Immunocytochemistry of gamma-aminobutyric acid, glutamate, serotonin, and histamine in Necturus taste buds

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.

Glutamate: Tastant and Neuromodulator in Taste Buds

In taste buds, glutamate plays a double role as a gustatory stimulus and neuromodulator. The detection of glutamate as a tastant involves several G protein-coupled receptors, including the heterodimer taste receptor type 1, member 1 and 3 as well as metabotropic glutamate receptors (mGluR1 and mGluR4). Both receptor types participate in the detection of glutamate as shown with knockout animals and selective antagonists. At the basal part of taste buds, ionotropic glutamate receptors [N-methyl-d-aspartate (NMDA) and non-NMDA] are expressed and participate in the modulation of the taste signal before its transmission to the brain. Evidence suggests that glutamate has an efferent function on taste cells and modulates the release of other neurotransmitters such as serotonin and ATP. This short article reviews the recent developments in the field with regard to glutamate receptors involved in both functions as well as the influence of glutamate on the taste signal.

GABA, its receptors, and GABAergic inhibition in mouse taste buds

Taste buds consist of at least three principal cell types that have different functions in processing gustatory signals: glial-like (type I) cells, receptor (type II) cells, and presynaptic (type III) cells. Using a combination of Ca2+ imaging, single-cell reverse transcriptase-PCR and immunostaining, we show that GABA is an inhibitory transmitter in mouse taste buds, acting on GABA(A) and GABA(B) receptors to suppress transmitter (ATP) secretion from receptor cells during taste stimulation. Specifically, receptor cells express GABA(A) receptor subunits β2, δ, and π, as well as GABA(B) receptors. In contrast, presynaptic cells express the GABA(A) β3 subunit and only occasionally GABA(B) receptors. In keeping with the distinct expression pattern of GABA receptors in presynaptic cells, we detected no GABAergic suppression of transmitter release from presynaptic cells. We suggest that GABA may serve function(s) in taste buds in addition to synaptic inhibition. Finally, we also defined the source of GABA in taste buds: GABA is synthesized by GAD65 in type I taste cells as well as by GAD67 in presynaptic (type III) taste cells and is stored in both those two cell types. We conclude that GABA is an inhibitory transmitter released during taste stimulation and possibly also during growth and differentiation of taste buds.

Expression of GABAergic receptors in mouse taste receptor cells

Background

Multiple excitatory neurotransmitters have been identified in the mammalian taste transduction, with few studies focused on inhibitory neurotransmitters. Since the synthetic enzyme glutamate decarboxylase (GAD) for gamma-aminobutyric acid (GABA) is expressed in a subset of mouse taste cells, we hypothesized that other components of the GABA signaling pathway are likely expressed in this system. GABA signaling is initiated by the activation of either ionotropic receptors (GABA_A and GABA_C) or metabotropic receptors (GABA_B) while it is terminated by the re-uptake of GABA through transporters (GATs).

Methodology/Principal Findings

Using reverse transcriptase-PCR (RT-PCR) analysis, we investigated the expression of different GABA signaling molecules in the mouse taste system. Taste receptor cells (TRCs) in the circumvallate papillae express multiple subunits of the GABA_A and GABA_B receptors as well as multiple GATs. Immunocytochemical analyses examined the distribution of the GABA machinery in the circumvallate papillae. Both GABA_A-and GABA_B- immunoreactivity were detected in the peripheral taste receptor cells. We also used transgenic mice that express green fluorescent protein (GFP) in either the Type II taste cells, which can respond to bitter, sweet or umami taste stimuli, or in the Type III GAD67 expressing taste cells. Thus, we were able to identify that GABAergic receptors are expressed in some Type II and Type III taste cells. Mouse GAT4 labeling was concentrated in the cells surrounding the taste buds with a few positively labeled TRCs at the margins of the taste buds.

Conclusions/Significance

The presence of GABAergic receptors localized on Type II and Type III taste cells suggests that GABA is likely modulating evoked taste responses in the mouse taste bud.

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.

