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

Physiology of Taste Processing in the Tongue, Gut, and Brain

The gustatory system detects and informs us about the nature of various chemicals we put in our mouth. Some of these have nutritive value (sugars, amino acids, salts, and fats) and are appetitive and avidly ingested, whereas others (atropine, quinine, nicotine) are aversive and rapidly rejected. However, the gustatory system is mainly responsible for evoking the perception of a limited number of qualities that humans taste as sweet, umami, bitter, sour, salty, and perhaps fat [free fatty acids (FFA)] and starch (malto-oligosaccharides). The complex flavors and mouthfeel that we experience while eating food result from the integration of taste, odor, texture, pungency, and temperature. The latter three arise primarily from the somatosensory (trigeminal) system. The sensory organs used for detecting and transducing many chemicals are found in taste buds (TBs) located throughout the tongue, soft palate esophagus, and epiglottis. In parallel with the taste system, the trigeminal nerve innervates the peri-gemmal epithelium to transmit temperature, mechanical stimuli, and painful or cooling sensations such as those produced by changes in temperature as well as from chemicals like capsaicin and menthol, respectively. This article gives an overview of the current knowledge about these TB cells' anatomy and physiology and their trigeminal induced sensations. We then discuss how taste is represented across gustatory cortices using an intermingled and spatially distributed population code. Finally, we review postingestion processing (interoception) and central integration of the tongue-gut-brain interaction, ultimately determining our sensations as well as preferences toward the wholesomeness of nutritious foods. © 2021 American Physiological Society. Compr Physiol 11:1-35, 2021.

Capacitance measurements of regulated exocytosis in mouse taste cells

Exocytosis, consisting of the merger of vesicle and plasma membrane, is a common mechanism used by different types of nucleated cells to release their vesicular contents. Taste cells possess vesicles containing various neurotransmitters to communicate with adjacent taste cells and afferent nerve fibers. However, whether these vesicles engage in exocytosis on a stimulus is not known. Since vesicle membrane merger with the plasma membrane is reflected in plasma membrane area fluctuations, we measured membrane capacitance (C(m)), a parameter linearly related to membrane surface area. To investigate whether taste cells undergo regulated exocytosis, we used the compensated tight-seal whole-cell recording technique to monitor depolarization-induced changes in C(m) in the different types of taste cells. To identify taste cell types, mice expressing green fluorescent protein from the TRPM5 promoter or from the GAD67 promoter were used to discriminate type II and type III taste cells, respectively. Moreover, the cell types were also identified by monitoring their voltage-current properties. The results demonstrate that only type III taste cells show significant depolarization-induced increases in C(m), which were correlated to the voltage-activated calcium currents. The results suggest that type III, but neither type II nor type I cells exhibit depolarization-induced regulated exocytosis to release transmitter and activate gustatory afferent nerve fibers.

The sweet taste quality is linked to a cluster of taste fibers in primates: lactisole diminishes preference and responses to sweet in S fibers (sweet best) chorda tympani fibers of M. fascicularis monkey

Background

Psychophysically, sweet and bitter have long been considered separate taste qualities, evident already to the newborn human. The identification of different receptors for sweet and bitter located on separate cells of the taste buds substantiated this separation. However, this finding leads to the next question: is bitter and sweet also kept separated in the next link from the taste buds, the fibers of the taste nerves? Previous studies in non-human primates, P. troglodytes, C. aethiops, M. mulatta, M. fascicularis and C. jacchus, suggest that the sweet and bitter taste qualities are linked to specific groups of fibers called S and Q fibers. In this study we apply a new sweet taste modifier, lactisole, commercially available as a suppressor of the sweetness of sugars on the human tongue, to test our hypothesis that sweet taste is conveyed in S fibers.

Results

We first ascertained that lactisole exerted similar suppression of sweetness in M. fascicularis, as reported in humans, by recording their preference of sweeteners and non- sweeteners with and without lactisole in two-bottle tests. The addition of lactisole significantly diminished the preference for all sweeteners but had no effect on the intake of non-sweet compounds or the intake of water. We then recorded the response to the same taste stimuli in 40 single chorda tympani nerve fibers. Comparison between single fiber nerve responses to stimuli with and without lactisole showed that lactisole only suppressed the responses to sweeteners in S fibers. It had no effect on the responses to any other stimuli in all other taste fibers.

Conclusion

In M. fascicularis, lactisole diminishes the attractiveness of compounds, which taste sweet to humans. This behavior is linked to activity of fibers in the S-cluster. Assuming that lactisole blocks the T1R3 monomer of the sweet taste receptor T1R2/R3, these results present further support for the hypothesis that S fibers convey taste from T1R2/R3 receptors, while the impulse activity in non-S fibers originates from other kinds of receptors. The absence of the effect of lactisole on the faint responses in some S fibers to other stimuli as well as the responses to sweet and non-sweet stimuli in non-S fibers suggest that these responses originate from other taste receptors.

