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PubMed1990

Analysis of taste bud innervation based on glycoconjugate and peptide neuronal markers.

Primary gustatory neurons and their peripheral and central processes were evaluated histochemically in the geniculate and petrosal cranial nerve ganglia, lingual fungiform taste buds, and the nucleus of the solitary tract (NST) using 1) the plant lectin Griffonia simplicifolia I-B4, which binds specifically to D-galactose residues and selectively labels primarily nonpeptide-containing peripheral somatosensory neurons, and 2) calcitonin gene-related peptide immunoreactivity (CGRP-IR), which labels most peptidergic somatosensory neurons. Lectin reactivity was expressed by the vast majority of geniculate and petrosal ganglion cells, while CGRP-IR labeled very few cells. Peripherally, gustatory intragemmal axons penetrating fungiform taste buds were labeled only by the lectin and were depleted following chorda tympani transection. However, both lectin-labeled and CGRP-IR subpopulations of somatosensory perigemmal axons surrounding the taste buds were observed and were eliminated by section of the lingual nerve. The differing brainstem projection patterns of lectin-reactive vs. CGRP-IR central axons reflected their distinct ganglionic origins and the differential distributions of lectin reactivity and CGRP-IR among taste buds. Central lectin-reactive terminals were found throughout the entire rostrocaudal extent of the NST, including its rostral lateral "gustatory" zone; the extensive lectin-reactive visceral afferent projection can be presumed to have originated mainly from the large proportion of lectin-labeled neurons in the nodose ganglion. The lectin also prominently and selectively labeled the area postrema. CGRP-IR central terminals, however, was relatively sparse and restricted primarily to the caudal and medial "visceral" divisions of the NST. The results are discussed with respect to the possible functional implications of cell surface glycoconjugate expression by gustatory axons innervating taste bud receptor cells of the tongue.

Animals↗

PubMed1977

Effect of the length of the distal stump of transected nerve upon the rate of degeneration of taste buds.

The present work was carried out to study the effect of the length of the distal stump of transected nerve upon the rate of degeneration and the time course of disappearance of mammalian taste buds. Twelve adult rabbits were anaesthetized and the glossopharyngeal nerves of both sides were exposed and transected so as to leave a long distal stump on the right and a short one on the left side. The animals were sacrificed at different post-operative periods ranging from 2 to 14 days and the circumvallate and foliate papillae of both sides were examined. The taste buds on the side of the short distal stump always showed a greater decrease in number, size and cell contents than those of the side of the long distal stump. The taste buds on the side of the short distal stump disappeared earlier than those of the side of the long distal stump. Vallate taste buds disappeared earlier than the foliate: possible reasons for this were put forwards. The validity of the neurohumoral theory to the mammalian taste buds was discussed.

Animals↗

PubMed1982

Monoamines of taste buds in the fungiform and foliate papillae of the mouse.

After administration of monoamine precursors, taste buds in the fungiform and foliate papillae of the mouse were observed by means of electron microscopy and fluorescence histochemistry. The taste buds in the fungiform papillae differed in the ultrastructure of their apical regions from those in the foliate papillae, which contained the same taste buds as those described in the circumvallate papillae. The gustatory cells in both the fungiform and foliate papillae were capable of taking up monoamine precursors, although this ability was greater in the latter papillae. The results suggest that, not only in the circumvallate papillae but also in both the foliate and fungiform papillae, monoamines might be involved in neurotransmission from the gustatory cells to the nerves.

Amines↗

PubMed1995

Distribution of taste buds on the epiglottis of the rat and house shrew, with special reference to air and food pathways.

We investigated the positioning of the epiglottis in the pharyngo-laryngeal region and the distribution of taste buds on the epiglottis in the rat and house shrew, animals which have different feeding habits. In the fixed samples of both species, when the mouth was closed or slightly opened, the epiglottis was found to protrude into the nasopharyngeal hiatus above the soft palate. But it retracted from its position when the mouth was widely opened. In omnivorous rats (n = 6), the mean number (mean density +/- s.d.) of taste buds was 52 (12.6 +/- 2.2/mm2) on the laryngeal surface but only 4 (1.3 +/- 1.0/mm2) on the oral surface. The three-dimensional view was reconstructed from serial sections. The taste buds were distributed most densely close to the caudal base and became fewer toward the more rostral tip. In insectivorous house shrews (n = 2), 4 taste buds on average were found only on the laryngeal surface of the epiglottis. Epiglottal taste buds may work as chemosensory detectors to initiate the reflex reaction to protect the airway from oral substances during swallowing and drinking.

