Lingual Epithelium Research Articles

The morphogenesis of mouse vallate gustatory epithelium and taste buds requires BDNF-dependent taste neurons

The developmental absence of brain-derived neurotrophic factor (BDNF) in null mutant mice caused three interrelated defects in the vallate gustatory papilla: sparse innervation, a reduction in the area of the gustatory epithelium, and fewer taste buds. On postnatal day 7, the stunted vallate papilla of bdnf null mutant mice was 30% narrower, the trench walls 35% reduced in area, and the taste buds 75% less abundant compared with wild-type controls. Quantitative assessment of innervation density was carried out to determine if the small trench walls and shortage of taste buds could be secondary consequences of the depletion of gustatory neurons. The diminished gustatory innervation was linearly associated with a reduced trench wall area ( r=+0.94) and fewer taste buds ( r=+0.96). Residual taste buds were smaller than normal and were innervated by a few surviving taste neurons. We conclude that BDNF-dependent taste neurons contribute to the morphogenesis of lingual gustatory epithelia and are necessary for both prenatal and postnatal mammalian taste bud formation. The gustatory system provides a conspicuous example of impaired sense organ morphogenesis that is secondary to sensory neuron depletion by neurotrophin gene null mutation.

Electron microscopic characterization of filiform papillae in the normal human tongue.

The fine structure of filiform papillae on the normal human tongue was examined level by level, from the basal layer to the surface, in specimens taken from the dorsal side of the lingual body. Human lingual epithelia showed three distinct regions: epithelia on the anterior and on the posterior sides of filiform papillae and an interpapillary epithelium. While the basal and the squamous cell layers were similar throughout these three regions, differences were noted in the granular and the horny layers. The interpapillary epithelium actually lacked both the granular and the horny layers. The epithelium on the anterior side of filiform papillae was characterized by alternating layers of granular cells and of cornified cells. Granular cells possessed three types of keratohyaline-like granules within their cytoplasm: uniformly electron dense, relatively less electron dense, and a heterogeneous type. While the number of the keratohyaline-like granules was remarkably diminished in the epithelium on the posterior side of filiform papillae, a considerable amount of tonofibrils was present in the cytoplasm. In the uppermost portion of the anterior side of filiform papillae, coherence between adjacent epithelial cells depended mainly on the interlocking of cytoplasmic villi and poorly developed desmosomes on villi. On the other hand, epithelial cells on the posterior side of filiform papillae appeared to be more tightly adhesive compared with those on the anterior side. This was due to focal thickening of the plasma membrane and to desmosomes at the interface between the granular and cornified cells, and to the formation of a marginal band and increased intercellular cement presumably derived from lamellar bodies in the horny layer. These findings demonstrate distinct differences between the anterior and the posterior sides of filiform papillae in the human tongue with respect to keratinization patterns, structures associated with cell-to-cell adhesion and the strength of cellular cohesion in the uppermost portion, and the turnover of cornified cells. These differences may contribute to the formation of the unique external configurations of filiform papillae.

MDR1 in taste buds of rat vallate papilla: functional, immunohistochemical, and biochemical evidence.

Multidrug resistance P-glycoprotein (MDR1) is a membrane protein of 150-170 kDa that catalyzes the ATP-driven efflux of hydrophobic xenobiotics, including fluorescent dyes, from cells. Expressed in many epithelial tissues and in the endothelia of the blood-brain barrier, the MDR1 protein provides major routes of detoxification. We found that taste cells of the rat vallate papilla (VP; posterior tongue) had only a slow increase in fluorescence due to uptake of the hydrophobic dye calcein acetoxymethyl ester. However, the development of fluorescence was accelerated two- to threefold by substrates and/or inhibitors of MDR1, such as verapamil, tamoxifen, and cyclosporin A, and by addition of the transport-blocking antibody to MDR1, UIC2. Western blots of vallate tissue rich in taste buds with the MDR1-specific monoclonal antibodies C219 and C494 revealed an immunoreactive protein at approximately 170 kDa. In contrast, the lingual epithelium surrounding the VP showed a much weaker band with these antibodies. Furthermore, using the antibodies C494 and UIC2 with tissue sections, MDR1-like immunoreactivity was found in taste cells. These results show that MDR1 is present and functional in vallate taste cells of the rat. MDR1-related transport may achieve active elimination of xenobiotics from the sensory cells and thereby protect the peripheral taste organs from potentially harmful molecules contained in an animal's food.

