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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.

Sugar detection in the oral cavity does not solely depend on the TAS1R2 + TAS1R3 sweet receptor. Similar to gut, pancreas, and hypothalamic neurons, in the tongue glucose transporters and ATP-sensitive K+ (KATP) channels are also involved in sugar detection. Among them, the KATP channel is the target for the antiobesity hormone leptin, which inhibits sugar-sensitive cells such as sweet taste cells, pancreatic β-cells, and hypothalamic orexigenic neurons. Sugar signals from the taste organ elicit cephalic-phase insulin release, and those from the gut contribute to sweet preference for caloric sugars. All of these systems are indispensable for maintaining energy homeostasis. Thus, an exquisite system for sugar detection/signaling to regulate energy homeostasis exists in our body.

Endocrine cells in the gastrointestinal tract secrete multiple hormones to maintain homeostasis in the body. In the present study, we generated intestinal organoids from the duodenum, jejunum, and ileum of Neurogenin 3 (Ngn3)-EGFP mice and examined how enteroendocrine cells (EECs) within organoid cultures resemble native epithelial cells in the gut. Transcriptome analysis of EGFP-positive cells from Ngn3-EGFP organoids showed gene expression pattern comparable to EECs in vivo. We also compared mRNAs of five major hormones, namely, ghrelin (Ghrl), cholecystokinin (Cck), Gip, secretin (Sct), and glucagon (Gcg) in organoids and small intestine along the longitudinal axis and found that expression patterns of these hormones in organoids were similar to those in native tissues. These findings suggest that an intestinal organoid culture system can be utilized as a suitable model to study enteroendocrine cell functions in vitro.

Taste-signaling proteins, which are expressed throughout the digestive tract, are involved in regulating metabolism and immunity. This study aimed to determine if these genes are expressed and altered in jejunal tissues from patients with extreme obesity who received bariatric surgery. Reverse transcription polymerase chain reaction revealed that phospholipase C beta 2 and transient receptor potential channel M5 expression was downregulated in the jejunum of patients with a body mass index above 50, whereas gustducin expression remained unchanged. Our data suggest that taste-signaling dysregulation might contribute to obesity.

Leptin is known to selectively suppress neural and taste cell responses to sweet compounds. The sweet suppressive effect of leptin is mediated by the leptin receptor Ob-Rb, and the ATP-gated K(+) (K(ATP)) channel expressed in some sweet-sensitive, taste receptor family 1 member 3 (T1R3)-positive taste cells. However, the intracellular transduction pathway connecting Ob-Rb to K(ATP) channel remains unknown. Here we report that phosphoinositide 3-kinase (PI3K) mediates leptin’s suppression of sweet responses in T1R3-positive taste cells. In in situ taste cell recording, systemically administrated leptin suppressed taste cell responses to sucrose in T1R3-positive taste cells. Such leptin’s suppression of sucrose responses was impaired by co-administration of PI3K inhibitors (wortmannin or LY294002). In contrast, co-administration of signal transducer and activator of transcription 3 inhibitor (Stattic) or Src homology region 2 domain-containing phosphatase-2 inhibitor (SHP099) had no effect on leptin’s suppression of sucrose responses, although signal transducer and activator of transcription 3 and Src homology region 2 domain-containing phosphatase-2 were expressed in T1R3-positive taste cells. In peeled tongue epithelium, phosphatidylinositol (3,4,5)-trisphosphate production and phosphorylation of AKT by leptin were immunohistochemically detected in some T1R3-positive taste cells but not in glutamate decarboxylase 67-positive taste cells. Leptin-induced phosphatidylinositol (3,4,5)-trisphosphate production was suppressed by LY294002. Thus, leptin suppresses sweet responses of T1R3-positive taste cells by activation of Ob-Rb–PI3K–K(ATP) channel pathway.

Aim We investigated potential neuron types that code sugar information and how sodium-glucose cotransporters (SGLTs) and T1Rs are involved. Methods Whole-nerve recordings in the chorda tympani (CT) and the glossopharyngeal (GL) nerves and single-fibre recordings in the CT were performed in T1R3-KO and wild-type (WT) mice. Behavioural response measurements were conducted in T1R3-KO mice using phlorizin (Phl), a competitive inhibitor of SGLTs. Results Results indicated that significant enhancement occurred in responses to sucrose and glucose (Glc) by adding 10 mmol/L NaCl but not in responses to KCl, monopotassium glutamate, citric acid, quinine sulphate, SC45647(SC) or polycose in both CT and GL nerves. These enhancements were abolished by lingual application of Phl. In single-fibre recording, fibres showing maximal response to sucrose could be classified according to responses to SC and Glc with or without 10 mmol/L NaCl in the CT of WT mice, namely, Phl-insensitive type, Phl-sensitive Glc-type and Mixed (Glc and SC responding)-type fibres. In T1R3-KO mice, Phl-insensitive-type fibres disappeared. Results from behavioural experiments showed that the number of licks and amount of intake for Glc with or without 10 mmol/L NaCl were significantly suppressed by Phl. Conclusion We found evidence for the contribution of SGLTs in sugar sensing in taste cells of mouse tongue. Moreover, we found T1R-dependent (Phl-insensitive) type, Glc-type and Mixed (SGLTs and T1Rs)-type fibres. SGLT1 may be involved in the latter two types and may play important roles in the glucose-specific cephalic phase of digestion and palatable food intake.

