Sweet taste responses of mouse chorda tympani neurons: existence of gurmarin-sensitive and -insensitive receptor components

Novel gurmarin-like peptides from Gymnema sylvestre and their interactions with the sweet taste receptor T1R2/T1R3

Gymnema sylvestre (GS) is a traditional medicinal plant known for its hypoglycemic and hypolipidemic effects. Gurmarin (hereafter Gur-1) is the only known active peptide in GS. Gur-1 has a suppressive sweet taste effect in rodents but no or only a very weak effect in humans. Here, 8 gurmarin-like peptides (Gur-2 to Gur-9) and their isoforms are reported in the GS transcriptome. The molecular mechanism of sweet taste suppression by Gur-1 is still largely unknown. Therefore, the complete architecture of human and mouse sweet taste receptors T1R2/T1R3 and their interaction with Gur-1 to Gur-9 were predicted by AlphaFold-Multimer (AF-M) and validated. Only Gur-1 and Gur-2 interact with the T1R2/T1R3 receptor. Indeed, Gur-1 and Gur-2 bind to the region of the cysteine-rich domain (CRD) and the transmembrane domain (TMD) of the mouse T1R2 subunit. In contrast, only Gur-2 binds to the TMD of the human T1R2 subunit. This result suggests that Gur-2 may have a suppressive sweet taste effect in humans. Furthermore, AF-M predicted that Gα-gustducin, a protein involved in sweet taste transduction, interacts with the intracellular domain of the T1R2 subunit. These results highlight an unexpected diversity of gurmarin-like peptides in GS and provide the complete predicted architecture of the human and mouse sweet taste receptor with the putative binding sites of Gur-1, Gur-2, and Gα-gustducin. In addition, gurmarin-like peptides may serve as promising drug scaffolds for the development of antidiabetic molecules.

Suppression of sweet taste-related responses by plant-derived bioactive compounds and eating. Part II: A systematic review in animals

This article, the second in a two-part series, continues the discussion on the nature of the relationship between the level of sweet taste suppression and eating behaviour, but in animal rather human subjects. In particular, the aim was to review the scientific literature on the impact that bioactive compounds that decrease oral sweet sensations have on intake, preference and physiological status in preclinical studies. This review was registered in the International Prospective Register of Systematic Reviews and conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and the Scottish Intercollegiate Guidelines Network and covered original papers included in Web of Science, PubMed, Scopus, Food Science Source and Food Science and technology abstracts. We identified 28 peer-reviewed English-language studies that fit the topic and met the inclusion criteria. We identified three plant species, Gymnema sylvestre, Hovenia dulcis, and Ziziphus jujuba, that possess acute sweetness-inhibitory properties. When administered orally, these plants reduced neural responses to sweet stimuli and decreased consumption. However, studies on the longer-term effects of antisweet activity remain to be conducted. Translating the valuable insights into the mechanisms underlying the relationship between sweet taste impairment and eating behaviour into practical clinical applications are discussed.

Chloride ions evoke taste sensations by binding to the extracellular ligand-binding domain of sweet/umami taste receptors

Salt taste sensation is multifaceted: NaCl at low or high concentrations is preferably or aversively perceived through distinct pathways. Cl^- is thought to participate in taste sensation through an unknown mechanism. Here, we describe Cl^- ion binding and the response of taste receptor type 1 (T1r), a receptor family composing sweet/umami receptors. The T1r2a/T1r3 heterodimer from the medaka fish, currently the sole T1r amenable to structural analyses, exhibited a specific Cl^- binding in the vicinity of the amino-acid-binding site in the ligand-binding domain (LBD) of T1r3, which is likely conserved across species, including human T1r3. The Cl^- binding induced a conformational change in T1r2a/T1r3LBD at sub- to low-mM concentrations, similar to canonical taste substances. Furthermore, oral Cl^- application to mice increased impulse frequencies of taste nerves connected to T1r-expressing taste cells and promoted their behavioral preferences attenuated by a T1r-specific blocker or T1r3 knock-out. These results suggest that the Cl^- evokes taste sensations by binding to T1r, thereby serving as another preferred salt taste pathway at a low concentration.

