Search

T1R3 is a class C G protein-coupled receptor family member that forms heterodimeric umami and sweet taste receptors with T1R1 and T1R2, respectively, in the taste cells of taste buds. T1R3 is expressed in 3T3-L1 cells in homomeric form and negatively regulates adipogenesis in a Gαs-dependent but cAMP-independent manner. Although T1R3 expression is markedly upregulated during adipogenesis, its physiological role in mature adipocytes remains obscure. Here, we show that stimulation of T1R3 with sucralose or saccharin induces microtubule disassembly in differentiated 3T3-L1 adipocytes. The effect was reproduced by treatment with cholera toxin or isoproterenol but not with forskolin. Treatment with sucralose or saccharin for 3 h inhibited insulin-stimulated glucose uptake by 32% and 45% in differentiated adipocytes, respectively, similar to the inhibitory effect of nocodazole (by 33%). Isoproterenol treatment inhibited insulin-stimulated glucose transport by 26%, whereas sucralose did not affect the intrinsic activity of the glucose transporter, indicating that it inhibited insulin-induced GLUT4 translocation to the plasma membrane. Immunostaining analysis showed that insulin-stimulated GLUT4 accumulation on the plasma membrane was abrogated in sucralose-treated cells, in association with depolymerization of microtubules. Sucralose-mediated inhibition of GLUT4 translocation was reversed by the overexpression of dominant-negative Gαs (Gαs-G226A) or knockdown of Gαs. Additionally, membrane fractionation analysis showed that sucralose treatment reduced GLUT4 levels in the plasma membrane fraction from insulin-stimulated adipocytes. We have identified a novel non-gustatory role for homomeric T1R3 in adipocytes, and activation of the T1R3 receptor negatively regulates insulin action of glucose transport via Gαs-dependent microtubule disassembly.

Heterogeneous tissue models require the assembly and co-culture of multiple types of cells. Our recent work demonstrated taste signal transmission from gustatory cells to neurons by grafting single-stranded DNA into the cell membrane to construct multicellular assemblies. However, the weak DNA linkage and low grafting density allowed the formation of large gustatory cell self-aggregates that cannot communicate with neurons efficiently. This article presents the construction of artificial taste buds exhibiting active intercellular taste signal transmission through the hybridization of gustatory-neuronal multicellular interfaces using bioorthogonal click chemistry. Hybrid cell clusters were formed by the self-assembly of neonatal gustatory cells displaying tetrazine with a precultured embryonic hippocampal neuronal network displaying trans-cyclooctene. A bitter taste signal transduction was provoked in gustatory cells using denatonium benzoate and transmitted to neurons as monitored by intracellular calcium ion sensing. In the multicellular hybrids, the average number of signal transmissions was five to six peaks per cell, and the signal transmission lasted for ∼5 min with a signal-to-signal gap time of 10-40 s. The frequent and extended intercellular signal transmission suggests that the cell surface modification by the bioorthogonal click chemistry is a promising approach to fabricating functional multicellular hybrid clusters potentially useful for cell-based biosensors, toxicity assays, and tissue regeneration.

The characterization of molecular mechanisms underlying the taste-sensing system of chickens will add to our understanding of their feeding behaviors in poultry farming. In the mammalian taste system, the heterodimer of taste receptor type 1 members 1/3 (T1R1/T1R3) functions as an umami (amino acid) taste receptor. Here, we analyzed the expression patterns of T1R1 and T1R3 in the taste cells of chickens, labeled by the molecular markers for chicken taste buds (vimentin and α-gustducin). We observed that α-gustducin was expressed in some of the chicken T1R3-positive taste bud cells but rarely expressed in the T1R1-positive and T2R7-positive taste bud cells. These results raise the possibility that there is another second messenger signaling system in chicken taste sensory cells. We also observed that T1R3 and α-gustducin were expressed mostly in the vimentin-positive taste bud cells, whereas T1R1 and bitter taste receptor (i.e., taste receptor type 2 member 7, T2R7) were expressed largely in the vimentin-negative taste bud cells in chickens. In addition, we observed that T1R1 and T1R3 were co-expressed in about 5% of chickens' taste bud cells, which express T1R1 or T1R3. These results suggest that the heterodimer of T1R1 and T1R3 is rarely formed in chickens’ taste bud cells, and they provide comparative insights into the expressional regulation of taste receptors in the taste bud cells of vertebrates.

Sex as a biological variable has been the focus of increasing interest. Relatively few studies have focused, however, on differences in peripheral taste function between males and females. Nonetheless, there are reports of sex-dependent differences in chemosensitivity in the gustatory system. The involvement of endogenous changes in ovarian hormones has been suggested to account for taste discrepancies. Additionally, whether sex differences exist in taste receptor expression, activation, and subsequent signaling pathways that may contribute to different taste responsiveness is not well understood. In this study, we show the presence of both the nuclear and plasma membrane forms of estrogen receptor (ER) mRNA and protein in mouse taste cells. Furthermore, we provide evidence that estrogen increases taste cell activation during the application of fatty acids, the chemical cue for fat taste, in taste receptor cells. We found that genes important for the transduction pathway of fatty acids vary between males and females and that these differences also exist across the various taste papillae. In vivo support for the effect of estrogens in taste cells was provided by comparing the fatty acid responsiveness in male, intact female, and ovariectomized (OVX) female mice with and without hormone replacement. In general, females detected fatty acids at lower concentrations, and the presence of circulating estrogens increased this apparent fat taste sensitivity. Taken together, these data indicate that increased circulating estrogens in the taste system may play a significant role in physiology and chemosensory cellular activation and, in turn, may alter taste-driven behavior. NEW & NOTEWORTHY Using molecular, cellular, and behavioral analyses, this study shows that sex differences occur in fat taste in a mouse model. Female mice are more responsive to fatty acids, leading to an overall decrease in intake and fatty acid preference. These differences are linked to sex hormones, as estradiol enhances taste cell responsiveness to fatty acids during periods of low circulating estrogen following ovariectomy and in males. Estradiol is ineffective in altering fatty acid signaling during a high-estrogen period and in ovariectomized mice on hormone replacement. Thus, taste receptor cells are a direct target for actions of estrogen, and there are multiple receptors with differing patterns of expression in taste cells.

New findings What is the central question of this study? Loss of taste or inability to distinguish between different tastes progresses with age. The purpose was to evaluate the age-dependent changes in taste by studying the electrophysiological properties of taste receptor cells. What is the main finding and its importance? Ageing decreased the voltage-gated Na+ and K+ current densities of type III cells (sour and/or salt receptor cells) but did not affect the current densities in type II cells. At the peripheral levels, the excitability of type III cells was reduced due to ageing, which may affect the signal transduction to taste nerves. Abstract The loss of taste due to normal ageing in mammals is assumed to be caused by the ageing of taste receptor cells. We examined the electrophysiological properties of taste receptor cells in the fungiform taste buds of ∼20-month-old mice in situ and subsequently identified their cell types with immunological markers: the inositol 1,4,5-trisphosphate (IP3 ) receptor (IP3 R3) for type II cells and a SNARE protein, synaptosomal-associated protein 25 (SNAP-25), for type III cells. Other cells are referred to as non-immunoreactive cells (non-IRCs). Cell types of some cells that could not be identified using cell-type markers were identified based on the electrophysiological feature of the respective cell types. All cell types generated action potentials and a variety of voltage-gated currents. The type II cells mainly expressed tetraethylammonium (TEA)-insensitive and slowly activating outwardly rectifying currents and generated tail currents in repolarization. In contrast, the type III cells expressed TEA-sensitive and faster activating K+ currents and did not generate tail currents. These cell type-specific characteristics of voltage-gated currents in ∼20-month-old mice were similar to their respective cell types in ∼2-month-old mice. Also, we showed an age-dependent decrease in Na+ and K+ current densities in type III cells and an age-dependent increase in outwardly rectifying current density in non-IRCs. Ageing did not affect the voltage-gated current densities in type II cells. The decreased Na+ and K+ current densities, i.e. the decreased excitability of type III cells, due to ageing may affect the signal transduction to taste nerves.

