibition in bittersweet taste mixtures.
      • Kroeze J.H.
      • Bartoshuk L.M.
      Bitterness suppression as revealed by split-tongue taste stimulation in humans.
      Although some mixture suppression effects likely have a peripheral origin,
      • Kroeze J.H.
      • Bartoshuk L.M.
      Bitterness suppression as revealed by split-tongue taste stimulation in humans.
      • Formaker B.K.
      • MacKinnon B.I.
      • Hettinger T.P.
      • Frank M.E.
      Opponent effects of quinine and sucrose on single fiber taste responses of the chorda tympani nerve.
      there are central contributions as well.
      • Lawless H.T.
      Evidence for neural inhibition in bittersweet taste mixtures.
      • Kroeze J.H.
      • Bartoshuk L.M.
      Bitterness suppression as revealed by split-tongue taste stimulation in humans.
      For example, if the sucrose solution is applied to one side of the tongue and the quinine solution to the other, the perceived intensity of the quinine is attenuated despite the stimulation of independent lingual receptor fields.
      • Kroeze J.H.
      • Bartoshuk L.M.
      Bitterness suppression as revealed by split-tongue taste stimulation in humans.
      Further, anesthetic block of the nerve that innervates the front of the tongue increases perceived bitterness of quinine applied to the back of the tongue, presumably preventing inhibition arising from anterior lingual taste signals.
      • Formaker B.K.
      • MacKinnon B.I.
      • Hettinger T.P.
      • Frank M.E.
      Opponent effects of quinine and sucrose on single fiber taste responses of the chorda tympani nerve.

      Taste Receptor Mechanisms and the T2R Family

      There are 2 general classes of taste receptor mechanisms: the G protein–coupled receptors (GPCRs), involved in mediating sweet, bitter, and umami taste, and the ion channel receptors, implicated in salt and sour taste.
      • Lehman C.D.
      • Bartoshuk L.M.
      • Catalanotto F.C.
      • et al.
      Effect of anesthesia of the chorda tympani nerve on taste perception in humans.
      • Kinnamon S.C.
      Taste receptor signalling—from tongues to lungs.
      • Niki M.
      • Jyotaki M.
      • Yoshida R.
      • Ninomiya Y.
      Reciprocal modulation of sweet taste by leptin and endocannabinoids.
      • Bigiani A.
      • Ghiaroni V.
      • Fieni F.
      Channels as taste receptors in vertebrates.
      • Chandrashekar J.
      • Hoon M.A.
      • Ryba N.J.
      • Zuker C.S.
      The receptors and cells for mammalian taste.
      The activities of some of these receptors and/or their downstream transduction intermediaries are thermally sensitive,
      • Ninomiya Y.
      • Fukami Y.
      • Yamazaki K.
      • Beauchamp G.K.
      Amiloride inhibition of chorda tympani responses to NaCl and its temperature dependency in mice.
      • Ohkuri T.
      • Yasumatsu K.
      • Horio N.
      • et al.
      Multiple sweet receptors and transduction pathways revealed in knockout mice by temperature dependence and gurmarin sensitivity.
      • Talavera K.
      • Yasumatsu K.
      • Voets T.
      • et al.
      Heat activation of TRPM5 underlies thermal sensitivity of sweet taste.
      making temperature a candidate strategy for modulating the taste of medicine.
      All of these receptor proteins are expressed in a variety of tissues in the body.
      • Kokrashvili Z.
      • Mosinger B.
      • Margolskee R.F.
      Taste signaling elements expressed in gut enteroendocrine cells regulate nutrient-responsive secretion of gut hormones.
      • Behrens M.
      • Meyerhof W.
      Gustatory and extragustatory functions of mammalian taste receptors.
      • Finger T.E.
      • Kinnamon S.C.
      Taste isn't just for taste buds anymore.
      • Rozengurt E.
      • Sternini C.
      Taste receptor signaling in the mammalian gut.
      • Kellenberger S.
      • Schild L.
      Epithelial sodium channel/degenerin family of ion channels: a variety of functions for a shared structure.
      For example, the GPCRs that serve as taste receptors are also found in the gut.
      • Kokrashvili Z.
      • Mosinger B.
      • Margolskee R.F.
      Taste signaling elements expressed in gut enteroendocrine cells regulate nutrient-responsive secretion of gut hormones.
      • Behrens M.
      • Meyerhof W.
      Gustatory and extragustatory functions of mammalian taste receptors.
      • Rozengurt E.
      • Sternini C.
      Taste receptor signaling in the mammalian gut.
      This has led to the term “gut taste,” which is more of a metaphor than a reality: as described below, “taste” results not from the receptors themselves but from the downstream neural consequences of the activation of these receptors. Bitter receptors are also expressed in the ciliated cells of the sinonasal epithelium and can trigger immune responses when stimulated with chemical signals from bacteria.
      • Lee R.J.
      • Xiong G.
      • Kofonow J.M.
      • et al.
      T2R38 taste receptor polymorphisms underlie susceptibility to upper respiratory infection.
      The GPCRs share certain transduction intermediaries in taste receptor cells, such as α-gustducin, phospholipase Cβ2 (PLCβ2), and TrpM5, which ultimately lead to release of the neurotransmitter (see the “Cell and Molecular” area in Figure 1).
      • Zhang Y.
      • Hoon M.A.
      • Chandrashekar J.
      • et al.
      Coding of sweet, bitter, and umami tastes: different receptor cells sharing similar signaling pathways.
      In some cells, the G-protein subunit α-gustducin helps mediate responses to both bitter- and sweet-tasting ligands.
      • McLaughlin S.K.
      • McKinnon P.J.
      • Margolskee R.F.
      Gustducin is a taste-cell-specific G protein closely related to the transducins.
      • Wong G.T.
      • Gannon K.S.
      • Margolskee R.F.
      Transduction of bitter and sweet taste by gustducin.
      Because GPCR transduction signaling components are shared by both bitter- and sweet-tasting ligands, they may not be selective targets for decreasing the bitterness of medications. However, although to our knowledge it is untested, the deactivation of these signaling components on a temporary basis could prove useful because even if sweetness is potentially attenuated, the decrease in bitterness could lead to an overall increase in the acceptability of the medicine.
      The T2R family of taste receptors was discovered a little more than a decade ago.
      • Adler E.
      • Hoon M.A.
      • Mueller K.L.
      • et al.
      A novel family of mammalian taste receptors.
      • Chandrashekar J.
      • Mueller K.L.
      • Hoon M.A.
      • et al.
      T2Rs function as bitter taste receptors.
      It consists of ∼25 GPCRs that serve as the principal receptors for mediating bitter taste. Although many of the receptors remain to be de-orphaned (ie, determine which ligands activate them), most T2Rs studied have binding profiles that involve several different bitter-tasting ligands.
      • Meyerhof W.
      • Batram C.
      • Kuhn C.
      • et al.
      The molecular receptive ranges of human TAS2R bitter taste receptors.
      • Behrens M.
      • Meyerhof W.
      Mammalian bitter taste perception.
      Likewise, a given bitter-tasting ligand can activate >1 T2R.
      • Meyerhof W.
      • Batram C.
      • Kuhn C.
      • et al.
      The molecular receptive ranges of human TAS2R bitter taste receptors.
      • Behrens M.
      • Meyerhof W.
      Mammalian bitter taste perception.
      As might be expected, there are some genetic variants in the receptors within and across species.
      • Behrens M.
      • Meyerhof W.
      Mammalian bitter taste perception.
      For example, a subset of the population, classified as “nontasters,” cannot detect the presence of the compounds propylthiouracil (PROP) and phenylthiocarbamide (PTC) at moderate concentrations that all others, referred to as “tasters,” find exceptionally bitter.
      • Fox A.L.
      The relationship between chemical constitution and taste.
      The nontaster phenotype is due to a haplotype involving polymorphisms at 3 amino acid positions in the hT2R38 protein, which is known to bind with these compounds.
      • Kim U.
      • Jorgenson E.
      • Coon H.
      • et al.
      Positional cloning of the human quantitative trait locus underlying taste sensitivity to phenylthiocarbamide.
      Likewise, genetic variants within another cluster of bitter receptor genes affect the ability to perceive the bitterness of quinine,
      • Reed D.R.
      • Zhu G.
      • Breslin P.A.
      • et al.
      The perception of quinine taste intensity is associated with common genetic variants in a bitter receptor cluster on chromosome 12.
      a bitter chemical used in the past to treat malaria. Thus, variation in the compliance of children to ingest particular liquid medications could be attributable to potential polymorphisms in these or other T2Rs that have yet to be revealed.
      Developing research indicates that receptors for stimuli generating different taste qualities are not coexpressed in taste bud cells.
      • Kinnamon S.C.
      Taste receptor signalling—from tongues to lungs.
      • Niki M.
      • Jyotaki M.
      • Yoshida R.
      • Ninomiya Y.
      Reciprocal modulation of sweet taste by leptin and endocannabinoids.
      • Chandrashekar J.
      • Hoon M.A.
      • Ryba N.J.
      • Zuker C.S.
      The receptors and cells for mammalian taste.
      • Adler E.
      • Hoon M.A.
      • Mueller K.L.
      • et al.
      A novel family of mammalian taste receptors.
      • Nelson G.
      • Hoon M.A.
      • Chandrashekar J.
      • et al.
      Mammalian sweet taste receptors.
      Thus, if a taste bud cell expresses the T1R2+T1R3 receptor responsible for mediating sweet taste, it will not express any of the T2Rs that serve as the receptors for bitter-tasting ligands. Although rodent studies first indicated that virtually all T2Rs were coexpressed on taste receptor cells responsive to bitter ligands,
      • Adler E.
      • Hoon M.A.
      • Mueller K.L.
      • et al.
      A novel family of mammalian taste receptors.
      • Chandrashekar J.
      • Mueller K.L.
      • Hoon M.A.
      • et al.
      T2Rs function as bitter taste receptors.
      later human studies revealed that most T2R-expressing cells possess only a subset of the T2R members.
      • Behrens M.
      • Meyerhof W.
      Mammalian bitter taste perception.
      Nevertheless, a consistent systematic pattern of this expression has not been identified. This lack of coexpression sets the stage for the flow of taste information that gives rise to different qualitative taste perceptions, although there is plenty of opportunity for convergence in the transmission of the signals through the brain.