Fine structure of the tongue and lingual papillae of the penguin.

The tongue in four species of penguin was investigated by light microscopy and by scanning electron microscopy, with special reference to the lingual papillae. 1) The middle of the penguin tongue contains a pair of long cartilages and long tendons accompanied by striated muscle bundles. 2) Large, spine-like, and caudally directed lingual papillae (filiform-like papillae) densely cover the dorsal surface of the tongue, apparently serving to catch fishes. 3) By light microscopy, the dorsal lingual epithelium with the lingual papillae are seen as a thick cornified layer, but the lateral and lower surfaces have a thinner cornified layer. 4) The connective tissue core (CTC) under the epithelium of the lingual papilla shows a stereo structure similar to but smaller than the external form of the papilla. The CTC contains some blood vessels and nerve fibers; the lingual glands are found in the submucous layer only in the posterior tongue. 5) Bundles of nerve fibers in the lamina propria of the tongue were immunohistochemically positive for PGP 9.5, and it appeared that Merkel corpuscles were distributed in the connective tissue closely beneath the epithelium of the finger-like papillae, though they were only weakly immunoreactive for PGP 9.5. 6) Numerous fine filaments of elastic fibers are found closely beneath the epithelial cell layer of the dorsal surface of the tongue including the lingual papillae, while there are very few of them on the lateral and lower sides of the tongue.

Hierarchical expression of desmosomal cadherins during stratified epithelial morphogenesis in the mouse

Desmosomes contain two heterogeneous families of specialized cadherins (desmogleins or Dsgs and desmocollins or Dscs), subtypes of which are known to be expressed in tissue-specific and differentiation-dependent patterns in adult epithelial tissues. To examine the temporal and spatial order in which the individual desmosomal cadherins are expressed during stratified epithelial development we have obtained partial cDNA clones of all six murine desmosomal cadherins and have carried out in situ hybridization analysis on E12.5 to E16.5 mouse embryos. The results indicate that the type 2, type 3 and type 1 desmosomal cadherin messages are not obligatorily expressed as pairs during stratified epithelial morphogenesis. Instead the individual genes appear to be transcribed in hierarchical, overlapping temporal and spatial patterns extending from DSG2 to DSC1. DSG2 was the most uniformly expressed message in all E12.5 epithelia, gradually becoming confined to the basal cell layers during epithelial stratification indicating that its transcription was restricted to undifferentiated cells. In contrast, DSC2 message was expressed variably in early epithelia and was strongly upregulated in the suprabasal cell layers during the stratification of wet-surfaced epithelia. DSC3 message was expressed before that of DSG3 in the dental and lingual epithelium where its spatial distribution matched that of DSG2, but after DSG3 in the non-glandular gastric epithelium. DSC3 transcripts became confined to the lower layers of stratifying epithelia but were usually less basally restricted than those of DSG2. Like DSC2, DSG3 mRNA was strongly upregulated in the suprabasal layers of wet-surfaced epithelia as they stratified. Upregulation of DSG1 message was temporally linked to that of DSG3 in all tissues apart from the non-glandular gastric epithelium. However, unlike DSG3, expression of DSG1 mRNA was maintained in the suprabasal layers of keratinizing epithelia and became restricted to the keratinizing inner root sheath in vibrissa follicles. DSC1 was always the last desmosomal cadherin message to be expressed and it was only detected in keratinizing tissues. Although there were tissue-specific differences in the temporal sequence of desmosomal cadherin transcription, the spatial order in which DSG and DSC messages were expressed during morphogenesis of all stratified epithelia was more consistent and correlated with the physical order of the genes at the desmosomal cadherin locus. These observations suggest the existence of an overall regulatory mechanism co-ordinating the hierarchical expression of DSG and DSC genes during epithelial differentiation and development.

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

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

Glutathione S-transferase pi-class as a tumour marker in lingual preneoplastic and neoplastic lesions of rats and humans.