Animals↗

PubMed1990

Synapsin I-like immunoreactivity in nerve fibers associated with lingual taste buds of the rat.

Immunoreactivity to synapsin I, a neuronal phosphoprotein, was localized in free-floating tissue sections prepared from lingual tissue of rats. Many nerve fibers within the tissue exhibited clear immunoreactivity including motor endplates on striated muscle, autonomic fibers innervating blood vessels or glands, and sensory fibers innervating muscles or the lingual epithelium including taste buds. Numerous immunoreactive fibers occurred within each taste bud, with fewer, fine fibers being dispersed in the epithelium between taste buds. The majority of the intragemmal immunoreactive fibers extended throughout the taste buds most of the distance outward from the basal lamina toward the surface of the epithelium. Fine, perigemmal fibers reached nearly to the epithelial surface. Ultrastructural analysis of the immunoreactive sensory fibers revealed that synapsin I-immunoreactivity occurred diffusely throughout the cytoplasm, and heavily in association with microvesicles. The synaptic vesicles at the taste receptor cell-to-afferent fiber synapse were, however, not immunoreactive for synapsin I, although these vesicles fall into the size class shown to be immunoreactive in other systems. This absence of synapsin I may be a common property of vesicles in axonless short receptor cells.

Animals↗

PubMed1984

Immunoelectron-microscopic study on the fine structure of substance-P-containing fibers in the taste buds of the rat.

The fine structure of substance-P-like immunoreactive [SPI] fibers in the taste buds of the circumvallate papillae of the rat tongue was investigated by means of electron microscopy using the unlabeled antibody-enzyme method. Outside the epithelium, SPI and non-SPI fibers are surrounded by the cytoplasm of Schwann cells. When the SPI fibers enter the epithelium, they immediately lose this cytoplasmic sheath and begin to traverse the taste buds. Though passing through the taste buds, no profiles suggesting clear synaptic contact between SPI fibers and underlying cells are identified. SPI terminals are filled with small synaptic vesicles and contain a few mitochondria. No SPI-positive structures are found in nerve endings that make synaptic contact with type III cells, the gustatory receptor cells.

Animals↗

PubMed2001

IP(3) receptor type 3 and PLCbeta2 are co-expressed with taste receptors T1R and T2R in rat taste bud cells.

The Ca(2+) signaling cascade has been reported to be activated by many tastants in vertebrate taste systems. Recently we have shown that G(i2) and phospholipase Cbeta2 (PLCbeta2) are co-expressed in a subset of taste bud cells and are possibly involved in Ca(2+) triggering of taste signaling in rats. We report here that, as a component downstream of PLCbeta2, the type 3 isoform of the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R3) is specifically expressed in the same cells as PLCbeta2 in rat taste buds. We also show that cells expressing rT2R9, a probable cycloheximide receptor, are included among PLCbeta2- and IP(3)R3-positive cells, as in the case of rT1R2, a different type of taste receptor. Our findings indicate that PLCbeta2 and IP(3)R3 co-localize together with G(i2) as downstream components of two different types of taste receptors, T1R and T2R, in taste bud cells.

Animals↗

PubMed1988

Collateral reinnervation of taste buds after chronic sensory denervation: a morphological study.

Peripheral transganglionic transport of horseradish peroxidase (HRP) was used to label afferent fibers in the taste buds and lingual epithelium 2-12 weeks after chronic chorda tympani or combined chorda tympani-lingual nerve lesions. From 4-12 weeks after a chronic chorda tympani lesion, taste buds could be found. These were innervated by fibers from the ipsilateral lingual nerve. From 8-12 weeks after a chronic chorda tympani-lingual nerve lesion, nerve fibers from the contralateral lingual nerve could be found in a few taste buds on the denervated side of the tongue. Thus, collateral sprouting took place over the midline in this instance. These findings indicate that intact gustatory axons do not sprout into denervated taste buds, but trigeminal fibers in the lingual nerve do have this ability.

