Taste receptor cells that discriminate between bitter stimuli

Autocrine and paracrine roles for ATP and serotonin in mouse taste buds

Receptor (type II) taste bud cells secrete ATP during taste stimulation. In turn, ATP activates adjacent presynaptic (type III) cells to release serotonin (5-hydroxytryptamine, or 5-HT) and norepinephrine (NE). The roles of these neurotransmitters in taste buds have not been fully elucidated. Here we tested whether ATP or 5-HT exert feedback onto receptor (type II) cells during taste stimulation. Our previous studies showed NE does not appear to act on adjacent taste bud cells, or at least on receptor cells. Our data show that 5-HT released from presynaptic (type III) cells provides negative paracrine feedback onto receptor cells by activating 5-HT(1A) receptors, inhibiting taste-evoked Ca(2+) mobilization in receptor cells, and reducing ATP secretion. The findings also demonstrate that ATP exerts positive autocrine feedback onto receptor (type II) cells by activating P2Y1 receptors and enhancing ATP secretion. These results begin to sort out how purinergic and aminergic transmitters function within the taste bud to modulate gustatory signaling in these peripheral sensory organs.

Bitter taste receptors and their cells

The molecular basis of human bitter taste perception is an area of intense research. Only 25 G protein-coupled receptors belonging to the hTAS2R gene family face the challenge to detect thousands of structurally different bitter compounds, most of which are plant metabolites. Since many natural bitter compounds are highly toxic, whereas others are part of our daily diets, bitter taste was crucial during evolution and still most likely affects our food selection. The article presented here addresses biosynthesis, functional analyses of TAS2Rs and TAS2R variants, as well as gustatory expression of hTAS2R genes.

Discrimination of taste qualities among mouse fungiform taste bud cells

Multiple lines of evidence from molecular studies indicate that individual taste qualities are encoded by distinct taste receptor cells. In contrast, many physiological studies have found that a significant proportion of taste cells respond to multiple taste qualities. To reconcile this apparent discrepancy and to identify taste cells that underlie each taste quality, we investigated taste responses of individual mouse fungiform taste cells that express gustducin or GAD67, markers for specific types of taste cells. Type II taste cells respond to sweet, bitter or umami tastants, express taste receptors, gustducin and other transduction components. Type III cells possess putative sour taste receptors, and have well elaborated conventional synapses. Consistent with these findings we found that gustducin-expressing Type II taste cells responded best to sweet (25/49), bitter (20/49) or umami (4/49) stimuli, while all GAD67 (Type III) taste cells examined (44/44) responded to sour stimuli and a portion of them showed multiple taste sensitivities, suggesting discrimination of each taste quality among taste bud cells. These results were largely consistent with those previously reported with circumvallate papillae taste cells. Bitter-best taste cells responded to multiple bitter compounds such as quinine, denatonium and cyclohexamide. Three sour compounds, HCl, acetic acid and citric acid, elicited responses in sour-best taste cells. These results suggest that taste cells may be capable of recognizing multiple taste compounds that elicit similar taste sensation. We did not find any NaCl-best cells among the gustducin and GAD67 taste cells, raising the possibility that salt sensitive taste cells comprise a different population.

Linking peripheral taste processes to behavior

The act of eating and drinking brings food-related chemicals into contact with taste cells. Activation of these taste cells, in turn, engages neural circuits in the central nervous system that help animals identify foods and fluids, determine what and how much to eat, and prepare the body for digestion and assimilation. Analytically speaking, these neural processes can be divided into at least three categories: stimulus identification, ingestive motivation, and digestive preparation. This review will discuss recent advances in peripheral gustatory mechanisms, primarily from rodent models, in the context of these three major categories of taste function.

Bitter-responsive brainstem neurons: characteristics and functions

The sensation that humans describe as "bitter" is evoked by a large group of chemically diverse ligands. Bitter stimuli are avoided by a range of species and elicit reflex rejection, behaviors considered adaptations to the toxicity of many of these compounds. We review novel evidence for neurons that are narrowly tuned to bitter ligands at the initial stages of central processing. These "B-best" neurons in the nucleus of the solitary tract (NST) and parabrachial nucleus (PBN) respond to multiple types of bitter stimuli and exhibit average responses to bitter tastants that are 6-8 times larger than to moderate concentrations of compounds representing other qualities. However, in the PBN B-best units are appreciably activated by intense salt and acid. Neurons broadly sensitive to salts and acids ("AN" neurons) also responded to bitter stimuli. This sensitivity appeared restricted to stronger intensities of ionic bitters, as cycloheximide remained ineffective across concentrations. In addition to chemosensitive profile, B-best neurons were also distinctive with regard to their posterior receptive fields, long latencies, slow firing rates and projection status. Compared to B-best NST cells, those in the PBN received increased convergence from anterior and posterior receptive fields and responded to a greater number of bitter stimuli. We conclude that B-best neurons likely contribute to pathways underlying gaping, aversive hedonic quality and taste coding. The differential responsiveness of B-best and AN neurons to ionic and nonionic bitter ligands also suggests a potential substrate for discrimination within this quality.

Bitter-responsive gustatory neurons in the rat parabrachial nucleus

Bitterness is a distinctive taste sensation, but central coding for this quality remains enigmatic. Although some receptor cells and peripheral fibers are selectively responsive to bitter ligands, central bitter responses are most typical in broadly tuned neurons. Recently we reported more specifically tuned bitter-best cells (B-best) in the nucleus of the solitary tract (NST). Most had glossopharyngeal receptive fields and few projected to the parabrachial nucleus (PBN), suggesting a role in reflexes. To determine their potential contribution to other functions, the present study investigated whether B-best neurons occur further centrally. Responses from 90 PBN neurons were recorded from anesthetized rats. Stimulation with four bitter tastants (quinine, denatonium, propylthiouracil, cycloheximide) and sweet, umami, salty, and sour ligands revealed a substantial proportion of B-best cells (22%). Receptive fields for B-best NST neurons were overwhelmingly foliate in origin, but in PBN, about half received foliate and nasoincisor duct input. Despite convergence, most B-best PBN neurons were as selectively tuned as their medullary counterparts and response profiles were reliable. Regardless of intensity, cycloheximide did not activate broadly tuned acid/sodium (AN) neurons but did elicit robust responses in B-best cells. However, stronger quinine activated AN neurons and concentrated electrolytes stimulated B-best cells, suggesting that B-best neurons might contribute to higher-order functions such as taste quality coding but work in conjunction with other cell types to unambiguously signal bitter-tasting ligands. In this ensemble, B-best neurons would help discriminate sour from bitter stimuli, whereas AN neurons might be more important in differentiating ionic from nonionic bitter stimuli.

