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PubMed2006

Espin cytoskeletal proteins in the sensory cells of rodent taste buds.

Espins are multifunctional actin-bundling proteins that are highly enriched in the microvilli of certain chemosensory and mechanosensory cells, where they are believed to regulate the integrity and/or dimensions of the parallel-actin-bundle cytoskeletal scaffold. We have determined that, in rats and mice, affinity purified espin antibody intensely labels the lingual and palatal taste buds of the oral cavity and taste buds in the pharyngo-laryngeal region. Intense immunolabeling was observed in the apical, microvillar region of taste buds, while the level of cytoplasmic labeling in taste bud cells was considerably lower. Taste buds contain tightly packed collections of sensory cells (light, or type II plus type III) and supporting cells (dark, or type I), which can be distinguished by microscopic features and cell type-specific markers. On the basis of results obtained using an antigen-retrieval method in conjunction with double immunofluorescence for espin and sensory taste cell-specific markers, we propose that espins are expressed predominantly in the sensory cells of taste buds. In confocal images of rat circumvallate taste buds, we counted 21.5 +/- 0.3 espin-positive cells/taste bud, in agreement with a previous report showing 20.7 +/- 1.3 light cells/taste bud when counted at the ultrastructural level. The espin antibody labeled spindle-shaped cells with round nuclei and showed 100% colocalization with cell-specific markers recognizing all type II [inositol 1,4,5-trisphosphate receptor type III (IP(3)R(3))(,) alpha-gustducin, protein-specific gene product 9.5 (PGP9.5)] and a subpopulation of type III (IP(3)R(3), PGP9.5) taste cells. On average, 72%, 50%, and 32% of the espin-positive taste cells were labeled with antibodies to IP(3)R(3), alpha-gustducin, and PGP9.5, respectively. Upon sectional analysis, the taste buds of rat circumvallate papillae commonly revealed a multi-tiered, espin-positive apical cytoskeletal apparatus. One espin-positive zone, a collection of approximately 3 mum-long microvilli occupying the taste pore, was separated by an espin-depleted zone from a second espin-positive zone situated lower within the taste pit. This latter zone included espin-positive rod-like structures that occasionally extended basally to a depth of 10-12 mum into the cytoplasm of taste cells. We propose that the espin-positive zone in the taste pit coincides with actin bundles in association with the microvilli of type II taste cells, whereas the espin-positive microvilli in the taste pore are the single microvilli of type III taste cells.

Adaptor Proteins, Vesicular Transport↗
PubMed2005

Confocal microscopy of the peripheral gustatory system: comparison between healthy subjects and patients suffering from taste disorders during radiochemotherapy.

OBJECTIVES: Laser-scanning microscopy (LSM) was used to compare taste buds and epithelia of fungiform papillae of healthy subjects with those of patients suffering from taste disorders during/after radiochemotherapy (RCT). Aim of the study was to investigate effects responsible for taste loss at a microscopic level. STUDY DESIGN: Prospective study. METHODS: Data from 12 healthy subjects (mean age 52.4, SD 9.5 years) were compared with those of 12 patients (mean age 54.7, SD 8.5 years) with head and neck cancer suffering from taste disorders during RCT. Four parameters from LSM were selected for analysis: 1) distance between the pore of the taste buds of fungiform papillae and the crest of the papillary vessels; 2) epithelial cells of each taste bud at 34 mum; 3), cell density, and 4) area of the taste pore at 4 mum. These data were correlated to measures of gustatory sensitivity obtained with both the validated "taste strips" test kit and electrogustometry. RESULTS: Patients complaining from taste disorders during RCT exhibited a significant decrease of taste function assessed with both natural and electric stimuli. In these patients, we found thicker epithelia and smaller areas of the taste pores compared with healthy subjects. In 30% of those patients, no taste pores were detectable; in deeper sections, however, normal taste buds were present. CONCLUSIONS: In conclusion, in RCT patients with taste disorders, LSM indicates changes of epithelia of fungiform papilla but no changes of the taste bud structure. Damage of the chorda tympani nerve by scattered rays, direct or indirect mucotoxic effects of chemotherapeutic agents, and covering of taste pores by epithelial cells are likely reasons for taste loss during RCT.