Neurochemical characterization of sea lamprey taste buds and afferent gustatory fibers: presence of serotonin, calretinin, and CGRP immunoreactivity in taste bud bi-ciliated cells of the earliest vertebrates

Neuroactive substances such as serotonin and other monoamines have been suggested to be involved in the transmission of gustatory signals from taste bud cells to afferent fibers. Lampreys are the earliest vertebrates that possess taste buds, although these differ in structure from taste buds in jawed vertebrates, and their neurochemistry remains unknown. We used immunofluorescence methods with antibodies raised against serotonin, tyrosine hydroxylase (TH), gamma-aminobutyric acid (GABA), glutamate, calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), calretinin, and acetylated alpha-tubulin to characterize the neurochemistry and innervation of taste buds in the sea lamprey, Petromyzon marinus L. For localization of proliferative cells in taste buds we used bromodeoxyuridine labeling and proliferating cell nuclear antigen immunohistochemistry. Results with both markers indicate that proliferating cells are restricted to a few basal cells and that almost all cells in taste buds are nonproliferating. A large number of serotonin-, calretinin-, and CGRP-immunoreactive bi-ciliated cells were revealed in lamprey taste buds. This suggests that serotonin participates in the transmission of gustatory signals and indicates that this substance appeared early on in vertebrate evolution. The basal surface of the bi-ciliated taste bud cells was contacted by tubulin-immunoreactive fibers. Some of the fibers surrounding the taste bud were calretinin immunoreactive. Lamprey taste bud cells or afferent fibers did not exhibit TH, GABA, glutamate, or NPY immunoreactivity, which suggests that expression of these substances evolved in taste buds of some gnathostomes lines after the separation of gnathostomes and lampreys.

Heterogeneous distribution of taste cells in facial and vagal nerve-innervated taste buds

Input from the three gustatory nerves of vertebrates is used to evaluate the nutritional quality of food. In some species, these cranial nerves are modified to accomplish additional specific functions. For example, the facial nerve innervated taste buds distributed over the body surface of catfish aid food search. Physiological studies indicate that this extra-oral taste pathway is more sensitive to amino acids than either the glossopharyngeal or vagal systems of the oral cavity. The current investigation seeks to determine if differences in taste cell subtypes might contribute to the observed differences in sensitivity. The distributions of five low molecular weight metabolites, L-alanine, L-aspartate, L-glutamate, GABA, taurine and the tripeptide glutathione, were examined in 2118 individual taste cells innervated by either the facial or vagal nerve of the channel catfish, Ictalurus punctatus. The metabolite profiles of these cells were determined immunocytochemically and subjected to a k-means clustering algorithm. Fifteen cell classes with quantitatively different patterns of metabolite co-localization were identified. All but one small class of two cells were found in both facial and vagal nerve-innervated taste buds. Four classes (9% of the total cells) had high, two classes (17%) had intermediate and the remaining nine classes (74%) had low levels of GABA immunoreactivity. While the functional significance of differences in metabolite profile remains to be determined, taste cell classes were not uniformly distributed across vagal and facial nerve innervated taste buds and may provide an anatomical basis for previously reported differences in gustatory sensitivity.

Morphological and biochemical heterogeneity in facial and vagal nerve innervated taste buds of the channel catfish, Ictalurus punctatus

In catfish, the facial nerve innervates taste buds distributed over the entire body including the barbels, while the glossopharyngeal and vagal nerves innervate oropharyngeal taste buds. Facial nerve innervated taste buds (FITBs) are thought to be involved in food detection and localization, while glossopharyngeal and vagal nerve innervated taste buds (VITBs) evaluate the palatability of food prior to ingestion. Physiological studies indicate that both oral and extra-oral taste buds detect sapid substances such as amino acids and nucleotides, but the facial taste system is more sensitive to some of these substances. The anatomical, molecular, and/or physiological mechanisms underlying the functional differences in these two gustatory pathways remain to be identified. In the current investigation we compare the basic morphological features of FITBs and VITBs and the distribution of the following metabolites: gamma-aminobutyric acid (GABA), glutamate, aspartate, alanine, taurine, and glutathione. Vagal innervated taste buds are significantly longer and narrower than FITBs, with fewer taste cells and a smaller nerve plexus. Each of the metabolites examined was heterogeneously distributed in taste cells with notably more GABA positive cells present in the VITBs. Patterns of metabolite colocalization suggest the presence of several taste cell subtypes. The morphological and metabolite differences noted between FITBs and VITBs provide a potential anatomical basis for the previously noted differences in physiological sensitivity.

