Diversity and evolution of serotonergic cells in taste buds of elasmobranchs and ancestral actinopterygian fish

A subset of gustatory cells are serotonin immunoreactive (ir) in the mammalian taste bud. In the taste bud of lamprey, elongated gustatory-like cells are also serotonin-ir. In contrast, flattened serotonin-ir cells are located only in the basal region of the taste buds in the teleosts and amphibians. These serotonin-ir cells are termed as basal cells. To evaluate the evolution and diversity of serotonergic cells in the taste bud of amniote animals, we explored the distribution and morphology of serotonin-ir cells in the taste buds of ancestral actinopterygian fish (spotted gar, sturgeon, Polypterus senegalus) and elasmobranch (stingray). In all examined animals, the taste buds contained serotonin-ir cells in their basal part. The number of serotonin-ir basal cells in each taste bud was different between these fish species. They were highest in the stingray and decreased in the order of the Polypterus, sturgeon, and gar. While serotonin immunoreactivity was observed only in the basal cells in the taste buds of the ancestral actinopterygian fish, some elongated cells were also serotonin-ir in addition to the basal cells in the stingray taste buds. mRNA of tryptophan hydroxylase 1 (tph1), a rate-limiting enzyme of the serotonin synthesis, is expressed in both the elongated and basal cells of stingray taste buds, indicating that these cells synthesize the serotonin by themselves. These results suggest that the serotonin-ir basal cells arose from the ancestor of the cartilaginous fish, and serotonin-ir cells in the elasmobranch taste bud exhibit an intermediate aspect between the lamprey and actinopterygian fish.

High Sox2 expression predicts taste lineage competency of lingual progenitors in vitro

Taste buds on the tongue contain taste receptor cells (TRCs) that detect sweet, sour, salty, umami and bitter stimuli. Like non-taste lingual epithelium, TRCs are renewed from basal keratinocytes, many of which express the transcription factor SOX2. Genetic lineage tracing has shown that SOX2+ lingual progenitors give rise to both taste and non-taste lingual epithelium in the posterior circumvallate taste papilla (CVP) of mice. However, SOX2 is variably expressed among CVP epithelial cells, suggesting that their progenitor potential may vary. Using transcriptome analysis and organoid technology, we show that cells expressing SOX2 at higher levels are taste-competent progenitors that give rise to organoids comprising both TRCs and lingual epithelium. Conversely, organoids derived from progenitors that express SOX2 at lower levels are composed entirely of non-taste cells. Hedgehog and WNT/β-catenin are required for taste homeostasis in adult mice. However, manipulation of hedgehog signaling in organoids has no impact on TRC differentiation or progenitor proliferation. By contrast, WNT/β-catenin promotes TRC differentiation in vitro in organoids derived from higher but not low SOX2+ expressing progenitors.

Ascl1-expressing cell differentiation in initially developed taste buds and taste organoids

Mammalian taste bud cells are composed of several distinct cell types and differentiated from surrounding tongue epithelial cells. However, the detailed mechanisms underlying their differentiation have yet to be elucidated. In the present study, we examined an Ascl1-expressing cell lineage using circumvallate papillae (CVP) of newborn mice and taste organoids (three-dimensional self-organized tissue cultures), which allow studying the differentiation of taste bud cells in fine detail ex vivo. Using lineage-tracing analysis, we observed that Ascl1 lineage cells expressed type II and III taste cell markers both CVP of newborn mice and taste organoids. However, the coexpression rate in type II cells was lower than that in type III cells. Furthermore, we found that the generation of the cells which express type II and III cell markers was suppressed in taste organoids lacking Ascl1-expressing cells. These findings suggest that Ascl1-expressing precursor cells can differentiate into both type III and a subset of type II taste cells.

Histological and immunohistochemical study of gilthead seabream tongue from the early stage of development: TRPV4 potential roles

BACKGROUND

Taste buds, the morphofunctional units for taste perception, transduce gustatory stimuli using G protein-coupled receptors, and a complex arrangement of ion channels, among which TRPV4, a member of the TRP superfamily. Studies on taste buds development on gilthead seabream are unknown, and the TRPV4 expression on fish taste cells studies were conducted only on zebrafish.

METHODS

In our study, we have investigated the histological features of the gilthead seabream tongue dorsal surface from the earliest stage of development using Masson trichrome with aniline blue staining. Additionally, TRPV4 expression pattern was studied by means of immunohistochemical labeling and quantitative RT-PCR.

