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SummaryT2R bitter receptors, encoded byTas2rgenes, are not only critical for bitter taste signal transduction but also important for defense against bacteria and parasites. However, little is known about whether and howTas2rgene expression are regulated. Here we show that, in an inflammation model mimicking bacterial infection, the expression of manyTas2rsare significantly up-regulated and mice displayed markedly increased neural and behavioral responses to bitter compounds. Using single-cell assays for transposase-accessible chromatin with sequencing (scATAC-seq), we found that the chromatin accessibility ofTas2rswas highly cell type specific and inflammation increased the accessibility of manyTas2rs. scATAC-seq also revealed substantial chromatin remodeling in immune response genes in taste tissue stem cells, suggesting potential long-term effects. Together, our results suggest an epigenetic mechanism connecting inflammation,Tas2rgene regulation, and altered bitter taste, which may explain heightened bitter taste that can occur with infections and cancer treatments.

Taste cells are maintained by continuous turnover throughout a lifetime, yet the mechanisms of taste cell differentiation, and how taste sensations remain constant despite this continuous turnover, remain poorly understood. Here, we report that a transcription factor Etv1 (also known as Er81) is involved in the differentiation of taste cells responsible for the preference for sweet, umami, and salty tastes. Molecular analyses revealed thatEtv1is expressed by a subset of taste cells that depend on Skn-1a (also known as Pou2f3) for their generation and express T1R genes (responsible for sweet and umami tastes) orScnn1a(responsible for amiloride-sensitive salty taste).Etv1CreERT2/CreERT2mice express Etv1 isoform(s) but not Etv1 in putative proprioceptive neurons as comparable to wild-type mice, yet lack expression of Etv1 or an isoform in taste cells. TheseEtv1CreERT2/CreERT2mice have the same population of Skn-1a-dependent cells in taste buds as wild-type mice but have altered gene expression in taste cells, with regional differences. They have markedly decreased electrophysiological responses of chorda tympani nerves to sweet and umami tastes and to amiloride-sensitive salty taste evoked by sodium cation, but they have unchanged responses to bitter or sour tastes. Our data thus show that Etv1 is involved in the differentiation of the taste cells responsible for sweet, umami, and salty taste preferences.

Taste buds are maintained via continuous turnover of taste bud cells derived from local epithelial stem cells. A transcription factor Skn-1a (also known as Pou2f3) is required for the generation of sweet, umami (savory), and bitter taste cells that commonly express TRPM5 and CALHM ion channels. Here we demonstrate that sodium–taste cells distributed only in the anterior oral epithelia and involved in evoking salty taste also require Skn-1a for their generation. We discovered taste cells in fungiform papillae and soft palate that show similar but not identical molecular feature with sweet, umami, and bitter taste-mediated type II cells. This novel cell population expresses Plcb2, Itpr3, Calhm3, Skn-1a, and ENaC alpha (also known as Scnn1a) encoding the putative amiloride-sensitive salty taste receptor, but lacks Trpm5 and Gnat3. Skn-1a-deficient taste buds are predominantly composed of putative non-sensory type I cells and sour-sensing type III cells, whereas wild-type taste buds include type II (i.e., sweet, umami, and bitter taste) cells and sodium–taste cells. Both Skn-1a and Calhm3-deficient mice have markedly decreased chorda tympani nerve responses to sodium chloride, and those decreased responses are attributed to the loss of the amiloride-sensitive salty taste response. Thus, amiloride-sensitive salty taste is mediated by Skn-1a–dependent taste cells, whereas amiloride-insensitive salty taste is mediated largely by type III sour taste cells and partly by bitter taste cells. Our results demonstrate that Skn-1a regulates differentiation toward all types of taste cells except sour taste cells. SIGNIFICANCE STATEMENT/Author summary Salty taste plays an important role in electrolyte homeostasis in body fluids. Other basic tastes are each mediated by specialized sensory cells and elicits either preference or avoidance; in contrast, salty taste elicits both behaviors, depending on concentrations, and is mediated by multiple mechanisms and cell types that are poorly defined. We report that a subset of cells that express ENaCa exhibit a gene expression profile similar but not identical to sweet, umami, and bitter taste cells. They mediate amiloride-sensitive sodium taste and rely on Skn-1a for their generation and the CALHM ion channel for neurotransmitter release. Amiloride-insensitive salty taste is partially mediated by sour taste cells, the only taste cells present in the Skn-1a knockout mice.