Residual Glucose Taste in T1R3 Knockout but not TRPM5 Knockout Mice

Knockout (KO) mice missing the sweet taste receptor subunit T1R3 or the signaling protein TRPM5 have greatly attenuated sweetener preferences. Yet both types of KO mice develop preferences for glucose but not fructose in 24-h tests, which has been attributed to the postoral reinforcing actions of glucose. Here we probed for residual sugar taste sensitivity in KO mice. Unlike wildtype (WT) mice, food-restricted T1R3 KO and TRPM5 KO mice displayed little attraction for 8% glucose and 8% fructose in 1-min, two-bottle choice tests. However, in 1-h tests about half of the T1R3 KO mice displayed a significant preference for glucose over fructose (78-84%), while WT mice showed either no or weak preferences (41-56%) for glucose. Following one-bottle training sessions, WT mice display greater glucose preferences although still weaker than those observed in T1R3 KO mice. In contrast, TRPM5 KO mice were indifferent to sugars in 1-h tests but developed a strong preference for glucose over fructose in 24-h tests. T1R3 taste cells contain the sodium glucose cotransporter 1 (SGLT1) and the ATP-gated K+ (K_ATP) metabolic sensor, which may mediate the unlearned glucose preference displayed by T1R3 KO mice. Unlike WT mice, many T1R3 KO mice strongly preferred glucose to a non-metabolizable glucose analog (α-methyl-D-glucopyranoside, MDG) in initial 1-h choice tests. Glucose and MDG are both ligands for SGLT1 which indicates that SGLT1 sensing does not mediate the glucose preference of T1R3 KO mice. Instead, K_ATP sensing and/or other oral sensors are implicated. The MDG findings also argue against postoral sensing as the primary source of the initial glucose preference displayed by T1R3 KO mice. Why only half of the T1R3 KO mice showed this preference in 1-h tests remains to be determined. All T1R3 KO mice preferred glucose to fructose in 24-h tests, which appears to be due to both oral and postoral glucose sensing.

Glucagon-like peptide-1 is specifically involved in sweet taste transmission

Five fundamental taste qualities (sweet, bitter, salty, sour, umami) are sensed by dedicated taste cells (TCs) that relay quality information to gustatory nerve fibers. In peripheral taste signaling pathways, ATP has been identified as a functional neurotransmitter, but it remains to be determined how specificity of different taste qualities is maintained across synapses. Recent studies demonstrated that some gut peptides are released from taste buds by prolonged application of particular taste stimuli, suggesting their potential involvement in taste information coding. In this study, we focused on the function of glucagon-like peptide-1 (GLP-1) in initial responses to taste stimulation. GLP-1 receptor (GLP-1R) null mice had reduced neural and behavioral responses specifically to sweet compounds compared to wild-type (WT) mice. Some sweet responsive TCs expressed GLP-1 and its receptors were expressed in gustatory neurons. GLP-1 was released immediately from taste bud cells in response to sweet compounds but not to other taste stimuli. Intravenous administration of GLP-1 elicited transient responses in a subset of sweet-sensitive gustatory nerve fibers but did not affect other types of fibers, and this response was suppressed by pre-administration of the GLP-1R antagonist Exendin-4(3-39). Thus GLP-1 may be involved in normal sweet taste signal transmission in mice.

Sweet-taste-suppressing compounds: current knowledge and perspectives of application

Sweet-tasting compounds are recognized by a heterodimeric receptor composed of the taste receptor, type 1, members 2 (T1R2) and 3 (T1R3) located in the mouth. This receptor is also expressed in the gut where it is involved in intestinal absorption, metabolic regulation, and glucose homeostasis. These metabolic functions make the sweet taste receptor a potential novel therapeutic target for the treatment of obesity and related metabolic dysfunctions such as diabetes. Existing sweet taste inhibitors or blockers that are still in development would constitute promising therapeutic agents. In this review, we will summarize the current knowledge of sweet taste inhibitors, including a sweet-taste-suppressing protein named gurmarin, which is only active on rodent sweet taste receptors but not on that of humans. In addition, their potential applications as therapeutic tools are discussed.