Adult↗
PubMed1998

Innervation of single fungiform taste buds during development in rat.

To determine whether the innervation of taste buds changes during postnatal development, the number of geniculate ganglion cells that innervated single fungiform taste buds were quantified in the tip- and midregions of the tongue of adult and developing rats. There was substantial variation in both the size of individual taste buds and number of geniculate ganglion cells that innervated them. Importantly, taste bud morphology and innervation were highly related. Namely, the number of labeled geniculate ganglion cells that innervated a taste bud was highly correlated with the size of the taste bud (r = 0.91, P < .0003): The larger the taste bud, the more geniculate ganglion cells that innervated it. The relationship between ganglion cell number and taste bud volume emerged during the first 40 days postnatal. Whereas there was no difference in the average number of ganglion cells that innervated individual taste buds in rats aged 10 days postnatal through adulthood, taste bud volumes increased progressively between 10 and 40 days postnatal, at which age taste bud volumes were similar to adults. The maturation of taste bud size was accompanied by the emergence of the relationship between taste bud volume and number of innervating neurons. Specifically, there was no correlation between taste bud size and number of innervating geniculate ganglion cells in 10-, 20-, or 30-day-old rats, whereas taste bud size and the number of innervating ganglion cells in 40-day-old rats were positively correlated (r = .80, P < .002). Therefore, the relationship between taste bud size and number of innervating ganglion cells develops over a prolonged postnatal period and is established when taste buds grow to their adult size.

Afferent Pathways↗
PubMed2002

Individual mouse taste cells respond to multiple chemical stimuli.

Sensory organs are specialized to detect and decode stimuli in terms of intensity and quality. In the gustatory system, the process of identifying and distinguishing taste qualities (e.g. bitter versus sweet) begins in taste buds. A central question in gustatory research is how information about taste quality is extracted by taste receptor cells. For instance, whether and how individual taste cells respond to multiple chemical stimuli is still a matter for debate. A recent study showed that taste cells expressing bitter-responsive taste receptors do not also express sweet-responsive taste receptors and vice versa. These results suggest that the gustatory system may use separate cellular pathways to process bitter and sweet signals independently. Results from electrophysiological studies, however, reveal that individual taste receptor cells respond to stimuli representing multiple taste qualities. Here we used non-invasive Ca(2+) imaging in slices of lingual tissue containing taste buds to address the issue of quality detection in murine taste receptor cells. We recorded calcium transients elicited by chemical stimuli representing different taste qualities (sweet, salty, sour and bitter). Many receptor cells (38 %) responded to multiple taste qualities, with some taste cells responding to both appetitive ("sweet") and aversive ("bitter") stimuli. Thus, there appears to be no strict and separate detection of taste qualities by distinct subpopulations of taste cells in peripheral gustatory sensory organs in mice.

Animals↗
PubMed2000

Taste perception after lingual nerve repair.