Immunocytochemical expression of the pi class glutathione S-transferase (GST) was investigated in preneoplastic and neoplastic lingual lesions in a 4-nitro-quinoline 1-oxide (4NQO)-induced rat genetic model [Wistar/Furth rats (WF) and Dark-Agouti rats (DA)] and in human surgical material [fibrous polyp, mild to moderate dysplasia, severe dysplasia, carcinoma in situ (CIS), squamous cell carcinoma (SCC)]. Two polyclonal antibodies raised against rat (GST-P) and human (GST-pi) antigens were used. In the rat model, DA and WF rats showed contrasting susceptibility to 4NQO, DA rats having a much higher tumour incidence and a significantly shorter survival time than WF rats. While the established lingual SCC in DA and WF rats all expressed GST-P, the number of GST-P+ foci in the preneoplastic lingual epithelium was significantly higher in DA (14.5 +/- 6.5) than in WF rats (5.5 +/- 2.6; P < 0.0001). In contrast, GST-pi epithelial staining in human specimens was more variable and the results overlapped in different groups. More frequent nuclear and/or basal cell staining was detected in severe dysplasia, CIS and SCC than in benign and mild to moderate dysplastic lesions. Although the pi class GST may be a useful marker for rat lingual carcinogenesis, its value in clinical applications is unclear. GST-pi staining patterns and their distribution may be helpful in identifying high-risk lingual lesions in humans.

Histological and ultrastructural study of the effects of cholinergic and adrenergic agonists on salivary secretion from the lingual epithelium and the lingual gland of the Tokyo Daruma pond frog

Comparative observations of the effects of cholinergic and adrenergic agonists (such as pilocarpine, phenylephrine and isoproterenol) on the secretion of salivary fluid and of secretory granules from the lingual epithelium and the lingual gland in the Tokyo Daruma pond frog, Rana porosa porosa, were made by light and transmission electron microscopy. The effect of pilocarpine on the loss of cytoplasm in both the lingual epithelium and the lingual gland was the strongest, and that of isoproterenol was the weakest. Transmission electron microscopy revealed that electron-dense granules, located in cells in both the lingual epithelium and the lingual gland, were discharged by exocytosis after stimulation by phenylephrine and isoproterenol. Immediately before secretion of these granules, they became somewhat larger and round or distorted in shape. However, after administration of pilocarpine, no obvious discharge of electron-dense granules was apparent and, instead, granules in some cells began deteriorating within the cells and those in other cells developed electron-dense and electron-lucent areas. A dot-like pattern was recognized in the electron-dense areas of these granules. These phenomena were assumed to be secondary effects accompanying the secretion of salivary fluid from cells of the lingual epithelium and the lingual gland. By contrast, mucous granules in the lingual gland were secreted by a holocrine process.

Isolation, partial purification, and ultrastructure of taste bud cells from rabbit foliate papillae

A method is described for obtaining large numbers of isolated taste bud cells from lingual epithelium of rabbit foliate papillae. The isolation method is based on isopyenic sedimentation in a Percoll gradient. The purification of taste bud cells was evaluated by electron microscopy and by immunohistochemistry using CK 20 antibody. The cytology of the isolated taste bud cells remained very similar to in situ cells. The type III cells, which are regarded as gustatory cells, retained their characteristic dense-cored granules in the cytoplasm. This method will permit study of various parameters of taste bud cell biology.

Epstein-Barr virus: biology and disease.

Epstein-Barr virus is a human herpes virus which, whilst found as a widespread asymptomatic infection, is also associated with certain tumours of lymphoid and epithelial origin including Burkitt's lymphoma (BL), immunoblastic lymphoma (IBL), Hodgkin's Disease (HD) and nasopharyngeal carcinoma (NPC). A unique characteristic of EBV is its ability to infect and transform primary resting B lymphocytes in vitro into permanently growing lymphoblastoid cell lines (LCLs); this effect is associated with constitutive expression of a limited set of viral genes. Interestingly, the pattern of EBV gene expression observed in LCLs in vitro is also a feature of IBLs, a tumour associated with immunosuppression. The other EBV associated tumours display a more restricted pattern of EBV latent protein expression. B cell lines can be activated in vitro into the virus replicative cycle, where a large number of viral genes associated with EBV DNA replication and virus assembly are synthesised. Whilst EBV can be detected in throat washings from seropositive individuals, the only in vivo situation where full virus replication can be reliably observed in hairy leukoplakia (HL), a benign lesion of lingual epithelium frequently found in AIDS patients. Thus, the relative contribution of lymphoid cells and epithelial cells to latent EBV infection/persistence vs replication in vivo remains controversial. Recent studies suggest that HL represents a focus of EBV replication in the absence of a truly latent infection and this supports the contention that EBV persistence resides in the lymphoid compartment. These aspects together with the role of EBV in oral diseases and the effect of certain EBV genes on the control of epithelial cell growth and differentiation will be discussed.

Ultrastructural study of the keratinization of the dorsal epithelium of the tongue of Middendorff's bean goose, Anser fabalis middendorffii (Anseres, Antidae).