Bitter taste receptors (hTAS2R) are expressed ectopically in various tissues, raising the possibility of a pharmacological exploitation. This seems of particular relevance in airways, since hTAS2Rs are involved in the protection of the aerial tissues from infections and in bronchodilation. The bis-guaianolide absinthin (1), one of the most bitter compounds known, targets the hTAS2R46 bitter receptor. Absinthin (1), an unstable compound, readily turns into anabsinthin (2) with substantial retention of the bitter properties, and this compound was used as a starting material to explore the chemical space around the bis-guaianolide bitter pharmacophore. Capitalizing on the chemoselective opening of the allylic lactone ring, the esters 3 and 4, and the nor-azide 6 were prepared and assayed on human bronchoepithelial (BEAS-2B) cells expressing hTAS2R46. Anti-inflammatory activity was evaluated by measuring the expression of MUC5AC, iNOS, and cytokines, as well as the production of superoxide anion, qualifying the methyl ester 3 as the best candidate for additional studies.

Inborn genetic differences in chemosensory receptors can lead to differences in perception and preference for foods and beverages. These differences can drive market segmentation for food products as well as contribute to nutritional status. This knowledge may be essential in the development of foods and beverages because the sensory profiles may not be experienced in the same way across individuals. Rather, distinct consumer groups may exist with different underlying genetic variations. Identifying genetic factors associated with individual variability can help better meet consumer needs through an enhanced understanding of perception and preferences. This review provides an overview of taste and chemesthetic sensations and their receptors, highlighting recent advances linking genetic variations in chemosensory genes to perception, food preference and intake, and health. With growing interest in personalized foods, this information is useful for both food product developers and nutrition health professionals alike.

Numerous fatty acid receptors have proven to play critical roles in normal physiology. Interactions among these receptor types and their subsequent membrane trafficking has not been fully elucidated, due in part to the lack of efficient tools to track these cellular events. In this study, we fabricated the surface-enhanced Raman scattering (SERS)-based molecular sensors for detection of two putative fatty acid receptors, G protein-coupled receptor 120 (GPR120) and cluster of differentiation 36 (CD36), in a spatiotemporal manner in single cells. These SERS probes allowed multiplex detection of GPR120 and CD36, as well as a peak that represented the cell. This multiplexed sensing system enabled the real-time monitoring of fatty acid-induced receptor activation and dynamic distributions on the cell surface, as well as tracking of the receptors’ internalization processes on the addition of fatty acid. Increased SERS signals were seen in engineered HEK293 cells with higher fatty acid concentrations, while decreased responses were found in cell line TBDc1, suggesting that the endocytic process requires innate cellular components. SERS mapping results confirm that GPR120 is the primary receptor and may work synergistically with CD36 in sensing polyunsaturated fatty acids and promoting Ca 2+ mobilization, further activating the process of fatty acid uptake. The ability to detect receptors’ locations and monitor fatty acid-induced receptor redistribution demonstrates the specificity and potential of our multiplexed SERS imaging platform in the study of fatty acid–receptor interactions and might provide functional information for better understanding their roles in fat intake and development of fat-induced obesity.

Salt taste is one of the 5 basic taste qualities. Depending on the concentration, table salt is perceived either as appetitive or aversive, suggesting the contribution of several mechanisms to salt taste, distinguishable by their sensitivity to the epithelial sodium channel (ENaC) blocker amiloride. A taste-specific knockout of the α-subunit of the ENaC revealed the relevance of this polypeptide for low-salt transduction, whereas the response to other taste qualities remained normal. The fully functional ENaC is composed of α-, β-, and γ-subunits. In taste tissue, however, the precise constitution of the channel and the cell population responsible for detecting table salt remain uncertain. In order to examine the cells and subunits building the ENaC, we generated mice carrying modified alleles allowing the synthesis of green and red fluorescent proteins in cells expressing the α- and β-subunit, respectively. Fluorescence signals were detected in all types of taste papillae and in taste buds of the soft palate and naso-incisor duct. However, the lingual expression patterns of the reporters differed depending on tongue topography. Additionally, immunohistochemistry for the γ-subunit of the ENaC revealed a lack of overlap between all potential subunits. The data suggest that amiloride-sensitive recognition of table salt is unlikely to depend on the classical ENaCs formed by α-, β-, and γ-subunits and ask for a careful investigation of the channel composition.

The current work gives concern to study the morphology of the Merluccius merluccius gills by using gross morphology, scanning electron microscopy (SEM), and light microscopy. The findings of the present study revealed that the gill system consisted of four pairs of gill arches which carry the gill filaments on the convex border and gill rakers on the concave border of them. SEM results revealed that the rakers and the spines distribution on the first gill arch differed from that of the other three gill arches on the lateral and medial surfaces. On the surface the gill filaments, there were longitudinal ridges that carried pores of chloride cells and mucous cells. The histological examination revealed that, the gill arch composed of hyaline cartilage that presented in the form of cups. Each cup consisted of central cartilagenous core and peripheral cartilagenous matrix. The gill filaments composed of cartilaginous bar of peripheral cartilaginous matrix and central cartilaginous core extended from the gill arches and covered by an epithelial layers with a few mucous cells permeate it, and chloride cells were straggly in the interlamellar epithelium. Each gill filament carried several leaves like secondary lamellae on both sides of it. The epithelium, which lined the secondary lamellae, composed of epithelial pavement cells, some mucous cells, and pillar cells.

Taste buds contain multiple cell types, two of which mediate transduction of specific taste qualities: Type III cells transduce sour while Type II cells transduce either sweet, or bitter or umami. In order to discern the degree of interaction between different cell types and specificity of connectivity with the afferent nerve fibers (NFs), we employed serial blockface scanning electron microscopy (sbfSEM) through five circumvallate mouse taste buds. Points of contact between Type II and Type III cells are rare and lack morphologically identifiable synapses, suggesting that interaction between these cell types does not occur via synapses. Of the 127 NFs that make synaptic contacts with taste cells in the sampling volume, ∼70% (n= 91) synapse with only one taste cell while 32 fibers synapse exclusively with multiple Type II cells or multiple Type III cells. Our data do not rule out multimodal fibers innervating Type II cells of separate taste qualities. Notably, four fibers (∼3%) synapse with both Type II and Type III cells, forming both mitochondrial and vesicular synapses on the different cell types. Since Type II and Type III cells transduce different taste qualities, these dual connected fibers are not consistent with a absolute labeled-line encoding system. Further, our data reveal considerable variation in both the number of synapses per cell/nerve pair and the number of innervating NFs per taste cell, both of which likely have consequences for encoding taste quality and concentration. Finally, we identify a subset of Type II cells which may represent an immature stage.SIGNIFICANCE STATEMENTTaste buds, the sensory end organs for the sense of taste, contain multiple types of sensory cells, with each responding to one of the primary tastes: salt, sweet, sour, bitter, and umami. In order to determine the degree of interaction between cell types and specificity of connectivity to afferent nerves, we employed serial blockface electron microscopy (EM) of mouse circumvallate taste buds. We find no synapses between cell types within the taste bud suggesting that any interactions are indirect. While the majority of nerve fibers (NFs) connect to a single type of taste cell, 3.1% of the fibers branch to receive input from taste cells of different specificities. Thus, taste cannot entirely be carried along NFs dedicated to single taste qualities.