Animals↗

PubMed1979

Epithelial differentiation and taste buds in the soft palate of the monkey, Macaca irus.

A combination of light, transmission and scanning electron microscopy was employed to demonstrate the occurrence, arrangement and structure of taste buds in the oral mucosa of the soft palate of monkeys (Macaca irus). Taste buds are found in aggregates confined to 0.15 to 0.3 mm wide, round islands of keratinizing epithelium embedded in the normally non-keratinizing integument. Topography, configuration and structure of these epithelial islands and their taste buds are described, and the question of a developmental and functional interrelationship between epithelial differentiation and properties, and taste bud function is discussed.

Animals↗

PubMed1989

Evidence for a novel mechanism of binding and release of stimuli in the primate taste bud.

In previous work, we showed that thaumatin, an intensely sweet protein, binds to certain formed elements in taste pores of Rhesus monkey foliate papillae, namely, microvilli and small vesicles shed from microvilli, in addition to amorphous secretions (Farbman et al., 1987). We suggested that the taste bud responds to a thaumatin stimulus by shedding the small vesicles containing fragments of microvillar membrane bearing the stimulus-binding site complex. To examine this hypothesis further, we used electron microscopy to examine taste pores of both vallate and foliate papillae from Rhesus monkeys before or after stimulation with thaumatin or sucrose. We also recorded the neural activity from the glossopharyngeal nerve during stimulation with thaumatin, sucrose, citric acid, and NaCl. The results indicate (1) with no stimulation, vesicles are found in pores of foliate papilla taste buds much more frequently than in pores of vallate papilla buds, (2) in both types of papillae, stimulation with sucrose has no apparent effect on the number of pores containing vesicles, (3) stimulation with thaumatin elicits release of vesicles into pores of both foliate and vallate buds, (4) repeated stimulation of taste buds with thaumatin results in a declining neural response, not seen after repeated stimulation with sucrose, citric acid, or NaCl, and (5) stimulation with thaumatin suppresses the neural response to sucrose, but the reverse does not occur. The combined morphological and physiological data support our original hypothesis that, in response to thaumatin stimulation, binding sites on taste microvillar membranes may be shed as a stimulus-receptor complex into the pore. Alternatively, the binding sites may in some way be altered by the shedding of part of the membrane. The data suggest further that the binding site for sucrose may be close to that for thaumatin because it too is lost or altered after thaumatin stimulation. The reduced neural response after repeated thaumatin stimulation indicates that cells may have suffered a net loss of functional receptors or at least a loss in functional capacity of the receptors. The data indicate that the cellular mechanism for handling the protein sweetener, thaumatin, is strikingly different from the adsorption-desorption response to saccharide sweeteners.

Adaptation, Physiological↗

PubMed1987

Taste bud development in chickens (Gallus gallus domesticus).

Oral epithelium in the anterior mandibular glands region was examined in embryonic, hatchling, and mature chickens to establish the timing of morphologic events during taste bud ontogeny. Hematoxylin-and-eosin-stained sections (10 microns) from 27 Anak (broiler breed) chickens were examined serially, and buds were quantified at 16-20 days of incubation (E) and, posthatch days 1 and 50-60. Taste buds were first recognized at the beginning of E17 as small clusters of cells in the basal epithelium. Only spherical-shaped buds were observed on E17 and E18, and these spherical clusters never penetrated to the surface of the stratified epithelial layer. E19 marked a transitional stage when mature bud features began to emerge: the buds assumed a more elongate shape, several kinds of cells comprising the bud were distinguishable and the first taste pores were observed. During the ensuing embryonic days, buds continued to elongate commensurate with the deepening oral epithelium and by hatching virtually all buds opened to the oral cavity. No marked morphological changes in taste bud structure were observed on the day of hatching and at 50-60 days posthatching. Taste bud numbers increased dramatically during E17 and E18, peaked on E19, and remained relatively constant thereafter. It is concluded that the morphological sequence of taste bud development in chickens is similar to that in mammals. The timing of bud ontogeny, though initiated only during the third trimester in ovo, essentially is completed by hatching, thus providing the precocial hatchling with the sensory apparatus essential for gustatory experience.