Optimization of the production of gurmarin, a sweet-taste-suppressing protein, secreted by the methylotrophic yeast Pichia pastoris

Gurmarin, a 35-residue polypeptide, is known to selectively inhibit responses to sweet substances in rodents without affecting responses to other basic taste stimuli, such as NaCl, HCl, and quinine. Here, we report the heterologous expression of gurmarin using the methylotrophic yeast Pichia pastoris. Gurmarin was secreted into the buffered minimal medium using the α-factor preprosequence without the EAEA spacer peptide of Saccharomyces cerevisiae and was under the control of the methanol-inducible alcohol oxidase promoter. We found that gurmarin accumulated in the yeast culture medium reaching 5 mg per liter of culture over an expression period of 4 days. To compare the production level and the signal peptide processing, the N-terminal amino acid of gurmarin was substituted by a glutamic acid residue. This construct resulted in a 6-fold increase in the level of gurmarin secretion leading to 30 mg of purified protein per liter of culture. Purified recombinant gurmarin resulting from both constructs was characterized using mass spectrometry. Circular dichroism and NMR spectroscopy revealed that recombinant gurmarin was properly folded and had secondary and tertiary structures. We also confirmed its capability to inhibit the rat heterodimeric sweet taste T1R2/T1R3 receptor by functional expression in human embryonic kidney HEK293T cells. The high level of fully active gurmarin obtained in P. pastoris makes this expression system attractive for fermentor growth and pharmacological investigations of taste receptor and gurmarin functions.

Gustatory signaling in the periphery: detection, transmission, and modulation of taste information

Gustatory signaling begins with taste receptor cells that express taste receptors. Recent molecular biological studies have identified taste receptors and transduction components for basic tastes (sweet, salty, sour, bitter, and umami). Activation of these receptor systems leads to depolarization and an increase in [Ca(2+)](i) in taste receptor cells. Then transmitters are released from taste cells and activate gustatory nerve fibers. The connection between taste cells and gustatory nerve fibers would be specific because there may be only limited divergence of taste information at the peripheral transmission. Recent studies have demonstrated that sweet taste information can be modulated by hormones or other endogenous factors that could act on their receptors in a specific group of taste cells. These peripheral modulations of taste information may influence preference behavior and food intake. This paper summarizes data on molecular mechanisms for detection and transduction of taste signals in taste bud cells, information transmission from taste cells to gustatory nerve fibers, and modulation of taste signals at peripheral taste organs, in particular for sweet taste, which may play important roles in regulating energy homeostasis.

Surface plasmon resonance analysis on interactions of food components with a taste epithelial cell model

A new device for evaluating the continuity of taste was developed with the use of surface plasmon resonance (SPR). The model of lingual cells was constructed with liposomes immobilized onto an L1 sensor chip for SPR. Using this device, we classified food components into three categories according to the sensorgram pattern and residual ratio on lipid bilayer. Samples in group A strongly interacted with lipid bilayer, those in group B poorly interacted, and those in group C belong to neither group A nor group B. Sweet proteins and gymnemic acids that prolonged sweet perception were categorized in group A. Almost all the carbohydrates investigated and aspartame, of which the taste perception does not continue, belonged to group B. This device made it possible to detect the interaction with lipid bilayer and dissected the mechanism of taste continuity.