Sweet taste perception is mediated by a heterodimeric receptor formed by the assembly of the TAS1R2 and TAS1R3 subunits. TAS1R2 and TAS1R3 are class C G-protein-coupled receptors whose members share a common topology, including a large extracellular N-terminal domain (NTD) linked to a seven transmembrane domain (TMD) by a cysteine-rich domain. TAS1R2-NTD contains the primary binding site for sweet compounds, including natural sugars and high-potency sweeteners, whereas the TAS1R2-TMD has been shown to bind a limited number of sweet tasting compounds. To understand the molecular mechanisms governing receptor–ligand interactions, we overexpressed the human TAS1R2 (hTAS1R2) in a stable tetracycline-inducible HEK293S cell line and purified the detergent-solubilized receptor. Circular dichroism spectroscopic studies revealed that hTAS1R2 was properly folded with evidence of secondary structures. Using size exclusion chromatography coupled to light scattering, we found that the hTAS1R2 subunit is a dimer. Ligand binding properties were quantified by intrinsic tryptophan fluorescence. Due to technical limitations, natural sugars have not been tested. However, we showed that hTAS1R2 is capable of binding high potency sweeteners with Kd values that are in agreement with physiological detection. This study offers a new experimental strategy to identify new sweeteners or taste modulators that act on the hTAS1R2 and is a prerequisite for structural query and biophysical studies.

The present study investigated the cellular components and afferent innervations of taste buds in the rat incisive papilla by immunohistochemistry using confocal scanning laser microscopy. Taste buds containing guanine nucleotide-binding protein G(t), subunit α3 (GNAT3)-imunoreactive cells were densely distributed in the lateral wall of incisive papilla forming the opening of nasoincisor ducts. GNAT3-immunoreactive cells in the taste buds were slender in shape and the tips of apical processes gathered at one point at the surface of the epithelium. The number of taste buds was 56.8 ± 4.5 in the incisive papilla. The incisive taste buds also contained ectonucleoside triphosphate diphosphohydrolase 2-immunoreactive cells and synaptotagmin-1-immunoreactive cells in addition to GNAT3-immunoreactive cells. Furthermore, GNAT3-immunoreactive cells were immunoreactive to taste transduction molecules such as phospholipase C, β2-subunit, and inositol 1,4,5-trisphosphate receptor, type 3. P2X3-immunoreactive subepithelial nerve fibers intruded into the taste buds and terminated with hederiform or calix-like nerve endings attached to GNAT3-immunoreactive cells and synaptosomal-associated protein, 25 kDa-immunoreactive cells. Some P2X3-immunoreactive endings were also weakly immunoreactive for P2X2. Furthermore, a retrograde tracing method using fast blue dye indicated that most of the P2X3-immunoreactive nerve endings originated from the geniculate ganglia (GG) of the facial nerve. These results suggest that incisive taste buds are morphologically and cellularly homologous to lingual taste buds and are innervated by P2X3-immunoreactive nerve endings derived from the GG. The incisive papilla may be the palatal taste papilla that transmits chemosensory information in the oral cavity to the GG via P2X3-immunoreactive afferent nerve endings.

In addition to the taste receptors corresponding to the six basic taste qualities—sweet, salty, sour, bitter, umami, and fatty—another type of taste receptor, calcium-sensing receptor (CaSR), is found in taste-bud cells. CaSR is called the ‘kokumi’ receptor because its agonists increase sweet, salty and umami tastes to induce ‘koku’, a Japanese word meaning the enhancement of flavor characters such as thickness, mouthfulness, and continuity. Koku is an important factor for enhancing food palatability. However, it is not well known whether other kokumi-receptors and substances exist. Here, we show that ornithine (L-ornithine but not D-ornithine) at low concentrations that do not elicit a taste of its own, enhances preferences to sweet, salty, umami, and fat taste solutions in mice. Increased preference to monosodium glutamate (MSG) was the most dominant effect. Antagonists of G-protein-coupled receptor family C group 6 subtype A (GPRC6A) abolished the additive effect of ornithine on MSG solutions. The additive effects of ornithine on taste stimuli are thought to occur in the oral cavity, and are not considered post-oral events because ornithine’s effects were confirmed in a brief-exposure test. Moreover, the additive effects of ornithine and the action of the antagonist were verified in electrophysiological taste nerve responses. Immunohistochemical analysis implied that GPRC6A was expressed in subsets of type II and type III taste cells of mouse circumvallate papillae. These results are in good agreement with those reported for taste modulation involving CaSR and its agonists. The present study suggests that ornithine is a kokumi substance and GPRC6A is a newly identified kokumi receptor.

Current understanding of functional characteristics and biochemical pathways in taste bud cells have been hindered due the lack of long-term cultured cells. To address this, we developed a holistic approach to fully characterise long term cultured bovine taste bud cells (BTBCs). Initially, cultured BTBCs were characterised using RT-PCR gene expression profiling, immunocytochemistry, flowcytometry and calcium imaging, that confirmed the cells were mature TBCs that express taste receptor genes, taste specific protein markers and capable of responding to taste stimuli, i.e., denatonium (2 mM) and quinine (462.30 μM). Gene expression analysis of forty-two genes implicated in taste transduction pathway (map04742) using custom-made RT-qPCR array revealed high and low expressed genes in BTBCs. Preliminary datamining and bioinformatics demonstrated that the bovine α-gustducin, gustatory G-protein, have higher sequence similarity to the human orthologue compared to rodents. Therefore, results from this work will replace animal experimentation and provide surrogate cell-based throughput system to study human taste transduction.

This review summarizes our understanding of ATP signaling in taste and describes new directions for research. ATP meets all requisite criteria to be considered a neurotransmitter: (1) presence in taste cells, as in all cells; (2) release upon appropriate taste stimulation; (3) binding to cognate purinergic receptors P2X2 and P2X3 on gustatory afferent neurons, and (4) after release, enzymatic degradation to adenosine and other nucleotides by the ectonucleotidase, NTPDase2, expressed on the Type I, glial-like cells in the taste bud. Importantly, double knockout of P2X2 and P2X3 or pharmacological inhibition of P2X3 abolishes transmission of all taste qualities. In Type II taste cells (those that respond to sweet, bitter, or umami stimuli), ATP is released non-vesicularly by a large conductance ion channel composed of CALHM1 and CALHM3, which form a so-called channel synapse at areas of contact with afferent taste nerve fibers. Although ATP release has been detected only from Type II cells, it is also required for the transmission of salty and sour stimuli, which are mediated primarily by the Type III taste cells. The source of the ATP required for Type III cell signaling to afferent fibers is still unclear and is a focus for future experiments. The ionotropic purinergic receptor, P2X3, is widely expressed on many sensory afferents and has been a therapeutic target for treating chronic cough and pain. However, its requirement for taste signaling has complicated efforts at treatment since patients given P2X3 antagonists report substantial disturbances of taste and become non-compliant.