PURPOSE: This retrospective study evaluates patients' perception of taste after lingual nerve injury and repair. It is hypothesized that return of taste is a distinct neurophysiologic phenomenon related to specialized taste physiology and it does not correlate with specific, objective sensory return. PATIENTS AND METHODS: During 1995 to 1996, 30 patients underwent microsurgical repair of lingual nerve injuries. Of these, 22 patients met the inclusion criteria for this study. Chart review documented the date and cause of injury, sensory examination, injury classification, procedures, operative findings, and postoperative sensory examination. A telephone questionnaire addressed whole mouth taste perception with normal daily eating. The questions asked were: 1) Was your sense of taste changed or abnormal after your nerve injury? and 2) Did your sense of taste recover after nerve repair? RESULTS: All patients had a postinjury, prerepair sensory deficit on levels A, B, and C testing: neurotmesis (n = 14); and axonotmesis (n = 8). The mean time from injury to repair was 16 weeks (range, 3 to 41 weeks). Operative findings confirmed 12 Sunderland Class IV (partial transection) injuries and 10 Class V (complete transection) injuries. All patients had primary microsurgical repair without a nerve graft or entubulization. Postoperatively, 18 patients showed marked improvement in sensory testing at levels A, B and C, and 4 patients showed no significant change. A telephone interview regarding whole mouth taste perception indicated that 20 of 22 patients perceived changed, abnormal taste postinjury and pre-repair. Two patients reported normal taste perception. Postrepair, only 7 of 20 patients reported an increase or return of taste perception to a more normal level. The mean follow-up time was 80 weeks. CONCLUSION: Most patients (20 of 22) with lingual nerve injuries in this study perceived whole mouth taste as abnormal. After nerve repair, although 82% (18 of 22) of patients had improvement in somatosensory function, whole mouth taste was perceived as improved by only 35% (7 of 20). It is proposed that the perception of whole mouth taste may not be related to the ability to perceive multiple sensory modalities, but rather to special sensory (taste) modality perception. Also, central changes may occur in the special sensory fibers that impact on the brainstem nucleus for taste (nucleus solitarius) and therefore the patient's perception of taste.

Cranial Nerve Injuries↗
PubMed2004

Umami taste responses are mediated by alpha-transducin and alpha-gustducin.

The sense of taste comprises at least five distinct qualities: sweet, bitter, sour, salty, and umami, the taste of glutamate. For bitter, sweet, and umami compounds, taste signaling is initiated by binding of tastants to G-protein-coupled receptors in specialized epithelial cells located in the taste buds, leading to the activation of signal transduction cascades. Alpha-gustducin, a taste cell-expressed G-protein alpha subunit closely related to the alpha-transducins, is a key mediator of sweet and bitter tastes. Alpha-gustducin knock-out (KO) mice have greatly diminished, but not entirely abolished, responses to many bitter and sweet compounds. We set out to determine whether alpha-gustducin also mediates umami taste and whether rod alpha-transducin (alpha(t-rod)), which is also expressed in taste receptor cells, plays a role in any of the taste responses that remain in alpha-gustducin KO mice. Behavioral tests and taste nerve recordings of single and double KO mice lacking alpha-gustducin and/or alpha(t-rod) confirmed the involvement of alpha-gustducin in bitter (quinine and denatonium) and sweet (sucrose and SC45647) taste and demonstrated the involvement of alpha-gustducin in umami [monosodium glutamate (MSG), monopotassium glutamate (MPG), and inosine monophosphate (IMP)] taste as well. We found that alpha(t-rod) played no role in taste responses to the salty, bitter, and sweet compounds tested or to IMP but was involved in the umami taste of MSG and MPG. Umami detection involving alpha-gustducin and alpha(t-rod) occurs in anteriorly placed taste buds, however taste cells at the back of the tongue respond to umami compounds independently of these two G-protein subunits.

Animals↗
PubMed1997

Differential expression of alpha-gustducin in taste bud populations of the rat and hamster.

The G-protein subunit alpha-gustducin, which is similar to rod transducin, has been implicated in the transduction of both sweet- and bitter-tasting substances. In rodents, there are differences in sensitivity to sweet and bitter stimuli in different populations of taste buds. Rat fungiform taste buds are more responsive to salts than to sweet stimuli, whereas those on the palate respond predominantly to sweet substances. In contrast, hamster fungiform taste buds are more sensitive to sweet-tasting stimuli. Taste buds in the vallate and foliate papillae of both species are sensitive to bitter compounds. These differences in sensitivity should be reflected in the numbers of gustducin-containing cells in different taste bud populations. We examined taste buds in the rat and hamster for immunoreactivity to an antibody against alpha-gustducin. Immunofluorescence of labeled taste cells was examined by confocal microscopy, and the cells were counted. Gustducin-positive cells were seen in all taste bud regions; they were spindle-shaped, with circular cross-sections and apical processes that extended to the taste pore. Cells with this characteristic shape in rat vallate taste buds are Type II (light) cells. In the rat, taste buds of the fungiform papillae had fewer gustducin-positive cells (3.1/taste bud) than those of other regions, including the posterior tongue and palate (>8.9/taste bud). Hamster fungiform taste buds contained twice as many gustducin-expressing cells (6.8/taste bud) as those of the rat. These data support the hypothesis that alpha-gustducin is involved in the transduction of both sweet- and bitter-tasting stimuli by mammalian taste receptor cells.