Responses of single chorda tympani taste fibers of the calf (Bos taurus)

In spite of a wealth of information on feed and nutrition in cattle, there little is published of what they actually can taste. Here, we attempt to remedy some of this deficiency by presenting recordings of the chorda tympani proper nerve of young Holstein calves during stimulation of approximately 30 compounds. Hierarchical cluster analysis of 46 single taste fibers separated 4 fiber clusters: N (salt best), H (sour best), and 2 clusters, which could not be related to any human taste quality. The N fibers responded best to LiCl, NaCl, urea, monosodium glutamate, and KCl, whereas the H fibers responded strongly to citric and ascorbic acid. Interestingly, propionic and butyric acid stimulated best the 3rd cluster, whereas the 4th cluster responded best to denatonium benzoate and only to a small extent to quinine hydrochloride. Sweeteners stimulated moderately all clusters. Beginning with the largest response to sweet, the order between the responses was: acesulfame-K, saccharin, D-phenylalanine, glycine, sucrose, fructose, erythritol, cyclamate, and lactose. Alitame, aspartame, and super-aspartame evoked no or little responses. Three and 5 M ethanol stimulated all clusters. Comparison with taste fibers in other species suggests that the taste world of cattle is quite different from other species'.

Genetically-increased taste cell population with G(alpha)-gustducin-coupled sweet receptors is associated with increase of gurmarin-sensitive taste nerve fibers in mice

Background

The peptide gurmarin is a selective sweet response inhibitor for rodents. In mice, gurmarin sensitivity differs among strains with gurmarin-sensitive C57BL and gurmarin-poorly-sensitive BALB strains. In C57BL mice, sweet-responsive fibers of the chorda tympani (CT) nerve can be divided into two distinct populations, gurmarin-sensitive (GS) and gurmarin-insensitive (GI) types, suggesting the existence of two distinct reception pathways for sweet taste responses. By using the dpa congenic strain (dpa CG) whose genetic background is identical to BALB except that the gene(s) controlling gurmarin sensitivity are derived from C57BL, we previously found that genetically-elevated gurmarin sensitivity in dpa CG mice, confirmed by using behavioral response and whole CT nerve response analyses, was linked to a greater taste cell population co-expressing sweet taste receptors and a Gα protein, Gα-gustducin. However, the formation of neural pathways from the increased taste cell population to nerve fibers has not yet been examined.

Results

Here, we investigated whether the increased taste cell population with Gα-gustducin-coupled sweet receptors would be associated with selective increment of GS fiber population or nonselective shift of gurmarin sensitivities of overall sweet-responsive fibers by examining the classification of GS and GI fiber types in dpa CG and BALB mice. The results indicated that dpa CG, like C57BL, possess two distinct populations of GS and GI types of sweet-responsive fibers with almost identical sizes (dpa CG: 13 GS and 16 GI fibers; C57BL: 16 GS and 14 GI fibers). In contrast, BALB has only 3 GS fibers but 18 GI fibers. These data indicate a marked increase of the GS population in dpa CG.

Conclusion

These results suggest that the increased cell population expressing T1r2/T1r3/Gα-gustducin in dpa CG mice may be associated with an increase of their matched GS type fibers, and may form the distinct GS sweet reception pathway in mice. Gα-gustducin may be involved in the GS sweet reception pathway and may be a key molecule for links between sweet taste receptors and cell type-specific-innervation by their matched fiber class.

Multiple sweet receptors and transduction pathways revealed in knockout mice by temperature dependence and gurmarin sensitivity