A morphological description of the sugar glider's (Petaurus breviceps) tongue was obtained from five individual sugar gliders and analysed under a scanning electron microscope and a light microscope using four methods of staining. The tongue has a noticeably elongated structure with three regions: apex, corpus and radix. Transition to the corpus is indicated by the sulcus medianus linguae and the frenulum on its ventral surface. The dorsal tongue displayed a raised mucous membrane manifesting as a type of papillae. Filiform papillae were differentiated into two subtypes: brush-like papillae, with a short base splitting into 5-6 processes; and flower bud filiform papillae that are shorter and wider than the former, and divided into a single main process and 4-5 secondary processes. Fungiform papillae with 1-2 taste buds are interspersed between filiform papillae and absent on the radix. Conical papillae cover the lateral margin of the corpus and increase in size when closer to the radix. Vallate papillae-three in number-are arranged as a triangular structure, with the middle papillae pointing towards the root of the tongue. A continuous groove wrapped around each papilla with several taste buds is found on its lateral surface. Two of the vallate papillae are located symmetrically on each side and are much smaller than the most caudal middle vallate that is directed posteriorly. A positive reaction using separate staining with periodic acid-Schiff and Alcian blue (pH 2.5) showed Weber's glands and von Ebner's glands appearing beneath the papillae, mainly on the muscle layer of the radix. Different reactions to the stains indicated that Weber's gland contains sialomucin and/or sulfomucin, whereas von Ebner's gland consists of serous cells secreting glycogen and/or neutral mucin. The collagen structure was identified using Masson's trichrome stain in the two bottom layers of the tongue from the apex, decreasing gradually towards the radix.

In mammalian taste buds, Type I cells comprise half of all cells. These are termed “glial-like” based on morphologic and molecular features, but there are limited studies describing their function. We tested whether Type I cells sense chemosensory activation of adjacent chemosensory (i.e., Types II and III) taste bud cells, similar to synaptic glia. UsingGad2;;GCaMP3 mice of both sexes, we confirmed by immunostaining that, within taste buds, GCaMP expression is predominantly in Type I cells (with no Type II and ≈28% Type III cells expressing weakly). In dissociated taste buds, GCaMP+ Type I cells responded to bath-applied ATP (10-100 μm) but not to 5-HT (transmitters released by Type II or III cells, respectively). Type I cells also did not respond to taste stimuli (5 μmcycloheximide, 1 mmdenatonium). In lingual slice preparations also, Type I cells responded to bath-applied ATP (10-100 μm). However, when taste buds in the slice were stimulated with bitter tastants (cycloheximide, denatonium, quinine), Type I cells responded robustly. Taste-evoked responses of Type I cells in the slice preparation were significantly reduced by desensitizing purinoceptors or by purinoceptor antagonists (suramin, PPADS), and were essentially eliminated by blocking synaptic ATP release (carbenoxolone) or degrading extracellular ATP (apyrase). Thus, taste-evoked release of afferent ATP from type II chemosensory cells, in addition to exciting gustatory afferent fibers, also activates glial-like Type I taste cells. We speculate that Type I cells sense chemosensory activation and that they participate in synaptic signaling, similarly to glial cells at CNS tripartite synapses.SIGNIFICANCE STATEMENTMost studies of taste buds view the chemosensitive excitable cells that express taste receptors as the sole mediators of taste detection and transmission to the CNS. Type I “glial-like” cells, with their ensheathing morphology, are mostly viewed as responsible for clearing neurotransmitters and as the “glue” holding the taste bud together. In the present study, we demonstrate that, when intact taste buds respond to their natural stimuli, Type I cells sense the activation of the chemosensory cells by detecting the afferent transmitter. Because Type I cells synthesize GABA, a known gliotransmitter, and cognate receptors are present on both presynaptic and postsynaptic elements, Type I cells may participate in GABAergic synaptic transmission in the manner of astrocytes at tripartite synapses.

Taste buds comprise four types of taste cells: 3 mature, elongate types: Type I, Type II, Type III; and basally-situated, immature post-mitotic Type IV cells. We employed serial blockface scanning electron microscopy to delineate the characteristics and interrelationships of the taste cells in the circumvallate papillae of adult mice. Type I cells have an indented, elongate nucleus with invaginations, folded plasma membrane, and multiple apical microvilli in the taste pore. Type I microvilli may be either restricted to the bottom of the pore or extend outward reaching midway up into the taste pore. Type II cells (aka receptor cells) are characterized by a large round or oval nucleus, a single apical microvillus extending through the taste pore, and specialized “atypical” mitochondria at functional points of contact with nerve fibers. Type III cells (aka “synaptic cells”) are elongate with an indented nucleus, possess a single, apical microvillus extending through the taste pore and are characterized by a small accumulation of synaptic vesicles at points of contact with nerve fibers. About one-quarter of Type III cells also exhibit an atypical mitochondrion amidst the presynaptic vesicle clusters at the synapse. Type IV cells (non-proliferative “basal cells”) have a nucleus in the lower quarter of the taste bud but have a foot process extending to the basement membrane often contacting nerve processes along the way. Type I cells represent just over 50% of the population, whereas Type II, Type III, and Type IV (basal cells) represent 19%, 15%, and 14% respectively.

Fungiform papillae house taste buds on the anterior dorsal tongue. Literature is inconclusive as to whether taste perception correlates with fungiform papillae density (FPD). Gustatory reflexes modulate the amount and composition of saliva subsequently produced, and thus may be a more physiologically objective measure of tastant-receptor interactions. Taste perception fluctuates with time but the stability of individual fungiform papillae is unclear. This study followed ten healthy volunteers longitudinally at baseline, one and six months. FPD, diameter and position were measured and participants rated intensity perception of sucrose, caffeine, menthol and capsaicin solutions. Salivary flow rate, protein concentration and relative changes in protein composition were measured following each tastant. FPD, diameter and position were unchanged at six months. FPD did not correlate with intensity rating for any taste. FPD did correlate with changes in salivary protein output following sucrose (ρ = 0.72, p = 0.02) and changes in levels of proline-rich protein and mucin 7 following capsaicin (ρ = 0.71, p = 0.02, ρ = 0.68, p = 0.04, respectively). These results suggest that over six months fungiform papillae are anatomically stable, playing a greater role in mediating the physiological salivary response to stimuli rather than determining the perceived intensity of taste.

The ocelot (Leopardus pardalis) is a Felidae of wide geographical distribution and food flexibility; therefore, it is essential to understand the morphology of the species. Thus, we aimed to describe its lingual morphology in order to gain information regarding the anatomy of this carnivore's digestive system. The tongues ​​were removed for ex situ macroscopic and morphometric analyses, as well as for light microscopy and scanning electron microscopy, of fragments of the different lingual regions. The tongue of L. pardalis had an elongated form that was subdivided into the apex, body and root, in which four papillary types were observed: filiform, fungiform, circumvallate and conical. It presented with a stratified, keratinized squamous epithelium, followed by loose and dense connective tissues, as well as a skeletal striated musculature that comprised most of the organ. In addition, in scanning electron microscopy the filiform papillae showed a complex with multiple layers of keratin with triangular shape projected caudally in oral cavity. The fungiform papillae were distributed among the filiform and showed a rounded shape with some gustatory pores, and are keratinized but in a lower intensity if compared with filiform. The vallate papillae, located in lingual root, showed an oval format, had a deep groove surrounded the papillae and some gustatory pores. The conical papillae are located in lingual root and are similar to the filiform. The tongue of L. pardalis resembles other carnivorous species, mainly among felids. However, it differed in relation to the quantity of vallate papillae and the absence of foliate papillae.