Synopsis Sweet and umami are the attractive tastes to detect nutritious compounds required for our health. TAS1Rs are major receptors for sweet and umami, but other receptors also contribute to these tastes. Information detected by these receptors is sent to the brain via taste cells and gustatory nerve fibers. At the peripheral level, some endocrine factors controlling food intake modulate taste sensitivity in concert with their central effects. This chapter will review the mechanisms for detection, modulation and transmission of sweet and umami tastes at the periphery.

In this study, we were challenging to identify characteristic compounds in breast cancer cell lines. GC analysis of extracts from the culture media of breast cancer cell lines (MCF-7, SK-BR-3, and YMB-1) using a solid-phase Porapak Q extraction revealed that two compounds of moderate volatility, 1-hexadecanol and 5-(Z)-dodecenoic acid, were detected with markedly higher amount than those in the medium of fibroblast cell line (KMST-6). Furthermore, LC-TOF/MS analysis of the extracts clarified that in addition to the above two fatty acids, the amounts of five unsaturated fatty acids [decenoic acid (C10:1), decadienoic acid (C10:2), 5-(Z)-dodecenoic acid (C12:1), 5-(Z)-tetradecenoic acid (C14:1), and tetradecadienoic acid (C14:2)] in MCF-7 medium were higher than those in medium of KMST-6. Interestingly, H2O2-oxidation of 5-(Z)-dodecenoic acid and 5-(Z)-tetradecenoic acid produced volatile aldehydes that were reported as specific volatiles in breath from various cancer patients, such as heptanal, octanal, nonanal, decanal, 2-(E)-nonenal, and 2-(E)-octenal. Thus, we concluded that these identified compounds over-produced in breast cancer cells in this study could serve as potential precursors producing reported cancer-specific volatiles.

Background Gymnemic acids, triterpene glycosides, are known to act as human-specific sweet inhibitors. The long-lasting effect of gymnemic acids is diminished by γ-cyclodextrin. Here, we focus on the molecular mechanisms underlying the interaction between gymnemic acids and sweet taste receptor and/or γ-cyclodextrin by a sweet taste receptor assay in transiently transfected HEK293 cells. Highlight Application of gymnemic acids inhibited intracellular calcium responses to sweet compounds in HEK293 cells expressing human TAS1R2+TAS1R3 but not in those expressing the mouse sweet receptor Tas1r2+Tas1r3 after application of gymnemic acids. The effect of gymnemic acids was reduced after rinsing cells with γ-cyclodextrin. Based on species-specific sensitivities to gymnemic acids, we showed that the transmembrane domain of hTAS1R3 is involved in the sensitivity to gymnemic acids. Point mutation analysis in the transmembrane domain of hTAS1R3 revealed that gymnemic acids shared the same binding pocket with another sweet inhibitor, lactisole. Sensitivity to sweet compounds was also reduced by mixtures of glucuronic acid, a common gymnemic acid. In our molecular models, gymnemic acids interacted with a binding site formed in the transmembrane domain of hTAS1R3. Conclusion Gymnemic acids inhibit sweet responses in humans through an interaction between the glucuronosyl group of gymnemic acids and the transmembrane domain of hTAS1R3. Our molecular model provides a foundation for the development of taste modifiers.

Expression of insulin and its receptor (IR) in rodent taste cells has been proposed, but exactly which types of taste cells express IR and the function of insulin signaling in taste organ have yet to be determined. In this study, we analyzed expression of IR mRNA and protein in mouse taste bud cells in vivo and explored its function ex vivo in organoids, using RT-PCR, immunohistochemistry, and quantitative PCR. In mouse taste tissue, IR was expressed broadly in taste buds, including in type II and III taste cells. With using 3-D taste bud organoids, we found insulin in the culture medium significantly decreased the number of taste cell and mRNA expression levels of many taste cell genes, including nucleoside triphosphate diphosphohydrolase-2 (NTPDase2), Tas1R3 (T1R3), gustducin, carbonic anhydrase 4 (CA4), glucose transporter-8 (GLUT8), and sodium-glucose cotransporter-1 (SGLT1) in a concentration-dependent manner. Rapamycin, an inhibitor of mechanistic target of rapamycin (mTOR) signaling, diminished insulin’s effects and increase taste cell generation. Altogether, circulating insulin might be an important regulator of taste cell growth and/or proliferation via activation of the mTOR pathway.