      Neural Response Profiles to T2R Ligands

      In addition to overlap among ligands for receptors and receptors for ligands,
      • Meyerhof W.
      • Batram C.
      • Kuhn C.
      • et al.
      The molecular receptive ranges of human TAS2R bitter taste receptors.
      • Behrens M.
      • Meyerhof W.
      Mammalian bitter taste perception.
      there is overlapping expression of the T2R members in the subset of taste bud cells responsive to bitter compounds. Surprisingly, however, imaging experiments of intracellular changes in calcium concentration in rat taste bud cells in situ in response to bitter stimuli indicate much narrower tuning properties: of the 374 cells tested, 69 responded to at least 1 of the 5 bitter ligands in the test panel, and of these, 45 cells responded to only 1 and 18 responded to only 2.
      • Caicedo A.
      • Roper S.D.
      Taste receptor cells that discriminate between bitter stimuli.
      Because single axons from a taste nerve branch are close to the tongue and innervate >1 taste cell, any selectivity present in taste receptor cells could be lost by early convergence in the system. The extent to which this occurs at the ganglion cell level remains understudied. Most previous studies used only quinine hydrochloride as the bitter stimulus rather than a diverse set of bitter ligands. Moreover, the vast majority of peripheral and central electrophysiological results in the literature are based on anterior tongue stimulation, reflecting the contribution of only ∼15% of the total taste bud population and circumventing the taste receptor field of the posterior tongue, which has the densest expression of T2Rs.
      • Adler E.
      • Hoon M.A.
      • Mueller K.L.
      • et al.
      A novel family of mammalian taste receptors.
      This is due, in part, to the difficulty in effectively perfusing the foliate and circumvallate trenches in the posterior tongue with stimulus solutions in an anesthetized preparation.
      Despite these difficulties, 2 studies stand out in this regard. Frank
      • Frank M.E.
      Taste-responsive neurons of the glossopharyngeal nerve of the rat.
      published the first comprehensive set of findings detailing the response properties of single fibers in the glossopharyngeal nerve. She inserted a pipette into the circumvallate papilla of the rat (rodent tongues have only 1 circumvallate papilla vs ~10 in the human tongue) and tested salts, acids, sugars, and quinine. Although in her previous studies quinine-responsive single fibers in the chorda tympani nerve innervating the front of the tongue responded best to acids and other electrolytes,
      • Frank M.E.
      • Contreras R.J.
      • Hettinger T.P.
      Nerve fibers sensitive to ionic taste stimuli in chorda tympani of the rat.
      in this study, a set of fibers was identified that responded selectively to quinine and not to the other stimuli. This indicates a segregation of quinine-evoked signals from those of other taste qualities, consistent with the so-called labeled-line model of neural coding, in which activity in a given class of neurons is necessary and sufficient for generating a specific taste quality.
      • Spector A.C.
      • Travers S.P.
      The representation of taste quality in the mammalian nervous system.
      In the other study, Dahl et al
      • Dahl M.
      • Erickson R.P.
      • Simon S.A.
      Neural responses to bitter compounds in rats.
      recorded single-fiber responses in the chorda tympani nerve (anterior tongue) and the glossopharyngeal nerve (posterior tongue) to a panel of bitter-tasting ligands. Not all ligands stimulated the same fibers, suggesting that signals may be present in the overall peripheral input that permits some discriminability among these particular bitter compounds. This has been taken as evidence of an ensemble or across-neuron model of neural coding.
      Once the signals from the peripheral nerves reach the brain, there is opportunity for further anatomic convergence. From a functional standpoint, the pattern of this convergence is key in terms of how the nervous system represents information about chemical compounds contacting the oral epithelium. Indeed, there is evidence that the breadth of tuning of taste-responsive neurons increases in the brain. However, some narrowly tuned neurons are still present in the population, and it is unclear to which taste function a given neuron contributes. Thus, some neurons might be responsive to the affective valence of the stimulus, whereas others might code for taste quality and contribute to stimulus identification.
      • Spector A.C.
      Linking gustatory neurobiology to behavior in vertebrates.
      The literature on responses of central taste neurons to bitter-tasting stimuli in rodent models is mixed. For many years the bitter stimulus quinine was included in many electrophysiologic studies of neuronal taste responses in several central gustatory structures, but despite its potent behavioral effects, neuronal responses were weak or nonexistent—possibly because many studies did not stimulate the posterior tongue. In recent years, however, some more robust responses of neurons in the gustatory zone of the NTS and PBN to a variety of bitter-tasting ligands have been revealed.
      • Geran L.C.
      • Travers S.P.
      Single neurons in the nucleus of the solitary tract respond selectively to bitter taste stimuli.
      • Geran L.C.
      • Travers S.P.
      Bitter-responsive gustatory neurons in the rat parabrachial nucleus.
      • Lemon C.H.
      • Smith D.V.
      Neural representation of bitter taste in the nucleus of the solitary tract.
      Indeed, a class of neurons has been identified that responds best to bitter compounds and little to compounds associated with other taste qualities.
      • Geran L.C.
      • Travers S.P.
      Single neurons in the nucleus of the solitary tract respond selectively to bitter taste stimuli.
      • Geran L.C.
      • Travers S.P.
      Bitter-responsive gustatory neurons in the rat parabrachial nucleus.
      Within this class, however, not all bitter compounds are equally effective stimuli for a given neuron. This may be due to idiosyncratic upstream connections originating from the specific T2Rs expressed in the taste receptor cells, or it may represent a fundamental distinction in organizing inputs from subclasses of bitter-tasting ligands. Some of the ionic bitter compounds can also stimulate neurons that respond best to acids and electrolytes, adding another layer of complexity to the unraveling of the neural representation of bitter taste.
      • Geran L.C.
      • Travers S.P.
      Single neurons in the nucleus of the solitary tract respond selectively to bitter taste stimuli.
      • Geran L.C.
      • Travers S.P.
      Bitter-responsive gustatory neurons in the rat parabrachial nucleus.
      In a set of recent results using a 2-photon imaging protocol to measure cellular calcium responses, anatomically distinct clusters of neurons were found in the insular cortex of the mouse that appeared to respond selectively to taste compounds associated with specific basic taste qualities, including a group that responded only to bitter compounds.
      • Chen X.
      • Gabitto M.
      • Peng Y.
      • et al.
      A gustotopic map of taste qualities in the mammalian brain.
      The disparity between these findings and the lack of evidence of explicit chemotopy from electrophysiological studies of central neuronal taste responses
      • Spector A.C.
      • Travers S.P.
      The representation of taste quality in the mammalian nervous system.
      has yet to be resolved. However, results from an earlier study using a less spatially precise optical imaging technique provide support for some degree of a spatial mapping of taste quality in this cortical region.
      • Accolla R.
      • Bathellier B.
      • Petersen C.C.
      • Carleton A.
      Differential spatial representation of taste modalities in the rat gustatory cortex.
      From all that we now know about bitter perception and its multiple receptors, it is not surprising that the bitter taste of oral pharmaceuticals is an ongoing formulation problem. The mechanics of bitter taste signaling suggest that it should be amenable to the methods of pharmacology.
      • Palmer R.K.
      The pharmacology and signaling of bitter, sweet, and umami taste sensing.
      However, the large number of bitter-tasting compounds and receptors makes blocking bitterness at the receptor level difficult because medicines may have multiple bitter compounds that stimulate multiple receptors, and each receptor may require its own antagonist. As mentioned above, the blockade of second-messenger signaling poses problems because several components of the bitter-taste transduction pathway are shared with those mediating sweet taste, and attenuation of both bitterness and sweetness may pose practical problems because sweeteners are a commonly used agents to reduce perceived bitterness. Nonetheless, temporary nonselective blockade of these taste transduction pathways could lead to an overall increase in the acceptability of the medicine.

      Linking the Neurobiology of Bitter Taste to Perception

      The previous discussion provides a cursory description of the “hardware” of the gustatory system, with a focus on neural mechanisms underlying bitter taste. Most of what we have learned about the molecular aspects of bitter taste transduction has been from experimental animal models, mostly rodents.
      However, without data defining the psychophysical properties of various taste compounds and their mixtures, we cannot link the underlying neurobiology with perception. In this regard, animal models are particularly useful because effects of very selective manipulations of the gustatory system can be studied in a highly systematic and quantitative way in a wide variety of tissues, including the nervous system, as well as on taste-related behavior. In such efforts, however, it is important to be mindful of several interpretive guidelines.
      • Spector A.C.
      Linking gustatory neurobiology to behavior in vertebrates.
      First, when most people talk about “taste,” they are actually referring to flavor. Flavor can be considered the perceptual integration of signals from the gustatory, olfactory, and trigeminal systems.
      • Rozin P.
      “Taste-smell confusions” and the duality of the olfactory sense.
      To the specialist, however, taste refers to the behavioral and physiologic consequences of stimulating taste receptor cells in the oral cavity. Accordingly, the potential for taste stimuli to activate nongustatory sensory systems, including those of a visceroceptive nature in the cases where the taste solutions are swallowed, must be considered.
      Second, perception cannot be measured directly; it must be inferred from behavior. The veracity of that inference depends heavily on the procedure used to measure the behavior, whether studying animals or humans.
      Third, taste function is multidimensional. The sensory/discriminative dimension encompasses stimulus identification, including the basic taste qualities of sweetness, sourness, saltiness, bitterness, and umami. The affective dimension involves the hedonic evaluation of taste stimuli, ultimately promoting or discouraging ingestion, which is perhaps most relevant to addressing the unpalatable nature of bitter medicines in children. Physiologic reflexes are also triggered by taste stimuli, such as salivation triggered by the oral sampling of a lemon. Thus, behavioral outcomes from a given gustatory manipulation need to be interpreted in light of the domain(s) being assessed.
      Finally, a neuron’s response to an orally applied chemical stimulus does not, in and of itself, reveal the functional domain(s) to which the cell contributes. In this sense, behavioral observations are indispensable in understanding the neurobiological mechanisms underlying taste function.

      Behavioral Assays in Animal Models

      Although behavioral procedures involving nonhuman subjects are time-consuming and resource intensive, their value is indisputable because they link the neurobiology of the gustatory system to behavior and, by inference, perception in the same animal model. Rodents are particularly useful animal models for studying taste perception; they are commensal with humans and thus have a similar sense of taste. Animal models also share other similarities with very young children; for both populations, behavior rather than language communicates important information about their sense of taste. The behavioral outcomes from animal models can then be compared with psychophysical results from similar experiments conducted with human subjects, providing a potential bridge between the animal neurobiological data and human taste perception.
      The most common behavioral procedure for assessing taste function in animal models (and in young infants, as described below) remains the 2-bottle preference test in which the animal is simultaneously presented with 2 liquid stimuli (eg, sucrose solution vs water) for a specified duration. Although these tests provide a reasonable first approximation of an animal’s taste responsiveness to a given compound and have the virtue of simplicity, their interpretation is limited because intake and choice can be influenced by nongustatory contributions, most notably, those arising from postingestive events (eg, satiety, nausea). Over the past several decades, however, a variety of behavioral procedures has been developed that assess taste function more selectively.
      • Spector A.C.
      Psychophysical evaluation of taste function in non-human mammals.
      These procedures could have great utility in testing various strategies for screening drugs in their early stages of development or for modulating the bitter taste of a drug based on more fundamental physiological or molecular research. In this section, we briefly summarize each of these methodologies in animal models.

      Brief-Access Test

      The brief-access taste test is an effective way to circumvent the limitations of intake tests by presenting small volumes of taste samples and measuring immediate behavioral responses. Generally, various concentrations of a given taste compound are presented in very brief trials, on the order of seconds, and licking responses are measured with the help of specialized testing devices.
      • Davis J.D.
      The effectiveness of some sugars in stimulating licking behavior in the rat.
      • O'Keefe G.B.
      • Schumm J.
      • Smith J.C.
      Loss of sensitivity to low concentrations of NaCl following bilateral chorda tympani nerve sections in rats.
      • St John S.J.
      • Garcea M.
      • Spector A.C.
      Combined, but not single, gustatory nerve transection substantially alters taste-guided licking behavior to quinine in rats.
      • Glendinning J.I.
      • Gresack J.
      • Spector A.C.
      A high-throughput screening procedure for identifying mice with aberrant taste and oromotor function.
      • Treesukosol Y.
      • Smith K.R.
      • Spector A.C.
      Behavioral evidence for a glucose polymer taste receptor that is independent of the T1R2+3 heterodimer in a mouse model.
      • Spector A.C.
      Gustatory parabrachial lesions disrupt taste-guided quinine responsiveness in rats.
      • Young P.T.
      • Trafton C.L.
      Activity contour maps as related to preference in four gustatory stimulus areas of the rat.
      • Smith J.C.
      The history of the “Davis Rig.”.
      • Smith J.C.
      • Davis J.D.
      • O'Keefe G.B.
      Lack of an order effect in brief contact taste tests with closely spaced test trials.
      This procedure is most commonly used to test rats and mice. With normally preferred taste stimuli, such as sucrose, the animals can be tested in either a nondeprived or a food-deprived state, and a monotonic increase in licking as a function of concentration is generally observed. With aversive stimuli, such as bitter-tasting ligands, animals are tested in a water-deprived state, and a monotonic decrease in licking as a function of concentration generally occurs.
      These responses are sensitive to gustatory manipulations. For example, mice in which the gene encoding the GPCR taste transduction intermediaries PLCβ2 and TRPM5 have been knocked out display relatively flat concentration-response curves to sweet and bitter stimuli compared with wild-type controls.
      • Zhang Y.
      • Hoon M.A.
      • Chandrashekar J.
      • et al.
      Coding of sweet, bitter, and umami tastes: different receptor cells sharing similar signaling pathways.
      • Damak S.
      • Rong M.
      • Yasumatsu K.
      • et al.
      Trpm5 null mice respond to bitter, sweet, and umami compounds.
      • Dotson C.D.
      • Roper S.D.
      • Spector A.C.
      PLCbeta2-independent behavioral avoidance of prototypical bitter-tasting ligands.
      Interestingly, the knockout mice still display some licking avoidance of very high concentrations of certain bitter-tasting compounds, such as quinine or denatonium, suggesting an alternative high-threshold taste transduction pathway(s) for these ligands that is independent of PLCβ2 and TRPM5.
      • Damak S.
      • Rong M.
      • Yasumatsu K.
      • et al.
      Trpm5 null mice respond to bitter, sweet, and umami compounds.
      • Dotson C.D.
      • Roper S.D.
      • Spector A.C.
      PLCbeta2-independent behavioral avoidance of prototypical bitter-tasting ligands.
      Although the brief-access test does not assess taste quality perception per se (eg, NaCl, citric acid, and quinine all produce decreasing licking functions), it is an effective measure of an animal’s affective responsiveness to a taste stimulus and has great potential utility for testing masking agents and other strategies to attenuate the aversiveness of medicines.