Amiloride-sensitive channels in type I fungiform taste cells in mouse

Background

Taste buds are the sensory organs of taste perception. Three types of taste cells have been described. Type I cells have voltage-gated outward currents, but lack voltage-gated inward currents. These cells have been presumed to play only a support role in the taste bud. Type II cells have voltage-gated Na⁺ and K⁺ current, and the receptors and transduction machinery for bitter, sweet, and umami taste stimuli. Type III cells have voltage-gated Na⁺, K⁺, and Ca²⁺ currents, and make prominent synapses with afferent nerve fibers. Na⁺ salt transduction in part involves amiloride-sensitive epithelial sodium channels (ENaCs). In rodents, these channels are located in taste cells of fungiform papillae on the anterior part of the tongue innervated by the chorda tympani nerve. However, the taste cell type that expresses ENaCs is not known. This study used whole cell recordings of single fungiform taste cells of transgenic mice expressing GFP in Type II taste cells to identify the taste cells responding to amiloride. We also used immunocytochemistry to further define and compare cell types in fungiform and circumvallate taste buds of these mice.

Results

Taste cell types were identified by their response to depolarizing voltage steps and their presence or absence of GFP fluorescence. TRPM5-GFP taste cells expressed large voltage-gated Na⁺ and K⁺ currents, but lacked voltage-gated Ca²⁺ currents, as expected from previous studies. Approximately half of the unlabeled cells had similar membrane properties, suggesting they comprise a separate population of Type II cells. The other half expressed voltage-gated outward currents only, typical of Type I cells. A single taste cell had voltage-gated Ca²⁺ current characteristic of Type III cells. Responses to amiloride occurred only in cells that lacked voltage-gated inward currents. Immunocytochemistry showed that fungiform taste buds have significantly fewer Type II cells expressing PLC signalling components, and significantly fewer Type III cells than circumvallate taste buds.

Conclusion

The principal finding is that amiloride-sensitive Na⁺ channels appear to be expressed in cells that lack voltage-gated inward currents, likely the Type I taste cells. These cells were previously assumed to provide only a support function in the taste bud.

Amiloride-sensitive channels in type I fungiform taste cells in mouse

BACKGROUND

Taste buds are the sensory organs of taste perception. Three types of taste cells have been described. Type I cells have voltage-gated outward currents, but lack voltage-gated inward currents. These cells have been presumed to play only a support role in the taste bud. Type II cells have voltage-gated Na+ and K+ current, and the receptors and transduction machinery for bitter, sweet, and umami taste stimuli. Type III cells have voltage-gated Na+, K+, and Ca2+ currents, and make prominent synapses with afferent nerve fibers. Na+ salt transduction in part involves amiloride-sensitive epithelial sodium channels (ENaCs). In rodents, these channels are located in taste cells of fungiform papillae on the anterior part of the tongue innervated by the chorda tympani nerve. However, the taste cell type that expresses ENaCs is not known. This study used whole cell recordings of single fungiform taste cells of transgenic mice expressing GFP in Type II taste cells to identify the taste cells responding to amiloride. We also used immunocytochemistry to further define and compare cell types in fungiform and circumvallate taste buds of these mice.

RESULTS

Taste cell types were identified by their response to depolarizing voltage steps and their presence or absence of GFP fluorescence. TRPM5-GFP taste cells expressed large voltage-gated Na+ and K+ currents, but lacked voltage-gated Ca2+ currents, as expected from previous studies. Approximately half of the unlabeled cells had similar membrane properties, suggesting they comprise a separate population of Type II cells. The other half expressed voltage-gated outward currents only, typical of Type I cells. A single taste cell had voltage-gated Ca2+ current characteristic of Type III cells. Responses to amiloride occurred only in cells that lacked voltage-gated inward currents. Immunocytochemistry showed that fungiform taste buds have significantly fewer Type II cells expressing PLC signalling components, and significantly fewer Type III cells than circumvallate taste buds.

CONCLUSION

The principal finding is that amiloride-sensitive Na+ channels appear to be expressed in cells that lack voltage-gated inward currents, likely the Type I taste cells. These cells were previously assumed to provide only a support function in the taste bud.

Qualitative and quantitative differences between taste buds of the rat and mouse

Background

Numerous electrophysiological, ultrastructural, and immunocytochemical studies on rodent taste buds have been carried out on rat taste buds. In recent years, however, the mouse has become the species of choice for molecular and other studies on sensory transduction in taste buds. Do rat and mouse taste buds have the same cell types, sensory transduction markers and synaptic proteins? In the present study we have used antisera directed against PLCβ2, α-gustducin, serotonin (5-HT), PGP 9.5 and synaptobrevin-2 to determine the percentages of taste cells expressing these markers in taste buds in both rodent species. We also determined the numbers of taste cells in the taste buds as well as taste bud volume.