Age Factors↗

PubMed1986

Ultrastructure of mouse vallate taste buds: II. Cell types and cell lineage.

The lifespan of cells in the mouse taste bud was examined with high-voltage electron microscopic (HVEM) autoradiography (ARG) after giving a single injection of 3H-thymidine. Animals were killed at 1 hour, 6 hours, 12 hours, 24 hours, and then daily up through 10 days postinjection. Lingual tissues were prepared for HVEM ARG so that we could identify and characterize labeled cells. Four categories of taste cells were identified: basal, dark, intermediate, and light cells. Basal cells were polygonal cells located near the basolateral sides of the taste buds and were characterized primarily by the presence of filaments attached to the nuclear envelope. Dark and light cells had the typical features described by previous authors. Intermediate cells had features in between those of dark and light cells. Over 90% of the cells labeled in the first 2 days following injection of 3H-thymidine were basal cells. Labeled dark cells appeared 6 hours after injection, reached their peak incidence at the fourth day postinjection, and then gradually decreased. Labeled intermediate cells were identified after the appearance of dark cells (12 hours) and reached a peak incidence at the fifth day after injection of 3H-thymidine. Lastly, labeled light cells were first observed on the fourth day postinjection and continued to increase until the tenth day, when they constituted 45% of the labeled cells. These data support the hypothesis that there is one cell line in the mouse vallate taste bud that undergoes morphological changes in its lifespan.

Animals↗

PubMed1984

Regeneration of taste buds after surgical excision of human vallate papilla.

We examined regenerative capabilities of vallate papilla and taste buds in a human adult after complete surgical excision of the papilla and underlying glands and muscle. Although trenchlike invaginations were observed in the healed epithelium biopsied 18 months after papilla removal, no normal papilla, characterized by definite boundaries and circular trench, could be found. A few regenerated taste buds, totaling slightly more than 7% of the number counted in the excised papilla, were present on the top surface of the regenerated epithelium. They were longer and narrower than the original buds, but otherwise appeared normal and most had distinct taste pores. Our findings confirm earlier reports that vallate papillae fail to reform if surgical excision is complete, but that taste buds can develop and will regenerate in tongue epithelium of nonhuman species without the presence of the original papilla.

Adult↗

PubMed2000

Co-expression of calcium signaling components in vertebrate taste bud cells.

In order to investigate the molecular mechanism of calcium signaling pathways common to the vertebrate gustatory systems, we have analyzed the expression of their molecular components. We first identified a phospholipase C (PLC) beta subtype expressed in the taste buds of pond loach (Misgurnus anguillicaudatus), designated DPLCbeta2, which is closely related to mammalian PLCbeta2 shown recently to be expressed in rat taste buds. The taste bud-specific expression of PLCbeta2 in a fish species as well as rat strongly suggests that PLCbeta2 mediates the tastant-induced second messenger response in taste buds, which is common to vertebrates. Next, we examined the correlation of gene expression of the candidate components leading to PLCbeta2 activation in rat circumvallate papillae, including G proteins, G(i2) and gustducin, and a G protein-coupled receptor, TR2. As a result, it was shown that the mRNAs for PLCbeta2 and G(i2) co-exist in the same cells, and PLCbeta2- and G(i2)-positive cells include both gustducin-positive cells and TR2-positive cells. However, no correlation was found between the expressions of TR2 and gustducin as reported previously. Our results thus indicate that a taste transduction pathway comprising TR2, G(i2) and PLCbeta2 occurs in a subset of taste cells.

Animals↗

PubMed2006

Dystonin deficiency reduces taste buds and fungiform papillae in the anterior part of the tongue.