Leptin is known to selectively suppress neural and behavioral responses to sweet-tasting compounds. However, the molecular basis for the effect of leptin on sweet taste is not known. Here, we report that leptin suppresses sweet taste via leptin receptors (Ob-Rb) and KATP channels expressed selectively in sweet-sensitive taste cells. Ob-Rb was more often expressed in taste cells that expressed T1R3 (a sweet receptor component) than in those that expressed glutamate-aspartate transporter (a marker for Type I taste cells) or GAD67 (a marker for Type III taste cells). Systemically administered leptin suppressed taste cell responses to sweet but not to bitter or sour compounds. This effect was blocked by a leptin antagonist and was absent in leptin receptor–deficient db/db mice and mice with diet-induced obesity. Blocking the KATP channel subunit sulfonylurea receptor 1, which was frequently coexpressed with Ob-Rb in T1R3-expressing taste cells, eliminated the effect of leptin on sweet taste. In contrast, activating the KATP channel with diazoxide mimicked the sweet-suppressing effect of leptin. These results indicate that leptin acts via Ob-Rb and KATP channels that are present in T1R3-expressing taste cells to selectively suppress their responses to sweet compounds.

Leptin is an anorexigenic mediator that reduces food intake by acting on hypothalamic receptor Ob-Rb. In contrast, endocannabinoids are orexigenic mediators that act via cannabinoid CB1 receptors in hypothalamus, limbic forebrain, and brainstem. In the peripheral taste system, leptin administration selectively inhibits behavioural, taste nerve and taste cell responses to sweet compounds. Opposing the action of leptin, endocannabinoids enhance sweet taste responses. However, potential roles of endogenous leptin and endocannabinoids in sweet taste remain unclear. Here, we used pharmacological antagonists (Ob-Rb: L39A/D40A/F41A (LA), CB1: AM251) and examined the effects of their blocking activation of endogenous leptin and endocannabinoid signalling on taste responses in lean control, leptin receptor deficient db/db, and diet-induced obese (DIO) mice. Lean mice exhibited significant increases in chorda tympani (CT) nerve responses to sweet compounds after LA administration, while they showed no significant changes in CT responses after AM251. In contrast, db/db mice showed clear suppression of CT responses to sweet compounds after AM251, increased endocannabinoid (2-arachidonoyl-sn-glycerol (2-AG)) levels in the taste organ, and enhanced expression of a biosynthesizing enzyme (diacylglycerol lipase α (DAGLα)) of 2-AG in taste cells. In DIO mice, the LA effect was gradually decreased and the AM251 effect was increased during the course of obesity. Taken together, our results suggest that circulating leptin, but not local endocannabinoids, may be a dominant modulator for sweet taste in lean mice; however, endocannabinoids may become more effective modulators of sweet taste under conditions of deficient leptin signalling, possibly due to increased production of endocannabinoids in taste tissue. Key points Potential roles of endogenous leptin and endocannabinoids in sweet taste were examined by using pharmacological antagonists and mouse models including leptin receptor deficient (db/db) and diet-induced obese (DIO) mice. Chorda tympani (CT) nerve responses of lean mice to sweet compounds were increased after administration of leptin antagonist (LA) but not affected by administration of cannabinoid receptor antagonist (AM251). db/db mice showed clear suppression of CT responses to sweet compounds after AM251, increased endocannabinoid levels in the taste organ, and enhanced expression of a biosynthesizing enzyme of endocannabinoids in taste cells. The effect of LA was gradually decreased and that of AM251 was increased during the course of obesity in DIO mice. These findings suggest that circulating leptin, but not local endocannabinoids, is a dominant modulator for sweet taste in lean mice and endocannabinoids become more effective modulators of sweet taste under conditions of deficient leptin signalling. Introduction Taste is primarily devoted to the selection of foods and required nutrients, and the avoidance of potentially harmful compounds. Sweet sensing is thought to be essential for the detection of carbohydrate sources of calories. Sensory input from sweet sensing cells to the brain may serve as a signal that can provoke palatability of food to stimulate food intake, and facilitate secretion of insulin in the pancreas and other factors in the gut involved in regulating nutrient absorption and energy homeostasis. Recent studies reveal that the peripheral sweet-sensing system is modulated by the anorexigenic mediator leptin (Kawai et al. 2000) and the orexigenic endocannabinoids 2-arachidonoyl-sn-glycerol (2-AG) and anandamide (N-arachidonoylethanolamine (AEA) (Yoshida et al. 2010). Leptin, an adipocyte-derived hormone, primarily acts on functional leptin receptors (Ob-Rb) in the hypothalamus and reduces food intake, increases energy expenditure, and regulates body weight (Friedman & Halaas, 1998; Friedman 2004). The db/db mice have a point mutation of the db gene, lack functional leptin receptors, and are hyperphagic, massively obese and diabetic (Lee et al. 1996). In the peripheral taste system, taste cells express Ob-Rb and administration of recombinant leptin selectively suppresses taste cell, taste nerve and behavioural responses to sweet compounds without affecting responses to other basic taste stimuli (salty, sour, bitter and umami compounds) in lean control mice (Kawai et al. 2000; Shigemura et al. 2004; Yoshida et al. 2013). The db/db mice show no such leptin inhibition on sweet taste responses, but instead exhibit greater gustatory neural sensitivities (Ninomiya et al. 1995, 1998; Sako et al. 1996) and higher behavioural preferences (Ninomiya et al. 1995) for various sweet compounds compared with lean control mice. Endocannabinoids act on cannabinoid receptor 1 (CB1 receptor) in the hypothalamus and limbic forebrain to induce appetite (Jamshidi & Taylor, 2001; Cota et al. 2003) and stimulate food intake, which opposes the action of leptin. Systemic administration of exogenous cannabinoids and endocannabinoids in rodents causes hyperphagia (Williams & Kirkham, 1999) and increases the preference for palatable substances, such as sucrose solution and sweetened food pellets (Higgs et al. 2003; Jarrett et al. 2005). Infusions of 2-AG into the pontine parabrachial nucleus, which contains third order gustatory neurons, increase intake of palatable foods, including sucrose pellets, without affecting the intake of normal chow (DiPatrizio & Simansky, 2008), suggesting that endocannabinoids may be related to hedonic aspects of sweet taste. In the peripheral taste system, comparable with their central action, systemic administration of 2-AG or AEA selectively increased taste cell, nerve and behavioural responses to sweeteners without affecting responses to salty, sour, bitter and umami compounds (Yoshida et al. 2010). Mice genetically lacking CB1 receptors showed no such enhancement of sweet taste responses by endocannabinoids and the sweet-enhancing effect was prevented by a CB1 antagonist (Yoshida et al. 2010), indicating that the effect may be mediated by CB1 receptors. Reciprocal regulation of peripheral sweet taste reception by endocannabinoids and leptin may, thus, contribute to their opposing actions on food intake and energy homeostasis. In our previous studies (Kawai et al. 2000; Yoshida et al. 2010), however, reciprocal modulation of sweet taste responses by leptin and endocannabinoids was found only in separate experiments after systemic administration of exogenous leptin and endocannabinoids to mice. Therefore, it remains unclear to what extent endogenous leptin or endocannabinoids tonically affect peripheral taste responsiveness in mice. In the hypothalamus, leptin inhibits production of endocannabinoids (Jo et al. 2005). Conversely, hypothalamic endocannabinoids are increased in genetically obese rodents lacking functional leptin (ob/ob mice) and leptin receptors (db/db mice and Zucker rats; Di Marzo et al. 2001). These studies suggest that endocannabinoids are normally under negative control by leptin, and might be involved in the hyperphagia and obesity that accompany defects in leptin signalling. A potential site for the interaction between leptin and endocannabinoids is reported in lateral hypothalamic neurons, where leptin inhibits voltage-gated calcium channels, preventing the influx of calcium into cells and blocking endocannabinoid release and synthesis (Jo et al. 2005), a mechanism that contributes to the appetite-suppressing effect of leptin. This evidence raises the possibility that such interaction between leptin and endocannabinds could also be involved in modulation of sweet responsiveness of taste cells and the sweet taste modulator could be switched from leptin in lean mice to endocannabinoid in obese mice with defects in leptin signalling. To answer these questions, we first investigated the effects of pharmacological inhibition of leptin or endocannabinoid receptors on peripheral taste responses in lean control mice with and without food deprivation, and in db/db mice. We treated mice with a leptin receptor antagonist, the leptin triple mutant L39A/D40A/F41A (LA), which is reported to increase food intake of normal chow-fed rats when infused into the lateral ventricle (Zhang et al. 2007), and the CB1 antagonist AM251, which is shown following systemic administration in mice to decrease the intake of a high-fat diet (Hildebrandt et al. 2003). Furthermore, we investigated the molecular basis for endocannabinoid actions on the taste organ. Finally, we investigated a proposed switch of the sweet taste modulator from leptin to endocannabinoids by comparing LA and AM251 effects on sweet taste responses at different stages of development of diet-induced obese (DIO) mice.