Results

There are significant differences (p < 0.05) between mouse and rat taste buds in the percentages of taste cells displaying immunoreactivity for all five markers. Rat taste buds display significantly more immunoreactivity than mice for PLCβ2 (31.8% vs 19.6%), α-gustducin (18% vs 14.6%), and synaptobrevin-2 (31.2% vs 26.3%). Mice, however, have more cells that display immunoreactivity to 5-HT (15.9% vs 13.7%) and PGP 9.5 (14.3% vs 9.4%). Mouse taste buds contain an average of 85.8 taste cells vs 68.4 taste cells in rat taste buds. The average volume of a mouse taste bud (42,000 μm³) is smaller than a rat taste bud (64,200 μm³). The numerical density of taste cells in mouse circumvallate taste buds (2.1 cells/1000 μm³) is significantly higher than that in the rat (1.2 cells/1000 μm³).

Conclusion

These results suggest that rats and mice differ significantly in the percentages of taste cells expressing signaling molecules. We speculate that these observed dissimilarities may reflect differences in their gustatory processing.

Discrete innervation of murine taste buds by peripheral taste neurons

The peripheral taste system likely maintains a specific relationship between ganglion cells that signal a particular taste quality and taste bud cells responsive to that quality. We have explored a measure of the receptoneural relationship in the mouse. By injecting single fungiform taste buds with lipophilic retrograde neuroanatomical markers, the number of labeled geniculate ganglion cells innervating single buds on the tongue were identified. We found that three to five ganglion cells innervate a single bud. Injecting neighboring buds with different color markers showed that the buds are primarily innervated by separate populations of geniculate cells (i.e., multiply labeled ganglion cells are rare). In other words, each taste bud is innervated by a population of neurons that only connects with that bud. Palate bud injections revealed a similar, relatively exclusive receptoneural relationship. Injecting buds in different regions of the tongue did not reveal a topographic representation of buds in the geniculate ganglion, despite a stereotyped patterned arrangement of fungiform buds as rows and columns on the tongue. However, ganglion cells innervating the tongue and palate were differentially concentrated in lateral and rostral regions of the ganglion, respectively. The principal finding that small groups of ganglion cells send sensory fibers that converge selectively on a single bud is a new-found measure of specific matching between the two principal cellular elements of the mouse peripheral taste system. Repetition of the experiments in the hamster showed a more divergent innervation of buds in this species. The results indicate that whatever taste quality is signaled by a murine geniculate ganglion neuron, that signal reflects the activity of cells in a single taste bud.

Modulation of taste peripheral signal through interpapillar inhibition in hamsters

Single taste buds from fungiform papillae were iontophoretically stimulated with chemicals filling glass microelectrodes while a single unit was recorded in the taste pore of a neighbor papilla. High signal-to-noise ratio responses were observed in the recorded papilla as antidromic action potentials. These responses were possibly modulated by the simultaneous stimulation of another adjacent papilla. A decrease in the frequency of firing and/or both decrementing spikes were observed during such dual papillae stimulations. These inhibitory effects were not modified by the section of the chordo-lingual nerve, suggesting the tongue is able to process the gustatory information thanks to interpapillar negative feedback, prior to transmitting the signal to the central nervous system. Branched chorda tympani fibers can account for responses observed for single papillae stimulations; inhibitions and decrementing spikes may suggest the contribution of another mechanism of interaction between two different single fibers.

Synaptobrevin-2-like immunoreactivity is associated with vesicles at synapses in rat circumvallate taste buds

Synaptobrevin is a vesicle-associated membrane protein (VAMP) that is believed to play a critical role with presynaptic membrane proteins (SNAP-25 and syntaxin) during regulated synaptic vesicle docking and exocytosis of neurotransmitter at the central nervous system. Synaptic contacts between taste cells and nerve processes have been found to exist, but little is known about synaptic vesicle docking and neurotransmitter release at taste cell synapses. Previously we demonstrated that immunoreactivity to SNAP-25 is present in taste cells with synapses. Our present results show that synaptobrevin-2-like immunoreactivity (-LIR) is present in approximately 35% of the taste cells in rat circumvallate taste buds. Synaptobrevin-2-LIR colocalizes with SNAP-25-, serotonin-, and protein gene product 9.5-LIR. Synaptobrevin-2-LIR also colocalizes with immunoreactivity for type III inositol 1,4,5-triphosphate receptor (IP3R3), a taste-signaling molecule in taste cells. All IP3R3-LIR taste cells express synaptobrevin-2-LIR. However, approximately 27% of the synaptobrevin-2-LIR taste cells do not display IP3R3-LIR. We believe, based on ultrastructural and biochemical features, that both type II and type III taste cells display synaptobrevin-2-LIR. All of the synapses that we observed from taste cells onto nerve processes express synaptobrevin-2-LIR, as well as some taste cells without synapses. By using colloidal gold immunoelectron microscopy, we found that synaptobrevin-2-LIR is associated with synaptic vesicles at rat taste cell synapses. The results of this study suggest that soluble NSF attachment receptor (SNARE) machinery may control synaptic vesicle fusion and exocytosis at taste cell synapses.