      Taste Reactivity

      Many animals, including rodents and humans, display reflex-like oromotor responses to taste stimuli,
      • Berridge K.C.
      Measuring hedonic impact in animals and infants: microstructure of affective taste reactivity patterns.
      which has been termed taste reactivity.
      • Grill H.J.
      • Norgren R.
      The taste reactivity test. I. Mimetic responses to gustatory stimuli in neurologically normal rats.
      • Spector A.C.
      • Breslin P.
      • Grill H.J.
      Taste reactivity as a dependent measure of the rapid formation of conditioned taste aversion: a tool for the neural analysis of taste-visceral associations.
      • Berridge K.C.
      Food reward: brain substrates of wanting and liking.
      • Grill H.J.
      • Spector A.C.
      • Schwartz G.J.
      • et al.
      Evaluating taste effects on ingestive behavior.
      This has been best studied in rats in which taste solutions are delivered directly into the oral cavity through surgically implanted cannulas. Normally preferred taste stimuli, such as sugars, elicit tongue and mouth movements directly proportional to the concentration of the solution. These are collectively referred to as ingestive behaviors. Normally avoided taste stimuli, such as quinine, elicit gapes, chin rubs, forelimb flails, and head shakes directly proportional to the concentration of the solution, and all of these are generally accompanied by active fluid ejection. These are collectively referred to as aversive behaviors. Transection of the glossopharyngeal nerve, which innervates the taste buds of the posterior tongue where T2Rs are densely expressed, virtually eliminates the characteristic aversive oromotor responses to intraorally delivered highly concentrated quinine solutions,
      • Travers J.B.
      • Grill H.J.
      • Norgren R.
      The effects of glossopharyngeal and chorda tympani nerve cuts on the ingestion and rejection of sapid stimuli: an electromyographic analysis in the rat.
      • Grill H.J.
      • Schwartz G.J.
      • Travers J.B.
      The contribution of gustatory nerve input to oral motor behavior and intake-based preference. I. Effects of chorda tympani or glossopharyngeal nerve section in the rat.
      • King C.T.
      • Garcea M.
      • Spector A.C.
      Glossopharyngeal nerve regeneration is essential for the complete recovery of quinine-stimulated oromotor rejection behaviors and central patterns of neuronal activity in the nucleus of the solitary tract in the rat.
      which return when the nerve regenerates.
      • King C.T.
      • Garcea M.
      • Spector A.C.
      Glossopharyngeal nerve regeneration is essential for the complete recovery of quinine-stimulated oromotor rejection behaviors and central patterns of neuronal activity in the nucleus of the solitary tract in the rat.
      Although bitter taste stimuli are often aversive, not all aversive tastes are bitter. Accordingly, taste reactivity does not assess taste quality but rather provides information about the acceptability of various taste stimuli. Nonetheless, these procedures could contribute significantly to developing ways to increase medication palatability.

      Conditioned Taste Generalization and Discrimination

      The procedures discussed below can more selectively assess taste quality independent of the inherent hedonic characteristics of the stimulus, by establishing a taste stimulus as a conditioned signal. For example, with the conditioned taste aversion procedure, commonly used in rodents
      • Nowlis G.H.
      • Frank M.E.
      • Pfaffmann C.
      Specificity of acquired aversions to taste qualities in hamsters and rats.
      • Tapper D.N.
      • Halpern B.P.
      Taste stimuli: a behavioral categorization.
      the ingestion of a specific taste stimulus is paired with administration of an agent that produces temporary visceral malaise (presumably nausea). On subsequent occasions, the animal will avoid ingesting the conditioned stimulus and others that have a similar qualitative taste, a phenomenon called generalization. When the test array includes sucrose, quinine, NaCl, and citric acid, inferences can be made about how sweet, bitter, salty, and sour the conditioned stimulus is. Although intake compared with nonconditioned control animals is often the primary dependent measure, brief-access tests and taste reactivity measures can also be used.
      One limitation of the use of this paradigm to assess qualitative characteristics of naturally aversive taste stimuli is that they are already unconditionally avoided. However, operant conditioning procedures can circumvent this shortcoming. In these procedures, a small volume of a taste compound serves as a cue in the presence of which a specific response is rewarded or punished. For example, using a specially designed gustometer, Grobe and Spector
      • Grobe C.L.
      • Spector A.C.
      Constructing quality profiles for taste compounds in rats: a novel paradigm.
      trained 1 group of thirsty rats to lick a specific drinking spout after sampling sucrose (the standard stimulus) and a different spout after sampling quinine, citric acid, or NaCl (the comparison stimuli). If the rats responded correctly, they were rewarded with water; if not, they were punished with a time out. Three other groups were trained with quinine, citric acid, and NaCl, respectively, as the standard stimulus and the remaining compounds as the comparison stimuli. Concentrations of all stimuli were varied, rendering intensity cues irrelevant. After all 4 groups learned the task, a test compound was randomly interjected during the taste trials. By observing which spout each animal went to after sampling the test stimulus, the experimenters were able to infer taste quality of the sample using the response profiles across all 4 groups: sweetness (sucrose standard group), bitterness (quinine standard group), saltiness (NaCl standard group), and sourness (citric acid standard group).
      A similar procedure can test how well rats and mice can discriminate between 2 specific compounds. There has been some debate about whether animals can discriminate among bitter-tasting compounds. As noted above, the coexpression of T2Rs in taste receptor cells, as well as their somewhat broad tuning profiles, predicts poor discriminability, whereas the calcium responses of taste bud cells predict good discriminability. The response profiles for central taste neurons can be used to support either prediction. Spector and Kopka
      • Spector A.C.
      • Kopka S.L.
      Rats fail to discriminate quinine from denatonium: implications for the neural coding of bitter-tasting compounds.
      tested whether rats could discriminate between quinine and denatonium, for which calcium imaging suggested a high degree of discriminability. The procedure was similar to the one described above: rats were rewarded for licking 1 spout when quinine was delivered and for licking the other when denatonium was presented; incorrect responses were punished with a time out. These rats could not be trained, but did subsequently learn to discriminate quinine from KCl.
      A second group of naive rats were first trained to discriminate quinine from KCl; then denatonium was substituted for quinine, and performance remained unperturbed on the very first session, suggesting that denatonium and quinine share similar qualitative properties. To show that any stimulus substitution does not necessarily result in unaltered performance, Spector and Kopka substituted NaCl for denatonium. In this case, performance dropped to chance levels on the first session and then subsequently improved across sessions as the animals learned the new discrimination task. Finally, these same highly trained rats were tested on the quinine versus denatonium task, and their performance remained at chance over 15 test sessions. Accordingly, if rats can discriminate quinine from denatonium, it is likely very difficult, suggesting that the 2 compounds produce a unitary qualitative taste perception that one could perhaps call bitterness. Whether other bitter-tasting ligands can be discriminated from one another remains to be tested. On a more conceptual level, failure to discriminate is always more compelling than is success, provided learning and intensity effects can be ruled out, because it suggests that an identity relationship exists somewhere along the sensory neuraxis.

      Behavioral Assays in Children

      A major challenge in formulating pharmaceuticals for children’s palates is the identification of methods to assess the acceptance of the taste of the medicines, once they are approved, and to determine in the long term which methods yield data that predict compliance to medication regimens.
      • Chambers E.
      Commentary: conducting sensory research in children.
      When conducting research in children, several methodological issues need to be addressed.
      First, young children are more prone to attention lapses and have shorter memory spans compared with adults. Therefore, any method relying on sustained attention that places demands on memory could yield spurious findings. Second, because young children tend to answer questions in the affirmative, a forced-choice categorization procedure is generally preferred. Age-appropriate tasks embedded in the context of a game that are fun for children and minimize the impact of language and the stage of cognitive development are particularly effective. Third, before actual testing and after a period of acclimation, the experimenter should ascertain whether the child comprehends the task. Training tools are needed to determine whether a given child has the ability to do the task. Reproducibility of the measures over time should be built into the design of the study. All of these special features must be considered when developing sensory methods for children.
      A variety of psychophysical methodologies have been used to assess taste perception and preference throughout infancy, childhood, and adolescence.

      Forestell CA, Mennella JA. The ontogeny of taste perception and preference throughout childhood. In: Doty RL, ed. Handbook of Olfaction and Gustation. 3rd ed. Boca Raton, Fla: CRC Press; in press.

      The method chosen depends on the objective of the study, as well as the age (and, in turn, cognitive and language abilities) of the participants under study. These psychophysical studies on taste provide data relevant to 2 separate aspects of sensation: (1) the sensitivity of the system to chemical stimuli and (2) the hedonic valence, or pleasantness, of the sensation.
      • Cowart B.J.
      Development of taste perception in humans: sensitivity and preference throughout the life span.
      • Cowart B.J.
      • Beauchamp G.K.
      • Mennella J.A.
      Development of taste and smell in the neonate.
      The century-long legacy of experimental research in taste has revealed that, like the other senses (sounds,
      • Schneider B.A.
      • Trehub S.E.
      • Morrongiello B.A.
      • Thorpe L.A.
      Developmental changes in masked thresholds.
      smells,
      • Dorries K.M.
      • Schmidt H.J.
      • Beauchamp G.K.
      • Wysocki C.J.
      Changes in sensitivity to the odor of androstenone during adolescence.
      • Odeigah P.G.
      • Obieze A.C.
      Differences in sodium chloride taste sensitivity in a rural and an urban population in Nigeria: implications for the incidence of hypertension.
      and irritants
      • Patil S.
      • Maibach H.I.
      Effect of age and sex on the elicitation of irritant contact dermatitis.
      ), children live in different sensory worlds than do adults. These age-related differences are especially striking for taste. Within hours after birth, infants have been shown to prefer sweet and umami tastes
      • Desor J.
      • Maller O.
      • Turner R.
      Taste in acceptance of sugars by human infants.
      • Steiner J.
      Facial expressions of the neonate infant indicating the hedonics of food-related chemical stimuli.
      • Rosenstein D.
      • Oster H.
      Differential facial responses to four basic tastes in newborns.
      and to reject bitter-tasting liquids,
      • Steiner J.E.
      • Glaser D.
      • Hawilo M.E.
      • Berridge K.C.
      Comparative expression of hedonic impact: affective reactions to taste by human infants and other primates.
      although adult-like sensitivity to salt does not emerge until the infant is approximately 4 months of age.
      • Beauchamp G.K.
      • Cowart B.J.
      • Moran M.
      Developmental changes in salt acceptability in human infants.
      Their dietary likes and dislikes provide further evidence of their stronger liking for foods and beverages that taste sweet,
      • Beauchamp G.K.
      • Cowart B.J.
      Development of sweet taste.
      salty,
      • Beauchamp G.K.
      • Moran M.
      Acceptance of sweet and salty tastes in 2-year-old children.
      and, in some cases, sour
      • Liem D.G.
      • Mennella J.A.
      Sweet and sour preferences during childhood: role of early experiences.
      and their profound dislike of all that tastes bitter. Children’s heightened liking for sweets and salts, relative to adults, probably reflects the need for energy or minerals, respectively, during periods of maximal growth, because many foods rich in energy (eg, mother’s milk, fruits) taste sweet. Thus, it is not surprising that many pediatric formulations taste sweet.
      Table I provides an overview of some of the psychophysical tools used to study bitter taste in children (for a more thorough review that includes the other basic tastes, see Forestell and Mennella

      Forestell CA, Mennella JA. The ontogeny of taste perception and preference throughout childhood. In: Doty RL, ed. Handbook of Olfaction and Gustation. 3rd ed. Boca Raton, Fla: CRC Press; in press.