The primary sweet sensor in mammalian taste cells for sugars and noncaloric sweeteners is the heteromeric combination of type 1 taste receptors 2 and 3 (T1R2+T1R3, encoded by Tas1r2 and Tas1r3 genes). However, in the absence of T1R2+T1R3 (e.g., in Tas1r3 KO mice), animals still respond to sugars, arguing for the presence of T1R-independent detection mechanism(s). Our previous findings that several glucose transporters (GLUTs), sodium glucose cotransporter 1 (SGLT1), and the ATP-gated K(+) (KATP) metabolic sensor are preferentially expressed in the same taste cells with T1R3 provides a potential explanation for the T1R-independent detection of sugars: sweet-responsive taste cells that respond to sugars and sweeteners may contain a T1R-dependent (T1R2+T1R3) sweet-sensing pathway for detecting sugars and noncaloric sweeteners, as well as a T1R-independent (GLUTs, SGLT1, KATP) pathway for detecting monosaccharides. However, the T1R-independent pathway would not explain responses to disaccharide and oligomeric sugars, such as sucrose, maltose, and maltotriose, which are not substrates for GLUTs or SGLT1. Using RT-PCR, quantitative PCR, in situ hybridization, and immunohistochemistry, we found that taste cells express multiple α-glycosidases (e.g., amylase and neutral α glucosidase C) and so-called intestinal "brush border" disaccharide-hydrolyzing enzymes (e.g., maltase-glucoamylase and sucrase-isomaltase). Treating the tongue with inhibitors of disaccharidases specifically decreased gustatory nerve responses to disaccharides, but not to monosaccharides or noncaloric sweeteners, indicating that lingual disaccharidases are functional. These taste cell-expressed enzymes may locally break down dietary disaccharides and starch hydrolysis products into monosaccharides that could serve as substrates for the T1R-independent sugar sensing pathways.

The taste system of animals is used to detect valuable nutrients and harmful compounds in foods. In humans and mice, sweet, bitter, salty, sour and umami tastes are considered the five basic taste qualities. Sweet and umami tastes are mediated by G-protein-coupled receptors, belonging to the T1R (taste receptor type 1) family. This family consists of three members (T1R1, T1R2 and T1R3). They function as sweet or umami taste receptors by forming heterodimeric complexes, T1R1+T1R3 (umami) or T1R2+T1R3 (sweet). Receptors for each of the basic tastes are thought to be expressed exclusively in taste bud cells. Sweet (T1R2+T1R3-expressing) taste cells were thought to be segregated from umami (T1R1+T1R3-expressing) taste cells in taste buds. However, recent studies have revealed that a significant portion of taste cells in mice expressed all T1R subunits and responded to both sweet and umami compounds. This suggests that sweet and umami taste cells may not be segregated. Mice are able to discriminate between sweet and umami tastes, and both tastes contribute to behavioural preferences for sweet or umami compounds. There is growing evidence that T1R3 is also involved in behavioural avoidance of calcium tastes in mice, which implies that there may be a further population of T1R-expressing taste cells that mediate aversion to calcium taste. Therefore the simple view of detection and segregation of sweet and umami tastes by T1R-expressing taste cells, in mice, is now open to re-examination.

Background Several studies have demonstrated that some gut peptides known to be important in energy metabolism are expressed in mouse taste bud cells. However, the functions of these peptides in taste cells are still largely unknown. In the gut, one of these peptides, glucagon-like peptide-1 (GLP-1), which is known as the insulinotropic gut peptide, is secreted from enteroendocrine L-cells, which express as many taste molecules as those on the tongue. These taste transduction molecules are suggested to be involved in GLP-1 secretion from L-cells in response to various nutrient stimuli. GLP-1 is reported to function as a neurotransmitter via activation of its receptors expressed on the vagus nerve, thereby regulating insulin secretion. Highlight Consistent with this evidence from the gastrointestinal tract, recent studies have demonstrated that GLP-1 is secreted from mouse taste cells in response to taste compounds such as sugars, artificial sweeteners, and long-chain fatty acids. GLP-1 secreted from taste cells may activate particular types of gustatory nerve fibers because they express GLP-1 receptors and respond to GLP-1 administered via the femoral vein. Conclusion GLP-1 released from taste cells may be involved in transmission of sweet and lipid signals, thereby impacting animals׳ feeding behavior in response to these important nutrient factors.

Aim To elucidate whether fatty acid taste has a quality that does not overlap with other primary qualities, we investigated potential neuron types coding fatty acid information and how GPR120 is involved. Methods Single fibre recordings in the chorda tympani (CT) nerve and behavioural response measurements using a conditioned taste aversion paradigm were performed in GPR120-knockout (KO) and wild-type (WT) mice. Results Single fibres can be classified into fatty acid (F)-, S-, M-, electrolyte (E)-, Q-, and N-type groups according to the maximal response among oleic acid, sucrose, monopotassium glutamate (MPG), HCl, quinine hydrochloride, and NaCl respectively. Among fibres, 4.0% in GPR120-KO and 17.9% in WT mice showed a maximal response to oleic acid (F-type). Furthermore, half or more of S- and M-type fibres showed responses to fatty acids in both mouse strains, although the thresholds in KO mice were significantly higher and impulse frequencies lower than those in WT mice. GPR120-KO mice conditioned to avoid linoleic acid showed generalized stimulus avoidances for MPG, indicating qualitative similarity between linoleic acid and MPG. The KO mice showed a higher generalization threshold for linoleic acid than that of WT mice. Conclusion Fatty acid taste is suggested to have a unique quality owing to the discovery of F-type fibres, with GPR120 involved in neural information pathways for a unique quality and palatable taste qualities in the mouse CT nerve. GPR120 plays roles in distinguishing fatty acid taste from other primary tastes and the detection of low linoleic acid concentrations.