Sweet taste transduction involves taste receptor type 1, member 2 (T1R2), taste receptor type 1, member 3 (T1R3), gustducin, and TRPM5. Because knockout (KO) mice lacking T1R3, gustducin's Galpha subunit (Galphagust), or TRPM5 exhibited greatly reduced, but not abolished responses of the chorda tympani (CT) nerve to sweet compounds, it is likely that multiple sweet transduction pathways exist. That gurmarin (Gur), a sweet taste inhibitor, inhibits some but not all mouse CT responses to sweet compounds supports the existence of multiple sweet pathways. Here, we investigated Gur inhibition of CT responses to sweet compounds as a function of temperature in KO mice lacking T1R3, Galphagust, or TRPM5. In T1R3-KO mice, responses to sucrose and glucose were Gur sensitive (GS) and displayed a temperature-dependent increase (TDI). In Galphagust-KO mice, responses to sucrose and glucose were Gur-insensitive (GI) and showed a TDI. In TRPM5-KO mice, responses to glucose were GS and showed a TDI. All three KO mice exhibited no detectable responses to SC45647, and their responses to saccharin displayed neither GS nor a TDI. For all three KO mice, the lingual application of pronase, another sweet response inhibitor, almost fully abolished responses to sucrose and glucose but did not affect responses to saccharin. These results provide evidence for 1) the existence of multiple transduction pathways underlying responses to sugars: a T1R3-independent GS pathway for sucrose and glucose, and a TRPM5-independent temperature sensitive GS pathway for glucose; 2) the requirement for Galphagust in GS sweet taste responses; and 3) the existence of a sweet independent pathway for saccharin, in mouse taste cells on the anterior tongue.

Cracking taste codes by tapping into sensory neuron impulse traffic

Insights into the biological basis for mammalian taste quality coding began with electrophysiological recordings from "taste" nerves and this technique continues to produce essential information today. Chorda tympani (geniculate ganglion) neurons, which are particularly involved in taste quality discrimination, are specialists or generalists. Specialists respond to stimuli characterized by a single taste quality as defined by behavioral cross-generalization in conditioned taste tests. Generalists respond to electrolytes that elicit multiple aversive qualities. Na(+)-salt (N) specialists in rodents and sweet-stimulus (S) specialists in multiple orders of mammals are well characterized. Specialists are associated with species' nutritional needs and their activation is known to be malleable by internal physiological conditions and contaminated external caloric sources. S specialists, associated with the heterodimeric G-protein coupled receptor T1R, and N specialists, associated with the epithelial sodium channel ENaC, are consistent with labeled line coding from taste bud to afferent neuron. Yet, S-specialist neurons and behavior are less specific than T1R2-3 in encompassing glutamate and E generalist neurons are much less specific than a candidate, PDK TRP channel, sour receptor in encompassing salts and bitter stimuli. Specialist labeled lines for nutrients and generalist patterns for aversive electrolytes may be transmitting taste information to the brain side by side. However, specific roles of generalists in taste quality coding may be resolved by selecting stimuli and stimulus levels found in natural situations. T2Rs, participating in reflexes via the glossopharynygeal nerve, became highly diversified in mammalian phylogenesis as they evolved to deal with dangerous substances within specific environmental niches. Establishing the information afferent neurons traffic to the brain about natural taste stimuli imbedded in dynamic complex mixtures will ultimately "crack taste codes."

The taste of sugars

Sugars evoke a distinctive perceptual quality ("sweetness" in humans) and are generally highly preferred. The neural basis for these phenomena is reviewed for rodents, in which detailed electrophysiological measurements have been made. A receptor has been identified that binds sweeteners and activates G-protein-mediated signaling in taste receptor cells, which leads to changes in neural firing rates in the brain, where perceptions of taste quality, intensity, and palatability are generated. Most cells in gustatory nuclei are broadly tuned, so quality perception presumably arises from patterns of activity across neural populations. However, some manipulations affect only the most sugar-oriented cells, making it useful to consider them as a distinct neural subtype. Quality perception may also arise partly due to temporal patterns of activity to sugars, especially within sugar-oriented cells that give large but delayed responses. Non-specific gustatory neurons that are excited by both sugars and unpalatable stimuli project to ventral forebrain areas, where neural responses provide a closer match with behavioral preferences. This transition likely involves opposing excitatory and inhibitory influences by different subgroups of gustatory cells. Sweeteners are generally preferred over water, but the strength of this preference can vary across time or between individuals, and higher preferences for sugars are often associated with larger taste-evoked responses.