      ). For preverbal children, the tools often focus on reflex-like responses (eg, orofacial responses) or consummatory responses—many of the experimental paradigms for this age group are similar to those used in animal model studies,
      • Glendinning J.I.
      • Gresack J.
      • Spector A.C.
      A high-throughput screening procedure for identifying mice with aberrant taste and oromotor function.
      • Grill H.J.
      • Norgren R.
      The taste reactivity test. I. Mimetic responses to gustatory stimuli in neurologically normal rats.
      • Steiner J.E.
      • Glaser D.
      • Hawilo M.E.
      • Berridge K.C.
      Comparative expression of hedonic impact: affective reactions to taste by human infants and other primates.
      • Spector A.C.
      • Smith J.C.
      • Hollander G.R.
      A comparison of dependent measures used to quantify radiation-induced taste aversion.
      • Berridge K.C.
      • Kringelbach M.L.
      Affective neuroscience of pleasure: reward in humans and animals.
      • Boughter J.D.J.
      • Bachmanov A.A.
      Behavioral genetics and taste.
      as reviewed above. Because virtually all of these measures can be associated with acceptance or rejection, they presumably involve a hedonic component. At least for human infants, sensitivity and hedonics are difficult to distinguish.
      • Cowart B.J.
      • Beauchamp G.K.
      • Mennella J.A.
      Development of taste and smell in the neonate.
      For older children, the psychophysical tools are more complex, but very little research has established at what age children can reliably perform these tasks.
      Table IExamples of types of psychophysical tools used to assess bitter taste and medication palatability in pediatric populations.
      MethodAge of SubjectsMeasuresOutcome MeasuresKey References
      Facial reactivityAll ages, but facial reactivity as an indicator becomes less reliable with age
      • Fridlund A.J.
      • Gilbert A.N.
      • Izard C.E.
      • Burdett A.N.
      Emotions and facial expression.
      The number of affective reactions infants express to a taste stimulus is quantified as a measure of the valence and intensity of an affective reaction. Facial expressions are dissected into constituent action units (AUs) via slow-motion video analysis by trained ratersBitter solutions elicit upper and midface facial expressions of distaste (including cheek raises and gaping).Oster and Rosenstein,
      • Rosenstein D.
      • Oster H.
      Differential facial responses to four basic tastes in newborns.
      Mennella et al,
      • Mennella J.A.
      • Forestell C.A.
      • Morgan L.K.
      • Beauchamp G.K.
      Early milk feeding influences taste acceptance and liking during infancy.
      Forestell and Mennella
      • Forestell C.A.
      • Mennella J.A.
      More than just a pretty face. The relationship between infant's temperament, food acceptance, and mothers' perceptions of their enjoyment of food.
      Brief-access testsInfancy (<1 y)Infants are provided with brief access to ≥2 bottles in succession containing various bitter-tasting liquids or diluentDecreased intake to bitter tasteDesor et al,
      • Desor J.A.
      • Maller O.
      • Andrews K.
      Ingestive responses of human newborns to salty, sour, and bitter stimuli.
      Kajiura et al
      • Kajiura H.
      • Cowart B.J.
      • Beauchamp G.K.
      Early developmental change in bitter taste responses in human infants.
      Brief-access tests, multiple daysInfancy (<1 y)Infants are fed food on multiple occasions; the days differ in the taste of the food presentedDecreased intake to bitter-flavored relative to plain cerealMennella et al
      • Mennella J.A.
      • Forestell C.A.
      • Morgan L.K.
      • Beauchamp G.K.
      Early milk feeding influences taste acceptance and liking during infancy.
      Suckling responseInfancy (<1 y)Patterning of suckling response measured while infant is feeding tastant vs diluent solutions; transducer may be embedded in nipple of bottleRetardation of suckling to bitter tasteKajiura et al
      • Kajiura H.
      • Cowart B.J.
      • Beauchamp G.K.
      Early developmental change in bitter taste responses in human infants.
      Suprathreshold taste thresholdsChildren ≥3 y of ageSubjects tasted (but did not swallow), solutions of propylthiouracil (PROP) in ascending concentrations, rinsing with water before and after each tasting. Subjects are then classified into groups based on which concentration, if any, tasted bitterVariation in sensitivity to PROP related to TAS2R38 genotype and children’s food likesAnliker et al,
      • Anliker J.A.
      • Bartoshuk L.
      • Ferris A.M.
      • Hooks L.D.
      Children's food preferences and genetic sensitivity to the bitter taste of 6-n-propylthiouracil (PROP).
      Mennella et al
      • Mennella J.A.
      • Pepino M.Y.
      • Reed D.R.
      Genetic and environmental determinants of bitter perception and sweet preferences.
      Scaling methodsChildren ≥3 y of ageA variety of scaling methods are used during taste testing to evaluate children’s hedonic responses after tasting 1 solution at a time. Typically, the child is presented with a line or other type of scale that contains pictorial (eg, faces ranging from frowns to smiles) or verbal descriptors to evaluate stimuli in a graded orderChildren (4–11 y) with kidney disease rated the taste of 2 different pulverized calcium channel blockers on a 5-point hedonic face scale to determine most palatable drug.
      • Milani G.
      • Ragazzi M.
      • Simonetti G.D.
      • et al.
      Superior palatability of crushed lercanidipine compared with amlodipine among children.
      Children (4–8 y) rated oral suspension of 4 antibiotics using a 5-point hedonic face scale to determine most preferred drug.
      • Powers J.L.
      • Gooch III, W.M.
      • Oddo L.P.
      Comparison of the palatability of the oral suspension of cefdinir vs. amoxicillin/clavulanate potassium, cefprozil and azithromycin in pediatric patients.
      Children (5–8 years) rated four antimicrobial agents using a 10-cm line with face labels placed above the line at ∼0, 2.5, 5, 7.5, and 10 cm to determine the most palatable drug preparation.
      • Angelilli M.L.
      • Toscani M.
      • Matsui D.M.
      • Rieder M.J.
      Palatability of oral antibiotics among children in an urban primary care center.
      Caveat: unclear at what age children can comprehend these tasks
      • Guinard J.X.
      Sensory and consumer testing with children.
      Powers et al,
      • Powers J.L.
      • Gooch III, W.M.
      • Oddo L.P.
      Comparison of the palatability of the oral suspension of cefdinir vs. amoxicillin/clavulanate potassium, cefprozil and azithromycin in pediatric patients.
      Angelilli et al,
      • Angelilli M.L.
      • Toscani M.
      • Matsui D.M.
      • Rieder M.J.
      Palatability of oral antibiotics among children in an urban primary care center.
      Milani et al,
      • Milani G.
      • Ragazzi M.
      • Simonetti G.D.
      • et al.
      Superior palatability of crushed lercanidipine compared with amlodipine among children.
      Guinard
      • Guinard J.X.
      Sensory and consumer testing with children.
      PROP, propylthiouracil.

      Taste Reactivity

      Some of the earliest investigations on taste in infants involved videotaping infants and then characterizing their oromotor reflexes when taste stimuli were placed on the tongue or in the oral cavity.
      • Steiner J.
      Facial expressions of the neonate infant indicating the hedonics of food-related chemical stimuli.
      • Rosenstein D.
      • Oster H.
      Differential facial responses to four basic tastes in newborns.
      • Steiner J.E.
      • Glaser D.
      • Hawilo M.E.
      • Berridge K.C.
      Comparative expression of hedonic impact: affective reactions to taste by human infants and other primates.
      • Beauchamp G.K.
      • Cowart B.J.
      • Moran M.
      Developmental changes in salt acceptability in human infants.
      • Beauchamp G.K.
      • Cowart B.J.
      Development of sweet taste.
      • Beauchamp G.K.
      • Moran M.
      Acceptance of sweet and salty tastes in 2-year-old children.
      • Liem D.G.
      • Mennella J.A.
      Sweet and sour preferences during childhood: role of early experiences.
      • Spector A.C.
      • Smith J.C.
      • Hollander G.R.
      A comparison of dependent measures used to quantify radiation-induced taste aversion.
      • Berridge K.C.
      • Kringelbach M.L.
      Affective neuroscience of pleasure: reward in humans and animals.
      • Boughter J.D.J.
      • Bachmanov A.A.
      Behavioral genetics and taste.
      • Mennella J.A.
      • Griffin C.E.
      • Beauchamp G.K.
      Flavor programming during infancy.
      • Forestell C.A.
      • Mennella J.A.
      More than just a pretty face. The relationship between infant's temperament, food acceptance, and mothers' perceptions of their enjoyment of food.
      • Forestell C.A.
      • Mennella J.A.
      Early determinants of fruit and vegetable acceptance.
      In 1988, Oster and Rosenstein
      • Rosenstein D.
      • Oster H.
      Differential facial responses to four basic tastes in newborns.
      developed a method for describing orofacial responses with the anatomically based Facial Action Coding System of Ekman and Friesen
      • Ekman P.
      • Friesen W.V.
      Facial Action Coding System: A Technique for the Measurement of Facial Movement.
      , which can dissect virtually any facial expression into its constituent action units. Video records are often analyzed in slow motion
      • Grill H.J.
      • Spector A.C.
      • Schwartz G.J.
      • et al.
      Evaluating taste effects on ingestive behavior.
      to quantify the actual number of affective reactions infants express to a taste stimulus, as a measure of valence and intensity.
      • Mennella J.A.
      • Forestell C.A.
      • Morgan L.K.
      • Beauchamp G.K.
      Early milk feeding influences taste acceptance and liking during infancy.
      This method requires trained individuals (preferably certified in the Facial Action Coding System) to analyze the video images and establish reliability across scores,
      • Forestell C.A.
      • Mennella J.A.
      More than just a pretty face. The relationship between infant's temperament, food acceptance, and mothers' perceptions of their enjoyment of food.
      which can be time-consuming and costly.

      Brief-Access Tests: Intake and Suckling Methods

      Ingestive and suckling responses have been used successfully to study response patterns as a function of individual and age-related differences in taste perception. Methods include delivering small quantities of taste solutions directly to the tongue or providing brief access to multiple bottles in succession that contain various taste or diluent solutions.
      • Desor J.
      • Maller O.
      • Turner R.
      Taste in acceptance of sugars by human infants.
      • Beauchamp G.K.
      • Cowart B.J.
      • Mennella J.A.
      • Marsh R.R.
      Infant salt taste: developmental, methodological, and contextual factors.
      • Kajiura H.
      • Cowart B.J.
      • Beauchamp G.K.
      Early developmental change in bitter taste responses in human infants.
      • Mennella J.A.
      • Lukasewycz L.D.
      • Castor SM
      • Beauchamp GK
      The timing and duration of a sensitive period in human flavor learning: a randomized trial.
      • Desor J.A.
      • Maller O.
      • Andrews K.
      Ingestive responses of human newborns to salty, sour, and bitter stimuli.
      In some cases, a transducer was embedded in the nipple of the bottle to measure the patterning of suckling in response to the tastant.
      • Maone T.R.
      • Mattes R.D.
      • Bernbaum J.C.
      • Beauchamp G.K.
      A new method for delivering a taste without fluids to preterm and term infants.
      • Mennella J.A.
      Infants’ suckling responses to the flavor of alcohol in mothers’ milk.
      In other cases, infants are tested on repeated days for their acceptance of a food (eg, cereal) that differs in taste quality,
      • Forestell C.A.
      • Mennella J.A.
      More than just a pretty face. The relationship between infant's temperament, food acceptance, and mothers' perceptions of their enjoyment of food.
      • Mennella J.A.
      • Forestell C.A.
      • Morgan L.K.
      • Beauchamp G.K.
      Early milk feeding influences taste acceptance and liking during infancy.
      which requires controlling for a number of variables, including time of day and time since the infant was last fed, and ensuring that appropriate controls are built into the study design. One can then infer from this research that infants detect a bitter taste solution (eg, urea solutions), and reject it more than the diluent, if they consume less (or suck less) of the bitter taste solution than of the diluent solution.
      • Kajiura H.
      • Cowart B.J.
      • Beauchamp G.K.
      Early developmental change in bitter taste responses in human infants.
      • Desor J.A.
      • Maller O.
      • Andrews K.
      Ingestive responses of human newborns to salty, sour, and bitter stimuli.