Voltage-gated channels involved in taste responses and characterizing taste bud cells in mouse soft palates

Taste bud cells (TBCs) on soft palates differ from those on tongues in innervation and chemosensitivity. We investigated voltage-gated channels involved in the taste responses of TBCs on mouse soft palates under in-situ tight-seal voltage/current-clamp conditions. Under the cell-attached mode, TBCs spontaneously fired action currents, which were blocked by application of 1 microM TTX to TBC basolateral membranes. Firing frequencies increased in response to taste substances applied to TBC receptor membranes. Under the whole-cell clamp mode, as expected, TBCs produced various voltage-gated currents such as TTX-sensitive Na+ currents (INa), outward currents (Iout) including TEA-sensitive and insensitive currents, inward rectifier K+ currents (Iir), and Ca2+ currents including T-type, P/Q-type, and L-type Ca2+ currents. We classified TBCs into three types based on the magnitude of their voltage-gated Na+ currents and membrane capacitance. HEX type (60% of TBCs examined) was significantly larger in Na+ current magnitude and smaller in membrane capacitance than LEX type (23%). NEX type (17%) had no Na+ currents. HEX type was equally distributed within single taste buds, while LEX type was centrally distributed, and NEX type was peripherally distributed. There were correlations between these electrophysiological cell types and morphological cell types determined by three-dimensional reconstruction. The present results show that soft palate taste buds contain TBCs with different electrophysiological properties, and suggest that their co-operation is required in taste transduction.

Bovine circumvallate taste buds: taste cell structure and immunoreactivity to alpha-gustducin

The taste buds of bovine circumvallate papillae were investigated under light and electron microscopy both by histological and immunohistochemical methods. Taste buds existed in the inner epithelium of the trench of the papillae. Under electron microscopy, two types of taste cells, type I and type II, could be classified according to the existence of dense-cored vesicles and cytoplasmic density. Type I had electron-lucent cytoplasm and possessed many electron-dense cored vesicles in the apical cytoplasm. It was considered that the electron-dense materials of the vesicles were released and constituted the pore substance. This type of cell possessed long and thick apical processes in the taste pore. Type II had denser electron cytoplasm compared with that of type I and possessed many electron-lucent vesicles in the apical cytoplasm. This type of cell possessed microvilli in the taste pore. To know the immunoreactivity to alpha-gustducin in bovine circumvallate taste buds, we used the immunoblotting method and the immunohistochemical method. The alpha-gustducin reaction band at 40 kDa was displayed in the specimen of Western blots. The immunohistochemical property of the antiserum to alpha-gustducin was investigated by using the avidin-biotin complex (ABC) method and the 1.4-nm gold and silver enhancement methods. A subset of taste cells showed the immunoreactivity under light microscopy. The electron microscopic specimens with the 1.4-nm gold and silver enhancement method revealed that only type II cells exhibited the alpha-gustducin immunoreactivity.

Acetylcholine increases intracellular Ca2+ in taste cells via activation of muscarinic receptors

Previous studies suggest that acetylcholine (ACh) is a transmitter released from taste cells as well as a transmitter in cholinergic efferent neurons innervating taste buds. However, the physiological effects on taste cells have not been established. I examined effects of ACh on taste-receptor cells by monitoring [Ca2+]i. ACh increased [Ca2+]i in both rat and mudpuppy taste cells. Atropine blocked the ACh response, but D-tubocurarine did not. U73122, a phospholipase C inhibitor, and thapsigargin, a Ca2+-ATPase inhibitor that depletes intracellular Ca2+ stores, blocked the ACh response. These results suggest that ACh binds to M1/M3/M5-like subtypes of muscarinic ACh receptors, causing an increase in inositol 1,4,5-trisphosphate and subsequent release of Ca2+ from the intracellular stores. A long incubation with ACh induced a transient response followed by a sustained phase of [Ca2+]i increase. In Ca2+-free solution, the sustained phases disappeared, suggesting that Ca2+ influx is involved in the sustained phase. Depletion of Ca2+ stores by thapsigargin alone induced Ca2+ influx. These findings suggest that Ca2+ store-operated channels may be present in taste cells and that they may participate in the sustained phase of [Ca2+]i increase. Immunocytochemical experiments indicated that the M1 subtype of muscarinic receptors is present in both rat and mudpuppy taste cells.