How taste buds detect NaCl remains poorly understood. Among other problems, applying taste-relevant concentrations of NaCl (50–500 mm) onto isolated taste buds or cells exposes them to unphysiological (hypo/hypertonic) conditions. To overcome these limitations, we used the anterior tongue of male and female mice to implement a slice preparation in which fungiform taste buds are in a relatively intact tissue environment and stimuli are limited to the taste pore. Taste-evoked responses were monitored using confocal Ca(2+) imaging via GCaMP3 expressed in Type 2 and Type 3 taste bud cells. NaCl evoked intracellular mobilization of Ca(2+) in the apical tips of a subset of taste cells. The concentration dependence and rapid adaptation of NaCl-evoked cellular responses closely resembled behavioral and afferent nerve responses to NaCl. Importantly, taste cell responses were not inhibited by the diuretic, amiloride. Post hoc immunostaining revealed that >80% of NaCl-responsive taste bud cells were of Type 2. Many NaCl-responsive cells were also sensitive to stimuli that activate Type 2 cells but never to stimuli for Type 3 cells. Ion substitutions revealed that amiloride-insensitive NaCl responses depended on Cl(−) rather than Na(+). Moreover, choline chloride, an established salt taste enhancer, was equally effective a stimulus as sodium chloride. Although the apical transducer for Cl(−) remains unknown, blocking known chloride channels and cotransporters had little effect on NaCl responses. Together, our data suggest that chloride, an essential nutrient, is a key determinant of taste transduction for amiloride-insensitive salt taste. SIGNIFICANCE STATEMENT Sodium and chloride are essential nutrients and must be regularly consumed to replace excreted NaCl. Thus, understanding salt taste, which informs salt appetite, is important from a fundamental sensory perspective and forms the basis for interventions to replace/reduce excess Na(+) consumption. This study examines responses to NaCl in a semi-intact preparation of mouse taste buds. We identify taste cells that respond to NaCl in the presence of amiloride, which is significant because much of human salt taste also is amiloride-insensitive. Further, we demonstrate that Cl(−), not Na(+), generates these amiloride-insensitive salt taste responses. Intriguingly, choline chloride, a commercial salt taste enhancer, is also a highly effective stimulus for these cells.

The metabolic hormone adiponectin is secreted into the circulation by adipocytes and mediates key biological functions, including insulin sensitivity, adipocyte development, and fatty acid oxidation. Adiponectin is also abundant in saliva, where its functions are poorly understood. Here we report that murine taste receptor cells (TRCs) express specific adiponectin receptors and may be a target for salivary adiponectin. This is supported by the presence of all three known adiponectin receptors in transcriptomic data obtained by RNA-seq analysis of purified circumvallate (CV) taste buds. As well, immunohistochemical analysis of murine CV papillae showed that two adiponectin receptors, ADIPOR1 and T-cadherin, are localized to subsets of TRCs. Immunofluorescence for T-cadherin was primarily co-localized with the Type 2 TRC marker phospholipase C β2, suggesting that adiponectin signaling could impact sweet, bitter, or umami taste signaling. However, adiponectin null mice showed no differences in behavioral lick responsiveness compared with wild-type controls in brief-access lick testing. AAV-mediated overexpression of adiponectin in the salivary glands of adiponectin null mice did result in a small but significant increase in behavioral lick responsiveness to the fat emulsion Intralipid. Together, these results suggest that salivary adiponectin can affect TRC function, although its impact on taste responsiveness and peripheral taste coding remains unclear.

In this study, a taste bud tissue biosensor was prepared by a starch-sodium alginate cross-linking fixation method. Capsaicin was used as a TRPV1 noxious ion channel activator to investigate the antagonism kinetics of six different substances on capsaicin. The results showed that capsazepine, AMG517, loureirin B, and tetrahydropalmatine were all competitive allosteric regulatory ligands for capsaicin, while aconitine and anandamide were mixed allosteric regulatory ligand that combines non-competition and competition effect. Through analyzing the kinetic parameters of capsaicin and its competitive allosteric regulatory ligands, and comparing the structures between spicy substances and endocannabinoids, the importance of amide groups and similar groups in the allosteric regulation of cannabinoids (CB) receptors and analgesic mechanism was elucidated. This indicates that vanilloid activators turn on the TRPV1 ion channel to transmit only pain and other nociceptive signals, while capsaicin and its competitive ligands are capable of activating intracellular G protein/PI3K/PIP2 signaling pathways by binding to endogenous cannabinoid receptors, and then increase intracellular PIP2 levels (the increasing PIP2 can competitively replace capsaicin and other vanilloid activators), thereby closing the TRPV1 channel and exerting the analgesic effect. The elucidation of this mechanism of pain and analgesia will lay the theoretical foundation and new ideas for investigating nociceptive signal and screening potential analgesic drugs.

Mammalian taste buds comprise types I, II, and III taste cells, with each type having specific characteristics: glia-like supporting cells (type I), taste receptor cells (type II), and presynaptic cells (type III). In this study, to characterize the peripheral taste-sensing systems in chickens, we analyzed the distributions of the mammalian types I, II, and III taste cell markers in chicken taste buds: glutamate-aspartate transporter (GLAST) for type I; taste receptor type 1 members 1 and 3 (T1R1 and T1R3), taste receptor type 2 member 7 (T2R7), and α-gustducin for type II; and synaptosomal protein 25 (SNAP25) and neural cell adhesion molecule (NCAM) for type III. We found that most GLAST+ taste cells expressed α-gustducin and SNAP25 and that high percentages of T1R3+ or α-gustducin+ taste cells expressed SNAP25 and NCAM. These results demonstrated a unique subset of chicken taste cells expressing multiple taste cell type marker proteins. Taken together, these results provide new insights into the taste-sensing mechanisms in vertebrate taste buds.

The ability of mammalian taste cells to respond to fatty acids (FAs) has garnered significant attention of late and has been proposed to represent a sixth primary taste. With few exceptions, studies on FA taste have centered exclusively on polyunsaturated FAs, most notably on linoleic acid. In the current study, we have identified an additional FA receptor, GPR84, in the gustatory system that responds to the medium chains saturated FAs (MCFAs) in male mice. GPR84 ligands activate both Type II and Type III taste cells in calcium imaging and patch clamp recording assays. MCFAs depolarize and lead to a rise in intracellular free [Ca2+] in mouse taste cells in a concentration-dependent fashion and the relative ligand specificity in taste cells is consistent with the response profile of GPR84 expressed in a heterologous system. A systemic Gpr84-/- mouse model reveals a specific deficit in both the neural (via chorda tympani recording) and behavioral responses to administration of oral MCFAs compared to wild type mice. Together, we show that the peripheral taste system can respond to an additional class of FAs, the saturated FAs, and that the cognate receptor necessary for this ability is GPR84. Significance Statement Recognition of fatty acids by taste receptor cells in the oral cavity has been suggested to underlie the taste cue for dietary fat. In vitro and in vivo studies in both animal models and humans supports the notion fat may represent an additional taste primer. However, virtually all these studies have focused on the ability of polyunsaturated fatty acids, like linoleic acid, to activate taste pathways. We have identified GPR84 in the taste system that underlies the ability of mice to recognize and respond to medium chain saturated fatty acids. The ability to sense the taste of saturated fats may be important for controlling the intake of this class of fats whose intake is a greater concern for human health.