      Forced-Choice Tracking Procedure/Thresholds

      Various methods have been used to measure how sensitive a child is to a particular tastant (eg, taste thresholds) and whether there are individual differences. Perhaps the most widely studied taste trait relates to the genetically determined ability to taste compounds containing an N − C = S (thio) group, such as PTC and its chemical relative PROP, in human populations.
      • Ishizaka T.
      • Okada S.
      • Tokuyama E.
      • et al.
      Suppression of bitterness and improvement of palatability of commercial prednisolone powder.
      • Roberts I.F.
      • Roberts G.J.
      Relation between medicines sweetened with sucrose and dental disease.
      • Hobson P.
      The treatment of medically handicapped children.
      • Feigal R.J.
      • Gleeson M.C.
      • Beckman T.M.
      • Greenwood M.E.
      Dental caries related to liquid medication intake in young cardiac patients.
      • Greenwood M.
      • Feigal R.
      • Messer H.
      Cariogenic potential of liquid medications in rats.
      As mentioned above, these chemicals taste bitter to “tasters,” whereas “nontasters” either cannot taste them or require high concentrations to recognize their presence.
      A variety of methods have been used to assess sensitivity to PROP and/or PTC.
      • Harris H.
      • Kalmus H.
      The measurement of taste sensitivity to phenylthiourea (P.T.C.).
      • Tepper B.J.
      • Christensen C.M.
      • Cao J.
      Development of brief methods to classify individuals by PROP taster status.
      • Mennella J.A.
      • Pepino M.Y.
      • Reed D.R.
      Genetic and environmental determinants of bitter perception and sweet preferences.
      • Bartoshuk L.M.
      • Duffy V.B.
      • Reed D.
      • Supertasting Williams A.
      earaches and head injury: genetics and pathology alter our taste worlds.
      Often, these include forced-choice procedures embedded in the context of a game. Based on the procedures of Anliker et al,
      • Anliker J.A.
      • Bartoshuk L.
      • Ferris A.M.
      • Hooks L.D.
      Children's food preferences and genetic sensitivity to the bitter taste of 6-n-propylthiouracil (PROP).
      children were presented, in succession, with samples of water and then 3 increasing concentrations of PROP (56, 180, and 560 μM) and were asked to taste the sample without swallowing.
      • Mennella J.A.
      • Pepino M.Y.
      • Reed D.R.
      Genetic and environmental determinants of bitter perception and sweet preferences.
      • Mennella J.A.
      • Pepino M.Y.
      • Duke F.F.
      • Reed D.R.
      Age modifies the genotype-phenotype relationship for the bitter receptor TAS2R38.
      • Mennella J.A.
      • Pepino M.Y.
      • Duke F.F.
      • Reed D.R.
      Psychophysical dissection of genotype effects on human bitter perception.
      If the solution tasted like “water” or “nothing,” then they were asked to give the sample to Big Bird, a popular television character. If the sample tasted “bad,” “yucky,” or “bitter,” children were asked to give it to Oscar the Grouch so he could throw it in his trash can. Children were grouped by the concentration of the first sample, if any, that was given to Oscar the Grouch. Children who were heterozygous at the TASR38 gene locus—that is, had 1 “taster” and 1 “nontaster” allele—were more sensitive to the taste of PROP than were heterozygous adults. The thresholds of heterozygous adolescents were intermediate,
      • Mennella J.A.
      • Pepino M.Y.
      • Duke F.F.
      • Reed D.R.
      Age modifies the genotype-phenotype relationship for the bitter receptor TAS2R38.
      and homozygous children and adults showed no difference in threshold.
      In other studies, children were presented with a series of pairs of solutions: water paired with an aqueous tastant (ie, paired comparisons). In some cases the aqueous tastants increased in concentration with each pair presented, and the child was asked to indicate which sample of the pair contained the tastant or tasted stronger. The lowest concentration successfully detected in 1 or 2 consecutive trials was recorded as the detection threshold.
      • James C.E.
      • Laing D.G.
      • Oram N.
      A comparison of the ability of 8–9-year-old children and adults to detect taste stimuli.

      Scaling Procedures

      Various types of scaling methods (ie, methods in which sensations to varying concentrations of suprathreshold stimuli are quantified) have been used to determine children’s preferences and sensitivity to tastes. Depending on age, children are presented with a line or other type of scale that contains pictorial or verbal descriptors in a graded order. Although there has been no systematic determination of what scaling test is most appropriate for children at what age, some researchers have concluded that use of scales in children younger than age 5 years of age can be problematic because they have not mastered the ability to rank things in order of magnitude.
      • Guinard J.X.
      Sensory and consumer testing with children.
      A variety of methods, including spontaneous verbal reports after dosing, time required for medication intake, 10-cm visual analog scales, and hedonic face scales, are used by pharmaceutical companies, marketing research firms, and other investigators when testing children.
      • Baguley D.
      • Lim E.
      • Bevan A.
      • et al.
      Prescribing for children—taste and palatability affect adherence to antibiotics: a review.
      Several different 5-point hedonic scales have been developed to assess taste acceptability of pharmaceuticals.
      • Angelilli M.L.
      • Toscani M.
      • Matsui D.M.
      • Rieder M.J.
      Palatability of oral antibiotics among children in an urban primary care center.
      • Cohen R.
      • de La Rocque F.
      • Lecuyer A.
      • et al.
      Study of the acceptability of antibiotic syrups, suspensions, and oral solutions prescribed to pediatric outpatients.
      • Holas C.
      • Chiu Y.L.
      • Notario G.
      • Kapral D.
      A pooled analysis of seven randomized crossover studies of the palatability of cefdinir oral suspension versus amoxicillin/clavulanate potassium, cefprozil, azithromycin, and amoxicillin in children aged 4 to 8 years.
      • Milani G.
      • Ragazzi M.
      • Simonetti G.D.
      • et al.
      Superior palatability of crushed lercanidipine compared with amlodipine among children.
      • Powers J.L.
      • Gooch III, W.M.
      • Oddo L.P.
      Comparison of the palatability of the oral suspension of cefdinir vs. amoxicillin/clavulanate potassium, cefprozil and azithromycin in pediatric patients.
      These scales typically consist of 5 different facial expressions accompanied by written labels and are used to evaluate children’s hedonic responses after tasting 1 medication at a time. Davies and Tuleu
      • Davies E.H.
      • Tuleu C.
      Medicines for children: a matter of taste.
      searched PubMed to identify 30 papers assessing medication palatability in children dating back to 1984 and found that half of the studies used a hedonic scale to rate palatability and that participants included children as young as 3 years of age. Although 5-point scales typically were used,
      • Powers J.L.
      • Gooch III, W.M.
      • Oddo L.P.
      Comparison of the palatability of the oral suspension of cefdinir vs. amoxicillin/clavulanate potassium, cefprozil and azithromycin in pediatric patients.
      scales ranged from 2 points
      • Motte J.
      • Pedespan J.M.
      • Sevestre M.
      • Chiron C.
      [Acceptability and tolerance of sodium valproate, a new sustained-action granule formulation, in monotherapy for epileptic children from 3 years old].
      to 10 points.
      • Guenther Skokan E.
      • Junkins Jr, E.P.
      • Corneli H.M.
      • Schunk J.E.
      Taste test: children rate flavoring agents used with activated charcoal.
      The use of such scales in young children is potentially problematic because it is not clear at what age children begin to use the entire scale versus just the 2 extremes.
      • Chambers E.
      Commentary: conducting sensory research in children.
      To date, only a few studies have examined the validity and reliability of hedonic scales in young children. We highlight some of these studies and how their findings provide insight into whether such methods are even valid for pediatric populations.
      Sjovall et al
      • Sjovall J.
      • Fogh A.
      • Huitfeldt B.
      • Karlsson G.
      • Nylen O.
      Methods for evaluating the taste of paediatric formulations in children: a comparison between the facial hedonic method and the patients’ own spontaneous verbal judgement.
      compared spontaneous verbal judgments and a 5-point facial hedonic scale in children given 5 different penicillin formulations. Although both methods successfully discriminated between pediatric formulations when used with older children, for children 6 years of age and younger, spontaneous verbal assessment discriminated between formulations better than did the facial hedonic scale.
      Leon et al
      • Leon F.
      • Couronne T.
      • Marcuz M.C.
      • Koster E.P.
      Measuring food liking in children: a comparison on non verbal methods.
      examined the reliability and validity of facial hedonic scales in children whose ages ranged from 4 to 10 years. Children in the study tasted biscuits covered with different flavors of jam. Children 4 and 5 years old rated the jams using a 2-point hedonic face scale (like vs dislike), whereas older children rated them using a 4-point hedonic face scale (like very much, like, dislike, dislike very much). For children younger than 5 years of age, intersession repeatability of results with the hedonic scale was poor (Kendall correlation = 0.18) and did not correlate with other measures of preference in the same children. In contrast, children older than 5 years could reliably use the 4-point hedonic scale, and results correlated with other measures of preference. These studies illustrate the difficulty of using hedonic scales in young children.