Three-dimensional high voltage electron microscopy of thick biological specimens

The procedures recently developed in our laboratory to observe three-dimensional structures of cell organelles in thick biological specimens by means of high voltage electron microscopy are reviewed. Thick biological specimens such as whole mount cultured cells seeded and grown on grid meshes in culture vessels or thick sections cut from embedded tissues and stained by histochemical reactions can be readily observed three-dimensionally by high voltage transmission electron microscopy at 400-1000kV. Cultured cells used were both primary cultures from animal tissues and established cell lines maintained in our laboratory. The livers of adult Wistar rats were isolated by collagenase perfusion, and hepatocytes were suspended in a Leibovitz medium and seeded on formval coated gold grid meshes in Petri dishes, incubated in a CO(2) incubator in a humidified atmosphere containing 5% CO(2) in air at 37 degrees C for a few days. Established cell lines, CHO-K1 cells, were cultured in Ham's F12 medium, while HeLa cells were cultured in Eagle's MEM under the same condition. Some of the cells were cultured under experimental conditions such as hepatocyte culture in the medium containing peroxisome proliferating agents such as clofibrate or bezafibrate and some of them were labeled with (3)H-thymidine, (3)H-uridine, (3)H-labeled precursors and (14)C-bezafibrate. Also some cells were incubated in medium containing HRP to induce pinocytosis. All the whole mount cultured cells on grid meshes were prefixed in buffered 2.5% glutaraldehyde, stained with various histochemical reactions and postfixed in 1% osmium tetroxide. The histochemical reactions used were glucose-6-phosphatase (G-6-Pase), thiamine pyrophosphatase (TPPase), cytochrome oxidase, acid phosphatase (AcPase), DAB, ZIO, PA-TCH-SP reactions and radioautography was performed after labeling with radiolabeled compounds. The whole mount cultured cells were dried in a critical point dryer and were observed with JEOL JEM-4000EX or Hitachi H-1250M high voltage electron microscopes at 400-1000kV. By tilting the specimens' stereo-pair micrographs were recorded and they were observed with stereoscopes. Rat liver, mouse intestine and pancreas tissues, fixed and stained as above, were embedded in Epoxy resin, thick sectioned at 1-2 microm and were observed as for the whole mount cultured cells at 1000kV. Stereo-pairs were further analyzed with an image analyzer JEOL JIM-5000 (JEOL, Tokyo, Japan), producing two contour lines plotted from the micrographs at a thickness of 0.2 microm and were observed with anaglyph type glasses, demonstrating the depth or heights of respective cell organelles. The results show that whole mount cultured cells and thick sections stained with histochemical reactions reveal cell organelles corresponding to marker enzymes, such as G-6-Pase in endoplasmic reticulum, TPPase and ZIO in Golgi apparatus, cytochrome oxidase in mitochondria, AcPase in lysosomes, DAB in peroxisomes and pinocytotic vesicles, PA-TCH-SP in secretory granules, (3)H-thymidine and (3)H-uridine in nuclei, (3)H-animo acids in endoplasmic reticulum and secretory granules, (14)C-bezafibrate around ER and peroxisomes. The ultrastructure of these cell organelles as well as the structural relationship between them can be demonstrated three-dimensionally with stereo-pair images. Overall, these procedures are useful for analyzing stereologically the ultrastructure of cell organelles in cells and tissues.

Acid and salt responses in mouse taste cells

Acid and salt responses of taste cells induced by natural stimulation have not been investigated with exception of early studies with conventional microelectrode method, due to the toxicity of high concentration of salt or low pH of acid stimuli applied to isolated taste cells. This indicates that the application of rapid and localized stimulation to the apical membrane of taste cells is necessary for recording of natural responses to salt or acid stimuli using patch clamp technique. Recently we have developed a procedure to accomplish the quasi-natural condition including rapid, localized stimuli to the apical receptive membrane and the maintenance of taste bud polarity. In this review, we present our recent results obtained under quasi-natural condition using patch clamp techniques, comparing with the previously proposed hypothesis. One of our major finding is the fact that the acid-induced responses of taste cells in the mouse fungiform papillae are never suppressed by amiloride but an apical proton-gated conductance and a basolateral Cl(-) conductance possibly contribute to sour transduction. On the other hand, salt-induced responses are suppressed by amiloride, although the salt-induced responses recorded from a single cell involve both amiloride-sensitive and -insensitive components. Furthermore, the amiloride-insensitive component of salt responses possibly consists of multiple subcomponents including an apical sodium-gated nonselective cation conductance and a basolateral Cl(-) conductance. Recent reports also support the hypothesis that both acid and salt responses require specific receptor mechanisms of inorganic cations such as H(+) and Na(+) at the apical receptive membrane.