Recovery of two independent sweet taste systems during regeneration of the mouse chorda tympani nerve after nerve crush

In rodents, section of the taste nerve results in degeneration of the taste buds. Following regeneration of the cut taste nerve, however, the taste buds reappear. This phenomenon can be used to study the functional reformation of the peripheral neural system responsible for sweet taste. In this study we examined the recovery of sweet responses by the chorda tympani (CT) nerve after nerve crush as well as inhibition of these responses by gurmarin (Gur), a sweet response inhibitor. After about 2 weeks of CT nerve regeneration, no significant response to any taste stimuli could be observed. At 3 weeks, responses to sweet stimuli reappeared but were not significantly inhibited by Gur. At 4 weeks, Gur inhibition of sweet responses reached statistically significant levels. Thus, the Gur-sensitive (GS) component of the sweet response reappeared about 1 week later than the Gur-insensitive (GI) component. Moreover, single CT fibers responsive to sucrose could be classified into distinct GS and GI groups at 4 weeks. After 5 weeks or more, responses to sweet compounds before and after treatment with Gur became indistinguishable from responses in the intact group. During regeneration, the GS and GI components of the sucrose response could be distinguished based on their concentration-dependent responses to sucrose. These results suggest that mice have two different sweet-reception systems, distinguishable by their sensitivity to Gur (the GS and GI systems). These two sweet-reception systems may be reconstituted independently during regeneration of the mouse CT nerve.

Taste Responsiveness of Fungiform Taste Cells With Action Potentials

It is known that a subset of taste cells generate action potentials in response to taste stimuli. However, responsiveness of these cells to particular tastants remains unknown. In the present study, by using a newly developed extracellular recording technique, we recorded action potentials from the basolateral membrane of single receptor cells in response to taste stimuli applied apically to taste buds isolated from mouse fungiform papillae. By this method, we examined taste-cell responses to stimuli representing the four basic taste qualities (NaCl, Na saccharin, HCl, and quinine-HCl). Of 72 cells responding to taste stimuli, 48 (67%) responded to one, 22 (30%) to two, and 2 (3%) to three of four taste stimuli. The entropy value presenting the breadth of responsiveness was 0.158 ± 0.234 (mean ± SD), which was close to that for the nerve fibers (0.183 ± 0.262). In addition, the proportion of taste cells predominantly sensitive to each of the four taste stimuli, and the grouping of taste cells based on hierarchical cluster analysis, were comparable with those of chorda tympani (CT) fibers. The occurrence of each class of taste cells with different taste responsiveness to the four taste stimuli was not significantly different from that of CT fibers except for classes with broad taste responsiveness. These results suggest that information derived from taste cells generating action potentials may provide the major component of taste information that is transmitted to gustatory nerve fibers.

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.

Mouse Strain Differences in Gurmarin-sensitivity of Sweet Taste Responses Are Not Associated with Polymorphisms of the Sweet Receptor Gene, Tas1r3

Gurmarin (Gur) is a peptide that selectively inhibits responses of the chorda tympani (CT) nerve to sweet compounds in rodents. In mice, the sweet-suppressing effect of Gur differs among strains. The inhibitory effect of Gur is clearly observed in C57BL/6 mice, but only slightly, if at all, in BALB/c mice. These two mouse strains possess different alleles of the sweet receptor gene, Sac (Tas1r3) (taster genotype for C57BL/6 and non-taster genotype for BALB/c mice), suggesting that polymorphisms in the gene may account for differential sensitivity to Gur. To investigate this possibility, we examined the effect of Gur in another Tas1r3 non-taster strain, 129 X 1/Sv mice. The results indicated that unlike non-taster BALB/c mice but similar to taster C57BL/6 mice, 129 X 1/Sv mice exhibited significant inhibition of CT responses to various sweet compounds by Gur. This suggests that the mouse strain difference in the Gur inhibition of sweet responses of the CT nerve may not be associated with polymorphisms of Tas1r3.