      Application of Methods to Study Bitter Taste in Children

      Some children refuse to take bitter medicines, whereas others comply readily.
      • Milne C.P.
      • Bruss J.B.
      The economics of pediatric formulation development for off-patent drugs.
      Likewise, not every child (or adult) is equally sensitive to the taste of bitter compounds.
      • Mennella J.A.
      • Pepino M.Y.
      • Reed D.R.
      Genetic and environmental determinants of bitter perception and sweet preferences.
      Many children are more sensitive to bitter tastes than are adults.
      • Mennella J.A.
      • Lukasewycz L.D.
      • Castor SM
      • Beauchamp GK
      The timing and duration of a sensitive period in human flavor learning: a randomized trial.
      • Mennella J.A.
      • Pepino M.Y.
      • Reed D.R.
      Genetic and environmental determinants of bitter perception and sweet preferences.
      However, because of the paucity of research on the ontogeny of bitter taste sensitivity, we do not know the full extent of the differences in perception between adults and children and how that relates to individual genotype. We hypothesize that the substantial degree of sequence diversity and variation that exists in taste receptor genes
      • Kim U.
      • Wooding S.
      • Ricci D.
      • et al.
      Worldwide haplotype diversity and coding sequence variation at human bitter taste receptor loci.
      may underlie individual differences in medication adherence in children related to taste. Although these individual differences arise for a variety of reasons (eg, temperament,
      • Macht M.
      • Mueller J.
      Increased negative emotional responses in PROP supertasters.
      experience,
      • Mela D.J.
      • Mattes R.D.
      • Tanimura S.
      • Garcia-Medina M.R.
      Relationships between ingestion and gustatory perception of caffeine.
      ethnicity/race
      • Guo S.W.
      • Reed D.R.
      The genetics of phenylthiocarbamide perception.
      ), the best-known example is person-to-person genetic variation. As described above, variations in perception of the bitter compound PROP are due in large part to “taster” and “nontaster” alleles of a particular bitter receptor.
      • Kim U.
      • Jorgenson E.
      • Coon H.
      • et al.
      Positional cloning of the human quantitative trait locus underlying taste sensitivity to phenylthiocarbamide.
      • Bufe B.
      • Breslin P.A.
      • Kuhn C.
      • et al.
      The molecular basis of individual differences in phenylthiocarbamide and propylthiouracil bitterness perception.
      Allele frequencies for this gene differ markedly by race; for example, high sensitivity to the bitterness of PROP and related compounds is more common in African populations.
      • Kim U.
      • Wooding S.
      • Ricci D.
      • et al.
      Worldwide haplotype diversity and coding sequence variation at human bitter taste receptor loci.
      A recent study explored the relationship between genotype of 1 of the 25 bitter receptor genes (TAS2R38) and medication history.
      • Lipchock S.V.
      • Reed D.R.
      • Mennella J.A.
      Relationship between bitter-taste receptor genotype and solid medication formulation usage among young children: a retrospective analysis.
      Children younger than 10 years of age who had at least 1 taster (P) allele (PP or AP genotype) were more likely to have taken medicine in solid formulation than were nontaster (AA genotype) children. We hypothesized that the resistance to taking bitter liquid formulations may relate to compliance and that bitter-sensitive children may be resistant to taking bitter liquid formulations and motivated to try medicine in pill form as an alternative. Although children were genotyped for only 1 of the 25 known bitter receptors, alleles of this particular receptor may be a proxy for general taste ability,
      • Bartoshuk L.M.
      • Duffy V.B.
      • Reed D.
      • Supertasting Williams A.
      earaches and head injury: genetics and pathology alter our taste worlds.
      or bitter receptor genes may occur in tightly linked clusters
      • Adler E.
      • Hoon M.A.
      • Mueller K.L.
      • et al.
      A novel family of mammalian taste receptors.
      such that genetic variation in this receptor may relate to variation in other receptors. This particular receptor may also respond more broadly than previously understood; drugs commonly used in children’s medications have not been widely tested in assays designed to understand such receptor-ligand interactions. Further study of the relationship between the TAS2R38 genotype and liquid formulation intake and compliance is warranted.
      Recent research revealed that cell-based assays are imperfect proxies of the human taste response. For example, TAS2R38 has 3 variant sites that give rise to several taster and nontaster haplotypes. When cell-based assays
      • Bufe B.
      • Breslin P.A.
      • Kuhn C.
      • et al.
      The molecular basis of individual differences in phenylthiocarbamide and propylthiouracil bitterness perception.
      of these haplotypes are compared with studies of people with those same haplotypes,
      • Mennella J.A.
      • Pepino M.Y.
      • Duke F.F.
      • Reed D.R.
      Psychophysical dissection of genotype effects on human bitter perception.
      there is agreement in many cases but not in all, especially for variants that may directly couple with the G protein. This study highlights the need for psychophysical as well as cell-based methods to understand the genotype-phenotype relationship for taste receptors.
      • Mennella J.A.
      • Pepino M.Y.
      • Duke F.F.
      • Reed D.R.
      Psychophysical dissection of genotype effects on human bitter perception.
      Research to further characterize how taste receptor genotype and other aspects of taste phenotypes relate to pediatric medication formulation and compliance is necessary to help us develop better medicines for pediatric populations. Such research could be incorporated into pediatric clinical trials to help understand individual compliance during the trial and to expand our understanding of the role of taste genetics in behavioral choices. Because children are more bitter-sensitive than are adults, and age-related changes in bitter perception are more common for people with particular genotypes, we need to study both adults and children and take genetic variation into account when interpreting the results.
      • Lipchock S.V.
      • Reed D.R.
      • Mennella J.A.
      Relationship between bitter-taste receptor genotype and solid medication formulation usage among young children: a retrospective analysis.
      Although we have discussed only a few examples of how bitter receptor genotype can affect bitter perception,
      • Reed D.R.
      • Zhu G.
      • Breslin P.A.
      • et al.
      The perception of quinine taste intensity is associated with common genetic variants in a bitter receptor cluster on chromosome 12.
      • Bufe B.
      • Breslin P.A.
      • Kuhn C.
      • et al.
      The molecular basis of individual differences in phenylthiocarbamide and propylthiouracil bitterness perception.
      • Hayes J.E.
      • Wallace M.R.
      • Knopik V.S.
      • et al.
      Allelic variation in TAS2R bitter receptor genes associates with variation in sensations from and ingestive behaviors toward common bitter beverages in adults.
      • Kuhn C.
      • Bufe B.
      • Winnig M.
      • et al.
      Bitter taste receptors for saccharin and acesulfame K.
      • Wooding S.
      • Gunn H.
      • Ramos P.
      • et al.
      Genetics and bitter taste responses to goitrin, a plant toxin found in vegetables.
      • Roudnitzky N.
      • Bufe B.
      • Thalmann S.
      • et al.
      Genomic, genetic, and functional dissection of bitter taste responses to artificial sweeteners.
      genotype, like age, it is an important determinant of perception and should be considered in all methods to evaluate the taste of medicine and compliance.
      Many investigators are developing a new generation of molecules to inhibit bitterness.
      • Palmer R.K.
      The pharmacology and signaling of bitter, sweet, and umami taste sensing.
      However, there are very few peer-reviewed studies on their effectiveness in adults (reviewed in Roy
      • Roy G.
      Modifying Bitterness: Mechanism, Ingredients, and Applications.
      ), and to our knowledge, only 1 study examined children.
      • Mennella J.A.
      • Pepino M.Y.
      • Beauchamp G.K.
      Modification of bitter taste in children.
      Nevertheless, because of the age-related differences in bitter taste perception, we suggest that research aimed at reducing the bitterness of medicine, such as evaluating the effectiveness of bitter blockers, should directly involve children rather than extrapolating from data collected from adults.

      Artificial Sensor Systems

      There is much debate in the literature on whether artificial sensors can be successful substitutes for the human palate and replace the use of sensory panelists because use of the latter is problematic in industry “due to the potential toxicity of drugs and subjectivity of taste panelists, problems in recruiting taste panelists, motivation and panel maintenance…when working with unpleasant products.”
      • Latha R.S.
      • Lakshmi P.K.
      Electronic tongue: an analytical gustatory tool.
      Furthermore, because FDA-unapproved drugs cannot be taste tested, use of artificial sensors, it has been argued, can provide important data regarding the taste of these drugs.
      • Latha R.S.
      • Lakshmi P.K.
      Electronic tongue: an analytical gustatory tool.
      • Woertz K.
      • Tissen C.
      • Kleinebudde P.
      • Breitkreutz J.
      Taste sensing systems (electronic tongues) for pharmaceutical applications.
      These artificial sensory devices typically are arrays of sensors, called “electronic noses” for arrays of gas sensors and “electronic tongues” for arrays of liquid sensors. Often these devices are designed to analyze the levels of various ingredients composing a fluid mixture and are used in a variety of applications involving product quality control.
      • Ferreira M.
      • Riul Jr, A.
      • Wohnrath K.
      • et al.
      High-performance taste sensor made from Langmuir-Blodgett films of conducting polymers and a ruthenium complex.
      But in recent years, these devices have been used as an analytical gustatory tool in evaluating pharmaceuticals.
      • Latha R.S.
      • Lakshmi P.K.
      Electronic tongue: an analytical gustatory tool.
      • Zheng J.Y.
      • Keeney M.P.
      Taste masking analysis in pharmaceutical formulation development using an electronic tongue.
      • Kayumba P.C.
      • Huyghebaert N.
      • Cordella C.
      • et al.
      Quinine sulphate pellets for flexible pediatric drug dosing: formulation development and evaluation of taste-masking efficiency using the electronic tongue.
      • Baldwin E.A.
      • Bai J.
      • Plotto A.
      • Dea S.
      Electronic noses and tongues: applications for the food and pharmaceutical industries.
      • Guhmann M.
      • Preis M.
      • Gerber F.
      • et al.
      Development of oral taste masked diclofenac formulations using a taste sensing system.
      It has been argued that this approach, whose advantages include its speed, relatively low cost, and lack of risk, will help develop more palatable pediatric formulations.
      • Walsh J.
      • Bickmann D.
      • Breitkreutz J.
      • Chariot-Goulet M.
      Delivery devices for the administration of paediatric formulations: overview of current practice, challenges and recent developments.
      • Breitkreutz J.
      European perspectives on pediatric formulations.
      • Anand V.
      • Kataria M.
      • Kukkar V.
      • et al.
      The latest trends in the taste assessment of pharmaceuticals.
      • Cram A.
      • Breitkreutz J.
      • Desset-Brethes S.
      • et al.
      Challenges of developing palatable oral paediatric formulations.
      Nevertheless, whether such artificial sensory systems will lead to significant insights that will address the heart of the problem in practice remains to be seen. Given the numerous and varied components of peripheral and central mechanisms involved in the mediation of bitter taste (summarized in Figure 1), the ability of an artificial sensor to model and predict the properties of this complex biological system is questionable. Thus, the utility of the electronic tongue to offer meaningful guidance in the development of strategies to increase the palatability of pediatric formulations is likely to be limited to simply providing a detailed analysis of the chemical constituents in the mixture. However, it is quite possible that, because this is an active area of research, these devices might be more useful in the future.

      Conclusions

      Like other sensory systems, taste is experienced through a “sensory window” that changes with age and experience and is partially defined by genetics. Children have well-developed sensory systems for detecting tastes, as well as smells and chemical irritants, and their rejection of unpalatable medications reflects their basic biological preferences for sweet, salty, and, to some extent, sour tastes and rejection of bitter tastes. Sugars, salt, acids, and other substances help reduce the perceived bitterness of several pharmaceuticals. Although adding pleasant flavor volatiles such as bubble gum may also help induce children to consume a medicine, such volatile compounds are often not very effective in suppressing the strong bitter tastes associated with many medications.
      This aversion to bitter creates a roadblock for oral formulations; undesirable chemosensory characteristics can hinder the acceptance and usefulness of many beneficial, safe, and efficacious drugs. The unpleasant taste of a medicine is often a sensory expression of its pharmacologic activity; in many cases, the more potent the drug, the more bitter it will be.
      • Fischer R.A.
      • Griffin F.
      Pharmacogenetic aspects of gustation.
      The more bitter, the more likely the drug will be rejected. Better-tasting medications may go a long way toward enhancing the ability of pediatric patients to adhere to drug therapy, especially when failure to consume may do harm and, in some cases, be life threatening.
      • Matsui D.
      Current issues in pediatric medication adherence.
      Thus, a primary challenge is to reduce the bitterness and other off flavors of pediatric formulations.
      Adult panelists who are sensitive to the pediatric palate, new techniques involving animal models, and even electronic detection devices are among the tools that can help evaluate the palatability of medications and predict compliance among pediatric populations. Further development of and consensus regarding which psychophysical tools are valid and appropriate for use with children will provide a better understanding of the sensory world of the child. Testing multiple strategies will help us refine methods that may be used to assess acceptance and compliance/adherence by pediatric populations of varying ages, which will allow for comparisons across studies. These methods then can be applied to clinical trials to obtain data that can help predict initial acceptance versus long-term compliance of a medication, and how medication use and disease state modify bitter taste perception of the drug in children. Although much of the research will by necessity focus on taste testing without swallowing, there are also receptors in the back of the throat
      • Miller I.J.
      • Bartoshuk L.M.
      Taste perception, taste bud distribution, and spatial relationship.
      that may be engaged primarily during swallowing of the liquid medication. The effect of these receptors on taste acceptance can be studied during clinical trials in which children not only taste but also swallow medicine.
      Although progress has been made in our current understanding of bitter taste, it is far from complete, and new ways to reduce the bitterness of certain medications may yet be discovered. Most of our knowledge on the neurobiological mechanisms of taste has been derived from animal models in which the gustatory system can be invasively manipulated and studied. As discussed in the preceding pages, a variety of behavioral techniques can be used to link taste perception to its underlying neurobiological processes. Accordingly, these model systems can be exploited to evaluate potential strategies to safely and effectively attenuate bitterness. Such an approach, coupled with psychophysical assessment of taste function in children, and ultimately clinical testing, should increase the chances of finding solutions to what has been the vexing problem of reducing the aversive taste of drugs to promote acceptance and compliance among pediatric populations. Understanding bitterness better may take the guesswork out of improving formulations.

      Dedication

      We dedicate this review article to the memory of Dr. Barry Davis, past director of the taste and smell program within the National Institute on Deafness and Other Communication Disorders, and a friend and mentor.

      Conflicts of Interest

      The authors have indicated that they have no conflicts of interest regarding the content of this article.

      Acknowledgments

      We acknowledge the valuable discussions with Dr. George Giacoia from the Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, DHHS, as well as other members of the Taste Working Group. Johan Lundström provided guidance about human taste brain areas. Preparation of this article was supported in part by NIH grants R01 DC01187 (J.M.), P30 DC011735 (D.R.), and the Washington Dental Service Endowed Professorship (S.C.). Funding sources had no role in the study design; in the collection, analysis, or interpretation of data; in the writing of the manuscript; or in the decision to submit the manuscript for publication. All authors contributed equally to this manuscript. We acknowledge the editorial assistance of Ms. Patricia Watson.