Taste receptor cells play a major role in detection of chemical compounds in the oral cavity. Information derived from taste receptor cells, such as sweet, bitter, salty, sour and umami is important for evaluating the quality of food components. Among five basic taste qualities, sweet taste is very attractive for animals and influences food intake. Recent studies have demonstrated that sweet taste sensitivity in taste receptor cells would be affected by leptin and endocannabinoids. Leptin is an anorexigenic mediator that reduces food intake by acting on leptin receptor Ob-Rb in the hypothalamus. Endocannabinoids such as anandamide [N-arachidonoylethanolamine (AEA)] and 2-arachidonoyl glycerol (2-AG) are known as orexigenic mediators that act via cannabinoid receptor 1 (CB1) in the hypothalamus and limbic forebrain to induce appetite and stimulate food intake. At the peripheral gustatory organs, leptin selectively suppresses and endocannabinoids selectively enhance sweet taste sensitivity via Ob-Rb and CB1 expressed in sweet sensitive taste cells. Thus leptin and endocannabinoids not only regulate food intake via central nervous systems but also modulate palatability of foods by altering peripheral sweet taste responses. Such reciprocal modulation of leptin and endocannabinoids on peripheral sweet sensitivity may play an important role in regulating energy homeostasis.

Summary Binding of sweet, umami, and bitter tastants to G protein-coupled receptors (GPCRs) in apical membranes of type II taste bud cells (TBCs) triggers action potentials that activate a voltage-gated nonselective ion channel to release ATP to gustatory nerves mediating taste perception. Although calcium homeostasis modulator 1 (CALHM1) is necessary for ATP release, the molecular identification of the channel complex that provides the conductive ATP-release mechanism suitable for action potential-dependent neurotransmission remains to be determined. Here we show that CALHM3 interacts with CALHM1 as a pore-forming subunit in a CALHM1/CALHM3 hexameric channel, endowing it with fast voltage-activated gating identical to that of the ATP-release channel in vivo . Calhm3 is co-expressed with Calhm1 exclusively in type II TBCs, and its genetic deletion abolishes taste-evoked ATP release from taste buds and GPCR-mediated taste perception. Thus, CALHM3, together with CALHM1, is essential to form the fast voltage-gated ATP-release channel in type II TBCs required for GPCR-mediated tastes.

The ability to perceive sweet compounds is important for animals to detect an external carbohydrate source of calories and has a critical role in the nutritional status of animals. In mice, a subset of sweet-sensitive taste cells possesses leptin receptors. Increase of plasma leptin with increasing internal energy storage in the adipose tissue suppresses sweet taste responses via this receptor. The data from recent studies indicate that leptin may also act as a modulator of sweet taste sensation in humans with a diurnal variation in sweet sensitivity. The plasma leptin level and sweet taste sensitivity are proposed to link with post-ingestive plasma glucose level. This leptin modulation of sweet taste sensitivity may influence an individual’s preference, ingestive behavior, and absorption of nutrients, thereby playing important roles in regulation of energy homeostasis. Keywords:: sweet taste, leptin, modulation, energy homeostasis, gastrointestinal tract

The present study summarized recent findings on roles of leptin and endocannabinoids as modulators of the peripheral components of sweet taste. The positive effect of endocannabinoids on sweet sensitivity was opposed to that of leptin which suppresses sweet sensitivity. Leptin and endocannabinoids, therefore, not only regulate food intake via central nervous systems but also may modulate palatability of foods by altering peripheral sweet taste responses via their cognate receptors. Orexigenic and anorexigenic factors such as endocannnabinoids and leptin may affect energy homeostasis by regulating taste sensitivity.