Comparative studies of ultrastructural features of tongues allow deductions to be made about relationships between structure and function, as reflected by an animal's feeding habits. The present study was performed to serve as a basis for further studies of avian feeding mechanisms and of relationships between the fine structure of the lingual epithelium and the development of the expression of keratins. The light microscope, scanning electron microscope, and transmission electron microscope were used. The dorsal surface of the tongue of Middendorff's bean goose, Anser fabalis middendorffii, has a distinctive anterior region that extends for five-sixths of its length and has a clear posterior region. The anterior region, when observed macroscopically and by scanning electron microscopy, is distinguished along its forward half by a clear median line. The back half of the anterior region has an indistinct median sulcus in some parts. There are no lingual papillae on the entire dorsal surface of the anterior and posterior regions. Giant conical papillae are located in a transverse row between the anterior and posterior regions. On both lateral sides of the anterior region for five-sixths of the length of the tongue, lingual hairs are compactly distributed, and small numbers of large cylindrical papillae are arranged at almost regular intervals between these lingual hairs. Examination of the dorsal lingual epithelium by light and transmission electron microscopy provided histological and cytological criteria for distinguishing the anterior and posterior regions, both of which were composed of stratified squamous epithelium. Basal cells were similar throughout the dorsal epithelium. The intermediate layer of cells in the anterior region contained numerous tonofibrils in electron-dense bundles composed of tonofilaments of 10 nm in diameter. The outer layer was composed of electron-dense, well-keratinized cells, with layers of electron-lucent cells on the outermost surface. The cells in the intermediate layer in the posterior region of the tongue were almost completely filled with unbundled tonofilaments. The surface layer exhibited features of parakeratinization. In all of the giant conical papillae, the large cylindrical papillae, and the lingual hairs, the epithelium was strongly keratinized. The three-dimensional microanatomy and cytological features of the dorsal lingual epithelium of avians seem to be related to the functional role and shape of the tongue of each species in feeding.

Effect of the leu-enkephalin analog dalargin on DNA synthesis in the myocardium and lingual epithelium of rats in early postnatal ontogeny

The DNA synthesis is studied in the myocardium and lingual epithelium of 4–5-day-old rats 24 h after a single administration of the leu-enkephalin analog dalargin in a dose of 10 or 100 µg/kg. At 10 µg/kg, dalargin significantly decreases the number of DNA-synthesizing nuclei in the left atrium, while at 100 µg/kg it significantly decreases the number of DNA-synthesizing nuclei in the left atrium and both ventricles. Dalargin does not change the DNA synthesis in the lingual epithelial cells.

Taste disks are induced in the lingual epithelium of salamanders during metamorphosis.

Morphological changes of oral cavity during metamorphosis with special reference to the taste organ were examined in Ezo salamanders (Hynobius retardatus) and axolotis (Ambystoma mexicanum), and compared with those in bullfrogs (Rana catesbeiana). The non-distensible tongue of salamanders changed the structure progressively during metamorphosis: a small area of the rostrum protruded and developed caudally with recession of the flat area of the tongue. The protrusion that developed on the tongue had numerous papillae, as seen in the frog tongue. The apical region of the papillae occasionally had a cell mass similar to the taste disk of frogs (termed a taste disk-like cell mass). On the flat area of the tongue, the barrel-shaped taste buds of larval salamanders were transformed into taste buds with a wider receptor area. The barrel-shaped taste buds decreased progressively during metamorphosis, while taste disk-like cell masses increased. Neuronal labeling with an antibody to neuron-specific enolase and fluorescent carbocyanine dye showed that the taste disk-like cell masses in metamorphosed salamanders were innervated by the glossopharyngeal nerve (nerve IX). Nerve IX responded to taste stimulation as well as mechanical stimulation applied to the rostral tongue. During metamorphosis the salamanders undergo transformation and rearrangement of taste organs on the tongue possibly as an adaptation to the terrestrial environment.

Initial innervation of embryonic rat tongue and developing taste papillae: nerves follow distinctive and spatially restricted pathways.