Alcohol Activates a Sucrose-Responsive Gustatory Neural Pathway

A strong positive association exists between the ingestion of alcohol and sweet-tasting solutions. The neural mechanisms underlying this relationship are unknown, although recent data suggest that gustatory substrates are involved. Here, we examined the role of sweet taste receptors and central neural circuits for sugar taste in the gustatory processing of ethanol. Taste responses to ethanol (3, 5, 10, 15, 25, and 40% vol/vol) and stimuli of different taste qualities (e.g., sucrose, NaCl, HCl, and quinine-HCl) were recorded from neurons of the nucleus of the solitary tract in anesthetized rats prior to and after oral application of the sweet receptor blocker gurmarin. The magnitude of ethanol-evoked activity was compared between sucrose-responsive ( n = 21) and sucrose-unresponsive ( n = 20) neurons and the central neural representation of ethanol taste was explored using multivariate analysis. Ethanol produced robust concentration-dependent responses in sucrose-responsive neurons that were dramatically larger than those in sucrose-unresponsive cells. Gurmarin selectively and similarly inhibited ethanol and sucrose responses, leaving NaCl, HCl, and quinine responses unaltered. Across-neuron patterns of response to ethanol were most similar to those evoked by sucrose, becoming increasingly more so as the ethanol concentration was raised. Results implicate taste receptors for sucrose as candidate receptors for ethanol and reveal that alcohol and sugar taste are represented similarly by gustatory activity in the CNS. These findings have important implications for the sensory and reward properties of alcohol.

The receptors for mammalian sweet and umami taste

Sweet and umami (the taste of monosodium glutamate) are the main attractive taste modalities in humans. T1Rs are candidate mammalian taste receptors that combine to assemble two heteromeric G-protein-coupled receptor complexes: T1R1+3, an umami sensor, and T1R2+3, a sweet receptor. We now report the behavioral and physiological characterization of T1R1, T1R2, and T1R3 knockout mice. We demonstrate that sweet and umami taste are strictly dependent on T1R-receptors, and show that selective elimination of T1R-subunits differentially abolishes detection and perception of these two taste modalities. To examine the basis of sweet tastant recognition and coding, we engineered animals expressing either the human T1R2-receptor (hT1R2), or a modified opioid-receptor (RASSL) in sweet cells. Expression of hT1R2 in mice generates animals with humanized sweet taste preferences, while expression of RASSL drives strong attraction to a synthetic opiate, demonstrating that sweet cells trigger dedicated behavioral outputs, but their tastant selectivity is determined by the nature of the receptors.

Differential gurmarin suppression of sweet taste responses in rat solitary nucleus neurons

We examined the effect of the sweet transduction blocker gurmarin on taste responses recorded from neurons in the rat solitary nucleus (NST) to determine how gurmarin sensitivity is distributed across neuronal type. Initially, responses evoked by washing the anterior tongue and palate with 0.5 M sucrose, 0.1 M NaCl, 0.01 M HCl, and 0.01 M quinine-HCl were recorded from 35 neurons. For some cells, responses to a sucrose concentration series (0.01-1.0 M) or an array of sweet-tasting compounds were also measured. Gurmarin (10 microg/ml, 2-4 ml) was then applied to the tongue and palate. Stimuli were reapplied after 10-15 min. Neurons were segregated into groups based on similarities among their initial response profiles using hierarchical cluster analysis (HCA). Results indicated that sucrose responses recorded from neurons representative of each HCA-defined class were suppressed by gurmarin. However, a disproportionate percentage of cells in each group displayed sucrose responses that were substantially attenuated after gurmarin treatment. Postgurmarin sucrose responses recorded from neurons that composed 57% of class S, 40% of class N, and 33% of class H were suppressed by >or=50% relative to control. On average, attenuation was statistically significant only in class S and N neurons. Although the magnitude of gurmarin-induced response suppression did not differ across sucrose concentration, responses to different sweet-tasting compounds were differentially affected. Responses to NaCl, HCl, or quinine were not suppressed by gurmarin. Results suggest that information from gurmarin-sensitive and -insensitive receptor processes converges onto single NST neurons.