Endocannabinoids such as anandamide [N-arachidonoylethanolamine (AEA)] and 2-arachidonoyl glycerol (2-AG) are known orexigenic mediators that act via CB 1 receptors in hypothalamus and limbic forebrain to induce appetite and stimulate food intake. Circulating endocannabinoid levels inversely correlate with plasma levels of leptin, an anorexigenic mediator that reduces food intake by acting on hypothalamic receptors. Recently, taste has been found to be a peripheral target of leptin. Leptin selectively suppresses sweet taste responses in wild-type mice but not in leptin receptor-deficient db/db mice. Here, we show that endocannabinoids oppose the action of leptin to act as enhancers of sweet taste. We found that administration of AEA or 2-AG increases gustatory nerve responses to sweeteners in a concentration-dependent manner without affecting responses to salty, sour, bitter, and umami compounds. The cannabinoids increase behavioral responses to sweet-bitter mixtures and electrophysiological responses of taste receptor cells to sweet compounds. Mice genetically lacking CB 1 receptors show no enhancement by endocannnabinoids of sweet taste responses at cellular, nerve, or behavioral levels. In addition, the effects of endocannabinoids on sweet taste responses of taste cells are diminished by AM251, a CB 1 receptor antagonist, but not by AM630, a CB 2 receptor antagonist. Immunohistochemistry shows that CB 1 receptors are expressed in type II taste cells that also express the T1r3 sweet taste receptor component. Taken together, these observations suggest that the taste organ is a peripheral target of endocannabinoids. Reciprocal regulation of peripheral sweet taste reception by endocannabinoids and leptin may contribute to their opposing actions on food intake and play an important role in regulating energy homeostasis.

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.

L-Glutamate and 5'-ribonucleotides are known to elicit a unique taste, “umami,” that is distinct from the tastes of sweet, salt, sour and bitter. Recent progress in molecular biology has identified several umami receptor candidates, such as the heterodimer T1R1/T1R3, and brain-expressed and taste-expressed type 1 and 4 metabotropic glutamate receptors (brain- and taste-mGluR1 and mGluR4). This paper summarizes recent findings on the receptor system for umami taste. Most of the findings support the idea that multiple receptors exist for umami taste, at least in mice. The accumulating evidence indicates that the potential role of the signal mediated by the transduction pathway involving T1R1/T1R3 may be different from that mediated by the pathway involving mGluRs. The former signal occurs mainly in the anterior tongue, and plays a major role in preference behavior, whereas the latter occurs mainly in the posterior tongue, is active in mice lacking T1R3, Gα-gustducin, IP3R3 or TRPM5, and contributes to behavioral discrimination between umami and other taste compounds. In humans, unlike in mice, T1R1/T1R3 acts as an umami-specific receptor that can discriminate between umami and other tastes, and thus account for umami-linked preferences or discrimination.

Leptin is an important hormone that regulates food intake and energy homeostasis by acting on central and peripheral targets. In the gustatory system, leptin is known to selectively suppress sweet responses by inhibiting the activation of sweet sensitive taste cells. Sweet taste receptor (T1R2+T1R3) is also expressed in gut enteroendocrine cells and contributes to nutrient sensing, hormone release and glucose absorption. Because of the similarities in expression patterns between enteroendocrine and taste receptor cells, we hypothesized that they may also share similar mechanisms used to modify/regulate the sweet responsiveness of these cells by leptin. Here, we used mouse enteroendocrine cell line STC-1 and examined potential effect of leptin on Ca(2+) responses of STC-1 cells to various taste compounds. Ca(2+) responses to sweet compounds in STC-1 cells were suppressed by a rodent T1R3 inhibitor gurmarin, suggesting the involvement of T1R3-dependent receptors in detection of sweet compounds. Responses to sweet substances were suppressed by ⩾1ng/ml leptin without affecting responses to bitter, umami and salty compounds. This effect was inhibited by a leptin antagonist (mutant L39A/D40A/F41A) and by ATP gated K(+) (KATP) channel closer glibenclamide, suggesting that leptin affects sweet taste responses of enteroendocrine cells via activation of leptin receptor and KATP channel expressed in these cells. Moreover, leptin selectively inhibited sweet-induced but not bitter-induced glucagon-like peptide-1 (GLP-1) secretion from STC-1 cells. These results suggest that leptin modulates sweet taste responses of enteroendocrine cells to regulate nutrient sensing, hormone release and glucose absorption in the gut.