Mammalian bitter taste perception

Bitter taste in mammals is achieved by a family of approximately 30 bitter taste receptor genes. The main function of bitter taste is to protect the organism against the ingestion of, frequently bitter, toxic food metabolites. The field of taste research has advanced rapidly during the last several years. This is especially true for the G-protein-coupled-receptor-mediated taste qualities, sweet, umami, and bitter. This review summarizes current knowledge of bitter taste receptor gene expression, signal transduction, the structure-activity relationship of bitter taste receptor proteins, as well as their variability leading to a high degree of individualization of this taste quality in mammals.

A study of the science of taste: On the origins and influence of the core ideas

Our understanding of the sense of taste is largely based on research designed and interpreted in terms of the traditional four “basic” tastes: sweet, sour, salty, and bitter, and now a few more. This concept of basic tastes has no rational definition to test, and thus it has not been tested. As a demonstration, a preliminary attempt to test one common but arbitrary psychophysical definition of basic tastes is included in this article; that the basic tastes are unique in being able to account for other tastes. This definition was falsified in that other stimuli do about as well as the basic words and stimuli. To the extent that this finding might show analogies with other studies of receptor, neural, and psychophysical phenomena, the validity of the century-long literature of the science of taste based on a few “basics” is called into question. The possible origins, meaning, and influence of this concept are discussed. Tests of the model with control studies are suggested in all areas of taste related to basic tastes. As a stronger alternative to the basic tradition, the advantages of the across-fiber pattern model are discussed; it is based on a rational data-based hypothesis, and has survived attempts at falsification. Such “population coding” has found broad acceptance in many neural systems.

Evidence for two populations of bitter responsive taste cells in mice

Taste receptor cells use multiple signaling mechanisms to detect different taste stimuli in the oral cavity. Ionic stimuli (sour, salty) interact directly with ion channels to elicit responses, whereas bitter, sweet, and umami tastants activate G protein-coupled receptors to initiate phospholipase C (PLC)-dependent release of calcium from intracellular stores. However, the precise role for PLC in taste responses remains unclear. One study reported that bitter, sweet, and umami detection is abolished in PLCbeta2 knock-out animals, indicating that the perception of these stimuli depends solely on PLCbeta2. In contrast, another study found that PLCbeta2 knock-out mice have a reduced, but not abolished, capacity to detect these taste qualities, suggesting a PLCbeta2-independent signaling pathway may be involved in the detection of taste stimuli. Since PLCbeta2-expressing taste cells do not have conventional synapses or express voltage-gated calcium channels (VGCCs), we sought to determine if any taste cells responding to bitter express VGCCs. We characterized calcium responses generated by bitter stimuli to activate the PLC pathway and 50 mM KCl to activate VGCCs. Comparisons of evoked calcium responses found that these two stimuli generated significantly different responses. Surprisingly, although most responsive taste cells responded to bitter or 50 mM KCl, some taste cells responded to both. Analysis of dual responsive cells found that bitter responses were inhibited by the PLC inhibitor U73122. Immunocytochemical analysis detected PLCbeta3 and IP(3)R1, indicating the presence of multiple PLC signaling pathways in taste cells.

Effects of quinine on the intracellular calcium level and membrane potential of PC 12 cultures

The mechanism for the perception of bitterness appears to be quite complicated, even for quinine, which is a model bitter substance, and thus has yet to be completely elucidated. To investigate the possibility of being able to predict the bitterness of quinine solutions, we examined the effects of quinine on intracellular calcium ion concentration ([Ca(2+)]i) and membrane potentials in PC 12 cultures. [Ca(2+)]i and membrane potentials were analysed by fluorescence confocal microscopic imaging using the Ca(2+)-sensitive probe Calcium Green 1/AM and the membrane potential-sensitive probe bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC(4)(3)). Quinine elicited an increase in the membrane potential along with a concentration-dependent increase in [Ca(2+)]i. These increases were inhibited by extracellular Ca(2+)-free conditions, thapsigargin, which is a Ca(2+)-pump inhibitor, and U73122, which is a phospholipase C inhibitor. The quinine-induced increase in [Ca(2+)]i levels was inhibited by nifedipine, an L-type Ca(2+)-channel blocker, omega-conotoxin, a T-type Ca(2+)-channel blocker, and BMI-40, which is a bitterness-masking substance. These results suggest that responses in PC 12 cultures may be used as a simple model of bitterness perception.

Gustatory expression pattern of the human TAS2R bitter receptor gene family reveals a heterogenous population of bitter responsive taste receptor cells

Human bitter taste is mediated by approximately 25 members of the human TAS2 receptor (hTAS2R) gene family. The hTAS2R genes are expressed in taste buds of gustatory papillae on the tongue surface. Because many naturally occurring bitter compounds are toxic, bitter taste receptors are believed to serve as warning sensors against the ingestion of toxic food compounds. An important question is whether bitter taste receptor cells are a homogeneous, broadly tuned population of cells, which uniformly express all bitter taste receptor genes, or not. Gene expression analyses in rodents demonstrated an essentially overlapping expression of TAS2R genes indicating a broad tuning, whereas functional in vivo analyses suggest a narrow tuning. The present study demonstrates the expression of all 25 human TAS2R genes in taste receptor cells of human circumvallate papillae. As shown by in situ hybridization experiments, the expression of hTAS2R genes differs in both the apparent level of expression and the number of taste receptor cells expressing these genes, suggesting a heterogeneous bitter taste receptor cell population. Differences in gene expression levels were verified by quantitative reverse transcription-PCR experiments for a subset of hTAS2R genes. Direct evidence for the heterogeneity of bitter taste receptor cells is provided by dual-labeling in situ hybridizations with selected pairs of hTAS2R gene-specific probes. Functional coexpression experiments in heterologous cells show competition among hTAS2Rs, indicating a possible biological reason for the observed expression pattern. From the data, we conclude that human bitter taste receptor cells are tuned to detect a limited subset of bitter stimuli.

How do predators cope with chemically defended foods?

Many prey species (including plants) deter predators with defensive chemicals. These defensive chemicals act by rendering the prey's tissues noxious, toxic, or both. Here, I explore how predators cope with the presence of these chemicals in their diet. First, I describe the chemosensory mechanisms by which predators (including herbivores) detect defensive chemicals. Second, I review the mechanisms by which predators either avoid or tolerate defensive chemicals in prey. Third, I examine how effectively free-ranging predators can overcome the chemical defenses of prey. The available evidence indicates that predators have mixed success overcoming these defenses. This conclusion is based on reports of free-ranging predators rejecting unpalatable but harmless prey, or voluntarily ingesting toxic prey.