Sweet taste thresholds are positively related to plasma leptin levels in normal weight humans: both show parallel diurnal variations and associations with postprandial glucose and insulin rises. Here, we tested whether this relationship also exists in overweight and obese (OW/Ob) individuals with hyperleptinemia. We tested 36 Japanese OW/Ob subjects (body mass index (BMI) > 25 kg/m2) for recognition thresholds for various taste stimuli at seven different time points from 8:00 a.m. to 10:00 p.m. using the staircase methodology, and measured plasma leptin, insulin, and blood glucose levels before each taste threshold measurement. We also used the homeostatic model assessment of insulin resistance (HOMA-IR) to evaluate insulin resistance. The results demonstrated that, unlike normal weight subjects, OW/Ob subjects showed no significant diurnal variations in the recognition thresholds for sweet stimuli but exhibited negative associations between the diurnal variations of both leptin and sweet recognition thresholds and the HOMA-IR scores. These findings suggest that in OW/Ob subjects, the basal leptin levels (~20 ng/mL) may already exceed leptin’s effective concentration for the modulation of sweet sensitivity and that this leptin resistance-based attenuation of the diurnal variations of the sweet taste recognition thresholds may also be indirectly linked to insulin resistance in OW/Ob subjects.

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.

Key points The taste receptor T1R1 + T1R3 heterodimer and metabotropic glutamate receptors (mGluR) may function as umami taste receptors. Here, we used mGluR4 knockout (mGluR4-KO) mice and examined the function of mGluR4 in peripheral taste responses of mice. The mGluR4-KO mice showed reduced responses to glutamate and l-AP4 (mGluR4 agonist) in the chorda tympani and glossopharyngeal nerves without affecting responses to other taste stimuli. Residual glutamate responses in mGluR4-KO mice were suppressed by gurmarin (T1R3 blocker) and AIDA (group I mGluR antagonist). The present study not only provided functional evidence for the involvement of mGluR4 in umami taste responses, but also suggested contributions of T1R1 + T1R3 and mGluR1 receptors in glutamate responses. Abstract Umami taste is elicited by l-glutamate and some other amino acids and is thought to be initiated by G-protein-coupled receptors. Proposed umami receptors include heterodimers of taste receptor type 1, members 1 and 3 (T1R1 + T1R3), and metabotropic glutamate receptors 1 and 4 (mGluR1 and mGluR4). Accumulated evidences support the involvement of T1R1 + T1R3 in umami responses in mice. However, little is known about the in vivo function of mGluR in umami taste. Here, we examined taste responses of the chorda tympani (CT) and the glossopharyngeal (GL) nerves in wild-type mice and mice genetically lacking mGluR4 (mGluR4-KO). Our results indicated that compared to wild-type mice, mGluR4-KO mice showed significantly smaller gustatory nerve responses to glutamate and l-(+)-2-amino-4-phosphonobutyrate (an agonist for group III mGluR) in both the CT and GL nerves without affecting responses to other taste stimuli. Residual glutamate responses in mGluR4-KO mice were not affected by (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (an antagonist for group III mGluR), but were suppressed by gurmarin (a T1R3 blocker) in the CT and (RS)-1-aminoindan-1,5-dicarboxylic acid (an antagonist for group I mGluR) in the CT and GL nerve. In wild-type mice, both quisqualic acid (an agonist for group I mGluR) and l-(+)-2-amino-4-phosphonobutyrate elicited gustatory nerve responses and these responses were suppressed by addition of (RS)-1-aminoindan-1,5-dicarboxylic acid and (RS)-alpha-cyclopropyl-4-phosphonophenylglycine, respectively. Collectively, the present study provided functional evidences for the involvement of mGluR4 in umami taste responses in mice. The results also suggest that T1R1 + T1R3 and mGluR1 are involved in umami taste responses in mice. Thus, umami taste would be mediated by multiple receptors.

Bitter taste serves as an important signal for potentially poisonous compounds in foods to avoid their ingestion. Thousands of compounds are estimated to taste bitter and presumed to activate taste receptor cells expressing bitter taste receptors (Tas2rs) and coupled transduction components including gustducin, phospholipase Cβ2 (PLCβ2) and transient receptor potential channel M5 (TRPM5). Indeed, some gustducin-positive taste cells have been shown to respond to bitter compounds. However, there has been no systematic characterization of their response properties to multiple bitter compounds and the role of transduction molecules in these cells. In this study, we investigated bitter taste responses of gustducin-positive taste cells in situ in mouse fungiform (anterior tongue) and circumvallate (posterior tongue) papillae using transgenic mice expressing green fluorescent protein in gustducin-positive cells. The overall response profile of gustducin-positive taste cells to multiple bitter compounds (quinine, denatonium, cyclohexamide, caffeine, sucrose octaacetate, tetraethylammonium, phenylthiourea, L-phenylalanine, MgSO4, and high concentration of saccharin) was not significantly different between fungiform and circumvallate papillae. These bitter-sensitive taste cells were classified into several groups according to their responsiveness to multiple bitter compounds. Bitter responses of gustducin-positive taste cells were significantly suppressed by inhibitors of TRPM5 or PLCβ2. In contrast, several bitter inhibitors did not show any effect on bitter responses of taste cells. These results indicate that bitter-sensitive taste cells display heterogeneous responses and that TRPM5 and PLCβ2 are indispensable for eliciting bitter taste responses of gustducin-positive taste cells.