The rat tongue has an extensive, complex innervation from four cranial nerves. However, the precise developmental time course and spatial routes of these nerves into the embryonic tongue are not known, although this knowledge is crucial for studying mechanisms that regulate development and innervation of the lingual taste organs, gustatory papillae and resident taste buds. We determined the initial spatial course of nerves in the developing tongue and papillae, and tested the hypothesis that sensory nerves first innervate the tongue homogeneously and then retract to more densely innervate papillae and taste buds. Antibodies to GAP-43 and neurofilaments were used to label nerve fibers in rat embryo heads from gestational day 11 through 16 (E11-E16). Serial sagittal sections were traced and reconstructed to follow paths of each nerve. In E11 rat, geniculate, trigeminal and petrosal ganglia were labeled and fibers left the ganglia and extended toward respective branchial arches. At E13 when the developing tongue is still a set of tissue swellings, the combined chorda/lingual, hypoglossal and petrosal nerves approached the lingual swellings from separate positions. Only the chorda/lingual entered the tongue base at this stage. At E14 and E15, the well-developed tongue was innervated by all four cranial nerves. However, the nerves maintained distinctive entry points and relatively restricted mesenchymal territories within the tongue, and did not follow one another in common early pathways. Furthermore, the chorda/lingual and glossopharyngeal nerves did not set up an obvious prepattern for gustatory papilla development, but rather seemed attracted to developing papillae which became very densely innervated compared to surrounding epithelium at E15. To effect this dense papilla innervation, sensory nerves did not first innervate the tongue in a homogeneous manner with subsequent retraction and/or extensive redirection of fibers into the taste organs. Results contribute to a set of working principles for development of tongue innervation. Points of entry and initial neural pathways are restricted from time of tongue formation through morphogenesis, suggesting distinctive lingual territories for each nerve. Thus, sensory and motor nerves distribute independently of each other, and sensory innervation to anterior and posterior tongue remains discrete. For taste organ innervation, gustatory papillae are not induced by a prepatterned nerve distribution. In fact, papillae might attract dense sensory innervation because neither chorda/lingual nor glossopharyngeal nerve grows homogeneously to the lingual epithelium and then redistributes to individual papillae.

Differential expression of brain-derived neurotrophic factor and neurotrophin 3 mRNA in lingual papillae and taste buds indicates roles in gustatory and somatosensory innervation.

Although many studies have demonstrated the dependency of taste bud function and/or survival on intact innervation, relatively few have dealt with the development of taste bud innervation. Using in situ hybridization histochemistry, we show that brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT3) mRNA are expressed in a specific pattern in the taste buds, tongue papillae, and lingual epithelium during development and that expression persists into adulthood. BDNF mRNA is expressed in a fraction of the taste cells of the developing and adult taste buds in rats, showing different labeling intensities among the labeled cells. NT3 and mRNA seems to be located in areas other than those where BDNF mRNA is expressed, mainly in the superior epithelial surfaces of circumvallate papillae, the outer surface epithelium of foliate papilae, the superior surface and the lateral epithelium of the fungiform papillae, and the epithelium of the filiform papillae. NT3 mRNA labeling is also observed among muscle and connective tissue of the tongue. The morphological appearance, expression of NT3 mRNA, and ramification of nerve fibers in defined epithelial structures in the posterior wall of the anterior filiform papillae suggest the existence of a mechanosensory apparatus in these papillae. Nerve growth factor and neurotrophin 4 probes did not give rise to selective labeling in tongue, although their presence cannot be totally excluded. Based on present and prior studies, we suggest that BDNF is needed during initiation and for maintenance of gustatory innervation of taste buds and gustatory papillae and that NT3 is mainly needed for somatosensory innervation of the tongue.

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

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

Basolateral amiloride-sensitive Na+ transport pathway in rat tongue epithelium.

1. Experiments were conducted to test for the presence of basolateral Na+ channels in the rat lingual epithelium. Researchers have proposed a model in which some lingual taste cells have Na+ channels in the basolateral membrane. That model is designed to account for the portion of the neural taste response and the portion of the transepithelial short-circuit current (Isc) in vitro that are insensitive to mucosal amiloride; some Na+ would diffuse across the tight junction into the cell via this lateral pathway, and would be transported out of the cell by Na+ pumps in the basal membrane. The model could also account for the differential effect of mucosal amiloride on Na+ salts of various anions, in which the neural taste responses to Na+ salts with anions larger than Cl- are more sensitive to mucosal amiloride than is the taste response to NaCl. 2. Voltage-clamp data were obtained from an in vitro preparation of the anterior-dorsal rat tongue epithelium in which the connective tissue was removed by enzyme digestion. Isc in a modified Ussing chamber was reduced by amiloride in the submucosal solution. 3. The pattern of sensitivity to submucosal amiloride differed in several respects from the pattern for mucosal amiloride. The inhibition constant (Ki) was 52 microM amiloride concentration, higher than for the apical amiloride-sensitive Na+ channel. The selectivity for Na+ over K+ was much less than for the response to mucosal amiloride; with 0.5 M NaCl or KCl on the mucosal side, the ratio of inhibition for the NaCl response to inhibition for the KCl response varied between 1 and 3. 4. As the concentration of NaCl in the mucosal solution was varied, submucosal amiloride caused little inhibition of Isc for mucosal NaCl below isosmotic concentration, with the percent inhibition increasing as mucosal salt concentration increased. With 0.5 M sodium gluconate in the mucosal solution, there was very little inhibition due to submucosal amiloride. 5. The results support the presence of amiloride-sensitive Na+ channels in the basolateral membranes of the dorsal tongue epithelium in rat, and are consistent with the proposed model in which these channels are present in taste cells.