Taste receptor genes

In the past several years, tremendous progress has been achieved with the discovery and characterization of vertebrate taste receptors from the T1R and T2R families, which are involved in recognition of bitter, sweet, and umami taste stimuli. Individual differences in taste, at least in some cases, can be attributed to allelic variants of the T1R and T2R genes. Progress with understanding how T1R and T2R receptors interact with taste stimuli and with identifying their patterns of expression in taste cells sheds light on coding of taste information by the nervous system. Candidate mechanisms for detection of salts, acids, fat, complex carbohydrates, and water have also been proposed, but further studies are needed to prove their identity.

Comparison of l-monosodium glutamate and l-amino acid taste in rats

T1R2/T1R3 heterodimers are selectively responsive to sweet substances whereas T1R1/T1R3 receptors are selective for umami substances, represented by monosodium glutamate (MSG), and for L-amino acids. If a single receptor is responsible for detection of umami and L-amino acids, then it would be predicted that MSG and L-amino acids elicit similar tastes in rats. The present study compared the taste profile of MSG with four amino acids (glycine, L-proline, L-serine and L-arginine) using conditioned taste aversion, detection threshold, and taste discrimination methods. These experiments were designed to either reduce or neutralize the taste of sodium associated with MSG and the other amino acids. Detection threshold studies showed that rats were most sensitive to L-arginine and least sensitive to L-proline. Glycine and L-serine thresholds were similar to those previously reported for MSG. Like MSG, a conditioned taste aversion to each of the four amino acids generalized to sucrose in the presence of amiloride, a sodium channel blocker. Rats showed moderate generalization of aversion between MSG and L-arginine, suggesting that these two amino acids taste only moderately alike. However, the taste aversion experiments indicated that glycine, L-serine, and L-proline elicit taste sensations similar to MSG when amiloride is present. Discrimination experiments further compared the tastes of these three amino acids with MSG. When the sodium taste associated with MSG was reduced or neutralized, glycine and L-proline elicited tastes very similar but not identical to the taste of MSG. Low (but not higher) concentrations of L-serine were also difficult for rats to discriminate from MSG. While there are taste qualities common to all of these amino acids, the perceptual differences found in this study, combined with previous reports, suggest either multiple taste receptors and/or multiple signaling pathways may be involved in umami and amino acid taste perception in rats.

Signal transduction and information processing in mammalian taste buds

The molecular machinery for chemosensory transduction in taste buds has received considerable attention within the last decade. Consequently, we now know a great deal about sweet, bitter, and umami taste mechanisms and are gaining ground rapidly on salty and sour transduction. Sweet, bitter, and umami tastes are transduced by G-protein-coupled receptors. Salty taste may be transduced by epithelial Na channels similar to those found in renal tissues. Sour transduction appears to be initiated by intracellular acidification acting on acid-sensitive membrane proteins. Once a taste signal is generated in a taste cell, the subsequent steps involve secretion of neurotransmitters, including ATP and serotonin. It is now recognized that the cells responding to sweet, bitter, and umami taste stimuli do not possess synapses and instead secrete the neurotransmitter ATP via a novel mechanism not involving conventional vesicular exocytosis. ATP is believed to excite primary sensory afferent fibers that convey gustatory signals to the brain. In contrast, taste cells that do have synapses release serotonin in response to gustatory stimulation. The postsynaptic targets of serotonin have not yet been identified. Finally, ATP secreted from receptor cells also acts on neighboring taste cells to stimulate their release of serotonin. This suggests that there is important information processing and signal coding taking place in the mammalian taste bud after gustatory stimulation.

Individual differences in perceived bitterness predict liking of sweeteners

Although recent molecular studies suggest that only one receptor and one signaling pathway are involved in the perception of sweetness, this seems to contradict everyday experience that people not only have different likes and dislikes of certain sweeteners but also perceive the sweeteners differently. One possible explanation is that variation in liking of sweeteners is due, in part, to variation across individuals in sensitivity to nonsweet tastes, such as bitterness, which are transduced by a variety of receptors. Fifty individuals were asked to rate intensities of several taste attributes of 10 sweeteners and to give hedonic assessments of each sweetener. Additionally, their sensitivity to 6-n-propyl-3-thiouracil (PROP) was determined. Results indicated that when matched for sweetness, the perception of bitterness and the sweetener compound were the 2 largest factors contributing to overall liking of a sweetener. Sensitivity to PROP did not contribute significantly to the model.

Architecture of the primary taste center ofDrosophila melanogasterlarvae

A simple nervous system combined with stereotypic behavioral responses to tastants, together with powerful genetic and molecular tools, have turned Drosophila larvae into a very promising model for studying gustatory coding. Using the Gal4/UAS system and confocal microscopy for visualizing gustatory afferents, we provide a description of the primary taste center in the larval central nervous system. Essentially, gustatory receptor neurons target different areas of the subesophageal ganglion (SOG), depending on their segmental and sensory organ origin. We define two major and two smaller subregions in the SOG. One of the major areas is a target of pharyngeal sensilla, the other one receives inputs from both internal and external sensilla. In addition to such spatial organization of the taste center, circumstantial evidence suggests a subtle functional organization: aversive and attractive stimuli might be processed in the anterior and posterior part of the SOG, respectively. Our results also suggest less coexpression of gustatory receptors than proposed in prior studies. Finally, projections of putative second-order taste neurons seem to cover large areas of the SOG. These neurons may thus receive multiple gustatory inputs. This suggests broad sensitivity of secondary taste neurons, reminiscent of the situation in mammals.

Signal transduction and information processing in mammalian taste buds

The molecular machinery for chemosensory transduction in taste buds has received considerable attention within the last decade. Consequently, we now know a great deal about sweet, bitter, and umami taste mechanisms and are gaining ground rapidly on salty and sour transduction. Sweet, bitter, and umami tastes are transduced by G-protein-coupled receptors. Salty taste may be transduced by epithelial Na channels similar to those found in renal tissues. Sour transduction appears to be initiated by intracellular acidification acting on acid-sensitive membrane proteins. Once a taste signal is generated in a taste cell, the subsequent steps involve secretion of neurotransmitters, including ATP and serotonin. It is now recognized that the cells responding to sweet, bitter, and umami taste stimuli do not possess synapses and instead secrete the neurotransmitter ATP via a novel mechanism not involving conventional vesicular exocytosis. ATP is believed to excite primary sensory afferent fibers that convey gustatory signals to the brain. In contrast, taste cells that do have synapses release serotonin in response to gustatory stimulation. The postsynaptic targets of serotonin have not yet been identified. Finally, ATP secreted from receptor cells also acts on neighboring taste cells to stimulate their release of serotonin. This suggests that there is important information processing and signal coding taking place in the mammalian taste bud after gustatory stimulation.