Animals↗
PubMed1994

Application of serial sectioning and three-dimensional reconstruction to the study of taste bud ultrastructure and organization.

The lingual taste buds of mammals are complex organs containing dozens of cells of varying morphology and numerous nerve fibers that are intermingled among the cellular processes. Some of the taste bud cells form synaptic contacts with these nerve fibers. Important questions remain to be answered regarding the structure and function of the cells of various types within taste buds and the means by which responses to gustatory stimuli are transmitted to the nerve fibers that communicate with the brain. Using both conventional and high voltage electron microscopy, we have examined serially sectioned taste buds from the tongues of mice and rabbits in order to address these issues and to obtain more complete information than that available from sampling of sections. The technique of computer-assisted 3-D reconstruction was used to generate models of whole taste buds and individual cellular and neural elements within taste buds from the serial sections. Analysis of serially sectioned taste buds from mice and rabbits has revealed that in both of these species relatively few (30% or less) of the cells within the taste buds form synaptic contacts with nerve fibers. In the foliate taste buds of rabbits, all of the cells that are presynaptic to nerve fibers are of a single morphological type (type III). The cells that are presynaptic to nerve fibers within the taste buds of mice are morphologically diverse. A pattern of synaptic connectivity exists within murine taste buds such that a given nerve fiber receives synaptic input only from taste cells that are ultrastructurally similar. In the taste buds of both mice and rabbits, we have observed both divergence and convergence of synaptic input from the putative taste receptor cells onto nerve fibers, suggesting that at the level of the taste bud there is some integration of the information generated by individual receptor cells. In addition to typical chemical synapses, other cytoplasmic specializations (such as subsurface cisternae and atypical mitochondria) may be involved in interactions between taste bud cells and nerve fibers.

Animals↗
PubMed1989

Anatomy and neurophysiology of the taste system in aged animals.

In summary, the recent data from quantitative studies of taste buds in old humans, rhesus monkeys, and rats complement neurophysiological data on taste responses from aged rats and lead to the general conclusion that the peripheral taste system is maintained structurally and functionally across the life span. Although some statistically significant differences were observed, the magnitude of these differences would not lead one to predict altered taste preferences or feeding behavior in old animals. The robust nature of the peripheral sense of taste in old age appears to be in sharp contrast to other sensory systems. Not only do receptors alter with age, but accessory organs, such as the lens in the visual system, also alter structurally so that sensory function is compromised. One factor contributing to maintenance of the receptor organ of taste bud and papilla is the turnover and replacement of taste bud cells and surrounding epithelial cells. Recent studies in aged mice indicate no differences in turnover time in epithelial mucosa lining the mouth. There are no data on turnover time, however, for taste bud cells or for gustatory papilla epithelium in old animals. Alterations in taste bud cell or membrane receptor turnover could affect function. In other cell types and receptors, age-related differences in membrane lipids, proteins, and fluidity characteristics are reported. Membrane changes such as these are proposed to account for age-related differences in response properties of the system of cardiac muscarinic receptors, a system in which density of receptors does not alter. The small, but statistically significant, differences in neurophysiological responses to taste stimuli from the chorda tympani nerve might well relate to age-related differences in membrane receptors. Not only is there an absence of data on turnover time or membrane characteristics for taste bud cells in old age, but there are no observations on the ultrastructure of taste buds in old animals. The taste bud is a complex structure, composed of many cells and different cell types, with a specific orientation and access to the oral cavity via the taste pore, and with extensive associations with the innervating afferent fibers. Ultrastructural observations could provide information on changes in synapses and other nerve/cell contacts--in microvilli and taste pore structure, and in numbers of various cell types within the bud. Such data will be essential to understanding the nature and extent of age-related alterations in the taste system.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗
PubMedDate not supplied

Comparative taste evaluation of aerosolized formulations of triamcinolone acetonide, flunisolide, and flunisolide with menthol.