Inflammatory cytokines are important regulators of metabolism and food intake. Over production of inflammatory cytokines during bacterial and viral infections leads to anorexia and reduced food intake. However, it remains unclear whether any inflammatory cytokines are involved in the regulation of taste reception, the sensory mechanism governing food intake. Previously, we showed that tumor necrosis factor (TNF), a potent proinflammatory cytokine, is preferentially expressed in a subset of taste bud cells. The level of TNF in taste cells can be further induced by inflammatory stimuli. To investigate whether TNF plays a role in regulating taste responses, in this study, we performed taste behavioral tests and gustatory nerve recordings in TNF knockout mice. Behavioral tests showed that TNF-deficient mice are significantly less sensitive to the bitter compound quinine than wild-type mice, while their responses to sweet, umami, salty, and sour compounds are comparable to those of wild-type controls. Furthermore, nerve recording experiments showed that the chorda tympani nerve in TNF knockout mice is much less responsive to bitter compounds than that in wild-type mice. Chorda tympani nerve responses to sweet, umami, salty, and sour compounds are similar between TNF knockout and wild-type mice, consistent with the results from behavioral tests. We further showed that taste bud cells express the two known TNF receptors TNFR1 and TNFR2 and, therefore, are potential targets of TNF. Together, our results suggest that TNF signaling preferentially modulates bitter taste responses. This mechanism may contribute to taste dysfunction, particularly taste distortion, associated with infections and some chronic inflammatory diseases.

Sweet taste perception is important for animals to detect carbohydrate source of calories and has a critical role in the nutritional status of animals. Recent studies demonstrated that sweet taste responses can be modulated by leptin and endocannabinoids [anandamide (N-arachidonoylethanolamine) and 2-arachidonoyl glycerol]. Leptin is an anorexigenic mediator that reduces food intake by acting on hypothalamic receptor, Ob-Rb. Leptin is shown to selectively suppress sweet taste responses in wild-type mice but not in leptin receptor-deficient db/db mice. In marked contrast, endocannabinoids are orexigenic mediators that act via CB(1) receptors in hypothalamus and limbic forebrain to induce appetite and stimulate food intake. In the peripheral taste system, endocannabinoids also oppose the action of leptin and enhance sweet taste sensitivities in wild-type mice but not in mice genetically lacking CB(1) receptors. These findings indicate that leptin and endocannabinoids not only regulate food intake via central nervous systems but also may modulate palatability of foods by altering peripheral sweet taste responses via their cognate receptors.

Sweet taste perception is important for animals to detect carbohydrate source of calories and has a critical role in the nutritional status of animals. Recent studies demonstrated that sweet taste responses can be modulated by leptin and endocannabinoids [anandamide (N-arachidonoylethanolamine) and 2-arachidonoyl glycerol]. Leptin is an anorexigenic mediator that reduces food intake by acting on hypothalamic receptor, Ob-Rb. Leptin is shown to selectively suppress sweet taste responses in wild-type mice but not in leptin receptor-deficient db/db mice. In marked contrast, endocannabinoids are orexigenic mediators that act via CB1 receptors in hypothalamus and limbic forebrain to induce appetite and stimulate food intake. In the peripheral taste system, endocannabinoids also oppose the action of leptin and enhance sweet taste sensitivities in wild-type mice but not in mice genetically lacking CB1 receptors. These findings indicate that leptin and endocannabinoids not only regulate food intake via central nervous systems but also may modulate palatability of foods by altering peripheral sweet taste responses via their cognate receptors.

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.

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.