Cycloheximide: no ordinary bitter stimulus

Cycloheximide (CyX), a toxic antibiotic with a unique chemical structure generated by the actinomycete, Streptomyces griseus, has emerged as a primary focus of studies on mammalian bitter taste. Rats and mice avoid it at concentrations well below the thresholds for most bitter stimuli and T2R G-protein-coupled receptors specific for CyX with appropriate sensitivity are identified for those species. Like mouse and rat, golden hamsters, Mesocricetus auratus, also detected and rejected micromolar levels of CyX, although 1mM CyX failed to activate the hamster chorda tympani nerve. Hamsters showed an initial tolerance for 500microM CyX, but after that, avoidance of CyX dramatically increased, plasticity not reported for rat or mouse. As the hamster lineage branches well before division of the mouse-rat lineage in evolutionary time, differences between hamster and mouse-rat reactions to CyX are not surprising. Furthermore, unlike hamster LiCl-induced learned aversions, the induced CyX aversion neither specifically nor robustly generalized to other non-ionic bitter stimuli; and unlike adverse reactions to other chemosensory stimuli, aversions to CyX were not mollified by adding a sweetener. Thus, CyX is unlike other bitter stimuli. The gene for the high-affinity CyX receptor is a member of a cluster of five orthologous T2R genes that are likely rodent-specific; this "CyX clade" is found in the mouse, rat and probably hamster, but not in the human or rabbit genome. The rodent CyX-T2R interaction may be one of multiple lineage-specific stimulus-receptor interactions reflecting a response to a particular environmental toxin. The combination of T2R multiplicity, species divergence and gene duplication results in diverse ligands for multiple species-specific T2R receptors, which confounds definition of 'bitter' stimuli across species.

Single neurons in the nucleus of the solitary tract respond selectively to bitter taste stimuli

Molecular data suggest that receptors for all bitter ligands are coexpressed in the same taste receptor cells (TRCs), whereas physiological results indicate that individual TRCs respond to only a subset of bitter stimuli. It is also unclear to what extent bitter-responsive neurons are stimulated by nonbitter stimuli. To explore these issues, single neuron responses were recorded from the rat nucleus of the solitary tract (NST) during whole mouth stimulation with a variety of bitter compounds: 10 microM cycloheximide, 7 mM propylthiouracil, 10 mM denatonium benzoate, and 3 mM quinine hydrochloride at intensities matched for behavioral effectiveness. Stimuli representing the remaining putative taste qualities were also tested. Particular emphasis was given to activating taste receptors in the foliate papillae innervated by the quinine-sensitive glossopharyngeal nerve. This method revealed a novel population of bitter-best (B-best) cells with foliate receptive fields and significant selectivity for bitter tastants. Across all neurons, multidimensional scaling depicted bitter stimuli as loosely clustered yet clearly distinct from nonbitter tastants. When neurons with posterior receptive fields were analyzed alone, bitter stimuli formed a tighter cluster. Nevertheless, responses to bitter stimuli were variable across B-best neurons, with cycloheximide the most, and quinine the least frequent optimal stimulus. These results indicate heterogeneity for the processing of ionic and nonionic bitter tastants, which is dependent on receptive field. Further, they suggest that neurons selective for bitter substances could contribute to taste coding.

Effect of sucrose on the properties of caffeine adsorption layers at the air/solution interface

Sweet and bitter tastes are known to be mediated by G-protein-coupled receptors. The relationship between the chemical structure of gustable molecules and their molecular organization in saliva (aqueous solution) near the surface of the tongue provides a useful tool for elucidating the mechanism of chemoreception. The interactions between stimulus and membrane receptors occur in an anisotropic system. They might be influenced by the molecular packing of gustable molecules within an aqueous solvent (saliva) close to the receptor protein. To investigate the molecular organization of a sweet molecule (sucrose), a bitter molecule (caffeine), and their mixture in an aqueous phase near a "wall", a hydrophobic phase, we modeled this using an air/liquid interface as an anisotropic system. The experimental (tensiometry and ellipsometry) data unambiguously show that caffeine molecules form an adsorption layer, whereas sucrose induces a desorption layer at the air/water interface. The adsorption of caffeine molecules at the air/water interface gradually increases with the volume concentration and is delayed when sucrose is added to the solution. Spectroscopic ellipsometry data show that caffeine in the adsorption layer has optical properties practically identical to those of the molecule in solution. The results are interpreted in terms of molecular association of caffeine with itself at the interface with and without sucrose in the subphase, using the theory of ideal gases.

The representation of taste quality in the mammalian nervous system

The process by which the mammalian nervous system represents the features of a sapid stimulus that lead to a perception of taste quality has long been controversial. The labeled-line (sparse coding) view differs from the across-neuron pattern (ensemble) counterpoint in proposing that activity in a given class of neurons is necessary and sufficient to generate a specific taste perception. This article critically reviews molecular, electro-physiological, and behavioral findings that bear on the issue. In the peripheral gustatory system, the authors conclude that most qualities appear to be signaled by labeled lines; however, elements of both types of coding characterize signaling of sodium salts. Given the heterogeneity of neuronal tuning functions in the brain, the central coding mechanism is less clear. Both sparse coding and neuronal ensemble models remain viable possibilities. Furthermore, temporal patterns of discharge could contribute additional information. Ultimately, until specific classes of neurons can be selectively manipulated and perceptual consequences assessed, it will be difficult to go beyond mere correlation and conclusively discern the validity of these coding models.

Separate populations of receptor cells and presynaptic cells in mouse taste buds

Taste buds are aggregates of 50-100 cells, only a fraction of which express genes for taste receptors and intracellular signaling proteins. We combined functional calcium imaging with single-cell molecular profiling to demonstrate the existence of two distinct cell types in mouse taste buds. Calcium imaging revealed that isolated taste cells responded with a transient elevation of cytoplasmic Ca2+ to either tastants or depolarization with KCl, but never both. Using single-cell reverse transcription (RT)-PCR, we show that individual taste cells express either phospholipase C beta2 (PLCbeta2) (an essential taste transduction effector) or synaptosomal-associated protein 25 (SNAP25) (a key component of calcium-triggered transmitter exocytosis). The two functional classes revealed by calcium imaging mapped onto the two gene expression classes determined by single-cell RT-PCR. Specifically, cells responding to tastants expressed PLCbeta2, whereas cells responding to KCl depolarization expressed SNAP25. We demonstrate this by two methods: first, through sequential calcium imaging and single-cell RT-PCR; second, by performing calcium imaging on taste buds in slices from transgenic mice in which PLCbeta2-expressing taste cells are labeled with green fluorescent protein. To evaluate the significance of the SNAP25-expressing cells, we used RNA amplification from single cells, followed by RT-PCR. We show that SNAP25-positive cells also express typical presynaptic proteins, including a voltage-gated calcium channel (alpha1A), neural cell adhesion molecule, synapsin-II, and the neurotransmitter-synthesizing enzymes glutamic acid decarboxylase and aromatic amino acid decarboxylase. No synaptic markers were detected in PLCbeta2 cells by either amplified RNA profiling or by immunocytochemistry. These data demonstrate the existence of at least two molecularly distinct functional classes of taste cells: receptor cells and synapse-forming cells.