      References

        • Roberts R.
        • Rodriguez W.
        • Murphy D.
        • Crescenzi T.
        Pediatric drug labeling: improving the safety and efficacy of pediatric therapies.
        JAMA. 2003; 290: 905-911
        • Milne C.P.
        • Bruss J.B.
        The economics of pediatric formulation development for off-patent drugs.
        Clin Ther. 2008; 30: 2133-2145
        • Giacoia G.P.
        • Taylor-Zapata P.
        • Mattison D.
        Eunice Kennedy Shriver National Institute of Child Health and Human Development Pediatric Formulation Initiative: selected reports from working groups.
        Clin Ther. 2008; 30: 2097-2101
        • Glendinning J.I.
        Is the bitter rejection response always adaptive?.
        Physiol Behav. 1994; 56: 1217-1227
        • Rodgers Jr, G.C.
        • Tenenbein M.
        The role of aversive bittering agents in the prevention of pediatric poisonings.
        Pediatrics. 1994; 93: 68-69
        • Schirm E.
        • Tobi H.
        • de Vries TW
        • et al.
        Lack of appropriate formulations of medicines for children in the community.
        Acta Paediatr. 2003; 92: 1486-1489
        • Ruark J.L.
        • McCullough G.H.
        • Peters R.L.
        • Moore C.A.
        Bolus consistency and swallowing in children and adults.
        Dysphagia. 2002; 17: 24-33
        • Sadrieh N.
        • Brower J.
        • Yu L.
        • et al.
        Stability, dose uniformity, and palatability of three counterterrorism drugs-human subject and electronic tongue studies.
        Pharm Res. 2005; 22: 1747-1756
        • Pawar S.
        • Kumar A.
        Issues in the formulation of drugs for oral use in children: role of excipients.
        Paediatr Drugs. 2002; 4: 371-379
        • Ishizaka T.
        • Okada S.
        • Tokuyama E.
        • et al.
        Suppression of bitterness and improvement of palatability of commercial prednisolone powder.
        Chem Pharm Bull. 2008; 56: 1395-1399
        • Roberts I.F.
        • Roberts G.J.
        Relation between medicines sweetened with sucrose and dental disease.
        BMJ. 1979; 2: 14-16
        • Hobson P.
        The treatment of medically handicapped children.
        Int Dent J. 1980; 30: 6-13
        • Feigal R.J.
        • Gleeson M.C.
        • Beckman T.M.
        • Greenwood M.E.
        Dental caries related to liquid medication intake in young cardiac patients.
        ASDC J Dent Child. 1984; 51: 360-362
        • Greenwood M.
        • Feigal R.
        • Messer H.
        Cariogenic potential of liquid medications in rats.
        Caries Res. 1984; 18: 447-449
        • Manley M.C.
        • Calnan M.
        • Sheiham A.
        A spoonful of sugar helps the medicine go down? Perspectives on the use of sugar in children's medicines.
        Soc Sci Med. 1994; 39: 833-840
        • Maguire A.
        • Rugg-Gunn A.J.
        Changes in the prescribing of liquid oral medicines (LOMs) in the northern region of England between 1987 and 1992 with special regard to sugar content and long-term use in children.
        Community Dent Health. 1997; 14: 31-35
        • Baqir W.
        • Maguire A.
        Consumption of prescribed and over-the-counter medicines with prolonged oral clearance used by the elderly in the northern region of England, with special regard to generic prescribing, dose form and sugars content.
        Public Health. 2000; 114: 367-373
        • Maguire A.
        • Baqir W.
        • Nunn J.H.
        Are sugars-free medicines more erosive than sugars-containing medicines? An in vitro study of paediatric medicines with prolonged oral clearance used regularly and long-term by children.
        Int J Paediatr Dent. 2007; 17: 231-238
        • Liem D.G.
        • Mennella J.A.
        Heightened sour preferences during childhood.
        Chem Senses. 2003; 28: 173-180
        • Allen Jr, L.V.
        Dosage form design and development.
        Clin Ther. 2008; 30: 2102-2111
        • Grenby T.H.
        • Phillips A.
        • Desai T.
        • Mistry M.
        Laboratory studies of the dental properties of soft drinks.
        Br J Nutr. 1989; 62: 451-464
        • Leung VW-H
        • Darvell B.W.
        Artificial salivas for in vitro studies of dental materials.
        J Dent. 1997; 25: 475-484
        • Thomson S.A.
        • Tuleu C.
        • Wong I.C.
        • et al.
        Minitablets: new modality to deliver medicines to preschool-aged children.
        Pediatrics. 2009; 123: e235-e238
        • Peyrot des Gachons C.
        • Beauchamp G.K.
        • Stern R.M.
        • et al.
        Bitter taste induces nausea.
        Curr Biol. 2011; 21: R247-R248
        • Bernstein I.L.
        Learned taste aversions in children receiving chemotherapy.
        Science. 1978; 200: 1302-1303
        • Nahata M.C.
        • Allen Jr, L.V.
        Extemporaneous drug formulations.
        Clin Ther. 2008; 30: 2112-2119
        • Mennella J.A.
        • Beauchamp G.K.
        Optimizing oral medications for children.
        Clin Ther. 2008; 30: 2120-2132
        • Glendinning J.I.
        Effect of salivary proline-rich proteins on ingestive responses to tannic acid in mice.
        Chem Senses. 1992; 17: 1-12
        • Cabras T.
        • Melis M.
        • Castagnola M.
        • et al.
        Responsiveness to 6-n-propylthiouracil (PROP) is associated with salivary levels of two specific basic proline-rich proteins in humans.
        PLoS One. 2012; 7: e30962
        • Miller Jr, I.J.
        Gustatory receptors of the palate.
        in: Katsuki Y. Sato M. Takagi S. Oomura Y. Food Intake and Chemical Senses. University of Tokyo Press, Tokyo, Japan1977: 173-186
        • Miller Jr, I.J.
        Anatomy of the peripheral gustatory system.
        in: Doty R.L. Handbook of Olfaction and Gustation. Dekker, New York1995: 521-547
        • Hamilton R.B.
        • Norgren R.
        Central projections of gustatory nerves in the rat.
        J Comp Neurol. 1984; 222: 560-577
        • May O.L.
        • Hill D.L.
        Gustatory terminal field organization and developmental plasticity in the nucleus of the solitary tract revealed through triple-fluorescence labeling.
        J Comp Neurol. 2006; 497: 658-669
        • Corson J.
        • Aldridge A.
        • Wilmoth K.
        • Erisir A.
        A survey of oral cavity afferents to the rat nucleus tractus solitarii.
        J Comp Neurol. 2012; 520: 495-527
        • Lundy R.F.
        • Norgren R.
        Gustatory system.
        The Rat Nervous System. 3rd ed. Elsevier, New York2004: 891-921
        • Zaidi F.N.
        • Todd K.
        • Enquist L.
        • Whitehead M.C.
        Types of taste circuits synaptically linked to a few geniculate ganglion neurons.
        J Comp Neurol. 2008; 511: 753-772
        • Travers S.P.
        • Travers J.B.
        Reflex topography in the nucleus of the solitary tract.
        Chem Senses. 2005; 30: i180-i181
        • Norgren R.
        Taste and the autonomic nervous system.
        Chem Senses. 1985; 10: 143-161
        • Pritchard T.C.
        • Norgren R.
        Gustatory system.
        in: Paxinos G. Mai J. The Human Nervous System. 2nd ed. Elsevier, New York, NY2004: 1171-1195
        • Lawless H.T.
        Evidence for neural inhibition in bittersweet taste mixtures.
        J Comp Physiol Psychol. 1979; 93: 538-547
        • Kroeze J.H.
        • Bartoshuk L.M.
        Bitterness suppression as revealed by split-tongue taste stimulation in humans.
        Physiol Behav. 1985; 35: 779-783
        • Formaker B.K.
        • MacKinnon B.I.
        • Hettinger T.P.
        • Frank M.E.
        Opponent effects of quinine and sucrose on single fiber taste responses of the chorda tympani nerve.
        Brain Res. 1997; 772: 239-242
        • Lehman C.D.
        • Bartoshuk L.M.
        • Catalanotto F.C.
        • et al.
        Effect of anesthesia of the chorda tympani nerve on taste perception in humans.
        Physiol Behav. 1995; 57: 943-951
        • Kinnamon S.C.
        Taste receptor signalling—from tongues to lungs.
        Acta Physiol (Oxf). 2012; 204: 158-168
        • Niki M.
        • Jyotaki M.
        • Yoshida R.
        • Ninomiya Y.
        Reciprocal modulation of sweet taste by leptin and endocannabinoids.
        Results Probl Cell Differ. 2010; 52: 101-114
        • Bigiani A.
        • Ghiaroni V.
        • Fieni F.
        Channels as taste receptors in vertebrates.
        Prog Biophys Mol Biol. 2003; 83: 193-225
        • Chandrashekar J.
        • Hoon M.A.
        • Ryba N.J.
        • Zuker C.S.
        The receptors and cells for mammalian taste.
        Nature. 2006; 444: 288-294
        • Ninomiya Y.
        • Fukami Y.
        • Yamazaki K.
        • Beauchamp G.K.
        Amiloride inhibition of chorda tympani responses to NaCl and its temperature dependency in mice.
        Brain Res. 1996; 708: 153-158
        • Ohkuri T.
        • Yasumatsu K.
        • Horio N.
        • et al.
        Multiple sweet receptors and transduction pathways revealed in knockout mice by temperature dependence and gurmarin sensitivity.
        Am J Physiol Regul Integr Comp Physiol. 2009; 296: R960-R971
        • Talavera K.
        • Yasumatsu K.
        • Voets T.
        • et al.
        Heat activation of TRPM5 underlies thermal sensitivity of sweet taste.
        Nature. 2005; 438: 1022-1025
        • Kokrashvili Z.
        • Mosinger B.
        • Margolskee R.F.
        Taste signaling elements expressed in gut enteroendocrine cells regulate nutrient-responsive secretion of gut hormones.
        Am J Clin Nutr. 2009; 90: 822S-825S
        • Behrens M.
        • Meyerhof W.
        Gustatory and extragustatory functions of mammalian taste receptors.
        Physiol Behav. 2011; 105: 4-13
        • Finger T.E.
        • Kinnamon S.C.
        Taste isn't just for taste buds anymore.
        F1000 Biol Rep. 2011; 3: 20
        • Rozengurt E.
        • Sternini C.
        Taste receptor signaling in the mammalian gut.
        Curr Opin Pharmacol. 2007; 7: 557-562
        • Kellenberger S.
        • Schild L.
        Epithelial sodium channel/degenerin family of ion channels: a variety of functions for a shared structure.
        Physiol Rev. 2002; 82: 735-767
        • Lee R.J.
        • Xiong G.
        • Kofonow J.M.
        • et al.
        T2R38 taste receptor polymorphisms underlie susceptibility to upper respiratory infection.
        J Clin Invest. 2012; 122: 4145-4159
        • Zhang Y.
        • Hoon M.A.
        • Chandrashekar J.
        • et al.
        Coding of sweet, bitter, and umami tastes: different receptor cells sharing similar signaling pathways.
        Cell. 2003; 112: 293-301
        • McLaughlin S.K.
        • McKinnon P.J.
        • Margolskee R.F.
        Gustducin is a taste-cell-specific G protein closely related to the transducins.
        Nature. 1992; 357: 563-569
        • Wong G.T.
        • Gannon K.S.
        • Margolskee R.F.
        Transduction of bitter and sweet taste by gustducin.
        Nature. 1996; 381: 796-800
        • Adler E.
        • Hoon M.A.
        • Mueller K.L.
        • et al.
        A novel family of mammalian taste receptors.
        Cell. 2000; 100: 693-702
        • Chandrashekar J.
        • Mueller K.L.
        • Hoon M.A.
        • et al.
        T2Rs function as bitter taste receptors.
        Cell. 2000; 100: 703-711
        • Meyerhof W.
        • Batram C.
        • Kuhn C.
        • et al.
        The molecular receptive ranges of human TAS2R bitter taste receptors.
        Chem Senses. 2010; 35: 157-170
        • Behrens M.
        • Meyerhof W.
        Mammalian bitter taste perception.
        Results Probl Cell Differ. 2009; 47: 203-220
        • Fox A.L.
        The relationship between chemical constitution and taste.
        Proc Natl Acad Sci U S A. 1932; 18: 115-120
        • Kim U.
        • Jorgenson E.
        • Coon H.
        • et al.
        Positional cloning of the human quantitative trait locus underlying taste sensitivity to phenylthiocarbamide.
        Science. 2003; 299: 1221-1225
        • Reed D.R.
        • Zhu G.
        • Breslin P.A.
        • et al.
        The perception of quinine taste intensity is associated with common genetic variants in a bitter receptor cluster on chromosome 12.
        Hum Mol Genet. 2010; 19: 4278-4285
        • Nelson G.
        • Hoon M.A.
        • Chandrashekar J.
        • et al.
        Mammalian sweet taste receptors.
        Cell. 2001; 106: 381-390
        • Caicedo A.
        • Roper S.D.
        Taste receptor cells that discriminate between bitter stimuli.
        Science. 2001; 291: 1557-1560
        • Frank M.E.
        Taste-responsive neurons of the glossopharyngeal nerve of the rat.
        J Neurophysiol. 1991; 65: 1452-1463
        • Frank M.E.
        • Contreras R.J.
        • Hettinger T.P.
        Nerve fibers sensitive to ionic taste stimuli in chorda tympani of the rat.
        J Neurophysiol. 1983; 50: 941-960
        • Spector A.C.
        • Travers S.P.
        The representation of taste quality in the mammalian nervous system.
        Behav Cogn Neurosci Rev. 2005; 4: 143-191
        • Dahl M.
        • Erickson R.P.
        • Simon S.A.
        Neural responses to bitter compounds in rats.
        Brain Res. 1997; 756: 22-34
        • Spector A.C.
        Linking gustatory neurobiology to behavior in vertebrates.
        Neurosci Biobehav Rev. 2000; 24: 391-416
        • Geran L.C.
        • Travers S.P.
        Single neurons in the nucleus of the solitary tract respond selectively to bitter taste stimuli.
        J Neurophysiol. 2006; 96: 2513-2527
        • Geran L.C.
        • Travers S.P.
        Bitter-responsive gustatory neurons in the rat parabrachial nucleus.
        J Neurophysiol. 2009; 101: 1598-1612
        • Lemon C.H.
        • Smith D.V.
        Neural representation of bitter taste in the nucleus of the solitary tract.
        J Neurophysiol. 2005; 94: 3719-3729
        • Chen X.
        • Gabitto M.
        • Peng Y.
        • et al.
        A gustotopic map of taste qualities in the mammalian brain.
        Science. 2011; 333: 1262-1266
        • Accolla R.
        • Bathellier B.
        • Petersen C.C.
        • Carleton A.
        Differential spatial representation of taste modalities in the rat gustatory cortex.
        J Neurosci. 2007; 27: 1396-1404
        • Palmer R.K.
        The pharmacology and signaling of bitter, sweet, and umami taste sensing.
        Mol Interv. 2007; 7: 87-98
        • Rozin P.
        “Taste-smell confusions” and the duality of the olfactory sense.
        Percept Psychophys. 1982; 31: 397-401
        • Spector A.C.
        Psychophysical evaluation of taste function in non-human mammals.
        in: Doty R.L. Handbook of Olfaction and Gustation. 2nd ed. Dekker, New York2003: 861-879
        • Davis J.D.
        The effectiveness of some sugars in stimulating licking behavior in the rat.
        Physiol Behav. 1973; 11: 39-45
        • O'Keefe G.B.
        • Schumm J.
        • Smith J.C.
        Loss of sensitivity to low concentrations of NaCl following bilateral chorda tympani nerve sections in rats.
        Chem Senses. 1994; 19: 169-184
        • St John S.J.
        • Garcea M.
        • Spector A.C.
        Combined, but not single, gustatory nerve transection substantially alters taste-guided licking behavior to quinine in rats.
        Behav Neurosci. 1994; 108: 131-140
        • Glendinning J.I.
        • Gresack J.
        • Spector A.C.
        A high-throughput screening procedure for identifying mice with aberrant taste and oromotor function.
        Chem Senses. 2002; 27: 461-474
        • Treesukosol Y.
        • Smith K.R.
        • Spector A.C.
        Behavioral evidence for a glucose polymer taste receptor that is independent of the T1R2+3 heterodimer in a mouse model.
        J Neurosci. 2011; 31: 13527-13534
        • Spector A.C.
        Gustatory parabrachial lesions disrupt taste-guided quinine responsiveness in rats.
        Behav Neurosci. 1995; 109: 79-90
        • Young P.T.
        • Trafton C.L.
        Activity contour maps as related to preference in four gustatory stimulus areas of the rat.
        J Comp Physiol Psychol. 1964; 58: 68-75
        • Smith J.C.
        The history of the “Davis Rig.”.
        Appetite. 2001; 36: 93-98
        • Smith J.C.
        • Davis J.D.
        • O'Keefe G.B.
        Lack of an order effect in brief contact taste tests with closely spaced test trials.
        Physiol Behav. 1992; 52: 1107-1111
        • Damak S.
        • Rong M.
        • Yasumatsu K.
        • et al.
        Trpm5 null mice respond to bitter, sweet, and umami compounds.
        Chem Senses. 2006; 31: 253-264
        • Dotson C.D.
        • Roper S.D.
        • Spector A.C.
        PLCbeta2-independent behavioral avoidance of prototypical bitter-tasting ligands.
        Chem Senses. 2005; 30: 593-600
        • Berridge K.C.
        Measuring hedonic impact in animals and infants: microstructure of affective taste reactivity patterns.
        Neurosci Biobehav Rev. 2000; 24: 173-198
        • Grill H.J.
        • Norgren R.
        The taste reactivity test. I. Mimetic responses to gustatory stimuli in neurologically normal rats.
        Brain Res. 1978; 143: 263-279
        • Spector A.C.
        • Breslin P.
        • Grill H.J.
        Taste reactivity as a dependent measure of the rapid formation of conditioned taste aversion: a tool for the neural analysis of taste-visceral associations.
        Behav Neurosci. 1988; 102: 942-952
        • Berridge K.C.
        Food reward: brain substrates of wanting and liking.
        Neurosci Biobehav Rev. 1996; 20: 1-25
        • Grill H.J.
        • Spector A.C.
        • Schwartz G.J.
        • et al.
        Evaluating taste effects on ingestive behavior.
        in: Toates F. Rowland N. Techniques in the Behavioral and Neural Sciences. Vol. 1. Feeding and Drinking. Elsevier, Amsterdam, the Netherlands1987: 151-188
        • Travers J.B.
        • Grill H.J.
        • Norgren R.
        The effects of glossopharyngeal and chorda tympani nerve cuts on the ingestion and rejection of sapid stimuli: an electromyographic analysis in the rat.
        Behav Brain Res. 1987; 25: 233-246
        • Grill H.J.
        • Schwartz G.J.
        • Travers J.B.
        The contribution of gustatory nerve input to oral motor behavior and intake-based preference. I. Effects of chorda tympani or glossopharyngeal nerve section in the rat.
        Brain Res. 1992; 573: 95-104
        • King C.T.
        • Garcea M.
        • Spector A.C.
        Glossopharyngeal nerve regeneration is essential for the complete recovery of quinine-stimulated oromotor rejection behaviors and central patterns of neuronal activity in the nucleus of the solitary tract in the rat.
        J Neurosci. 2000; 20: 8426-8434
        • Nowlis G.H.
        • Frank M.E.
        • Pfaffmann C.
        Specificity of acquired aversions to taste qualities in hamsters and rats.
        J Comp Physiol Psychol. 1980; 94: 932-942
        • Tapper D.N.
        • Halpern B.P.
        Taste stimuli: a behavioral categorization.
        Science. 1968; 161: 708-710
        • Grobe C.L.
        • Spector A.C.
        Constructing quality profiles for taste compounds in rats: a novel paradigm.
        Physiol Behav. 2008; 95: 413-424
        • Spector A.C.
        • Kopka S.L.
        Rats fail to discriminate quinine from denatonium: implications for the neural coding of bitter-tasting compounds.
        J Neurosci. 2002; 22: 1937-1941
        • Chambers E.
        Commentary: conducting sensory research in children.
        J Sensory Stud. 2005; 20: 90-92
      1. Forestell CA, Mennella JA. The ontogeny of taste perception and preference throughout childhood. In: Doty RL, ed. Handbook of Olfaction and Gustation. 3rd ed. Boca Raton, Fla: CRC Press; in press.