Irradiation impairment of umami taste in patients with head and neck cancer

BACKGROUND

Previous studies have reported the alteration of the four basic tastes (sweet, salty, sour and bitter) in patients with head and neck cancer (HNC) during radiotherapy. However, there has been no investigation on the function of umami taste, a novel basic taste recognized recently, during head and neck irradiation.

OBJECTIVE

To investigate the recognition threshold of umami and the four basic tastes at different irradiation dose intervals during radiotherapy for head and neck cancer.

METHODS

In 30 patients with malignant neoplasm of head and neck, objective taste thresholds were examined by use of a whole-mouth method at pre-radiotherapy, irradiation doses at 15, 30, 45, and 60 Gy, respectively. Subjective taste loss and some distresses were recorded simultaneously.

RESULTS

Sweet, sour, salty, and bitter tastes showed temporarily and slightly increased thresholds during the treatment, but no statistical difference was found between the threshold at pre-radiotherapy and that at 15, 30, 45, and 60 Gy in any taste quality (all P > 0.05). Significantly impaired threshold of umami taste was revealed at 30 Gy (P < 0.05) and remained throughout the following treatment (at 45 and 60 Gy, both P < 0.01). Subjective taste impairment, appetite loss and satisfaction with the current state tended to deteriorate significantly during the irradiation. Mean body weight of the patients experienced a continuous loss, decreasing from an average of 60.4 kg before treatment to 57.3 kg at 60 Gy (P < 0.01). Scores of satisfaction with current state showed a significant correlation with umami taste thresholds (P = 0.035).

CONCLUSION

The clinical impairment pattern of umami taste is different from that of the other four basic tastes in HNC patients during radiotherapy. Impaired umami taste acuity plays an important role in impacting the quality of life of the patients irradiated to the head and neck.

Glutamate-induced cobalt uptake reveals non-NMDA receptors in developing rat taste buds

Non-NMDA type glutamate receptors are present in rat taste buds. However, the function of those receptors is not yet known. Developmental changes in the glutamate receptors in taste cells may provide clues to their functional role. We used a cobalt staining technique to determine at which stage in development functional non-NMDA glutamate receptors first appeared. Cobalt-stained taste bud cells first appeared in 20-day-old rats. The number of cobalt-stained cells increased with age and reached a maximum at 45 days. The shape of stained cells looked similar at all age groups. Cobalt-labeled cells appeared to be correlated with synaptic, not taste, glutamate receptors.

Biophysical properties and responses to neurotransmitters of petrosal and geniculate ganglion neurons innervating the tongue

The properties of afferent sensory neurons supplying taste receptors on the tongue were examined in vitro. Neurons in the geniculate (GG) and petrosal ganglia (PG) supplying the tongue were fluorescently labeled, acutely dissociated, and then analyzed using patch-clamp recording. Measurement of the dissociated neurons revealed that PG neurons were significantly larger than GG neurons. The active and passive membrane properties of these ganglion neurons were examined and compared with each other. There were significant differences between the properties of neurons in the PG and GG ganglia. The mean membrane time constant, spike threshold, action potential half-width, and action potential decay time of GG neurons was significantly less than those of PG neurons. Neurons in the PG had action potentials that had a fast rise and fall time (sharp action potentials) as well as action potentials with a deflection or hump on the falling phase (humped action potentials), whereas action potentials of GG neurons were all sharp. There were also significant differences in the response of PG and GG neurons to the application of acetylcholine (ACh), serotonin (5HT), substance P (SP), and GABA. Whereas PG neurons responded to ACh, 5HT, SP, and GABA, GG neurons only responded to SP and GABA. In addition, the properties of GG neurons were more homogeneous than those of the PG because all the GG neurons had sharp spikes and when responses to neurotransmitters occurred, either all or most of the neurons responded. These differences between neurons of the GG and PG may relate to the type of receptor innervated. PG ganglion neurons innervate a number of receptor types on the posterior tongue and have more heterogeneous properties, while GG neurons predominantly innervate taste buds and have more homogeneous properties.