Comparison of the responses of the chorda tympani and glossopharyngeal nerves to taste stimuli in C57BL/6J mice

BACKGROUND

Recent progress in discernment of molecular pathways of taste transduction underscores the need for comprehensive phenotypic information for the understanding of the influence of genetic factors in taste. To obtain information that can be used as a base line for assessment of effects of genetic manipulations in mice taste, we have recorded the whole-nerve integrated responses to a wide array of taste stimuli in the chorda tympani (CT) and glossopharyngeal (NG) nerves, the two major taste nerves from the tongue.

RESULTS

In C57BL/6J mice the responses in the two nerves were not the same. In general sweeteners gave larger responses in the CT than in the NG, while responses to bitter taste in the NG were larger. Thus the CT responses to cyanosuosan, fructose, NC00174, D-phenylalanline and sucrose at all concentrations were significantly larger than in the NG, whereas for acesulfame-K, L-proline, saccharin and SC45647 the differences were not significant. Among bitter compounds amiloride, atropine, cycloheximide, denatonium benzoate, L-phenylalanine, 6-n-propyl-2-thiouracil (PROP) and tetraethyl ammonium chloride (TEA) gave larger responses in the NG, while the responses to brucine, chloroquine, quinacrine, quinine hydrochloride (QHCl), sparteine and strychnine, known to be very bitter to humans, were not significantly larger in the NG than in the CT.

CONCLUSION

These data provide a comprehensive survey and comparison of the taste sensitivity of the normal C57BL/6J mouse against which the effects of manipulations of its gustatory system can be better assessed.

A candidate taste receptor gene near a sweet taste locus

The mechanisms underlying sweet taste in mammals have been elusive. Although numerous studies have implicated G proteins in sweet taste detection, the expected G protein-coupled receptors have not been found. Here we describe a candidate taste receptor gene, T1r3, that is located at or near the mouse Sac locus, a genetic locus that controls the detection of certain sweet tastants. T1R3 differs in amino acid sequence in mouse strains with different Sac phenotypes ('tasters' versus 'nontasters'). In addition, a perfect correlation exists between two different T1r3 alleles and Sac phenotypes in recombinant inbred mouse strains. The T1r3 gene is expressed in a subset of taste cells in circumvallate, foliate and fungiform taste papillae. In circumvallate and foliate papillae, most T1r3-expressing cells also express a gene encoding a related receptor, T1R2, raising the possibility that these cells recognize more than one ligand, or that the two receptors function as heterodimers.

Leptin as a modulator of sweet taste sensitivities in mice

Leptin acts as a potent inhibitory factor against obesity by regulating energy expenditure, food intake, and adiposity. The obese diabetic db/db mouse, which has defects in leptin receptor, displays enhanced neural responses and elevated behavioral preference to sweet stimuli. Here, we show the effects of leptin on the peripheral taste system. An administration of leptin into lean mice suppressed responses of peripheral taste nerves (chorda tympani and glossopharyngeal) to sweet substances (sucrose and saccharin) without affecting responses to sour, salty, and bitter substances. Whole-cell patch-clamp recordings of activities of taste receptor cells isolated from circumvallate papillae (innervated by the glossopharyngeal nerve) demonstrated that leptin activated outward K(+) currents, which resulted in hyperpolarization of taste cells. The db/db mouse with impaired leptin receptors showed no such leptin suppression. Taste tissue (circumvallate papilla) of lean mice expressed leptin-receptor mRNA and some of the taste cells exhibited immunoreactivities to antibodies of the leptin receptor. Taken together, these observations suggest that the taste organ is a peripheral target for leptin, and that leptin may be a sweet-sensing modulator (suppressor) that may take part in regulation of food intake. Defects in this leptin suppression system in db/db mice may lead to their enhanced peripheral neural responses and enhanced behavioral preferences for sweet substances.

The evolution of neural coding ideas in the chemical senses

A brief review of the evolution of hypotheses about neural coding in the chemical senses provides some perspective on the current status of these fields, and implications for further development.