Characterization and long-term maintenance of rat taste cells in culture

Taste cells have a limited life span and are replaced from a basal cell population, although the specific factors involved in this process are not well known. Short- and long-term cultures of other sensory cells have facilitated efforts to understand the signals involved in proliferation, differentiation, and senescence, yet few studies have reported successful primary culture protocols for taste cells. Furthermore, no studies have demonstrated both proliferation and differentiation in vitro. In this study, we have developed an in vitro culture system to maintain and utilize rat primary taste cells for more than 2 months without losing key molecular and biochemical features. Gustducin, phospholipase C-beta2 (PLC-beta2), T1R3, and T2R5 mRNA were detected in the cultured cells by reverse transcriptase-polymerase chain reaction. Western blot analysis demonstrated gustducin and PLC-beta2 expression in the same samples, which was confirmed by immunocytochemistry. Labeling with bromo-2-deoxyuridine (BrdU) demonstrated proliferation, and a subset of BrdU-labeled cells were also immunoreactive for either gustducin or PLC-beta2, indicating differentiation of newly generated cells in vitro. Cultured cells also exhibited increases in intracellular calcium in response to several taste stimuli. These results indicate that taste cells from adult rats can be generated and maintained under the described conditions for at least 2 months. This system will enable further studies of the processes involved in proliferation, differentiation, and function of mammalian taste receptor cells in an in vitro preparation.

Taste recognition: food for thought

The ability to identify food that is nutrient-rich and avoid toxic substances is essential for an animal's survival. Although olfaction and vision contribute to food detection, the gustatory system acts as a final checkpoint control for food acceptance or rejection behavior. Recent studies with model organisms such as mice and Drosophila have identified candidate taste receptors and examined the logic of taste coding in the periphery. Despite differences in terms of gustatory anatomy and taste-receptor families, these gustatory systems share a basic organization that is different from other sensory systems. This review will summarize our current understanding of taste recognition in mammals and Drosophila, highlighting similarities and raising several as yet unanswered questions.

Differential covariation in taste responsiveness to bitter stimuli in rats

Variation exists in the sensitivity of individual rodents and humans to different bitter tastants. An absence of uniform correlation in responsiveness to different bitter substances across individuals within a species suggests heterogeneity in the mechanisms underlying stimulus processing within this taste modality. Here, we examined taste responsiveness of individual rats to three bitter compounds (quinine hydrochloride, denatonium benzoate, and cycloheximide) in short-term lick tests to determine the magnitude of covariation among responses to these stimuli and infer commonalities in their receptor and neural mechanisms. Rats were tested with a given pair of bitter stimuli during three sessions comprising randomized trial blocks of six concentrations of each stimulus + deionized water. Psychophysical functions were generated for individual rats for respective stimulus pairs, and concentrations of each stimulus that produced equivalent lick suppression relative to water were correlated across animals. Behavioral taste responsiveness to quinine hydrochloride strongly covaried with responsiveness to denatonium benzoate (r = +0.82). Lick responsiveness to quinine was less robustly correlated with that to cycloheximide (r = +0.44), and denatonium and cycloheximide responses failed to correlate. These results imply substantial overlap in the bitter taste coding mechanisms for quinine and denatonium but some degree of independence in the mechanisms responsible for gustatory processing of cycloheximide. More generally, these data reinforce the notion that bitter taste processing is not a homogeneous event.

Neural representation of bitter taste in the nucleus of the solitary tract

Based on the molecular findings that many bitter taste receptors (T2Rs) are expressed within the same receptor cells, it has been proposed that bitter taste is encoded by the activation of discrete neural elements. Here we examined how a variety of bitter stimuli are represented by neural activity in central gustatory neurons. Taste responses (spikes/s) evoked by bathing the tongue and palate with intensity-matched concentrations (in M) of 2 sugars (0.32 sucrose and 0.5 D-fructose), ethanol (40%), 4 salts (0.01 NaCl, 0.008 NaNO(3), 0.01 MgCl(2), and 0.05 KCl), 2 acids (0.003 HCl and 0.005 citric acid), and 10 bitter ligands (0.007 quinine-HCl, 0.015 denatonium benzoate, 0.003 l-cysteine, 0.001 nicotine, 0.005 strychnine-HCl, 0.04 tetraethylammonium chloride, 0.03 atropine-SO(4), 0.005 brucine-SO(4), 0.03 papaverine-HCl, and 0.009 sparteine) were recorded from 51 neurons in the nucleus of the solitary tract of anesthetized rats. Cluster analysis was used to categorize neurons into types based on responses to sucrose, NaCl, HCl, and quinine-HCl. Three groupings emerged: type S (responded optimally to sweets), type N (sodium-optimal), and type H/Q (responded robustly to bitters, acids, and salts). Multivariate analyses revealed that across-neuron patterns of response among bitter stimuli were strongly correlated. However, neural type H/Q, which was most responsive to bitter tastants, was not differentially sensitive to bitter stimuli and Na(+) salts, which rats perceive as distinct. Thus central neurons most responsive to bitter substances receive significant input from receptors that mediate other tastes, indicating that bitter stimuli are not represented by activity in specifically tuned neurons.

Genomic organization, expression, and function of bitter taste receptors (T2R) in mouse and rat

Mammalian type 2 taste receptors (T2R) are a family of G protein-coupled receptors that mediate bitter signals in taste cells. In the present study, we compared the genomic organization of rodent T2R genes based on the recently completed mouse and rat genomes and examined tissue- and cell-specific expression of T2Rs. Both mouse and rat T2R families consist of 36 intact genes and at least 7 pseudogenes that are mapped to mouse chromosomes 15, 2, and 6 and to rat chromosomes 2, 3, and 4, respectively. All but two T2R genes are clustered on mouse chromosome 6 and rat chromosome 4 with virtually identical genomic organization. The orthologs of the first human T2R gene identified, mT2R119 and rT2R1, are located on mouse chromosome 15 and rat chromosome 2, whereas the novel rodent-specific T2R genes, mT2R134 and rT2R34, are located on mouse chromosome 2 and rat chromosome 3, respectively. Our results, using RT-PCR, demonstrate the presence of transcripts corresponding to the putative denatonium benzoate (DB) and phenylthiocarbamide (PTC) receptors in the antrum, fundus, and duodenum as well as in STC-1 and AR42J cells. The novel rodent-specific T2R gene (mT2R134 and rT2R34) was also expressed in these tissues and cell lines. The addition of DB, PTC, or cycloheximide to AR42J cells induced a rapid increase in the intracellular Ca(2+) concentration. The specificity of these effects is shown by the fact that these bitter stimuli did not induce any detectable Ca(2+) signaling in many other rodent or human cells that do not express receptors or G proteins implicated in bitter taste signaling. These results demonstrate that mouse and rat T2R genes are highly conserved in terms of genomic organization and tissue expression, suggesting that rodent T2Rs are evolved under similar dietary pressure and share bitter sensing functions in the lingual and gastrointestinal systems.