RESULTS

We have recorded for the first time on gilthead seabream lingual dorsal surface the presence of, stage specific, three types of taste buds: type I, type II and type III in larvae, juvenile and adults respectively. At 40 days post hatching, taste buds were mature-looking. TRPV4 expression was detected in a subpopulation of taste cells of larvae, juveniles, and adults. Furthermore, TRPV4 was expressed in the basal epithelial cells of the tongue at the larvae and juvenile stage, while this expression pattern was more diffused within all the epithelial cell layers on the adult.

CONCLUSION

Our findings presume a taste sensory role of TRPV4 in the three stage-specific taste buds and oral epithelia of gilthead seabream. In addition to its sensory role on the epithelial cell layers, we hypothesize that TRPV4 is implicated in epithelial cells differentiation and membrane protection.

Purinergic neurotransmission in the gustatory system

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.

Implications of the specific localization of YAP signaling on the epithelial patterning of circumvallate papilla

Circumvallate papilla (CVP) is a distinctively structured with dome-shaped apex, and the surrounding trench which contains over two hundred taste buds on the lateral walls. Although CVP was extensively studied to determine the regulatory mechanisms during organogenesis, it still remains to be elucidated the principle mechanisms of signaling regulations on morphogenesis including taste buds formation. The key role of Yes-associated protein (YAP) in the regulation of organ size and cell proliferation in vertebrates is well understood, but little is known about the role of this signaling pathway in CVP development. We aimed to determine the putative roles of YAP signaling in the epithelial patterning during CVP morphogenesis. To evaluate the precise localization patterns of YAP and other related signaling molecules, including β-catenin, Ki67, cytokeratins, and PGP9.5, in CVP tissue, histology and immunohistochemistry were employed at E16 and adult mice. Our results suggested that there are specific localization patterns of YAP and Wnt signaling molecules in developing and adult CVP. These concrete localization patterns would provide putative involvements of YAP and Wnt signaling for proper epithelial cell differentiation including the formation and maintenance of taste buds.

Mash1-expressing cells may be relevant to type III cells and a subset of PLCβ2-positive cell differentiation in adult mouse taste buds

Mammalian taste bud cells have a limited lifespan and differentiate into type I, II, and III cells from basal cells (type IV cells) (postmitotic precursor cells). However, little is known regarding the cell lineage within taste buds. In this study, we investigated the cell fate of Mash1-positive precursor cells utilizing the Cre-loxP system to explore the differentiation of taste bud cells. We found that Mash1-expressing cells in Ascl1^CreERT2::CAG-floxed tdTomato mice differentiated into taste bud cells that expressed aromatic _L-amino acid decarboxylase (AADC) and carbonic anhydrase IV (CA4) (type III cell markers), but did not differentiate into most of gustducin (type II cell marker)-positive cells. Additionally, we found that Mash1-expressing cells could differentiate into phospholipase C β2 (PLCβ2)-positive cells, which have a shorter lifespan compared with AADC- and CA4-positive cells. These results suggest that Mash1-positive precursor cells could differentiate into type III cells, but not into most of type II cells, in the taste buds.

Bile acid receptor TGR5 is critically involved in preference for dietary lipids and obesity

We investigated the implication of Takeda G protein-coupled receptor 5 (TGR5) in fat preference and fat sensing in taste bud cells (TBC) in C57BL/6 wild-type (WT) and TGR5 knock out (TGR5^-/^-) male mice maintained for 20 weeks on a high-fat diet (HFD). We also assessed the implication of TGR5 single nucleotide polymorphism (SNP) in young obese humans. The high-fat diet (HFD)-fed TGR5^-/^- mice were more obese, marked with higher liver weight, lipidemia and steatosis than WT obese mice. The TGR5^-/^- obese mice exhibited high daily food/energy intake, fat mass and inflammatory status. WT obese mice lost the preference for dietary fat, but the TGR5^-/^- obese mice exhibited no loss towards the attraction for lipids. In lingual TBC, the fatty acid-triggered Ca²⁺ signaling was decreased in WT obese mice; however, it was increased in TBC from TGR5^-/^- obese mice. Fatty acid-induced in vitro release of GLP-1 was higher, but PYY concentrations were lower, in TBC from TGR5^-/^- obese mice than those in WT obese mice. We noticed an association between obesity and variations in TGR5 rs11554825 SNP. Finally, we can state that TGR5 modulates fat eating behavior and obesity.