The taste characteristics of aerosolized formulations of triamcinolone acetonide, flunisolide, and flunisolide with menthol flavoring were compared in 102 adult asthmatic patients. During a 2-hour test period, study participants evaluated the taste of each of the three inhaled corticosteroids, one at a time, in a randomly assigned sequence. The extent to which they liked the taste of each preparation and the taste intensity (strength of taste) of each product were rated on 100-point scales. Patients also characterized the predominant taste of each inhaled corticosteroid as "no taste," "salty," "sour," "sweet," or "bitter." Assessments were made immediately after inhalation and 2 minutes after inhalation. At both assessment times, subjects liked the taste of triamcinolone acetonide significantly more than the taste of flunisolide or flunisolide with menthol, and they liked the taste of flunisolide with menthol better than that of flunisolide without menthol. The taste intensity of triamcinolone acetonide was rated significantly less than that of the two flunisolide preparations at both evaluation times. There was no significant difference in the taste-intensity ratings for flunisolide with and without menthol. Triamcinolone acetonide was most frequently described as having no taste, whereas the taste of both flunisolide and flunisolide with menthol was most frequently described as bitter. Because of the possible adverse impact of an unpleasant taste on patient compliance with prescribed therapy, differences in the taste of inhaled corticosteroids should be an important consideration in selecting and recommending an aerosolized steroidal preparation for the control of asthma.

Administration, Inhalation↗
PubMed2006

Umami responses in mouse taste cells indicate more than one receptor.

A number of gustatory receptors have been proposed to underlie umami, the taste of L-glutamate, and certain other amino acids and nucleotides. However, the response profiles of these cloned receptors have not been validated against responses recorded from taste receptor cells that are the native detectors of umami taste. We investigated umami taste responses in mouse circumvallate taste buds in an intact slice preparation, using confocal calcium imaging. Approximately 5% of taste cells selectively responded to L-glutamate when it was focally applied to the apical chemosensitive tips of receptor cells. The concentration-response range for L-glutamate fell approximately within the physiologically relevant range for taste behavior in mice, namely 10 mm and above. Inosine monophosphate enhanced taste cell responses to L-glutamate, a characteristic feature of umami taste. Using pharmacological agents, ion substitution, and immunostaining, we showed that intracellular pathways downstream of receptor activation involve phospholipase C beta2. Each of the above features matches those predicted by studies of cloned and expressed receptors. However, the ligand specificity of each of the proposed umami receptors [taste metabotropic glutamate receptor 4, truncated metabotropic glutamate receptor 1, or taste receptor 1 (T1R1) and T1R3 dimers], taken alone, did not appear to explain the taste responses observed in mouse taste cells. Furthermore, umami responses were still observed in mutant mice lacking T1R3. A full explanation of umami taste transduction may involve novel combinations of the proposed receptors and/or as-yet-undiscovered taste receptors.

Animals↗
PubMed1996

Development of voltage-dependent currents in taste receptor cells.

Taste buds, the specialized end organs of gustation, comprise a renewing sensory epithelium. Undifferentiated basal cells become taste receptor cells by elongating and extending processes apically toward the taste pore. Mature taste cells are electrically excitable and express voltage-dependent Na+ Ca2+, and K+ currents, whereas basal stem cells exhibit only slowly activating K+ currents. The question we have addressed in the present study is whether contact with the taste pore is required for expression of voltage-dependent inward currents in Necturus taste cells. Mature taste cells were distinguished from developing cells by labeling the apical surface of the cells with fluorescein-isothiocyanate-conjugated wheat germ agglutinin (FITC-WGA), while the tissue was still intact. Elongate cells lacking FITC-WGA staining were interpreted as developing taste cells that had not yet reached the taste pore. Giga-seal whole-cell recording revealed that most developing taste cells lacked inward currents. Although some developing cells expressed inward currents, they were much smaller than those of mature cells, and the activation kinetics of the K+ currents were slower than in mature cells. Electron microscopy confirmed the identity of labeled and unlabeled cells. All FITC-WGA-labeled cells exhibited the ultrastructural characteristics of mature taste receptor cells, whereas most unlabeled taste cells had a characteristic morphology that was markedly different from mature taste receptor cells or basal stem cells. These data suggest that contact with the taste pore is required for the development of inward currents in taste cells.