Psychophysical investigations of cetylpyridinium chloride in rats: its inherent taste and modifying effects on salt taste

Salts are transduced by at least 2 mechanisms: (a) antagonized by amiloride and (b) antagonized by cetylpyridinium chloride (CPC). The authors report on 4 behavioral experiments in rats that characterize the orosensory properties of CPC itself as well as its effect in suppressing the intensity of NaCl and KCl taste. Experiments 1 and 2 indicated that CPC has a quinine-like taste quality. Experiments 3 and 4 demonstrated that the recognition of KCl, but not NaCl, is modestly reduced by mixture with CPC. However, control experiments call into question the mechanism of the salt suppression of CPC, because both CPC-salt and quinine-salt mixtures had similar effects. The relevance of these studies for understanding salt and bitter taste coding is discussed.

The receptors and coding logic for bitter taste

The sense of taste provides animals with valuable information about the nature and quality of food. Bitter taste detection functions as an important sensory input to warn against the ingestion of toxic and noxious substances. T2Rs are a family of approximately 30 highly divergent G-protein-coupled receptors (GPCRs) that are selectively expressed in the tongue and palate epithelium and are implicated in bitter taste sensing. Here we demonstrate, using a combination of genetic, behavioural and physiological studies, that T2R receptors are necessary and sufficient for the detection and perception of bitter compounds, and show that differences in T2Rs between species (human and mouse) can determine the selectivity of bitter taste responses. In addition, we show that mice engineered to express a bitter taste receptor in 'sweet cells' become strongly attracted to its cognate bitter tastants, whereas expression of the same receptor (or even a novel GPCR) in T2R-expressing cells resulted in mice that are averse to the respective compounds. Together these results illustrate the fundamental principle of bitter taste coding at the periphery: dedicated cells act as broadly tuned bitter sensors that are wired to mediate behavioural aversion.

Elucidation of mammalian bitter taste

A family of approximately 30 TAS2R bitter taste receptors has been identified in mammals. Their genes evolved through adaptive diversification and are linked to chromosomal loci known to influence bitter taste in mice and humans. The agonists for various TAS2Rs have been identified and all of them were established as bitter tastants. TAS2Rs are broadly tuned to detect multiple bitter substances, explaining, in part, how mammals can recognize numerous bitter compounds with a limited set of receptors. The TAS2Rs are expressed in a subset of taste receptor cells, which are distinct from those mediating responses to other taste qualities. However, cells devoted to the detection of sweet, umami, and bitter stimuli share common signal transduction components. Transgenic expression of a human TAS2R in sweet or bitter taste receptor-expressing cells of mice induced either strong attraction or aversion to the receptor's cognate bitter tastant. Thus, dedicated taste receptor cells appear to function as broadly tuned detectors for bitter substances and are wired to elicit aversive behavior.

Effect of humulus lupulus on Gastric Secretion in a Rat Pylorus-Ligated Model

In this study, we investigated the pharmacological effect of humulus lupulus (hops) on gastric juice volume and acidity using a rat pylorus-ligated model. In an intraorally administered experiment, hops clearly increased gastric juice volume without affecting acidity. On the other hand, hops had no influence on gastric juice volume when it was intragastrically administered. A cholinergic agonist, carbachol, increased gastric juice volume without affecting acidity, whereas histamine increased gastric juice volume and acidity. The increase of gastric juice volume induced by carbachol was completely inhibited by atropine. On the other hand, atropine did not inhibit the increase in gastric juice volume induced by histamine. The increase in gastric juice volume induced by hops was completely inhibited by atropine. These results suggested that the increase in gastric juice volume induced by intraorally administered hops could be mediated by the cholinergic nervous system.

Nerve regeneration-induced recovery of quinine avoidance after complete gustatory deafferentation of the tongue

The concentration-dependent decrease in quinine licking by rats is substantially attenuated by combined bilateral transection of the chorda tympani (CT) and glossopharyngeal (GL) nerves, but transection of either nerve alone produces marginal impairments at most. Here we tested whether regeneration of one or both of these nerves after combined transection would result in recovery of taste avoidance. Water-restricted rats were presented with a series of brief-access (5 s) taste trials (water and 0.003–3.0 mM quinine-HCl) in a 5-day test block of 40-min sessions both before nerve transection and starting 75–77 days after transection. Licking avoidance returned to presurgical levels when both nerves were allowed to regenerate. When only the GL was allowed to regenerate, performance did not differ from that of sham-transected animals. This suggests that even after considerable gustatory deafferentation, regeneration has the capacity to restore normal taste-guided behavior. Surprisingly, when only the CT was allowed to regenerate, avoidance behavior was severely impaired and was not different from that of rats in which regeneration of both nerves was prevented. Taking into account prior findings, it appears that the absence of the GL in the presence of an intact CT is fundamentally different from the absence of the GL in the presence of a regenerated CT with respect to some taste functions. This represents the first reported instance to our knowledge in which the capacity of a regenerated nerve to maintain taste-guided behavior was distinctly different from that of an intact nerve in a rodent model.

Acid-sensing ion channel-2 is not necessary for sour taste in mice

The acid-sensitive cation channel acid-sensing ion channel-2 (ASIC2) is widely believed to be a receptor for acid (sour) taste in mammals on the basis of its physiological properties and expression in rat taste bud cells. Using reverse transcriptase-PCR, we detected expression of ASIC1 and ASIC3, but not ASIC4, in mouse and rat taste buds and nonsensory lingual epithelium. Surprisingly, we did not detect mRNA for ASIC2 in mouse taste buds, although we readily observed its expression in rat taste buds. Furthermore, in Ca2+ imaging experiments, ASIC2 knock-out mice exhibited normal physiological responses (increases in intracellular Ca2+ concentrations) to acid taste stimuli. Our results indicate that ASIC2 is not required for acid taste in mice, and that if a universal mammalian acid taste transduction mechanism exists, it likely uses other acid-sensitive receptors or ion channels.