Chronic exposure to liquid sucrose and dry sucrose diet have differential effects on peripheral taste responses in female rats

Sugar-sweetened beverages are the major source of added calories in the Western diet and their prevalence is associated with obesity and metabolic disruption. Despite the critical role of the taste system in determining food selection and consumption, the effects of chronic sucrose consumption on the peripheral taste system in mammals have received limited attention. We offered female Sprague Dawley rats free access to water and one of three diets for up to 40 days: (1) sucrose-free chow or "NS" diet; (2) a high-sucrose dry diet or "HS"; or (3) 30% sucrose solution and the NS diet, designated "LiqS" diet. Sucrose consumption by LiqS rats gradually increased and by day 14 was equal to that of HS rats. Food intake decreased in LiqS rats, but their energy intake remained higher than for NS or HS rats. There was no significant difference in weight gain of the groups during the study. Recordings from the chorda tympani nerve (CT), which innervates taste buds on the anterior tongue, revealed decreased responses to 1 M sucrose in both LiqS and HS rats and to acesulfame K and salt tastants in LiqS rats after 40 days on diet. Umami, bitter, and acid response magnitudes were unchanged in both groups. These results demonstrate that chronic sucrose exposure inhibits taste responses to higher concentrations of sweet stimuli. More surprisingly, CT responses to NaCl and 0.5M NaAc were significantly reduced in rats on the LiqS diet. Thus, the physical form of the diet influences taste responsiveness to salt and sweet taste function. These data suggest that taste buds are previously unappreciated targets of chronic sucrose consumption.

AAV-Mediated Gene Delivery to Taste Cells of the Tongue

Taste sensation is initiated in sensory cells within the taste buds (taste cells), in which the cooperation of many signaling molecules leads to the coding and transmission of information on the quality and intensity of taste to the afferent gustatory nerves. Here, we describe our method for inducing foreign gene expression in taste cells of fungiform papillae in a living mouse using a recombinant adeno-associated virus (AAV) vector, enabling us to study and control the function of a gene product in vivo. Among the serotypes tested to date, only AAV-DJ, a synthetic serotype, can transduce taste cells in vivo. We also describe how to validate intragemmal foreign gene expression in fungiform taste buds using an immunohistochemical approach.

Early taste buds are from Shh+ epithelial cells of tongue primordium in distinction from mature taste bud cells which arise from surrounding tissue compartments

Mammalian taste buds emerge perinatally and most become mature 3-4 weeks after birth. Mature taste bud cells in rodents are known to be renewed by the surrounding K14+ basal epithelial cells and potentially other progenitor source(s), but the dynamics between initially developed taste buds and surrounding tissue compartments are unclear. Using the K14-Cre and Dermo1-Cre mouse lines to trace epithelial and mesenchymal cell lineages, we found that early taste buds in E18.5 and newborn mouse tongues are not derived from either lineage. At E11.5 when the tongue primordia (i.e., lingual swellings) emerge, the relatively homogeneous sonic hedgehog-expressing (Shh+) epithelial cells express Keratin (K) 8, a marker that is widely used to label taste buds. Mapping lineage of E11.0 Shh+ epithelium of the tongue rudiment with Shh-CreERT2/RFP mice demonstrated that both the early taste buds and the surrounding lingual epithelium are from the same population of progenitors - Shh+ epithelial cells of the tongue primordium. In combination with previous reports, we propose that Shh+K8+ cells in the homogeneous epithelium of tongue primordium at early embryonic stages are programmed to become taste papilla and taste bud cells. Switching off Shh and K8 expression in the Shh+ epithelial cells of the tongue primordium transforms the cells to non-gustatory cells surrounding papillae, including K14+ basal epithelial cells which will eventually contribute to the cell renewal of mature taste buds.