Gymnemic acids are triterpene glycosides that selectively suppress taste responses to various sweet substances in humans but not in mice. This sweet-suppressing effect of gymnemic acids is diminished by rinsing the tongue with γ-cyclodextrin (γ-CD). However, little is known about the molecular mechanisms underlying the sweet-suppressing effect of gymnemic acids and the interaction between gymnemic acids versus sweet taste receptor and/or γ-CD. To investigate whether gymnemic acids directly interact with human (h) sweet receptor hT1R2 + hT1R3, we used the sweet receptor T1R2 + T1R3 assay in transiently transfected HEK293 cells. Similar to previous studies in humans and mice, gymnemic acids (100 μg/ml) inhibited the [Ca(2+)]i responses to sweet compounds in HEK293 cells heterologously expressing hT1R2 + hT1R3 but not in those expressing the mouse (m) sweet receptor mT1R2 + mT1R3. The effect of gymnemic acids rapidly disappeared after rinsing the HEK293 cells with γ-CD. Using mixed species pairings of human and mouse sweet receptor subunits and chimeras, we determined that the transmembrane domain of hT1R3 was mainly required for the sweet-suppressing effect of gymnemic acids. Directed mutagenesis in the transmembrane domain of hT1R3 revealed that the interaction site for gymnemic acids shared the amino acid residues that determined the sensitivity to another sweet antagonist, lactisole. Glucuronic acid, which is the common structure of gymnemic acids, also reduced sensitivity to sweet compounds. In our models, gymnemic acids were predicted to dock to a binding pocket within the transmembrane domain of hT1R3.

In the oral cavity, taste receptor cells dedicate to detecting chemical compounds in foodstuffs and transmitting their signals to gustatory nerve fibers. Heretofore, five taste qualities (sweet, umami, bitter, salty and sour) are generally accepted as basic tastes. Each of these may have a specific role in the detection of nutritious and poisonous substances; sweet for carbohydrate sources of calories, umami for protein and amino acid contents, bitter for harmful compounds, salty for minerals and sour for ripeness of fruits and spoiled foods. Recent studies have revealed molecular mechanisms for reception and transduction of these five basic tastes. Sweet, umami and bitter tastes are mediated by G-protein coupled receptors (GPCRs) and second-messenger signaling cascades. Salty and sour tastes are mediated by channel-type receptors. In addition to five basic tastes, taste receptor cells may have the ability to detect fat taste, which is elicited by fatty acids, and calcium taste, which is elicited by calcium. Taste compounds eliciting either fat taste or calcium taste may be detected by specific GPCRs expressed in taste receptor cells. This review will focus on transduction mechanisms and cellular characteristics responsible for each of basic tastes, fat taste and calcium taste.

The systemic renin-angiotensin system (RAS) is an important regulator of body fluid and sodium homeostasis. Angiotensin II (AngII) is a key active product of the RAS. We previously revealed that circulating AngII suppresses amiloride-sensitive salt taste responses and enhances the responses to sweet compounds via the AngII type 1 receptor (AT1) expressed in taste cells. However, the molecular mechanisms underlying the modulation of taste function by AngII remain uncharacterized. Here we examined the expression of three RAS components, namely renin, angiotensinogen, and angiotensin-converting enzyme-1 (ACE1), in mouse taste tissues. We found that all three RAS components were present in the taste buds of fungiform and circumvallate papillae and co-expressed with &alpha, ENaC (epithelial sodium channel &alpha, subunit, a salt taste receptor) or T1R3 (taste receptor type 1 member 3, a sweet taste receptor component). Water-deprived mice exhibited significantly increased levels of renin expression in taste cells (p &lt, 0.05). These results indicate the existence of a local RAS in the taste organ and suggest that taste function may be regulated by both locally-produced and circulating AngII. Such integrated modulation of peripheral taste sensitivity by AngII may play an important role in sodium/calorie homeostasis.

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.