Animals↗
PubMed2001

Development and maturation of taste buds of the palatal epithelium of the rat: histological and immunohistochemical study.

Palatal taste buds are intriguing partners in the mediation of taste behavior and their spatial distribution is functionally important for suckling behavior, especially in the neonatal life. Their prenatal development has not been previously elucidated in the rat, and the onset of their maturation remains rather controversial. We delineated the development and frequency distribution of the taste buds as well as the immunohistochemical expression of alpha-gustducin, a G protein closely related to the transduction of taste stimuli, in the nasoincisor papilla (NIP) and soft palate (SP) from the embryonic day 17 (E17) till the postnatal day 70 (PN70). The main findings in the present study were the development of a substantial number of taste pores in the SP of fetal rats (60.3 +/- 1.7 out of 122.8 +/- 5.5; mean +/- SD/animal at E19) and NIP of neonatal rats (9.8 +/- 1.0 out of 44.8 +/- 2.2 at PN4). alpha-gustducin-like immunoreactivity (-LI) was not expressed in the pored taste buds of either prenatal or newborn rats. The earliest expression of alpha-gustducin-LI was demonstrated at PN1 in the SP (1.5 +/- 0.5 cells/taste bud; mean +/- SD) and at PN4 in the NIP (1.4 +/- 0.5). By age the total counts of pored taste buds continuously increased and their morphological features became quite discernible. They became pear in shape, characterized by distinct pores, long subporal space, and longitudinally oriented cells. Around the second week, a remarkable transient decrease in the total number of taste buds was recorded in the oral epithelium of NIP and SP, which might be correlated with the changes of ingestive behaviors. The total counts of cells showing alpha-gustducin-LI per taste bud gradually increased till the end of our investigation (14.1 +/- 2.7 in NIP and 12.4 +/- 2.5 in SP at PN70). We conclude that substantial development of taste buds began prenatally in the SP, whereas most developed entirely postnatal in the NIP. The present study provides evidence that the existence of a taste pore which is considered an important criterion for the morphological maturation of taste buds is not enough for the onset of the taste transduction, which necessitates also mature taste cells. Moreover, the earlier maturation of palatal taste buds compared with the contiguous populations in the oral cavity evokes an evidence of their significant role in the transmission of gustatory information, especially in the early life of rat.

Animals↗
PubMed1988

Ultrastructure of mouse vallate taste buds: III. Patterns of synaptic connectivity.

We have used serial high voltage electron micrographs and computer-generated, three-dimensional reconstructions to study morphological relationships and patterns of synaptic connectivity in taste buds from the circumvallate papillae of the mouse. The intragemmal arborizations of 40 sensory nerve fibers were examined from 7 taste buds that were sectioned serially. We identified the synaptic connections from taste cells onto the reconstructed nerve fibers and classified the presynaptic taste cells based on previously established ultrastructural criteria. From these data we were able to extract the following information for the reconstructed nerve fibers: (1) the morphology of intragemmal nerve fibers and their arborizations within the taste bud, (2) the total number of synaptic connections from taste bud cells onto the nerve fibers, and (3) the taste cell types associated with each of the synapses. Fifty-six synapses were studied. Synapses were often found to be located at either the branch points or terminations of nerve fiber processes. The maximum number of taste cells observed to synapse onto a single nerve fiber was 5. Several nerve fibers had no apparent synapses. Dark cells (type I), intermediate cells, and light cells (type II) all formed synaptic connections with sensory nerve fibers. In no cases did dark cells and light cells synapse onto the same sensory nerve fiber. Our observation that any given nerve fiber receives its synaptic input from morphologically similar taste cells provides evidence for specificity in taste bud synaptic connections. We speculate that the observed pattern of synaptic connections is related to taste bud function. Since all of the synapses onto a given nerve fiber are from morphologically similar taste cells, we postulate that there is a correlation between taste cell morphology and sensory responsiveness. Intracellular electrophysiological studies on taste cells, in which responses to focally applied chemical stimuli are followed by characterization of the ultrastructural features of the same taste cells, will prove or disprove this hypothesis.