SOX2 regulation by hedgehog signaling controls adult lingual epithelium homeostasis

Adult tongue epithelium is continuously renewed from epithelial progenitor cells, a process that requires hedgehog (HH) signaling. In mice, pharmacological inhibition of the HH pathway causes taste bud loss within a few weeks. Previously, we demonstrated that sonic hedgehog (SHH) overexpression in lingual progenitors induces ectopic taste buds with locally increased SOX2 expression, suggesting that taste bud differentiation depends on SOX2 downstream of HH. To test this, we inhibited HH signaling in mice and observed a rapid decline in Sox2 and SOX2-GFP expression in taste epithelium. Upon conditional deletion of Sox2, differentiation of both taste and non-taste epithelial cells was blocked, and progenitor cell number increased. In contrast to basally restricted proliferation in controls, dividing cells were overabundant and spread to suprabasal epithelial layers in mutants. SOX2 loss in progenitors also led non-cell-autonomously to taste cell apoptosis, dramatically shortening taste cell lifespans. Finally, in tongues with conditional Sox2 deletion and SHH overexpression, ectopic and endogenous taste buds were not detectable; instead, progenitor hyperproliferation expanded throughout the lingual epithelium. In summary, we show that SOX2 functions downstream of HH signaling to regulate lingual epithelium homeostasis.

Association of tongue brushing with the number of fungiform taste buds and taste perception: A preliminary study using confocal laser scanning microscopy in combination with a filter-paper disc method

OBJECTIVE

The aim of this study was to investigate the association of tongue brushing with the number of fungiform taste buds and taste perception using a confocal laser scanning microscopy in combination with a filter-paper disc method (FPDM).

METHODS

Twenty-four subjects with or without a habit of tongue brushing (11 males and 13 females, 20-46 years old) participated in this study. Nine of the 24 subjects had no habit of tongue brushing (Group 1, n=9). Fifteen subjects had a habit of tongue brushing, and the brushing regions of the tongue were as follows: central region (Group 2, n=7), or entire region (Group 3, n=8) of the tongue dorsum. Using confocal laser scanning microscopy, the average number of taste buds per fungiform papilla (FP) was counted. Taste perception was evaluated using an FPDM. These observations were performed in the midlateral region of the tongue since the distribution of fungiform papillae is large in the midlateral region compared to that in the central region.

RESULTS

The subjects in Group 3 showed a significantly decreased number of fungiform taste buds compared to Group 1 and Group 2. Group 3 also showed significantly higher FPDM scores than the other two groups.

CONCLUSIONS

Excessive tongue brushing of the entire tongue dorsum, including the midlateral region, may have an association with the decreased number of FP and taste buds and decreased taste sensation. To avoid these conditions, instituting proper tongue brushing methods, such as limiting it to the central region of the tongue and using a light touch, is suggested and is important for the subjects who are eager to participate in tongue brushing.

Epithelial cell behaviours during neurosensory organ formation

Perception of the environment in vertebrates relies on a variety of neurosensory mini-organs. These organs develop via a multi-step process that includes placode induction, cell differentiation, patterning and innervation. Ultimately, cells derived from one or more different tissues assemble to form a specific mini-organ that exhibits a particular structure and function. The initial building blocks of these organs are epithelial cells that undergo rearrangements and interact with neighbouring tissues, such as neural crest-derived mesenchymal cells and sensory neurons, to construct a functional sensory organ. In recent years, advances in in vivo imaging methods have allowed direct observation of these epithelial cells, showing that they can be displaced within the epithelium itself via several modes. This Review focuses on the diversity of epithelial cell behaviours that are involved in the formation of small neurosensory organs, using the examples of dental placodes, hair follicles, taste buds, lung neuroendocrine cells and zebrafish lateral line neuromasts to highlight both well-established and newly described modes of epithelial cell motility.

TRPC1, TRPC3, and TRPC4 in Rat Orofacial Structures

TRPC (transient receptor potential cation channel subfamily C) members are nonselective monovalent cation channels and control Ca2+ inflow. In this study, immunohistochemistry for TRPC1, TRPC3, and TRPC4 was performed on rat oral and craniofacial structures to elucidate their distribution and function in the peripheries. In the trigeminal ganglion (TG), 56.1, 84.1, and 68.3% of sensory neurons were immunoreactive (IR) for TRPC1, TRPC3, and TRPC4, respectively. A double immunofluorescence method revealed that small to medium-sized TG neurons co-expressed TRPCs and calcitonin gene-related peptide. In the superior cervical ganglion, all sympathetic neurons showed TRPC1 and TRPC3 immunoreactivity. Parasympathetic neurons in the submandibular ganglion, tongue, and parotid gland were TRPC1, TRPC3, and TRPC4 IR. Gustatory and olfactory cells were also IR for TRPC1, TRPC3, and/or TRPC4. In the musculature, motor endplates expressed TRPC1 and TRPC4 immunoreactivity. It is likely that TRPCs are associated with sensory, autonomic, and motor functions in oral and craniofacial structures.