        • Cowart B.J.
        Development of taste perception in humans: sensitivity and preference throughout the life span.
        Psychol Bull. 1981; 90: 43-73
        • Cowart B.J.
        • Beauchamp G.K.
        • Mennella J.A.
        Development of taste and smell in the neonate.
        in: Polin R.A. Fox W.W. Abman S.H. Fetal and Neonatal Physiology, Vol. 2. 3rd ed. Saunders, Philadelphia, Pa2004: 1819-1827
        • Schneider B.A.
        • Trehub S.E.
        • Morrongiello B.A.
        • Thorpe L.A.
        Developmental changes in masked thresholds.
        J Acoust Soc Am. 1989; 86: 1733-1742
        • Dorries K.M.
        • Schmidt H.J.
        • Beauchamp G.K.
        • Wysocki C.J.
        Changes in sensitivity to the odor of androstenone during adolescence.
        Dev Psychobiol. 1989; 22: 423-435
        • Odeigah P.G.
        • Obieze A.C.
        Differences in sodium chloride taste sensitivity in a rural and an urban population in Nigeria: implications for the incidence of hypertension.
        East Afr Med J. 1986; 63: 236-243
        • Patil S.
        • Maibach H.I.
        Effect of age and sex on the elicitation of irritant contact dermatitis.
        Contact Dermatitis. 1994; 30: 257-264
        • Desor J.
        • Maller O.
        • Turner R.
        Taste in acceptance of sugars by human infants.
        J Comp Phys Psychol. 1973; 84: 496-501
        • Steiner J.
        Facial expressions of the neonate infant indicating the hedonics of food-related chemical stimuli.
        in: Weiffenbach J.M. Taste and Development: The Genesis of Sweet Preference. U.S. Government Printing Office, Washington, DC1977: 173-188
        • Rosenstein D.
        • Oster H.
        Differential facial responses to four basic tastes in newborns.
        Child Dev. 1988; 59: 1555-1568
        • Steiner J.E.
        • Glaser D.
        • Hawilo M.E.
        • Berridge K.C.
        Comparative expression of hedonic impact: affective reactions to taste by human infants and other primates.
        Neurosci Biobehav Rev. 2001; 25: 53-74
        • Beauchamp G.K.
        • Cowart B.J.
        • Moran M.
        Developmental changes in salt acceptability in human infants.
        Dev Psychobiol. 1986; 19: 17-25
        • Beauchamp G.K.
        • Cowart B.J.
        Development of sweet taste.
        in: Dobbing J. Sweetness. Springer, London, UK1987: 127-138
        • Beauchamp G.K.
        • Moran M.
        Acceptance of sweet and salty tastes in 2-year-old children.
        Appetite. 1984; 5: 291-305
        • Liem D.G.
        • Mennella J.A.
        Sweet and sour preferences during childhood: role of early experiences.
        Dev Psychobiol. 2002; 41: 388-395
        • Spector A.C.
        • Smith J.C.
        • Hollander G.R.
        A comparison of dependent measures used to quantify radiation-induced taste aversion.
        Physiol Behav. 1981; 27: 887-901
        • Berridge K.C.
        • Kringelbach M.L.
        Affective neuroscience of pleasure: reward in humans and animals.
        Psychopharmacology (Berl). 2008; 199: 457-480
        • Boughter J.D.J.
        • Bachmanov A.A.
        Behavioral genetics and taste.
        BMC Neurosci. 2007; 8: S3
        • Mennella J.A.
        • Griffin C.E.
        • Beauchamp G.K.
        Flavor programming during infancy.
        Pediatrics. 2004; 113: 840-845
        • Forestell C.A.
        • Mennella J.A.
        More than just a pretty face. The relationship between infant's temperament, food acceptance, and mothers' perceptions of their enjoyment of food.
        Appetite. 2012; 58: 1136-1142
        • Forestell C.A.
        • Mennella J.A.
        Early determinants of fruit and vegetable acceptance.
        Pediatrics. 2007; 120: 1247-1254
        • Ekman P.