It has recently been shown that taste receptors, in addition to being present in the oral cavity, exist in various extra-oral organs and tissues such as the thyroid, lungs, skin, stomach, intestines, and pancreas. Although their physiological function is not yet fully understood, it appears that they can help regulate the body's homeostasis and provide an additional defense function against pathogens. Since the vast majority of drugs are bitter, the greatest pharmacological interest is in the bitter taste receptors. In this review, we describe how bitter taste 2 receptors (TAS2Rs) induce bronchodilation and mucociliary clearance in the airways, muscle relaxation in various tissues, inhibition of thyroid stimulating hormone (TSH) in thyrocytes, and release of glucagon-like peptide-1 (GLP-1) and ghrelin in the digestive system. In fact, substances such as dextromethorphan, chloroquine, methimazole and probably glimepiride, being agonists of TAS2Rs, lead to these effects. TAS2Rs and taste 1 receptors (TAS1R2/3) are G protein-coupled receptors (GPCR). TAS1R2/3 are responsible for sweet taste perception and may induce GLP-1 release and insulin secretion. Umami taste receptors, belonging to the same superfamily of receptors, perform a similar function with regard to insulin. The sour and salty taste receptors work in a similar way, both being channel receptors sensitive to amiloride. Finally, gene-protein coupled receptor 40 (GPR40) and GPR120 for fatty taste perception are also protein-coupled receptors and may induce GLP-1 secretion and insulin release, similar to those of other receptors belonging to the same superfamily.

Taste buds consist of specialized epithelial cells which detect particular tastants and synapse onto the afferent taste nerve innervating the endorgan. The nature of the neurotransmitter released by taste cells onto the nerve fiber was enigmatic early in this century although neurotransmitters for other sensory receptor cell types, e.g. hair cells, photoreceptors, was known for at least a decade. A 1999 paper by Burnstock and co-workers (Bo et al., 1999) showing the presence of P2X receptors on the afferent nerves served as a springboard for research that ultimately led to the discovery of ATP as the crucial neurotransmitter in the taste system (Finger et al., 2005). Subsequent work showed that a subpopulation of taste cells utilize a unique release channel, CALHM1/3, to release ATP in a voltage-dependent manner. Despite these advances, several aspects of purinergic transmission in this system remain to be elucidated.

(Background) We investigated the effect of dietary supplementation with monosodium glutamate (MSG) on chemotherapy-induced downregulation of the T1R3 taste receptor subunit expression in the tongue of patients with advanced head and neck cancer. (Methods) Patients undergoing two rounds of chemoradiotherapy were randomly allocated to a control or intervention group (dietary supplementation with MSG at 2.7 g/day during the second round of chemotherapy). The relative expression of T1R3, a subunit of both umami and sweet taste receptors, in the tongue was assessed by quantitative polymerase chain reaction analysis. Dysgeusia was assessed with a visual analog scale and daily energy intake was evaluated. (Results) T1R3 expression levels in the tongue, taste sensitivity, and daily energy intake were significantly reduced after the first round of chemotherapy compared with before treatment. Furthermore, these parameters significantly decreased after the second round of chemotherapy, but the extent of decrease was significantly attenuated in the MSG group compared with the control group. (Conclusions) MSG supplementation suppresses chemotherapy-induced dysgeusia, possibly due to the inhibition of the T1R3-containing taste receptor downregulation in the tongue, thereby increasing energy intake in patients with advanced head and neck cancer.

Key points Taste transduction occurs in taste buds in the tongue epithelium.The Ca2+‐activated Cl– channels TMEM16A and TMEM16B play relevant physiological roles in several sensory systems.Here, we report that TMEM16A, but not TMEM16B, is expressed in the apical part of taste buds.Large Ca2+‐activated Cl− currents blocked by Ani‐9, a selective inhibitor of TMEM16A, are measured in type I taste cells but not in type II or III taste cells.ATP indirectly activates Ca2+‐activated Cl– currents in type I cells through TMEM16A channels.These results indicate that TMEM16A is functional in type I taste cells and contribute to understanding the largely unknown physiological roles of these cells. Abstract The Ca2+‐activated Cl– channels TMEM16A and TMEM16B have relevant roles in many physiological processes including neuronal excitability and regulation of Cl– homeostasis. Here, we examined the presence of Ca2+‐activated Cl– channels in taste cells of mouse vallate papillae by using immunohistochemistry and electrophysiological recordings. By using immunohistochemistry we showed that only TMEM16A, and not TMEM16B, was expressed in taste bud cells where it largely co‐localized with the inwardly rectifying K+ channel KNCJ1 in the apical part of type I cells. By using whole‐cell patch‐clamp recordings in isolated cells from taste buds, we measured an average current of −1083 pA at −100 mV in 1.5 μm Ca2+ and symmetrical Cl– in type I cells. Ion substitution experiments and blockage by Ani‐9, a specific TMEM16A channel blocker, indicated that Ca2+ activated anionic currents through TMEM16A channels. We did not detect any Ca2+‐activated Cl– currents in type II or III taste cells. ATP is released by type II cells in response to various tastants and reaches type I cells where it is hydrolysed by ecto‐ATPases. Type I cells also express P2Y purinergic receptors and stimulation of type I cells with extracellular ATP produced large Ca2+‐activated Cl− currents blocked by Ani‐9, indicating a possible role of TMEM16A in ATP‐mediated signalling. These results provide a definitive demonstration that TMEM16A‐mediated currents are functional in type I taste cells and provide a foundation for future studies investigating physiological roles for these often‐neglected taste cells., Key points Taste transduction occurs in taste buds in the tongue epithelium.The Ca2+‐activated Cl– channels TMEM16A and TMEM16B play relevant physiological roles in several sensory systems.Here, we report that TMEM16A, but not TMEM16B, is expressed in the apical part of taste buds.Large Ca2+‐activated Cl− currents blocked by Ani‐9, a selective inhibitor of TMEM16A, are measured in type I taste cells but not in type II or III taste cells.ATP indirectly activates Ca2+‐activated Cl– currents in type I cells through TMEM16A channels.These results indicate that TMEM16A is functional in type I taste cells and contribute to understanding the largely unknown physiological roles of these cells.

Kokumi taste is a well-accepted and characterised taste modality and is described as a sensation of enhancement of sweet, salty, and umami tastes. The Calcium Sensing Receptor (CaSR) has been designated as the putative kokumi taste receptor for humans, and a number of kokumi-active ligands of CaSR have been discovered recently with activity confirmed both in vivo and in vitro. Domestic cats (Felis catus) are obligate carnivores and accordingly, their diet is abundant in proteins, peptides, and amino acids. We hypothesised that CaSR is a key taste receptor for carnivores, due to its role in the detection of different peptides and amino acids in other species. Using in silico, in vitro and in vivo approaches, here we compare human CaSR to that of a model carnivore, the domestic cat. We found broad similarities in ligand specificity, but differences in taste sensitivity between the two species. Indeed our in vivo data shows that cats are sensitive to CaCl2 as a kokumi compound, but don’t show this same activity with Glutathione, whereas for humans the reverse is true. Collectively, our data suggest that kokumi is an important taste modality for carnivores that drives the palatability of meat-derived compounds such as amino acids and peptides, and that there are differences in the perception of kokumi taste between carnivores and omnivores.

