Neonatal chorda tympani transection permanently disrupts fungiform taste bud and papilla structure in the rat

The present report examined the morphology of fungiform papillae in adult rats that received bilateral chorda tympani transection at 10 days of age. Tongue tissue was examined using surface-structure analysis. Counts were made of fungiform papillae with a pore, fungiform papillae with no pore and fungiform papillae with a keratinized conical surface; a feature referred to as "filiform-like. " Neonatal chorda tympani nerve transection resulted not only in a loss of taste buds but also in a permanent loss in numbers of fungiform papillae. Compared with an average of 152 fungiform papillae in sham-operated control rats, there was an average of only 54 fungiform papillae after neonatal chorda tympani transection. Nearly 80% of these fungiform papillae in neonatal chorda tympani transected rats were filiform-like. No filiform-like papillae were noted in sham-operated rats. These results suggest that the chorda tympani nerve is necessary during an early postnatal period of development to maintain normal fungiform papillae morphology.     

Neural control of ectopic filiform spines in adult tongue

The tongue surface directly above a fungiform taste bud is flat, thinly keratinized, and free of filiform spines. We examined fungiform papillae in serial sections of rat and gerbil tongues after unilateral transection of the chorda-lingual nerve had caused many fungiform taste buds to degenerate. Such empty fungiform papillae often formed a solitary keratinized outgrowth that closely resembled the spine of an ordinary filiform papilla. By six months an ectopic spine was found on 61% of empty fungiform papillae, but never on fungiform papillae that contained a taste bud. Experimental innervation of the tongue reduced the incidence of ectopic filiform spines in proportion to the cross-sectional area of the trigeminal nerve branches tested (the mylohyoid nerve, the lingual nerve, lingual + mylohyoid or lingual + auriculotemporal nerves). The chorda tympani nerve was 60 times more effective than trigeminal nerves in preventing ectopic filiform spines. We suggest that positive and negative trophic actions are normal characteristics of taste axons, for they promote the formation of taste buds and prevent the expression of ectopic filiform spines. By preventing the outgrowth of ectopic spines on fungiform papillae, taste axons maintain a thinly keratinized apical surface that can be breached by the taste receptor cells.     

Distribution of taste and general sensory nerve endings in fungiform papillae of the hamster

Sensory endings of chorda tympani and lingual (trigeminal) nerve fibers were identified by selective denervation and localized within specific regions of fungiform pipillae in the hamster. The chorda tympani was resected from the middle ear and the peripheral fibers were allowed to degenerate for 1, 3, or 8 days prior to perfusion-fixation and electron-microscopic examination of the anterior tongue. Taste buds were virtually devoid of intact nerves by 3 days following chorda tympani denervation. Remnants of the fibers were restricted to taste buds. Lingual fibers, on the other hand, persist in normal numbers after chorda tympani resection and populate perigemmal areas of connective tissue and extragemmal areas located apically in the squamous, nontaste epithelium surrounding the taste bud. This study provides evidence of a segregation of chorda tympani fibers in the taste bud and lingual nerve fibers in the apical fungiform papilla. The lingual nerve-epithelial arrangement and superficial location, near the least cornified area of the tongue, may be well suited for relatively sensitive somatosensation, possibly mechanoreception. Thus, the apical fungiform papilla appears to be a site where both taste and tactile oral stimuli interact with receptors.     

Long-term effects of gustatory neurectomy on fungiform papillae in the young rat

Recent evidence from mature hamster fungiform papillae indicates that following denervation taste buds are present from 21 to 330 days in the absence of discernible intragemmal nerve fibers. In contrast, most prior taste bud degeneration studies focused on shorter survival times. The present inquiry in young rats examined the issue of postneurectomy buds, in which regeneration of the resected chorda tympani or facial nerves was prevented and anterior tongue tissue examined over a range of relatively long survival times (30-90 days). Conditions for observing potential taste buds used three histologic stains and a definition of the taste bud not necessarily requiring pore identification. In each case, serial section examination of the anterior-most 2-3 mm of lingual epithelium revealed 29-56 bud-containing fungiform papillae on the unoperated side. In contrast, ipsilateral to the neurectomy, only zero-7 medially-placed, mature-looking buds were observed per case, as well as zero-3 more laterally situated fungiform papillae containing small clusters of cells in basal epithelium that lacked the vertical organization and cytoplasmic staining intensity of mature taste buds. These cell aggregates were distributed evenly across survival time and stain used. Therefore, in young rats following gustatory neurectomy, longer survival times, per se, would not appear to be a prerequisite for sustaining fungiform taste buds. The appearance of "midline" buds postsurgery may be attributed to either normal contralateral or a net bilateral innervation, and/or ipsilateral denervation and bud loss inducing neural sprouting from the contralateral side.     