Key points • The taste receptor heterodimer T1R1 + T1R3, metabotropic glutamate receptors (mGluRs) and/or their variants may function as umami taste receptors. • Here, we used newly developed T1R1−/− mice and examined the role of T1R1 and mGluRs in taste detection. • The T1R1−/− mice exhibited seriously diminished synergistic responses to glutamate and inosine monophosphate but not to glutamate alone and significantly smaller responses to sweeteners. • Addition of mGluR antagonists significantly inhibited responses to glutamate in both T1R1−/− and heterozygous T1R1+/− mice. • Taken together, these results suggest that T1R1 mainly contributes to umami taste synergism and partly to sweet sensitivity, while mGluRs are involved in the detection of umami compounds. Abstract The T1R1 receptor subunit acts as an umami taste receptor in combination with its partner, T1R3. In addition, metabotropic glutamate receptors (brain and taste variants of mGluR1 and mGluR4) are thought to function as umami taste receptors. To elucidate the function of T1R1 and the contribution of mGluRs to umami taste detection in vivo, we used newly developed knock-out (T1R1−/−) mice, which lack the entire coding region of the Tas1r1 gene and express mCherry in T1R1-expressing cells. Gustatory nerve recordings demonstrated that T1R1−/− mice exhibited a serious deficit in inosine monophosphate-elicited synergy but substantial residual responses to glutamate alone in both chorda tympani and glossopharyngeal nerves. Interestingly, chorda tympani nerve responses to sweeteners were smaller in T1R1−/− mice. Taste cell recordings demonstrated that many mCherry-expressing taste cells in T1R1+/− mice responded to sweet and umami compounds, whereas those in T1R1−/− mice responded to sweet stimuli. The proportion of sweet-responsive cells was smaller in T1R1−/− than in T1R1+/− mice. Single-cell RT-PCR demonstrated that some single mCherry-expressing cells expressed all three T1R subunits. Chorda tympani and glossopharyngeal nerve responses to glutamate were significantly inhibited by addition of mGluR antagonists in both T1R1−/− and T1R1+/− mice. Conditioned taste aversion tests demonstrated that both T1R1−/− and T1R1+/− mice were equally capable of discriminating glutamate from other basic taste stimuli. Avoidance conditioned to glutamate was significantly reduced by addition of mGluR antagonists. These results suggest that T1R1-expressing cells mainly contribute to umami taste synergism and partly to sweet sensitivity and that mGluRs are involved in the detection of umami compounds.

CCK is secreted by endocrine cells of the proximal intestine in response to dietary components, including amino acids. CCK plays a variety of roles in digestive processes, including inhibition of food intake, consistent with a role in satiety. In the lingual epithelium, the sensing of a broad spectrum of l-amino acids is accomplished by the heteromeric amino acid (umami) taste receptor (T1R1-T1R3). T1R1 and T1R3 subunits are also expressed in the intestine. A defining characteristic of umami sensing by T1R1-T1R3 is its potentiation by IMP or GMP. Furthermore, T1R1-T1R3 is not activated by Trp. We show here that, in response to l-amino acids (Phe, Leu, Glu, and Trp), but not d-amino acids, STC-1 enteroendocrine cells and mouse proximal small intestinal tissue explants secrete CCK and that IMP enhances Phe-, Leu-, and Glu-induced, but not Trp-induced, CCK secretion. Furthermore, small interfering RNA inhibition of T1R1 expression in STC-1 cells results in significant diminution of Phe-, Leu-, and Glu-stimulated, but not Trp-stimulated, CCK release. In STC-1 cells and mouse intestine, gurmarin inhibits Phe-, Leu-, and Glu-induced, but not Trp-stimulated, CCK secretion. In contrast, the Ca2+-sensing receptor antagonist NPS2143 inhibits Phe-stimulated CCK release partially and Trp-induced CCK secretion totally in mouse intestine. However, NPS2143 has no effect on Leu- or Glu-induced CCK secretion. Collectively, our data demonstrate that functional characteristics and cellular location of the gut-expressed T1R1-T1R3 support its role as a luminal sensor for Phe-, Leu-, and Glu-induced CCK secretion.

Nordihydroguaiaretic acid (NDGA) is known to have prominent anticancer activity against several cancers, and is also known to be an inhibitor of 5-lipoxygenase (5-LO). In this study, we investigated the regulatory function of NDGA on inflammatory bone destruction mediated by osteoclasts. NDGA markedly inhibited receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL)-induced formation of osteoclasts in cultures of murine osteoclast precursor cell line RAW-D cells and primary bone marrow-derived macrophages culture systems. The inhibitory effect of NDGA on osteoclastogenesis did not arise from the inhibition of 5-LO activity. NDGA did not affect MAPKs, such as p38, JNK, and NF-κB, but significantly inhibited the induction of NFATc1, a key transcription factor for osteoclastogenesis. NDGA also suppressed activation of ERK in osteoclast precursors. RANKL-induced calcium oscillation observed in osteoclast precursors was completely diminished by the addition of NDGA. In mature osteoclasts, RANKL-induced nuclear translocation of NFATc1 was clearly inhibited by NDGA treatment. Finally, in vivo studies demonstrated that administration of NDGA significantly reduced severe bone destruction and osteoclast recruitment in the ankle joint of rats with adjuvant-induced arthritis. These results indicate the potential utility of NDGA as a therapeutic agent for ameliorating inflammatory bone destruction in rheumatoid arthritis.