The objective of this study was to investigate the effects of chronic administration of caffeine on the anatomical characteristics of taste buds, the expression level of taste receptor protein in mice, and preference for a palatable solution. We found that following a 21-day administration of caffeine, mice showed increased behavioural responses to sweet stimuli (acesulfame-K solution). Mirroring this behavioural change, chronic caffeine treatment evidently decreased the maximal cross-sectional area and height of the longitudinal axis of fungiform taste buds, the number of taste cells per fungiform taste bud, and the expression of G protein α-gustducin, while the expression of the sweet taste receptors T1R2 and T1R3 was reversed. Our findings demonstrate that chronic administration of caffeine has an impact on taste sensitivity and changes in taste bud features, which may contribute to the alteration of taste behaviour.

The lingual surface potential (LSP), which hyperpolarizes in response to salt and bitter stimuli, is thought to be a bioelectrical signal associated with taste transduction in humans. In contrast, a recent study reported sweet and sour stimuli to evoke a depolarization of the LSP. We questioned the origin of such a depolarization because liquid junction potentials (JPs), which arise at the interfaces of recording electrode and taste solutions, are neglected in the report. We recorded the LSPs to sucrose and NaCl solutions on the human tongue using an Ag/AgCl electrode. To estimate JPs generated by each taste solution, we made an agar model to simulate the human tongue. The lingual surface was rinsed with a 10 mM NaCl solution that mimics the sodium content of the lingual fluid. In the human tongue, sucrose dissolved in distilled water evoked a depolarizing LSP that could be attributed to JPs, resulting from the change in electrolyte concentration of the taste solution. Sucrose dissolved in 10 mM NaCl solution evoked a hyperpolarizing LSP which became more negative in a concentration-dependent manner (300–1500 mM). Lactisole (3.75 mM), an inhibitor of sweet taste, significantly reduced the LSPs and decreased perceived intensity of sweetness by human subjects. The negative JPs generated by 100 mM NaCl in the agar model were not different from the LSPs to 100 mM NaCl. When the electrolyte environment on the lingual surface is controlled for JPs, the bioelectrical signal associated with sweet taste transduction is a hyperpolarizing potential.

Taste buds are localized in fungiform (FF), foliate (FL), and circumvallate (CV) papillae on the tongue, and taste buds also occur on the soft palate (SP). Mature elongate cells within taste buds are constantly renewed from stem cells and classified into three cell types, Types I, II, and III. These cell types are generally assumed to reside in respective taste buds in a particular ratio corresponding to taste regions. A variety of cell-type markers were used to analyze taste bud cells. NCAM is the first established marker for Type III cells and is still often used. However, NCAM was examined mainly in the CV, but not sufficiently in other regions. Furthermore, our previous data suggested that NCAM may be transiently expressed in the immature stage of Type II cells. To precisely assess NCAM expression as a Type III cell marker, we first examined Type II and III cell-type markers, IP3R3 and CA4, respectively, and then compared NCAM with them using whole-mount immunohistochemistry. IP3R3 and CA4 were segregated from each other, supporting the reliability of these markers. The ratio between Type II and III cells varied widely among taste buds in the respective regions (Pearson’s r = 0.442 [CV], 0.279 [SP], and − 0.011 [FF]), indicating that Type II and III cells are contained rather independently in respective taste buds. NCAM immunohistochemistry showed that a subset of taste bud cells were NCAM(+)CA4(−). While NCAM(+)CA4(−) cells were IP3R3(−) in the CV, the majority of them were IP3R3(+) in the SP and FF.

Action potentials play an important role in neurotransmitter release in response to taste. Here, I have investigated voltage-gated Na+ channels, a primary component of action potentials, in respective cell types of mouse fungiform taste bud cells (TBCs) with in situ whole-cell clamping and single-cell RT-PCR techniques. The cell types of TBCs electrophysiologically examined were determined immunohistochemically using the type III inositol 1,4,5-triphoshate receptor as a type II cell marker and synaptosomal-associated protein 25 as a type III cell marker. I show that type II cells, type III cells, and TBCs not immunoreactive to these markers (likely type I cells) generate voltage-gated Na+ currents. The recovery following inactivation of these currents was well fitted with double exponential curves. The time constants in type III cells (~20 ms and ~ 1 s) were significantly slower than respective time constants in other cell types. RT-PCR analysis indicated the expression of Nav1.3, Nav1.5, Nav1.6, and β1 subunit mRNAs in TBCs. Pharmacological inhibition and single-cell RT-PCR studies demonstrated that type II and type III cells principally express tetrodotoxin (TTX)-sensitive Nav1.3 channels and that ~ 30% of type I cells express TTX-resistant Nav1.5 channels. The auxiliary β1 subunit that modulates gating kinetics was rarely detected in TBCs. As the β1 subunit co-expressed with an α subunit is known to accelerate the recovery from inactivation, it is likely that voltage-gated Na+ channels in TBCs may function without β subunits. Slow recovery from inactivation, especially in type III cells, may limit high-frequency firing in response to taste substances.

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.

Taste buds, the receptor organs for taste, contain 50-100 taste bud cells. Although these cells undergo continuous turnover, the structural and functional integrity of taste buds is maintained. The molecular mechanisms by which synaptic connectivity between taste buds and afferent fibers is formed and maintained remain ambiguous. In the present study, we examined the localization of N-cadherin in the taste buds of the mouse circumvallate papillae because N-cadherin, one of the classical cadherins, is important for the formation and maintenance of synapses. At the light microscopic level, N-cadherin was predominantly detected in type II cells and nerve fibers in the connective tissues in and around the vallate papillae. At the ultrastructural level, N-cadherin immunoreactivity appears along the cell membrane and in the intracellular vesicles of type II cells. N-cadherin immunoreactivity also is evident in the membranes of afferent terminals at the contact sites to N-cadherin-positive type II cells. At channel type synapses between type II cells and nerve fibers, N-cadherin is present surrounding, but not within, the presumed neurotransmitter release zone, identified by large mitochondria apposed to the taste cells. The present results suggest that N-cadherin is important for the formation or maintenance of type II cell afferent synapses in taste buds.

Calcium homeostasis modulator 1 (calhm1) proteins form an outwardly rectifying nonselective ion channel having exceedingly slow kinetics and low sensitivity to voltage that is shifted by lowering extracellular Ca2+ ([Ca2+]e). Here we found that physiological temperature dramatically facilitates the voltage-dependent activation of the calhm1 current (Icalhm1); increased amplitude (Q10, 7-15) and fastened speed of activation. Also, the leftward shift of the half-activation voltage (V1/2) was similary observed in the normal and lower [Ca2+]e. Since calhm1 is highly expressed in the brain and taste cells, the thermosensitivity should be considered in their electrophysiology.