Afferent Pathways↗
PubMed1995

Localization of serotonin in taste buds: a comparative study in four vertebrates.

To investigate monoaminergic synaptic mechanisms in taste buds, we examined taste buds of mice, rats, rabbits, and mudpuppies for the presence of the neurotransmitter candidate, serotonin. Immunocytochemistry revealed serotonin-like immunostaining in cells in mammalian taste buds and Merkel-like basal cells in taste buds of mudpuppies. In untreated mudpuppies and in mammals injected with the precursor to serotonin, L-tryptophan, certain taste cells showed serotonin-like immunoreactivity, although in mammalian taste buds the immunostaining was relatively weak. After pretreating mammals with 5-hydroxytryptophan (5-HTP), the intermediate precursor between L-tryptophan and serotonin, several taste cells showed strong immunoreactivity for serotonin. These findings indicate that mammalian taste cells normally contain serotonin and that taste cells can take up 5-HTP and convert it to serotonin. Immunocytochemistry on wholemount preparations demonstrated that serotonergic cells of mudpuppies (i.e., Merkel-like basal cells) were disposed in a ring at the periphery of taste buds. Similarly, serotonergic cells in mammalian taste buds tended to be located at the periphery of taste buds. Based on the position of serotonergic cells in the taste bud and on recent physiological studies on the actions of serotonin in taste buds, we postulate that serotonin functions as a neuromodulator or neurotransmitter in vertebrate taste buds.

Animals↗
PubMed2002

Differential involvement of cortical muscarinic and NMDA receptors in short- and long-term taste aversion memory.

In conditioned taste aversion, an animal avoids a taste previously associated with toxic effects, and this aversive memory formation requires an intact insular cortex. In this paper, we investigated the possible differential involvement of cholinergic and glutamatergic receptors in the insular cortex in short-term memory (STM) and long-term memory (LTM) of taste aversion in rats. Taste aversion was induced by intraperitoneal administration of lithium chloride (a malaise-inducing drug) 15 min after experience with an unfamiliar taste. In order to test STM and LTM of taste aversion, taste stimulus was again presented 4 h and 72 h after lithium injection, respectively. During the acquisition, microinjection of the muscarinic antagonist, scopolamine, in the insular cortex before, but not after, the presentation of the new taste, abolished STM as well as LTM. Blockade of the NMDA receptor, in the insular cortex, by AP5 before, but not after, the presentation of the taste stimulus, impaired LTM but left STM intact. Moreover, when injected 1 h after malaise induction (i.e., during taste-illness association), AP5 disrupted both STM and LTM. These results suggest that activation of muscarinic receptors in the insular cortex is involved in the acquisition of taste memory, whereas NMDA receptors participate in taste memory consolidation. These data demonstrate that different neurochemical mechanisms subserve different memory phases. NMDA receptors are also probably involved in processing the visceral input, thus allowing subsequent taste-illness association. This indicates that in the same cortical area the same neurotransmitter system can be involved in distinct processes: taste memory consolidation vs. taste-illness association.

2-Amino-5-phosphonovalerate↗
PubMed1989

Information processing in the taste system of primates.