Localization of the glutamate-aspartate transporter, GLAST, in rat taste buds

A number of putative neurotransmitter substances have been found in vertebrate taste buds. Amongst these glutamate has been localized in fibres innervating the buds and uptake of glutamate has been shown to occur into receptor cells. It is therefore possible that, in common with other sensory systems, glutamate is a neurotransmitter in taste buds. In the inner ear and retina of mammals, the membranes of supporting cells have been shown to contain the glial glutamate transporter GLAST. In the brain, this protein is involved in glutamate re-uptake into glial cells where the glutamate is converted into glutamine for recycling into glutamatergic terminals. In this study, the presence of GLAST has been investigated in taste buds in the rat vallate papilla and its distribution compared with that of glutamine to determine whether there are cells in this system that play a glia-like role in glutamate handling. Immunofluorescent labelling showed that a subset of cells in the taste bud contains GLAST. Immunogold labelling indicated that it occurs in the plasma membranes of supporting cells, especially on the fine cytoplasmic processes of dark cells towards the basal region of the bud. A protein of molecular mass similar to that of cerebellar GLAST was detected in immunoblots of excised papillae. Double labelling and semiquantitative analysis of glutamine and GLAST immunoreactivity showed that the GLAST-positive cells have a higher level of cytoplasmic glutamine than the adjacent cells. It is proposed that these GLAST-positive cells play a glia-like role in the uptake of glutamate following its release at synapses within the taste bud although the precise location of the latter remains uncertain. The GLAST-positive cells may also be involved in its subsequent conversion to glutamine in a glutamate/glutamine cycle similar to that described in the brain.

The taste of ethanol in a primate model. II. Glossopharyngeal nerve response in Macaca mulatta

The glossopharyngeal nerve (NG) mediates taste from the posterior part of the tongue. Here, we studied the effects of ethanol on the tongue in recordings from both the whole NG and individual taste fibers of the rhesus monkey, Macaca mulatta. The results show that the nerve activity increased at 0.7 M ethanol, reaching half maximum at around 4 M alcohol. Previously, we identified three types of taste fibers in the rhesus monkey NG: S fibers predominantly responding to sweeteners, Q fibers responding to bitter, such as quinine hydrochloride (QHCl), and M fibers responding best to monosodium glutamate, NaCl and acids [Hellekant, G., Danilova, V., & Ninomiya, Y. (1997). Primate sense of taste: behavioral and single chorda tympani and glossopharyngeal nerve fiber recordings in the rhesus monkey, Macaca mulatta. J Neurophysiol 77, 978-993]. Here, this fiber classification was used to elucidate the oral effects of ethanol and ethanol mixtures with NaCl, sucrose, citric acid and QHCl. One and three molar concentrations of ethanol stimulated all fiber types. Mixtures of ethanol with QHCl elicited a smaller response in Q fibers than did QHCl alone. In S fibers, mixtures of ethanol with sucrose gave a larger response than did sucrose alone. The variability of M fibers was too large to allow a conclusion about the effect of ethanol. These results suggest that ethanol suppresses the taste of QHCl. Similarly, the taste of sucrose might be enhanced by adding ethanol to sucrose. These effects and conclusions corroborate an earlier ethanol study of the chorda tympani (CT) nerve [Hellekant, G., Danilova, V., Roberts, T., & Ninomiya, Y. (1997). The taste of ethanol in a primate model: I. Chorda tympani nerve response in Macaca mulatta. Alcohol 14, 473-484].