The hypothalamic feeding center plays an important role in energy homeostasis. In the feeding center, whole-body energy signals including hormones and nutrients are sensed, processed, and integrated. As a result, food intake and energy expenditure are regulated. Two types of glucose-sensing neurons exist in the hypothalamic arcuate nucleus (ARC): glucose-excited neurons and glucose-inhibited neurons. While some molecules are known to be related to glucose sensing in the hypothalamus, the mechanism underlying glucose sensing in the hypothalamus are not fully understood. The sweet taste receptor is a heterodimer of taste type 1 receptor 2 (T1R2) and taste type 1 receptor 3 (T1R3) and senses sweet tastes. T1R2 and T1R3 receptors are distributed in multiple organs including the tongue, pancreas, adipose tissue, and hypothalamus. However, the role of sweet taste receptors in the ARC remains to be clarified. To examine the role of sweet taste receptors in the ARC, cytosolic Ca2+ concentration ([Ca2+]i) in isolated single ARC neurons were measured using Fura-2 fluorescent imaging. An artificial sweetener, sucralose at 10-5 M-10-2 M dose dependently increased [Ca2+]i in 12-16% of ARC neurons. The sucralose-induced [Ca2+]i increase was suppressed by a sweet taste receptor inhibitor, gurmarin. The sucralose-induced [Ca2+]i increase was inhibited under an extracellular Ca2+-free condition and in the presence of an L-type Ca2+ channel blocker, nitrendipine. Sucralose-responding neurons were activated by high-concentration of glucose. This response to glucose was markedly suppressed by gurmarin. More than half of sucralose-responding neurons were activated by leptin but not ghrelin. Percentage of proopiomelanocortin (POMC) neurons among sucralose-responding neurons and sweet taste receptor expressing neurons were low, suggesting that majority of sucralose-responding neurons are non-POMC neurons. These data suggest that sweet taste receptor-mediated cellular activation mainly occurs on non-POMC leptin-responding neurons and contributes to glucose responding. Endogenous sweet molecules including glucose may regulate energy homeostasis through sweet taste receptors on glucose-and leptin-responsive neurons in the ARC.

Acidification of the glycoprotein, miraculin (MCL), induces sweet taste in humans, but not in mice. The sweet taste induced by MCL is more intense when acidification occurs with weak acids as opposed to strong acids. MCL interacts with the human sweet receptor subunit hTAS1R2, but the mechanisms by which the acidification of MCL activates the sweet taste receptor remain largely unexplored. The work reported here speaks directly to this activation by utilizing a sweet receptor TAS1R2 + TAS1R3 assay. In accordance with previous data, MCL-applied cells displayed a pH dependence with citric acid (weak acid) being right shifted to that with hydrochloric acid (strong acid). When histidine residues in both the intracellular and extracellular region of hTAS1R2 were exchanged for alanine, taste-modifying effect of MCL was reduced or abolished. Stronger intracellular acidification of HEK293 cells was induced by citric acid than by HCl and taste-modifying effect of MCL was proportional to intracellular pH regardless of types of acids. These results suggest that intracellular acidity is required for full activation of the sweet taste receptor by MCL.

The sense of taste conveys crucial information about the quality and nutritional value of foods before it is ingested. Taste signaling begins with taste cells via taste receptors in oral cavity. Activation of these receptors drives the transduction systems in taste receptor cells. Then particular transmitters are released from the taste cells and activate corresponding afferent gustatory nerve fibers. Recent studies have revealed that taste sensitivities are defined by distinct taste receptors and modulated by endogenous humoral factors in a specific group of taste cells. Such peripheral taste generations and modifications would directly influence intake of nutritive substances. This review will highlight current understanding of molecular mechanisms for taste reception, signal transduction in taste bud cells, transmission between taste cells and nerves, regeneration from taste stem cells, and modification by humoral factors at peripheral taste organs.

Taste receptor cells sense various chemical compounds in foods and transmit these signals through gustatory nerve fibers to the central nervous system. These sensory signals are vitally important for life; they provide information about which prospective foods are nutritious and warnings as to those that are noxious. Recent studies have revealed the involvement of multifarious bioactive peptides, many of which are primarily identified organs such as the gastrointestinal tract, in the modulation of taste responses. These peptides affect peripheral taste responsiveness of animals and play important roles in the regulation of feeding behavior and the maintenance of homeostasis. In this chapter, we discuss the various functions of peptide signaling in the peripheral taste system.

The heteromeric sweet taste receptor T1R2-T1R3 is expressed on the luminal membrane of certain populations of enteroendocrine cells. Sensing of sugars and other sweet compounds by this receptor activates a pathway in enteroendocrine cells, resulting in secretion of a number of gut hormones, including glucagon-like peptide 2 (GLP-2). This subsequently leads to upregulation in the expression of intestinal Na+/glucose cotransporter, SGLT1, and increased intestinal glucose absorption. On the basis of the current information available on the horse genome sequence, it has been proposed that the gene for T1R2 ( Tas1R2) is absent in the horse. We show here, however, that horses express both the mRNA and protein for T1R2. Equine T1R2 is most closely homologous to that in the pig and the cow. T1R2 protein, along with T1R3, α-gustducin, and GLP-2 proteins are coexpressed in equine intestinal endocrine cells. Intravenous administration of GLP-2, in rats and pigs, leads to an increase in the expression of SGLT1 in absorptive enterocytes and enhancement in blood glucose concentrations. GLP-2 receptor is expressed in enteric neurons, excluding the direct effect of GLP-2 on enterocytes. However, electric stimulation of enteric neurons generates a neural response leading to SGLT1 upregulation, suggesting that sugar in the intestine activates a reflex increase in the functional expression of SGLT1. Horses possess the ability to upregulate SGLT1 expression in response to increased dietary carbohydrates, and to enhance the capacity of the gut to absorb glucose. The gut sweet receptor provides an accessible target for manipulating the equine gut to absorb glucose (and water), allowing greater energy uptake and hydration for hard-working horses.