1. Analysis of the activity of single neurones in the gustatory pathways in primates (cynomolgus monkeys) shows that the tuning of neurones to the four prototypical stimuli 1.0 mol l-1 glucose, 1.0 mol l-1 NaCl, 0.001 mol l-1 quinine-HCl and 0.01 mol l-1 HCl becomes sharper as information progresses through the taste system from the first central relay in the brainstem, the nucleus of the solitary tract, via the thalamus to the primary taste cortex in the frontal operculum and insula to reach a secondary cortical taste area in the caudolateral orbitofrontal cortex. 2. Feeding monkeys to satiety with glucose has no effect on the gustatory responses of neurones in the nucleus of the solitary tract, the frontal opercular taste cortex or the insular taste cortex, but decreases the magnitude of the neuronal responses of orbitofrontal cortex taste neurones which respond to glucose to zero. 3. The responses of orbitofrontal cortex taste neurones decrease to foods on which the monkey is fed to satiety, but continue to foods which have not just been eaten, that is, they reflect sensory-specific satiety, the phenomenon in which the pleasantness of the taste of a food and its acceptability, but not those of of other foods, is decreased by eating that food to satiety. 4. It has been found that the orbitofrontal cortex taste area or areas receives inputs from different modalities: single neurones with unimodal responses to taste (47%), olfactory (12%) and visual (10%) stimuli are found in close proximity to each other. Moreover, some single neurones show multimodal convergence, responding, for example, to taste and visual inputs (17%), taste and olfactory inputs (10%), and olfactory and visual inputs (4%). Some of these multimodal single neurones have corresponding sensitivities in the two modalities: they respond best to sweet tastes (e.g. 1 mol l-1 glucose), and respond more in a visual discrimination task to the visual stimulus which signifies sweet fruit juice than to that which signifies saline; or they respond to sweet taste and, in an olfactory discrimination task, to fruit juice odour. These results suggest that multimodal representations are formed in the orbitofrontal cortex secondary and related taste areas.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗
PubMed1988

An immunohistochemical study of cellular and nervous elements in the taste organ of the bullfrog, Rana catesbeiana.

The cellular and nervous elements of the bullfrog taste organ were examined by immunohistochemical methods using various antibodies. The immunoreactivity for spot 35 protein, a soluble protein isolated from bovine cerebellum, was found in numerous taste cells located at the middle or slightly lower levels within the gustatory cell layer. The immunoreactive cells possessed cytoplasmic processes rising upward the free surface and also issued branched processes to the base of the epithelium. The immunoreaction for spot 35 protein was found diffusely throughout the cytoplasm from the apical to the basal parts of the taste cells. NSE-immunoreactive taste cells were located at the upper or middle levels within the gustatory cell layer in the taste organ. The fact that the cells were smaller in number and size than spot 35 protein-reactive cells and further differed in localization distinguished the NSE-taste cells from the spot 35 protein cells. Serotonin-like immunoreactivity was detectable in the basal cells localized at the base of the taste epithelium. The immunoreactive cells were arranged in a circle at the periphery of the taste organ, each extending a slender process toward the center. The terminal portion of this process spread leaf-like; numerous fine projections protruded from its margin. The serotonin-immunoreactive cells appear to coincide with the monoamine-containing basal cells, which have been previously reported. Substance P-, calcitonin gene-related peptide (CGRP)-, vasoactive intestinal polypeptide (VIP)-, peptide HI (PHI)- or gastrin releasing peptide (GRP)-immunoreactive nerve fibers with varicosities were demonstrated within the taste organ. Some substance P-fibers ran along the bottom of the taste organ epithelium. A few thinner substance P-fibers ascended among the epithelial cells of the organ and terminated closely below the free surface. CGRP-fibers were found to correspond to substance P-fibers from their evidencing a double immunostaining. VIP- and PHI-fibers formed a meshwork in the basal area of the taste epithelium. Abundant substance P- and/or CGRP-fibers formed a meshwork among the ciliated cells located at the periphery of the taste organ. However, PHI- and GRP-fibers were detected less than substance P- and/or CGRP-fibers, though VIP-fibers were rarely present in the same region. Neurofilament protein- or tyrosine hydroxylase-like immunoreactivities were found in thick nerve fibers in the taste organ, whereas no immunoreactivities were present in cellular elements within the taste organ. The relationship between cellular and nervous elements in the taste organ was examined by double immunostainings.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