Taste cells with synapses in rat circumvallate papillae display SNAP-25-like immunoreactivity

SNAP-25 is a 25 kDa protein believed to be involved in the processes of membrane fusion and exocytosis associated with neurotransmitter release. In the present study we present evidence that SNAP-25-like immunoreactivity can be used as a marker for taste cells with synapses in rat circumvallate papillae. SNAP-25 immunoreactivity is present in most intragemmal nerve processes and a small subset of taste cells. Intense immunoreactivity is associated with the nerve plexus located below the base of the taste bud. Of a total of 87 taste cells with synapses onto nerve processes, 80 of the presynaptic taste cells had SNAP-25 immunoreactivity. The association of SNAP-25 immunoreactivity with taste cells possessing synapses suggests that these cells may be gustatory receptor cells. Because this SNAP-25 antibody can label taste cells with synapses, it may also serve as a useful tool for future studies correlating structure with function in the taste bud.

Taste cell function. Structural and biochemical implications

Maintenance of constant relations between receptor cell types and branching from a single gustatory nerve fiber during normal cell turnover and regeneration requires cell-cell recognition likely mediated by timed expression of molecules at surfaces of taste bud cells, nerve endings, and in extracellular matrix. These processes assure stability of gustatory quality representation during intragemmal remodeling. Coincidentally, features of gemmal cell lifespan, including elongation, differentiation, and migration prior to apoptosis, must also be orchestrated by molecular signals. This article reviews the potential roles played by a variety of molecular markers for some relevant classes of proteins, peptides, and enzymes, which were presumed to be important for carrying out these gustatory cellular functions.

Organization of geniculate and trigeminal ganglion cells innervating single fungiform taste papillae: a study with tetramethylrhodamine dextran amine labeling

Single gustatory nerve fibers branch and innervate several taste buds. In turn, individual taste buds may receive innervation from numerous gustatory nerve fibers. To evaluate the pattern of sensory innervation of fungiform papilla-bearing taste buds, we used iontophoretic fluorescent injection to retrogradely label the fibers that innervate single taste papillae in the hamster. For each animal, a single taste papilla was injected through the gemmal pore with 3.3% tetramethylrhodamine dextran amine. Fungiform papillae either at the tongue tip (0.5-1.5 mm from the tip) or more posteriorly (1.5-3.0 mm from the tip) were injected. After one to seven days survival, the geniculate and trigeminal ganglia and the tongue were sectioned and examined for labeled cells and fibers, respectively. Analysis of the number and topographic distribution of geniculate cells innervating single taste papillae, shows that: (i) 15 +/- 4 (S.D.) ganglion cells converge to innervate a single fungiform taste bud; (ii) more ganglion cells innervate anterior- (range: 13-35 cells) than posterior-lying buds (range: five to 12 cells), which, in part, may be related to bud volume (microm3); and (iii) ganglion somata innervating a single taste bud are scattered widely within the geniculate ganglion. Analysis of labeled fibers in the tongue demonstrated that two to eight taste buds located within 2 mm of the injected taste bud share collateral innervation with the injected taste bud. Since all buds with labeled fibers were located in close proximity (within a 2-mm radius), widely dispersed geniculate ganglion cells converge to innervate closely spaced fungiform taste buds. Trigeminal ganglion (mandibular division) cells were also labeled in every case and, as with the geniculate ganglion, a dispersed cell body location and collateralization pattern among papillae were observed. This study shows that iontophoresis of tetramethylrhodamine dextran amine, selectively applied to individual peripheral receptor end-organs, effectively locates sensory ganglion cells in two different ganglia that project to these sites. Moreover, the marker demonstrates collateral branches of sensory afferents associated with the labeled fibers and the nearby receptor areas innervated by these collaterals. The labeling of single or clusters of receptor cells, as well as identified sensory afferents, affords future possibilities for combining this technique with immunocytochemistry to establish the relationships of innervation patterns with neurotransmitters and neurotropic substances within identified cells.