Biophysical properties and responses to glutamate receptor agonists of identified subpopulations of rat geniculate ganglion neurons

The goal of the current study was to evaluate the electrophysiological properties and responses to glutamate receptor agonists of rat geniculate ganglion (GG) neurons innervating the tongue. Subpopulations of GG neurons were labeled by injecting Fluoro-Gold (FG) or True Blue chloride into the anterior tongue and soft palate (AT and SP neurons) and applying FG crystals to the posterior auricular branch of the facial nerve (PA neurons). Three to 12 days later, the GG neurons were acutely isolated and patch clamped. Although many biophysical properties of the AT, SP and PA neurons were similar, significant differences were found among these groups in properties related to cell excitability. For example, the average amount of current necessary to elicit an action potential was 61 pA in AT neurons (n=55), 90 pA in SP neurons (n=41) and 189 pA in PA neurons (n=35, P<0.001). In addition, AT neurons tended to fire significantly more action potentials during depolarization as well as following hyperpolarizing pulses than SP or PA neuron types. Most GG neurons responded to application of glutamate receptor agonists. The neurons responded with a depolarization accompanied by a reduction in input resistance. These results suggest that subpopulations of neurons in the geniculate ganglion have distinct biophysical properties and express functional glutamate receptors. The differing biophysical properties of GG neurons is possibly related to their functional heterogeneity and glutaminergic neurotransmission may function in the processing of gustatory, and other sensory information, within the geniculate ganglion and its projections.

Glutamate and the UMAMI taste: sensory, metabolic, nutritional and behavioural considerations. A review of the literature published in the last 10 years

Monosodium glutamate (MSG) is used increasingly often in processed foods and in home cooking in the Western world. This substance is responsible for a pleasurable taste sensation, the Umami taste. This review covers recent developments in sensory studies of glutamate effects, and traces the Umami taste from sensory receptors on the tongue to the brain. The metabolism of glutamic acid, as revealed from recent literature, is described. A specific section is devoted to safety issues. In addition, effects of glutamic salts on nutrition and ingestive behaviours are shown to be potent. Animal and human works are treated separately, with special attention to the specific methods used in both cases. Future areas of research include further investigation of sensory physiology, role of glutamate as an excitatory substance in the brain, acquisition of food likes and impact on long-term food selection, food intake, and body weight control.

Roles of chemical mediators in the taste system

Recent advances in neural mechanisms of taste are reviewed with special reference to neuroactive substances. In the first section, taste transduction mechanisms of basic tastes are explained in two groups, whether taste stimuli directly activate ion channels in the taste cell membrane or they bind to cell surface receptors coupled to intracellular signaling pathways. In the second section, putative transmitters and modulators from taste cells to afferent nerves are summarized. The candidates include acetylcholine, catecholamines, serotonin, amino acids and peptides. Studies favor serotonin as a possible neuromodulator in the taste bud. In the third section, the role of neuroactive substances in the central gustatory pathways is introduced. Excitatory and inhibitory amino acids (e.g., glutamate and GABA) and peptides (substance P and calcitonin gene-related peptide) are proved to play roles in transmission of taste information in both the brainstem relay and cortical gustatory area. In the fourth section, conditioned taste aversion is introduced as a model to study gustatory learning and memory. Pharmacobehavioral studies to examine the effects of glutamate receptor antagonists and protein kinase C inhibitors on the formation of conditioned taste aversion show that both glutamate and protein kinase C in the amygdala and cortical gustatory area play essential roles in taste aversion learning. Recent molecular and genetic approaches to disclose biological mechanisms of gustatory learning are also introduced. In the last section, behavioral and pharmacological approaches to elucidate palatability, taste pleasure, are described. Dopamine, benzodiazepine derivatives and opioid substances may play some roles in evaluation of palatability and motivation to ingest palatable edibles.

Neural cell adhesion molecule, neuron-specific enolase and calcitonin gene-related peptide immunoreactivity in hamster taste buds after chorda tympani/lingual nerve denervation

Hamster fungiform papilla taste buds persist in an atrophic form following sensory denervation. While atrophic and innervated taste buds are morphologically similar, it is not known whether their gemmal cells have similar molecular characteristics. Three neurochemicals, neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide have been implicated in trophic phenomena, synaptogenesis and cell recognition in neurons and sensory neuroepithelia. The present study uses immunocytochemical localization of these molecular markers to characterize normal and denervated fungiform taste buds following unilateral chorda tympani/lingual nerve denervation in hamsters. In normal taste buds, immunoreactivity to neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide was present in a group of cells located centrally in the bud as well as in fungiform nerve fibres and endings. After denervation, gemmal cell immunoreactivity to all three markers was reduced and often confined to a single or a few bud cell(s). Also, fibre staining was absent except for sparse calcitonin gene-related peptide-immunoreactive fibres associated with blood vessels and within the fungiform papillae. These remaining fibres may be autonomic or somatomotor in origin. These results indicate that sensory denervation of hamster taste buds reduces, but does not wholly eliminate the immunoreactivity of surviving gemmal cells to neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide. While the number of taste bud cells expressing the markers appears to be nerve-dependent, immunoreactivity in sensory-denervated bud cells of hamster may reflect the influence of local tissue factors.