BDNF is required for taste axon regeneration following unilateral chorda tympani nerve section

Taste nerves readily regenerate to reinnervate denervated taste buds; however, factors required for regeneration have not yet been identified. When the chorda tympani nerve is sectioned, expression of brain-derived neurotrophic factor (BDNF) remains high in the geniculate ganglion and lingual epithelium, despite the loss of taste buds. These observations suggest that BDNF is present in the taste system after nerve section and may support taste nerve regeneration. To test this hypothesis, we inducibly deleted Bdnf during adulthood in mice. Shortly after Bdnf gene recombination, the chorda tympani nerve was unilaterally sectioned causing a loss of both taste buds and neurons, irrespective of BDNF levels. Eight weeks after nerve section, however, regeneration was differentially affected by Bdnf deletion. In control mice, there was regeneration of the chorda tympani nerve and taste buds reappeared with innervation. In contrast, few taste buds were reinnervated in mice lacking normal Bdnf expression such that taste bud number remained low. In all genotypes, taste buds that were reinnervated were normal-sized, but non-innervated taste buds remained small and atrophic. On the side of the tongue contralateral to the nerve section, taste buds for some genotypes became larger and all taste buds remained innervated. Our findings suggest that BDNF is required for nerve regeneration following gustatory nerve section.

Nerve-independent and ectopically additional induction of taste buds in organ culture of fetal tongues

An improved organ culture system allowed to observe morphogenesis of mouse lingual papillae and taste buds relatively for longer period, in which fetal tongues were analyzed for 6 d. Taste cells were defined as eosinophobic epithelial cells expressing CK8 and Sox2 within lingual epithelium. Addition of glycogen synthase kinase 3 beta inhibitor CHIR99021 induced many taste cells and buds in non-gustatory and gustatory stratified lingual epithelium. The present study clearly demonstrated induction of taste cells and buds ectopically and without innervation.

Distribution of α-Gustducin and Vimentin in premature and mature taste buds in chickens

The sensory organs for taste in chickens (Gallus sp.) are taste buds in the oral epithelium of the palate, base of the oral cavity, and posterior tongue. Although there is not a pan-taste cell marker that labels all chicken taste bud cells, α-Gustducin and Vimentin each label a subpopulation of taste bud cells. In the present study, we used both α-Gustducin and Vimentin to further characterize chicken taste buds at the embryonic and post-hatching stages (E17-P5). We found that both α-Gustducin and Vimentin label distinct and overlapping populations of, but not all, taste bud cells. A-Gustducin immunosignals were observed as early as E18 and were consistently distributed in early and mature taste buds in embryos and hatchlings. Vimentin immunoreactivity was initially sparse at the embryonic stages then became apparent in taste buds after hatch. In hatchlings, α-Gustducin and Vimentin immunosignals largely co-localized in taste buds. A small subset of taste bud cells were labeled by either α-Gustducin or Vimentin or were not labeled. Importantly, each of the markers was observed in all of the examined taste buds. Our data suggest that the early onset of α-Gustducin in taste buds might be important for enabling chickens to respond to taste stimuli immediately after hatch and that distinctive population of taste bud cells that are labeled by different molecular markers might represent different cell types or different phases of taste bud cells. Additionally, α-Gustducin and Vimentin can potentially be used as molecular markers of all chicken taste buds in whole mount tissue.

Developmental Plasticity of Patterned and Regenerating Oral Organs

In many aquatic vertebrates, including bony and cartilaginous fishes, teeth and taste buds colocalize on jaw elements. In these animals, taste buds are renewed continuously throughout life, whereas teeth undergo cycled whole-organ replacement by various means. Recently, studies of cichlid fishes have yielded new insights into the development and regeneration of these dental and sensory oral organs. Tooth and taste bud densities covary positively across species with different feeding strategies, controlled by common regions of the genome and integrated molecular signals. Developing teeth and taste buds share a bipotent epithelium during early patterning stages, from which dental and taste fields are specified. Moreover, these organs share a common epithelial ribbon that supports label-retaining cells during later stages of regeneration. During both patterning and regeneration stages, dental organs can be converted to taste bud fate by manipulation of BMP signaling. These observations highlight a surprising long-term plasticity between dental and sensory organ types. Here, we review these findings and discuss the implications of developmental plasticity that spans the continuum of craniofacial organ patterning and regeneration.