Electrophysiological characterization of a putative supporting cell isolated from the frog taste disk

Chemosensory cells in vertebrate taste organs have two obvious specializations: an apical membrane with access to the tastants occurring in food, and synapses with sensory axons. In many species, however, certain differentiated taste cells have access to the tastants but lack any synaptic contacts with axons, and a supportive rather than chemosensory function has been attributed to them. Until now, no functional data are available for these taste cells. To begin to understand their role in taste organ physiology, we have characterized with patch-clamp recording techniques the electrophysiological properties of a putative supporting cell-the so-called wing cell-isolated from frog taste disks. Wing cells were distinguished from chemosensory elements by the presence of a typical, sheet-like apical process. Their resting potential was approximately -52 mV, and the average input resistance was 4.8 GOmega. Wing cells possessed voltage-gated Na+ currents sensitive to TTX, and an inactivating, voltage-gated K+ current sensitive to TEA. Current injections elicited single action potentials but not repetitive firing. We found no evidence for voltage-gated Ca2+ currents under various experimental conditions. Amiloride-sensitive Na+ channels, thought to be involved in Na+ chemotransduction, were present in wing cells. Many of the membrane properties of wing cells have been also reported for chemosensory taste cells. The presence of ion channels in wing cells might be suggestive of a role in controlling the microenvironment inside the taste organs or the functioning of chemosensory cells or both. In addition, they might participate directly in the sensory transduction events by allowing loop currents to flow inside the taste organs during chemostimulation.

A rapid method for combined laser scanning confocal microscopic and electron microscopic visualization of biocytin or neurobiotin-labeled neurons

Intracellular recording and dye filling are widely used to correlate the morphology of a neuron with its physiology. With laser scanning confocal microscopy, the complex shapes of labeled neurons in three dimensions can be reconstructed rapidly, but this requires fluorescent dyes. These dyes are neither permanent nor electron dense and therefore do not allow investigation by electron microscopy. Here we report a technique that quickly and easily converts a fluorescent label into a more stable and electron-dense stain. With this technique, a neuron is filled with Neurobiotin or biocytin, reacted with fluorophore-conjugated avidin, and imaged by confocal microscopy. To permit long-term storage or EM study, the fluorescent label is then converted to a stable electron-dense material by a single-step conversion using a commercially available ABC kit. We find that the method, which apparently relies on recognition of avidin's excess biotin binding sites by the biotin-peroxidase conjugate, is both faster and less labor intensive than photo-oxidation procedures in common use. The technique is readily adaptable to immunocytochemistry with biotinylated probes, as we demonstrate using anti-serotonin as an example.

In situ tight-seal recordings of taste substance-elicited action currents and voltage-gated Ba currents from single taste bud cells in the peeled epithelium of mouse tongue

We investigated the taste responses of single taste-bud cells (TBCs) in mice by applying stimuli only on receptor membranes acclimated to deionized water under tight-seal cell-attached voltage-clamp conditions, while their basolateral membranes were irrigated with a physiological saline solution. For this irrigation, we developed a new method: a peeled-tongue epithelium with TBCs mounted on a recording chamber where the peeled epithelium separated the irrigating solutions for each membrane as it separated in situ. Although no quinine-elicited action potentials had been reported, TBCs elicited a long-lasting train of biphasic currents derived from the action potentials in response to 10 mM quinine, in addition to responses to 10 mM HCl, or 200 mM NaCl dissolved in deionized water. These results indicate that quinine as well as HCl and NaCl depolarizes TBCs and generate action potentials. Under whole-cell recording conditions, TBCs generated action potentials, and voltage-gated currents such as LVA and HVA Ca currents, TTX-sensitive Na currents, and TEA/4-AP-sensitive K currents on depolarization. These voltage-gated channels were shown to exist predominantly on the basolateral membranes. We discussed the receptor mechanisms and the role of taste substance-elicited action potentials.

Responses to glutamate in rat taste cells

We studied taste transduction in sensory receptor cells. Specifically, we examined the actions of glutamate, a significant taste stimulus, on the membrane properties of taste cells by applying whole cell patch-clamp techniques to cells in rat taste buds isolated from foliate and vallate papillae. In 55 of 91 taste cells, bath-applied glutamate, at concentrations that elicit taste responses in the intact animal (10-20 mM), produced one of two different responses when the cell membrane was held near its presumed resting potential, -85 mV. "Sustained" glutamate responses were observed in the majority of taste cells (51 of 55) and consisted of an outward current (reduction of the maintained inward current). Sustained glutamate responses were voltage dependent, were decreased by membrane depolarization, and were accompanied by a reduction in membrane conductance. An analysis of the reversal potential of sustained responses in different ionic conditions and the effect of ion substitutions suggested that the currents were carried by cations. The data suggest that sustained responses are mediated by the closure of nonselective cation channels. Other taste cells (4 of 55) responded to glutamate with a transient inward current--so-called "transient" responses. Transient glutamate responses were voltage dependent and Na+ dependent, and appeared to be generated by nonspecific cation channels activated by glutamate. L(+)-2-amino-4-phosphonobutyric acid (L-AP4), a specific agonist of a metabotropic glutamate receptor (mGluR4) recently identified in rat taste cells and believed to be involved in taste transduction, mimicked the sustained glutamate responses. These findings indicate that glutamate, at concentrations at or slightly above threshold for taste in rats, produces two different membrane currents. The properties of these two responses suggest that there may be two different sets of nonspecific cation channels in taste cells, one closed by glutamate (sustained response) and the other opened (transient response). Our findings on the effect of L-AP4 suggest that the sustained response is the membrane mechanism mediating, at least in part, taste transduction for glutamate.