Serotonin inhibits calcium-activated K+ current in rat taste receptor cells

Little is definitively known of the identity or actions of neurotransmitters utilized within mammalian taste buds. Serotonin has been immunocytochemically localized to taste cells of several species but its physiological actions are unknown. Using whole-cell patch clamp recordings on dissociated posterior rat taste cells, data are presented to suggest that exogenously applied serotonin inhibits a calcium-activated potassium current by up to 50%. This current, best visualized at depolarized holding potentials, is both apamin- and charybdotoxin-sensitive. Approximately 60% of the tested taste cells were serotonin sensitive. This inhibition was mimicked by N-(3-trifluoromethylphenyl)piperazine (TFMPP), a general serotonin receptor agonist, by 8-hydroxy-dipropylaminotetralin (8-OH-DPAT), a selective 5-HT1A receptor agonist, but not by phenylbiguanide, a 5-HT3 receptor agonist. These are the first data to establish a physiological effect of serotonin on mammalian taste cells.

GABAergic neurotransmission in rat taste buds: immunocytochemical study for GABA and GABA transporter subtypes

Gamma-aminobutyric acid (GABA) is known to be a candidate for the neurotransmitter involved in the sense of taste. We hereby studied GABA and its termination system, GABA transporters, in rat taste buds by immunocytochemical approaches. Immunoblot analysis of three GABA transporter subtypes (GAT1, GAT2 and GAT3) revealed that the immunoreactive bands of GAT2 and GAT3, but not GAT1, were detected in the tongue. GAT3-immunoreactive band was recognized only in the circumvallate papilla containing a large number of taste buds while GAT2-immunoreactive bands were seen in all areas of the tongue. GAT2 immunoreactivity appeared to be specifically in the nerve fibers beneath the lingual epithelium. Both GAT3 and GABA immunoreactivities were detected only in taste buds. A few GAT3-immunoreactive cells were found in a cross-section of each taste bud but most GAT3-immunoreactive cells were localized in the margin of the taste bud. GAT3 was predominantly concentrated in the distal portion of the GAT3-immunoreactive cells. In contrast, GABA-immunoreactive cells were seen more frequently within each taste bud and the immunoreactivity was distributed throughout the perikarya of the cells. These results suggest that the GABA-uptake system is present in the taste buds and the GABAergic neurotransmission involved in the sensation of taste is terminated by the uptake of GABA into certain taste cells via GAT3.

Neurochemical markers of human fungiform papillae and taste buds

The presence of distribution of several neurochemical markers in human fungiform papillae and taste buds were investigated by the immunohistochemical technique. The gustatory cells of the taste buds are in synaptic contact with sensory nerve endings, and considering the taste buds strictly as specialized sensory organs, the amounts and distribution of some of the neurochemical markers were different to what we expected. For example, few structures showed immunoreactivity to the tachykinins substance P (SP), calcitonin gene-related peptide (CGRP), and neurokinin A (NKA) also for the peptides vasoactive intestinal polypeptide (VIP), neuropeptide tyrosine (NPY) and galanin, low amounts of immunoreactivity occurred. On the other hand, using antibodies to protein gene product 9.5 (PGP 9.5), protein S-100, and glutamate, numerous nerve fibres and/or immunoreactive cells were found in the fungiform papillae, in the epithelium, in the connective tissue and around blood vessels, as well as in or near taste buds. Incubation with the antibodies against somatostatin, enkephalin, bombesin, peptide histidine isoleucine amide (PHI), cholecystokinin (CCK)/gastrin and dopamine-beta-hydroxylase (DBH) was negative for the fungiform papillae. In conclusion, the present study has shown several immunoreactive structures using antibodies against certain neurochemical markers. Further investigations will hopefully correlate these morphological findings with functional taste perception data. Future studies of patients with taste disorders or other pathological changes correlated with taste and tongue will also be of utmost importance.