Localization of substance P NK-1 receptors in rat tongue

In rat tongue, neurons containing substance P terminate in connective tissue, in taste buds, and in lingual epithelium surrounding taste buds in fungiform, foliate and circumvallate papillae. Although many functions have been attributed to these neurons, virtually nothing is known about their physiological function. As a step towards this end, immunocytochemical methods were used to identify the NK-1 receptors (SPR) in rat tongue. SPR-IR was found in the basolateral membranes of taste cells in fungiform, circumvallate and foliate papillae. SPR-IR was not found in the dorsal epithelium or in any structure that could be clearly identified as a neuron. SPR-IR was also found in von Ebner's glands in circumvallate and foliate papillae and in blood vessels in connective tissue in all three papillae. These data suggest that substance P may play a role in taste and/or in oral pain.

Biochemistry of sweet taste transduction

Recent biochemical and biophysical studies point to mechanistic hypotheses for sweet taste transduction involving either second messengers or stimulus-gated ion channels. Biochemical studies have shown that sweet tasting stimuli enhance the production of the second messenger, cyclic AMP, in a GTP-dependent manner in taste tissue homogenates. The cyclic AMP thus produced apparently stimulates a protein kinase A which may phosphorylate ion channels, leading ultimately to depolarization, an increase in intracellular calcium ion activity, and release of neurotransmitter. Recent studies show that some sweeteners also induce the production of inositol 1,4,5-trisphosphate (IP 3). Other sweet transduction processes not associated with second messenger production may exist. For example, evidence for a stimulus-gated type ion channel for sweet taste can be inferred from ion transport studies on lingual epithelia and from psychophysics. A recent study demonstrated that certain amphiphilic sweeteners are capable of directly stimulating purified G proteins in an in vitro assay. Perhaps these and other amphiphilic intense sweeteners cross the plasma membrane and directly stimulate the G protein, inducing production of second messenger and bypassing the receptor. A number of sweeteners are capable of forming ion channels or of simply perturbing the membrane, actions which could operate during stimulation of the sweet receptor cell. This type of action could explain the relatively longer response times and lingering taste intensity associated with many amphiphilic sweeteners.

Experimental trigeminal nerve injury.

The successful reinnervation of peripheral targets after injury varies with the axonal population of the nerve that is injured and the extent of the dislocation of its central component from the peripheral endoneurial tube. Larger-diameter axons such as those supplying mechanoreceptors recover more readily than narrower axons such as those supplying taste. A complex, bi-directional interaction between lingual epithelium and sprouting nerve results in the redifferentiation of taste buds after denervation. Dentin and the dental pulp provide a strong attraction to sprouting nerves and will become reinnervated from collateral sources if recovery of the original innervation is blocked. The most effective repair technique for transected lingual nerves is one which brings the cut ends together rather than one that provides a temporary bridge. Injuries can result in cell death in the trigeminal ganglion but only if the injury is severe and recovery is prevented. Lesser damage results in chromatolysis and the increased expression of neuropeptides. All nerve injuries bring about changes in the trigeminal nucleus. These occur as changes in receptive field and the incidence of spontaneously active neurons, effects which are consistent with the unmasking of existing afferents. These functional changes are short-lived and reversible. Morphologically, nerve injury results in terminal degeneration in the nuclei and an increased expression of the c-Fos gene and some neuropeptides. Only a chronic constriction injury induces behavioral changes. The adult trigeminal system retains considerable plasticity that permits it to respond successfully to nerve injury. Much remains to be learned about this response, particularly of the trophic factors that control peripheral recovery and the central response to more severe injuries.