Taste-2 receptors (TAS2Rs), which belong to the G-protein coupled receptor (GPCR) family, are receptors for bitter taste perception. The aim of this study was to investigate whether zinc deficiency affects the expression of TAS2R genes. The promoter activity of the TAS2R7, TAS2R8, and TAS2R42 genes were determined in Ca9-22 oral squamous cell carcinoma cells cultured in the presence or absence of zinc. Luciferase reporter assays showed that zinc deprivation inhibited TAS2R8 promoter activity, but not the promoter activity of the other two genes. Treatment of the cells with N,N,N',N'-tetrakis(2-pyridinylmethyl)-1,2-ethanediamine (TPEN), an intracellular chelator of Zn2+, in the presence of 10% fetal bovine serum reduced TAS2R8 promoter activity. Truncation/deletion mutants of TAS2R8 promoter-luciferase constructs showed that the region from nucleotide -1152 to nucleotide -925 was critical for intracellular zinc dependency and contained a CCCTC-binding factor (CTCF) binding motif. A chromatin immunoprecipitation (ChiP) assay showed that CTCF bound specifically to this region, a binding abrogated by zinc deficiency, suggesting that CTCF plays a critical role in zinc-dependent bitter taste perception through TAS2R8.

The human sweet taste receptor (T1R2) monomer—a member of the G-protein coupled receptor family that detects a wide variety of chemically and structurally diverse sweet tasting molecules, is known to pose a significant threat to human health. Protein that lack crystal structure is a challenge in structure-based protein design. This study focused on the interaction of the T1R2 monomer with rebaudioside A (Reb-A), a steviol glycoside with potential use as a natural sweetener using in-silico and biosensing methods. Herein, homology modelling, docking studies, and molecular dynamics simulations were applied to elucidate the interaction between Reb-A and the T1R2 monomer. In addition, the electrochemical sensing of the immobilised T1R2-Reb-A complex with zinc oxide nanoparticles (ZnONPs) and graphene oxide (GO) were assessed by testing the performance of multiwalled carbon nanotube (MWCNT) as an adsorbent experimentally. Results indicate a strong interaction between Reb-A and the T1R2 receptor, revealing the stabilizing interaction of the amino acids with the Reb-A by hydrogen bonds with the hydroxyl groups of the glucose moieties, along with a significant amount of hydrophobic interactions. Moreover, the presence of the MWCNT as an anchor confirms the adsorption strength of the T1R2-Reb-A complex onto the GO nanocomposite and supported with electrochemical measurements. Overall, this study could serve as a cornerstone in the development of electrochemical immunosensor for the detection of Reb-A, with applications in the food industry.

The variety of taste sensations, including sweet, umami, bitter, sour, and salty, arises from diverse taste cells, each of which expresses specific taste sensor molecules and associated components for downstream signal transduction cascades. Recent years have witnessed major advances in our understanding of the molecular mechanisms underlying transduction of basic tastes in taste buds, including the identification of the bona fide sour sensor H(+) channel OTOP1, and elucidation of transduction of the amiloride-sensitive component of salty taste (the taste of sodium) and the TAS1R-independent component of sweet taste (the taste of sugar). Studies have also discovered an unconventional chemical synapse termed “channel synapse” which employs an action potential-activated CALHM1/3 ion channel instead of exocytosis of synaptic vesicles as the conduit for neurotransmitter release that links taste cells to afferent neurons. New images of the channel synapse and determinations of the structures of CALHM channels have provided structural and functional insights into this unique synapse. In this review, we discuss the current view of taste transduction and neurotransmission with emphasis on recent advances in the field.

Detection of NaCl by the gustatory system is fundamental for salt intake and tissue homeostasis. Yet, signal transduction mechanisms for salty taste have remained obscure. In this issue of Neuron, Nomura et al. (2020) report that the epithelial sodium channel ENaC, which serves as the salty receptor, is co-expressed with the voltage-activated ATP release channel CALHM1/3 in a subset of taste cells and that these cells mediate amiloride-sensitive salty taste.

Kokumi taste substances exemplified by g-glutamyl peptides and Maillard Peptides modulate salt and umami tastes. However, the underlying mechanism for their action has not been delineated. Here, we investigated the effects of a kokumi taste active and inactive peptide fraction (500-10,000 Da) isolated from mature (FIIm) and immature (FIIim) Ganjang, a typical Korean soy sauce, on salt and umami taste responses in humans and rodents. Only FIIm (0.1&ndash, 1.0%) produced a biphasic effect in rat chorda tympani (CT) taste nerve responses to lingual stimulation with 100 mM NaCl + 5 mM benzamil, a specific epithelial Na+ channel blocker. Both elevated temperature (42 &deg, C) and FIIm produced synergistic effects on the NaCl + benzamil CT response. At 0.5% FIIm produced the maximum increase in rat CT response to NaCl + benzamil, and enhanced salt taste intensity in human subjects. At 2.5% FIIm enhanced rat CT response to glutamate that was equivalent to the enhancement observed with 1 mM IMP. In human subjects, 0.3% FIIm produced enhancement of umami taste. These results suggest that FIIm modulates amiloride-insensitive salt taste and umami taste at different concentration ranges in rats and humans.

The gustation system for sweeteners is well-known to be regulated by nutritional and metabolic conditions, but there is no or little information on the underlying mechanism. Here, we examined whether elevation of the blood glucose level was involved in alteration of the expression of sweet taste receptors in circumvallate papillae (CP) and sweet taste sensitivity in male Sprague-Dawley rats. Rats under 4 h-fed conditions following 18 h-fasting exhibited elevated blood glucose levels and decreased pancreatic T1R3 expression, compared to rats after 18 h-fasting treatment, and they exhibited increased protein expression of sweet taste receptors T1R2 and T1R3 in CP. Under streptozotocin (STZ)-induced diabetes mellites (DM) conditions, the protein expression levels of T1R2 and T1R3 in CP were higher than those under control conditions, and these DM rats exhibited increased lick ratios in a low sucrose concentration range in a brief access test with a mixture of sucrose and quinine hydrochloride (QHCl). These findings indicate that the elevation of blood glucose level is a regulator for an increase in sweet taste receptor protein expression in rat CP, and such alteration in STZ-induced DM rats is involved in enhancement of their sweet taste sensitivity.

The chorda tympani (CT) nerve is exceptionally responsive to NaCl. Amiloride, an epithelial Na+ channel (ENaC) blocker, consistently and significantly decreases the NaCl responsiveness of the CT but not the glossopharyngeal (GL) nerve in the rat. Here, we examined whether amiloride would suppress the NaCl responsiveness of the CT when it cross-reinnervated the posterior tongue (PT). Whole-nerve electrophysiological recording was performed to investigate the response properties of the intact (CTsham), regenerated (CTr), and cross-regenerated (CT-PT) CT in male rats to NaCl mixed with and without amiloride and common taste stimuli. The intact (GLsham) and regenerated (GLr) GL were also examined. The CT responses of the CT-PT group did not differ from those of the GLr and GLsham groups, but did differ from those of the CTr and CTsham groups for some stimuli. Importantly, the responsiveness of the cross-regenerated CT to a series of NaCl concentrations was not suppressed by amiloride treatment, which significantly decreased the response to NaCl in the CTr and CTsham groups and had no effect in the GLr and GLsham groups. This suggests that the cross-regenerated CT adopts the taste response properties of the GL as opposed to those of the regenerated CT or intact CT. This work replicates the 5 decade-old findings of Oakley and importantly extends them by providing compelling evidence that the presence of functional ENaCs, essential for sodium taste recognition in regenerated taste receptor cells, depends on the reinnervated lingual region and not on the reinnervating gustatory nerve, at least in the rat.