Amino acids are known to elicit complex taste, but most human psychophysical studies on the taste of amino acids have focused on a single basic taste, such as umami (savory) taste, sweetness, or bitterness. In this study, we addressed the potential relationship between the structure and the taste properties of amino acids by measuring the human gustatory intensity and quality in response to aqueous solutions of proteogenic amino acids in comparison to D-enantiomers. Trained subjects tasted aqueous solution of each amino acid and evaluated the intensities of total taste and each basic taste using a category-ratio scale. Each basic taste of amino acids showed the dependency on its hydrophobicity, size, charge, functional groups on the side chain, and chirality of the alpha carbon. In addition, the overall taste of amino acid was found to be the combination of basic tastes according to the partial structure. For example, hydrophilic non-charged middle-sized amino acids elicited sweetness, and L-enantiomeric hydrophilic middle-sized structure was necessary for umami taste. For example, L-serine had mainly sweet and minor umami taste, and D-serine was sweet. We further applied Stevens' psychophysical function to relate the total-taste intensity and the concentration, and found that the slope values depended on the major quality of taste (e.g., bitter large, sour small).

The distinctive umami taste elicited by l-glutamate and some other amino acids is thought to be initiated by G-protein-coupled receptors. Proposed umami receptors include heteromers of taste receptor type 1, members 1 and 3 (T1R1+T1R3), and metabotropic glutamate receptors 1 and 4 (mGluR1 and mGluR4). Multiple lines of evidence support the involvement of T1R1+T1R3 in umami responses of mice. Although several studies suggest the involvement of receptors other than T1R1+T1R3 in umami, the identity of those receptors remains unclear. Here, we examined taste responsiveness of umami-sensitive chorda tympani nerve fibres from wild-type mice and mice genetically lacking T1R3 or its downstream transduction molecule, the ion channel TRPM5. Our results indicate that single umami-sensitive fibres in wild-type mice fall into two major groups: sucrose-best (S-type) and monopotassium glutamate (MPG)-best (M-type). Each fibre type has two subtypes; one shows synergism between MPG and inosine monophosphate (S1, M1) and the other shows no synergism (S2, M2). In both T1R3 and TRPM5 null mice, S1-type fibres were absent, whereas S2-, M1- and M2-types remained. Lingual application of mGluR antagonists selectively suppressed MPG responses of M1- and M2-type fibres. These data suggest the existence of multiple receptors and transduction pathways for umami responses in mice. Information initiated from T1R3-containing receptors may be mediated by a transduction pathway including TRPM5 and conveyed by sweet-best fibres, whereas umami information from mGluRs may be mediated by TRPM5-independent pathway(s) and conveyed by glutamate-best fibres.

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.

There are no credible data to support the notion that individual taste qualities have dedicated pathways leading from the tongue to the end of the pathway in the brain. Moreover, the insular cortex is activated not only by taste but also by non-taste information from oral stimuli. These responses are invariably excitatory, and it is difficult to determine whether they are sensory, motor, or proprioceptive in origin. Furthermore, umami is a more unfamiliar and complex taste than other basic tastes. Considering these issues, it may be effective to minimize somatosensory stimuli, oral movement, and psychological effects in a neuroimaging study to elicit cerebral activity by pure umami on the human tongue. For this purpose, we developed an original taste delivery system for functional magnetic resonance imaging (fMRI) studies for umami. Then, we compared the results produced by two authorized models, namely, the block design model and event-related design model, to decide the appropriate model for detecting activation by umami. Activation by the umami taste was well localized in the insular cortex using our new system and block design model analysis. The peaks of the activated areas in the middle insular cortex by umami were very close to another prototypical taste quality (salty). Although we have to carefully interpret the perceiving intensities and brain activations by taste from different sessions, this study design might be effective for detecting the accession area in the cortex of pure umami taste on the tongue.

Only some taste cells fire action potentials in response to sapid stimuli. Type II taste cells express many taste transduction molecules but lack well-elaborated synapses, bringing into question the functional significance of action potentials in these cells. We examined the dependence of adenosine triphosphate (ATP) transmitter release from taste cells on action potentials. To identify type II taste cells we used mice expressing a green fluorescence protein (GFP) transgene from the α-gustducin promoter. Action potentials were recorded by an electrode basolaterally attached to a single GFP-positive taste cell. We monitored ATP release from gustducin-expressing taste cells by collecting the electrode solution immediately after tastant-stimulated action potentials and using a luciferase assay to quantify ATP. Stimulation of gustducin-expressing taste cells with saccharin, quinine, or glutamate on the apical membrane increased ATP levels in the electrode solution; the amount of ATP depended on the firing rate. Increased spontaneous firing rates also induced ATP release from gustducin-expressing taste cells. ATP release from gustducin-expressing taste cells was depressed by tetrodotoxin and inhibited below the detection limit by carbenoxolone. Our data support the hypothesis that action potentials in taste cells responsive to sweet, bitter, or umami tastants enhance ATP release through pannexin 1, not connexin-based hemichannels.