Anatomical and physiological studies of bigheaded carps demonstrate that the epibranchial organ functions as a pharyngeal taste organ

The epibranchial organ (EO) is an enigmatic tubular organ found in the pharyngeal cavity of many filter-feeding fishes. We investigated whether it might function as a taste organ that mediates aggregation and ingestion of planktonic food within the buccal cavity. The EO and associated structures of bighead and silver carps, two successful and invasive planktivorous fishes, were examined using histological and electrophysiological techniques. Both species possess finely structured gill rakers that extend directly via a series of protrusions into each of the four blind canals which are organized as the muscular EO, suggesting that the gill rakers and EO probably function in an integrated manner. Both the interior and exterior surfaces of the EOs of both species are covered with high densities of taste buds and solitary chemosensory cells (SCCs) as well as mucous cells. Conversely, taste buds are scarce in both the buccal cavities and external portions of the head and mouth of both species. Electrophysiological recordings from a caudal branch of the vagus nerve (cranial nerve X) found to innervate the EO showed it to be sensitive to chemicals found in a planktonic diet. l-Amino acids accounted for some, but not all of the neural activity. We conclude that taste buds and SCCs located on the EO and gill rakers probably serve to chemically detect food particles, which the EO then aggregates by mucus secretion before eventually expelling them onto the floor of the pharynx for ingestion. This specialized, pharyngeal chemosensory structure may explain the feeding success of these, and perhaps other planktivorous, filter-feeding fishes.

The glossopharyngeal nerve controls epithelial expression of Sprr2a and Krt13 around taste buds in the circumvallate papilla

Tastants reach the tip of taste bud cells through taste pores which are openings in the epithelium. We found Sprr2a is selectively expressed in the upper layer of the epithelium surrounding taste buds in the circumvallate papilla (CV) where the epithelium is organized into taste pores. Sprr2a is a member of a small proline-rich protein family, which is suggested to be involved in the restitution/migration phase of epithelial wound healing. The expression of Sprr2a was restricted to the upper layer and largely segregated with Ptch1 expression that is restricted to the basal side of the epithelium around the taste buds. Denervation resulted in the gradual loss of Sprr2a-expressing cells over 10 days similarly to that of taste bud cells which is in contrast to the rapid loss of Ptch1 expression. We also found that denervation caused an increase of Keratin (Krt)13 expression around taste buds that corresponded with the disappearance of Sprr2a and Ptch1 expression. Taste buds were surrounded by Krt13-negative cells in the CV in control mice. However, at 6 days post-denervation, taste buds were tightly surrounded by Krt13-positive cells. During taste bud development, taste bud cells emerged together with Krt13-negtive cells, and Sprr2a expression was increased along with the progress of taste bud development. These results demonstrate that regional gene expression surrounding taste buds is associated with taste bud formation and controlled by the innervating taste nerve.

Type II and III Taste Bud Cells Preferentially Expressed Kainate Glutamate Receptors in Rats

Glutamate-induced cobalt uptake reveals that non-NMDA glutamate receptors (GluRs) are present in rat taste bud cells. Previous studies involving glutamate induced cobalt staining suggest this uptake mainly occurs via kainate type GluRs. It is not known which of the 4 types of taste bud cells express subunits of kainate GluR. Circumvallate and foliate papillae of Sprague-Dawley rats (45~60 days old) were used to search for the mRNAs of subunits of non-NMDA GluRs using RT-PCR with specific primers for GluR1-7, KA1 and KA2. We also performed RT-PCR for GluR5, KA1, PLCbeta2, and NCAM/SNAP 25 in isolated single cells from taste buds. Taste epithelium, including circumvallate or foliate papilla, express mRNAs of GluR5 and KA1. However, non-taste tongue epithelium expresses no subunits of non-NMDA GluRs. Isolated single cell RT-PCR reveals that the mRNAs of GluR5 and KA1 are preferentially expressed in Type II and Type III cells over Type I cells.