The frog taste disc: a prototype of the vertebrate gustatory organ

The frog taste disc (TD) is apparently the largest gustatory organ found in vertebrates and seems to differentiate into a specialized variety of the prototypic scheme of the taste bud. An explanation for this unusual organization is lacking although it is possible to speculate the existence of environmental and nutritional requirements. Up to the present time, the most common model of the TD was based on two main cell types (sensory and sustentacular). This model may oversimplify the morphology of this structure since more numerous cell types have been described. We now propose a new model of the TD, based on comprehensive data on the ultrastructure of the organ obtained in the last 20 years. The main conclusions are the following: (1) the TD is a pluristratified epithelium with a general organization similar to that of the olfactory and vomeronasal epithelium; (2) it has skeleton composed of three different types of epithelial cells; (3) the chemoreceptorial surface is covered by different microenvironments; (4) three different types of neuro-epithelial systems are present; the type II is an 'open' sensory cell with axonal contacts devoid of vesicles; the type III is an 'open' sensory cell with synaptic-like junctions; the type i.v. is a 'closed' sensory cell with a 'Merkel-neurite complex'; (5) the nerve fibers in the basal plexus are mostly cholinergic while the peridiscal nerve fibers are mostly peptidergic. The presence of several cell types in the TD must be considered using these large receptors in electrophysiological studies or as a source of isolated cells, and their complexity must induce caution in the interpretation of the data. Text books of histology usually describe the peripheral structures associated with taste as very simple: an idea that probably must be revised. A taste organ is a highly complex structure composed of several sensory systems and a comparative approach can aid comprehension of its general organization. The study of the 'large taste organs' present in some species of amphibians can provide useful data for knowledge of the gustatory system of vertebrates.

Electron microscopic immunocytochemistry of glutamate-containing nerve fibers in the taste bud of mudpuppy (Necturus maculosus)

The presence of glutamate immunoreactivity (glu-IR) in the nerve fibers of the mudpuppy taste bud was investigated by electron microscopy. Pre-embedding staining with avidin-biotin-peroxidase complex (ABC) and post-embedding staining with 5 nm colloid gold conjugates were used separately to identify immuno-stained structures. We have found the following: 1) the majority of the nerve fibers innervating the mudpuppy taste bud are unmyelinated; 2) about 85% of nerve fibers located at the base of the taste bud and about 60% of the nerve fibers located between the taste cells show glu-IR by pre-embedding staining; 3) there is a preferential staining of the glu-IR in the nerve fibers of the mudpuppy taste bud; and 4) the distribution of the colloidal gold particles in the nerve fibers is 1.5 to 2 times denser than that of the staining in the connective tissue background or cellular profiles of taste cells. From the distribution and pattern of the nerve fibers obtained in the thick and thin sections, we conclude that the mudpuppy taste bud is innervated by glutamate-containing unmyelinated nerve fibers.

Immunocytochemical survey of putative neurotransmitters in taste buds from Necturus maculosus

To investigate synaptic mechanisms in taste buds and collect information about synaptic transmission in these sensory organs, we have examined taste buds of the mudpuppy, Necturus maculosus for the presence of neurotransmitters and neuromodulators. Immunocytochemical staining at the light microscopic level revealed the presence of serotonin-like and cholecystokinin-like (CCK) immunoreactivity in basal cells in the taste bud. Nerve fibers innervating taste buds were immunoreactive for vasoactive intestinal peptide-like (VIP), substance P-like, and calcitonin gene-related peptide-like (CGRP) or compounds closely related to these substances. Immunoreactivity for tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT) in the taste cells and nerve fibers was absent. These data suggest that serotonin, CCK, VIP, substance P, and CGRP are involved in synaptic transmission or neuromodulation in the peripheral organs of taste. No evidence was found for cholinergic or adrenergic mechanisms on the basis of the absence of immunocytochemical staining for key enzymes involved in these two transmitter systems.