Saccharin stimulates the "deterrent" cell in the blowfly: behavioral and electrophysiological evidence

In the attempt to gain more information on the mechanisms underlying bitter and/or sweet taste reception, we have investigated the responses of labellar chemosensilla in the blowfly Protophormia terraenovae to Na-saccharin, as compared to sweet stimuli (sucrose or fructose) and bitter stimuli (denatonium benzoate or amiloride). Electrophysiological and behavioral results indicate that the sweetener Na-saccharin inhibits the "sugar" cell in the labellar taste sensilla of the blowfly P. terraenovae. In multichoice preference tests, flies ingested more of the solutions containing sugar to those with sugar+Na-saccharin. This finding is in good agreement with the spike frequency reduction observed for the "sugar" cell activity. Analysis of the spike discharges also shows a positive dose-response for the "deterrent" cell following stimulation with Na-saccharin and denatonium benzoate. Flies drank any of the Na-saccharin solutions, regardless of their concentration, less than water, thus indicating a weak deterring effect on water drinking. The prevailing activation of the "deterrent" cell by stimulation with Na-saccharin is not directly coupled with a coherent behavioral output. Cross adaptation was found to occur between responses to Na-saccharin and denatonium benzoate or amiloride regardless of the order of adapting stimuli. In the case of sweet stimuli, cross adaptation occurred when the adapting stimulus was Na-saccharin, but it did not when the adapting stimuli were sucrose or fructose. Addition of Na-saccharin to both sugars significantly depressed the spike firing frequency, while an increase was observed with denatonium benzoate or amiloride.

The distinctiveness of ionic and nonionic bitter stimuli

The diverse chemical structures of stimuli that are bitter to humans suggest a need for multiple bitter receptors. Reactions of golden hamsters (Mesocricetus auratus) to 1 mM quinine hydrochloride, 3 mM denatonium benzoate, 180 mM magnesium sulfate, 30-100 mM caffeine, and 1-1.5 mM sucrose octaacetate (SOA) were studied to address whether there are multiple sensations elicited by bitter stimuli. Methods included behavioral generalization of LiCl-induced conditioned taste aversions (CTAs), intake preference tests, and electrophysiological recordings from the chorda tympani (CT) nerve. The five compounds, all bitter to humans, were all innately aversive to hamsters. CTA for the ionic quinine.HCl, denatonium benzoate, and MgSO(4) mutually cross-generalized and these ionic compounds were effective CT stimuli. Yet, the hamsters were much less sensitive to denatonium than humans, requiring a 100,000 times higher concentration for detection. CTA for nonionic caffeine and SOA did not cross-generalize to quinine or the other two ionic stimuli and these nonionic compounds were not effective CT stimuli. SOA and caffeine may elicit aversive reflexes or systemic reactions rather than taste sensations in the animals. Thus, the three ionic and two nonionic compounds form separate aversive stimulus classes in hamsters, neither of which appears to be a close homologue of the human bitter taste.

Acid-sensitive two-pore domain potassium (K2P) channels in mouse taste buds

Sour (acid) taste is postulated to result from intracellular acidification that modulates one or more acid-sensitive ion channels in taste receptor cells. The identity of such channel(s) remains uncertain. Potassium channels, by regulating the excitability of taste cells, are candidates for acid transducers. Several 2-pore domain potassium leak conductance channels (K(2)P family) are sensitive to intracellular acidification. We examined their expression in mouse vallate and foliate taste buds using RT-PCR, and detected TWIK-1 and -2, TREK-1 and -2, and TASK-1. Of these, TWIK-1 and TASK-1 were preferentially expressed in taste cells relative to surrounding nonsensory epithelium. The related TRESK channel was not detected, whereas the acid-insensitive TASK-2 was. Using confocal imaging with pH-, Ca(2+)-, and voltage-sensitive dyes, we tested pharmacological agents that are diagnostic for these channels. Riluzole (500 microM), selective for TREK-1 and -2 channels, enhanced acid taste responses. In contrast, halothane (< or = approximately 17 mM), which acts on TREK-1 and TASK-1 channels, blocked acid taste responses. Agents diagnostic for other 2-pore domain and voltage-gated potassium channels (anandamide, 10 microM; Gd(3+), 1 mM; arachidonic acid, 100 microM; quinidine, 200 microM; quinine, 100 mM; 4-AP, 10 mM; and TEA, 1 mM) did not affect acid responses. The expression of 2-pore domain channels and our pharmacological characterization suggest that a matrix of ion channels, including one or more acid-sensitive 2-pore domain K channels, could play a role in sour taste transduction. However, our results do not unambiguously identify any one channel as the acid taste transducer.

Cooperation between Drosophila flies in searching behavior

In Drosophila melanogaster food search behaviour, groups of flies swarm around and aggregate on patches of food. We wondered whether flies explore their environment in a cooperative way as interactions between individual flies within a population might influence the flies' ability to locate food sources. We have shown that the food search behavior in the fruit fly Drosophila is a two-step process. Firstly, 'primer' flies search the environment and randomly land on different food patches. Secondly, the remaining group of flies move to the most favorable food source and aggregate there. We call this a 'search-aggregation' cycle. Our data demonstrate that flies do not individually assess all available food resources. Rather, social interactions between flies appear to affect their choice of a specific food patch. A genetic analysis of this 'search-aggregation' behavior shows that flies carrying mutations in specific genes (for example, the dunce (dnc) gene which codes for a phosphodiesterase) were defective in this search-aggregation behavior when compared to normal flies. Future investigations of the neuronal signaling involved in this behavior will help us to understand the complexities of this aspect of Drosophila social behaviour.

Physiological phenotyping of cholecystokinin-responsive rat taste receptor cells

The recent discovery that subsets of rat taste receptor cells (TRCs) express the peptide cholecystokinin (CCK) and that subsets of TRCs respond to CCK with altered potassium currents or elevated intracellular calcium via CCK-A receptor has lead to the hypothesis that CCK may play a novel signaling role within the taste bud, perhaps modifying tastant responses by co-transmission with a classic transmitter. To better understand this phenomenon, CCK-responsive TRCs were characterized for sensitivity to two bitter stimuli, quinine or caffeine, or to the neurotransmitter ACh using a ratiometric procedure with the calcium sensitive dye fura-2. In characterizing TRC responses to quinine, it was observed that quinine-induced elevations of intracellular calcium were not due to endogenous fluorescence of the quinine molecule. Most (60-70%) CCK-responsive cells were also sensitive to either bitter stimuli or to cholinergic stimulation. These data suggest that TRCs expressing CCK-receptors also express receptors to bitter stimuli and/or muscarinic receptors. They further support the notion of a putative modulatory role of CCK with convergence of multiple inputs occurring at the level of intracellular calcium.