The anterior part of the tongue was examined in wild type and dystonia musculorum mice to assess the effect of dystonin loss on fungiform papillae. In the mutant mouse, the density of fungiform papillae and their taste buds was severely decreased when compared to wild type littermates (papilla, 67% reduction; taste bud, 77% reduction). The mutation also reduced the size of these papillae (17% reduction) and taste buds (29% reduction). In addition, immunohistochemical analysis demonstrated that the dystonin mutation reduced the number of PGP 9.5 and calbindin D28k-containing nerve fibers in fungiform papillae. These data together suggest that dystonin is required for the innervation and development of fungiform papillae and taste buds.

Animals↗

PubMed1996

The taste of monosodium glutamate: membrane receptors in taste buds.

Receptor proteins for photoreception have been studied for several decades. More recently, putative receptors for olfaction have been isolated and characterized. In contrast, no receptors for taste have been identified yet by molecular cloning. This report describes experiments aimed at identifying a receptor responsible for the taste of monosodium glutamate (MSG). Using reverse transcriptase (RT)-PCR, we found that several ionotropic glutamate receptors are present in rat lingual tissues. However, these receptors also could be detected in lingual tissue devoid of taste buds. On the other hand, RT-PCR and RNase protection assays indicated that a G-protein-coupled metabotropic glutamate receptor, mGluR4, also is expressed in lingual tissues and is limited only to taste buds. In situ hybridization demonstrated that mGluR4 is detectable in 40-70% of vallate and foliate taste buds but not in surrounding nonsensory epithelium, confirming the localization of this metabotropic receptor to gustatory cells. Expression of mGluR4 in taste buds is higher in preweaning rats compared with adult rats. This may correspond to the known higher sensitivity to the taste of MSG in juvenile rodents. Finally, behavioral studies have indicated that MSG and L-2-amino-4-phosphonobutyrate (L-AP4), a ligand for mGluR4, elicit similar tastes in rats. We conclude that mGluR4 may be a chemosensory receptor responsible, in part, for the taste of MSG.

Amino Acid Sequence↗

PubMed1998

The timing of alpha-gustducin expression during cell renewal in rat vallate taste buds.

The G protein subunit alpha-gustducin is expressed in a subset of light (Type II) but not in dark (Type I) cells in rat vallate taste buds. The thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) is incorporated into DNA during the S-phase of the cell cycle and can be used to determine the time of origin of a cell. In this study, 31 rats were injected with BrdU (50 mg/kg i.p.) and perfused at various times, from 2.5 to 10.5 days, following BrdU administration. Vallate papillae were embedded in polyester wax, cut into 4 microm transverse sections, and characterized with antibodies to BrdU and alpha-gustducin. Sections were processed for indirect immunofluorescence or with an immunoperoxidase procedure. From immunoperoxidase material on 21 rats, counts of alpha-gustducin- and BrdU-labeled cells were obtained from 300-800 taste bud profiles at each survival time; a total of 4122 taste bud profiles were examined. Cells with nuclei immunoreactive for BrdU occurred within the taste buds at 2.5 days and double-labeled cells were clearly evident at 3.5 days; a small number of double-labeled cells were seen as early as 2.5 days. Double-labeled cells reached a peak at 6.5 days and did not decline significantly by 10.5 days. Cells labeled for BrdU but not alpha-gustducin peaked at 5.5 days and showed a significant decline by 8.5 days. These latter cells included light cells not expressing alpha-gustducin and dark cells, which have previously been shown to have a shorter life span than light cells. These data suggest that expression of alpha-gustducin appears very early in a cell's life span and that these cells are longer lived than many of the cells that do not express this G protein.

Analysis of Variance↗

PubMed1984

[Morphological changes in different cells of the taste bud].

Application of colchicine to the glossopharyngeal nerve produces the diminution of the area of foliate papillae taste buds and the cell number in them. At the same time the correlation of morphologically different cells of the taste bud changes, since the content of intermediate cells increases and that of dark cells descends. The data obtained do not confirm the hypothesis about the appurtenance of the morphologically different cells of the taste bud to the same cell type.

Animals↗