Light and dark cells of rat vallate taste buds are morphologically distinct cell types

Cells of mammalian taste buds have been classified into morphological types based on ultrastructural criteria, but investigators have disagreed as to whether these are distinct cell types or the extremes of a continuum. To address this issue, we examined taste buds from rat vallate papillae that had been sectioned transversely, rather than longitudinally, to their longest axis. In these transverse sections, dark (Type I) and light (Type II) cells were easily distinguished by their relative electron density, shape and topological relationships. Cells with electron-lucent cytoplasm (light cells) were circular or oval in outline, while those with electron-dense cytoplasm (dark cells) had an irregular outline with sheetlike cytoplasmic projections that separated adjacent light cells. A hierarchical cluster analysis of 314 cells across five morphological parameters (cell shape and area, and nuclear ellipticity, electron density and invagination) revealed two distinct groups of cells, which largely corresponded to the dark and light cells identified visually. These cells were not continuously distributed within a principal components factor solution. Differences in the means for dark and light cells were highly significant for each morphological parameter, but within either cell type, changes in one parameter correlated little with changes in any other. These analyses all failed to reveal cells with a consistent set of intermediate characteristics, suggesting that dark and light cells of rat vallate taste buds are distinct cell types rather than extremes of a continuum. Sections of taste buds were stained with antibodies to several carbohydrates, then observed by indirect immunofluorescence. Optical sections taken with a confocal laser-scanning microscope showed that the Lewis antigen was present only on spindle-shaped cells with circular or oval outlines and lacking transverse projections; these characteristic shapes matched those of light cells seen by electron microscopy. The H blood group antigen and the 2B8 epitope appeared at most cell-cell interfaces in the bud and are present on dark cells and possibly on some light cells. These findings relate molecular markers to morphological phenotypes and should facilitate future studies of taste cell turnover, development and regeneration.

Embryonic and early fetal development of human taste buds: a transmission electron microscopical study

BACKGROUND

Taste buds are assemblies of slender epithelial cells that receive chemical stimuli from the outer (oral) environment. In contrast to the large and well documented information on the morphology of taste buds in adult humans and animals, there are only a few reports on fetal ones, and ultrastructural studies of prenatal human taste buds are lacking completely. Therefore, the present investigation has been carried out to study the taste bud primordium, its morphological changes including synaptogenesis, cell differentiation, and taste pore formation from the time of the onset of taste bud formation around the 8th week until the 15th postovulatory week.

METHODS

Taste bud primordia of 42 human embryonic/fetal tongues have been examined by means of transmission electron microscopy.

RESULTS

Nerve fibers approach the lingual epithelium between the 6th and 7th postovulatory week. They penetrate the basal lamina during the 8th week and form synapses with poorly differentiated, elongated, epithelial cells. By the 12th week, more differentiated cell types are seen: 1) electron-dense cells resembling type III cells of the adult taste bud containing large numbers of dense-cored vesicles (80-150 nm in diameter); 2) electron-dark cells with well developed endoplasmic reticulum and many apical mitochondria, being candidates for type II cells. Basally, these cells have foot-like processes containing dense-cored vesicles (120-200 nm in diameter), but they do not synapse to nerve fibers. Type I cells, characterized by apically located dense secretory granules, are not observed. First shallow grooves above the taste bud primordium are found around the 10th week. Untypically differentiated apical cellular processes extend onto the surface. Most of the taste pores develop around the 14th to 15th week. In the taste pit, mucous material is not present during the first 15 weeks of gestation. Synapses between cells and afferent nerve fibers were found by the 8th week, reaching a maximum around the 12th to 13th week.

CONCLUSIONS

The early presence of taste bud cells containing dense-cored vesicles suggests an at least dual function of embryonic/ fetal taste buds: First, from the 8th until the 14th week, non-gustatory, paracrine functions should be considered. After the 14th week of gestation, when typical taste pores are present, the taste buds possibly start their gustatory function. Differentiated marginal cells are possibly involved in the formation of the taste pore. The lack of type I cells producing the mucous material in the taste pit indicates that the taste bud has not achieved a fully developed function until the 15th week of gestation.