Two antagonistic gustatory receptor neurons responding to sweet-salty and bitter taste inDrosophila

In Drosophila, gustatory receptor neurons (GRNs) occur within hair-like structures called sensilla. Most taste sensilla house four GRNs, which have been named according to their preferred sensitivity to basic stimuli: water (W cell), sugars (S cell), salt at low concentration (L1 cell), and salt at high concentration (L2 cell). Labellar taste sensilla are classified into three types, l-, s-, and i-type, according to their length and location. Of these, l- and s-type labellar sensilla possess these four cells, but most i-type sensilla house only two GRNs. In i-type sensilla, we demonstrate here that the first GRN responds to sugar and to low concentrations of salt (10-50 mM NaCl). The second GRN detects a range of bitter compounds, among which strychnine is the most potent; and also to salt at high concentrations (over 400 mM NaCl). Neither type of GRN responds to water. The detection of feeding stimulants in i-type sensilla appears to be performed by one GRN with the combined properties of S+L1 cells, while the other GRN detects feeding inhibitors in a similar manner to bitter-sensitive L2 cells on the legs. These sensilla thus house two GRNs having an antagonistic effect on behavior, suggesting that the expression of taste receptors is segregated across them accordingly.

Role of the G-protein subunit alpha-gustducin in taste cell responses to bitter stimuli

Many bitter stimuli are believed to bind to specific G-protein-coupled membrane receptors on taste cells. Despite the compelling evidence for its pivotal role in bitter taste sensation, a direct involvement of the G-protein subunit alpha-gustducin in bitter taste transduction in taste cells has not been demonstrated in situ at the cellular level. We recorded activation of taste cells by bitter stimuli using Ca2+ imaging in lingual slices and examined alpha-gustducin immunoreactivity in the same cells. In mice vallate papillae, many, but not all, bitter-responsive cells expressed alpha-gustducin. In agreement with this correlation, the incidence of cells responding to bitter stimuli was reduced by 70% in mutant mice lacking alpha-gustducin. Nevertheless, some taste cells lacking alpha-gustducin responded to bitter stimuli, suggesting that other G-protein alpha subunits are involved. We found that the G-protein alpha subunit Galpha(i2) is present in most bitter-responsive cells and thus may also play a role in bitter taste transduction. The reduced behavioral sensitivity to bitter stimuli in alpha-gustducin knock-out mice thus appears to be the consequence of a reduced number of bitter-activated taste cells, as well as reduced sensitivity.

Channels as taste receptors in vertebrates

Taste reception is fundamental for proper selection of food and beverages. Chemicals detected as taste stimuli by vertebrates include a large variety of substances, ranging from inorganic ions (e.g., Na(+), H(+)) to more complex molecules (e.g., sucrose, amino acids, alkaloids). Specialized epithelial cells, called taste receptor cells (TRCs), express specific membrane proteins that function as receptors for taste stimuli. Classical view of the early events in chemical detection was based on the assumption that taste substances bind to membrane receptors in TRCs without permeating the tissue. Although this model is still valid for some chemicals, such as sucrose, it does not hold for small ions, such as Na(+), that actually diffuse inside the taste tissue through ion channels. Electrophysiological, pharmacological, biochemical, and molecular biological studies have provided evidence that indeed TRCs use ion channels to reveal the presence of certain substances in foodstuff. In this review, we focus on the functional and molecular properties of ion channels that serve as receptors in taste transduction.

An artificial taste sensor based on conducting polymers

Pure and composite nanostructured films of conducting polymers were used as individual sensing units constituting an electronic tongue. The use of extremely thin films for signal transduction via impedance spectroscopy measurements in the frequency range 10-1 MHz allows the detection of trace amounts of tastants and inorganic contaminants in liquid systems. In addition, the sensor could detect the suppression of sourness by sweetness displaying similarities with the biological system. Brands of several commercial beverages could be easily distinguished without complex analysis, including the discrimination of waters, tastants and wines.

Identification and characterization of human taste receptor genes belonging to the TAS2R family

The sense of taste is a chemosensory system responsible for basic food appraisal. Humans distinguish between five primary tastes: bitter, sweet, sour, salty and umami. The molecular events in the perception of bitter taste are believed to start with the binding of specific water-soluble molecules to G-protein-coupled receptors encoded by the TAS2R/T2R family of taste receptor genes. TAS2R receptors are expressed at the surface of taste receptor cells and are coupled to G proteins and second messenger pathways. We have identified, cloned and characterized 11 new bitter taste receptor genes and four new pseudogenes that belong to the human TAS2R family. Their encoded proteins have between 298 and 333 amino acids and share between 23 and 86% identity with other human TAS2R proteins. Screening of a mono-chromosomal somatic cell hybrid panel to assign the identified bitter taste receptor genes to human chromosomes demonstrated that they are located in chromosomes 7 and 12. Including the 15 sequences identified, the human TAS2R family is composed of 28 full-length genes and 16 pseudogenes. Phylogenetic analyses suggest a classification of the TAS2R genes in five groups that may reflect a specialization in the detection of specific types of bitter chemicals.

Differential changes in taste perception induced by benzoic acid prickling

Benzoic acid (Bz) is a prickling compound used to preserve foods. However, its effects on taste are unknown. This work examines Bz-taste interaction using psychophysical methods [magnitude estimation (ME) and paired comparison (PC)] to measure taste intensity in aqueous solutions of pure tastants (T) and their respective mixtures with 10 mM Bz (Mix). Prototypical tastants induced basic taste qualities (mM): sucrose [90-1440, sweetness (Sw)], citric acid [1-64, sourness (So)], NaCl [15-960, saltiness (Sa)], quinine [0.01-0.64, bitterness (Bitt)], KCl (12.5-400, Sa and Bitt). MEs were analysed using Steven's and Beidler's equations. Bz increased Sw (all concentrations) and ionic tastes (low concentrations) and Bz effects were reduced by concentration increase according with quality and tastant Bz reduced Bitt(Quinine) (high concentrations). Bz reduced taste slopes (percentage decrease): Sw 45% (P<.02), So 34% (P<.01), Sa 35% or 41% (NaCl or KCl, P<.03), Bitt 33% or 60% (quinine P<.01 or KCl P<.04). Bz reduced K(diss) (affinity(-1)) (percentage reduction): Sw 79% (P<.0002), So 40% (P<.03), Sa(NaCl) 63% (P<.005), Sa(KCl) 48% (P<.04), Bitt(KCl) 64% (P<.04). Bz reduced ME(max) (percentage reduction): Sw 31% (P<.004), Bitt(Quinine) 29% (P<.03). PCs confirmed taste increases by Bz (percentage of 'Mix(intensity)>T(intensity)' answers/total answers): Sw 79-69% (90-1440 mM sucrose), So 75% (1 mM citric acid) and 71% (2 mM citric acid), Sa 75-71% (15-120 mM NaCl). Negative concentration dependence of taste increases by Bz suggests different levels of interaction. Biophysical and neurophysiological changes are discussed in relation with Bz properties and mechanism of interaction with taste.