Biochemical changes in the sublingual and submandibular glands after interruption of chorda tympani

The left chorda tympani nerve was interrupted through meatus acusticus externus in ten dogs. In total, 40 dog salivary glands (20 submandibular and 20 sublingual) innervated via chorda tympani were examined. Twenty glands (10 submandibular and 10 sublingual) on the left side were deprived of parasympathetic innervation by chordectomy, whereas contraleteral glands, on the right side, served as controls. Biochemical analysis showed that the interruption of chorda tympani did not cause any significant changes in the concentrations of eight enzymes investigated, i.e. lactate dehydrogenase, alkaline phosphatase, acid phosphatase, amylase, aspartate aminotransferase, alanine aminotransferase, gamma-glutamyltransferase and creatine kinase. There were no significant changes in the concentrations of most important extracellular ions (sodium, potassium, chloride and phosphorus) in the right glands, but the loss of parasympathetic innervation in the left glands was found to cause a statistically significant decrease in the concentration of potassium as intracellular cation and of phosphorus as extracellular anion.     

The nature of the substance P-containing nerve fibres in taste papillae of the rat tongue

The nature of the association of substance P (SP) with taste buds in the rat tongue was investigated by immunohistochemical and radioimmunoassay techniques. Both the circumvallate and fungiform papillae were found to receive a rich innervation by substance P-containing fibres. Although these fibres were closely associated with the taste buds in these structures, they assumed a perigemmal rather than an intragemmal location. Bilateral lesions of the glossopharyngeal nerve resulted in the depletion of taste buds from the vallate papilla and a large reduction in substance P immunoreactive fibres in this area. Lesions of the chorda tympani, which led to the degeneration of taste buds in fungiform papillae, had no effect on the immunohistochemical appearance of substance P in these papilla or on the substance P levels in the anterior part of the tongue. Lesions of the mandibular division of the trigeminal nerve or neonatal capsaicin treatment had no effect on the structural integrity of taste buds in fungiform papillae but led to the depletion of substance P-immunoreactive fibres from these papillae. Both of these procedures caused a 71% reduction in the substance P content of the anterior tongue, ipsilaterally after the nerve lesion and bilaterally after capsaicin treatment. The results are discussed in relation to the possible functional role of substance P-containing fibres within nerves supplying taste structures of the tongue.     

Chronic restraint stress in rats suppresses sweet and umami taste responses and lingual expression of T1R3 mRNA

Effects of chronic restraint stress on the taste responses to five basic taste qualities were investigated electrophysiologically in the rat chorda tympani. In addition, the mRNA expression for T1R3, the common G-protein-coupled receptor (GPCR) for sweet and umami tastes, was studied quantitatively by RT-PCR after such stress. Rats were restrained in a small cylindrical restrainer made of steel wire for 8h daily for 14 successive days. The integrated responses to sweet and umami tastes, as recorded from the chorda tympani, were significantly suppressed after such stress, but the other three basic taste responses were unaffected. Expression of T1R3 mRNA in the fungiform papillae, as estimated by RT-PCR, was slightly reduced by the stress, and a quantitative real time RT-PCR study revealed a significant suppression of T1R3 mRNA expression in the stress group. These results suggest that the observed stress-induced changes in taste sensation could be caused by a peripheral disorder of the transduction mechanism in taste-receptor cells, involving in particular a stress-induced inhibition of T1R3 expression.     

Kinins, beta-adrenergic receptors and functional vasodilatation in the submaxillary gland of the cat

1. The close arterial infusion of bradykinin into the submaxillary gland of the cat produced a pronounced hyperaemia that could be blocked by simultaneous perfusion of the gland with blood containing carboxypeptidase B. Carboxypeptidase B, however, failed to reduce the vasodilatation of chorda tympani nerve stimulation suggesting that the kinins are not involved in the regulation of submaxillary gland blood flow.2. Isoproterenol injections produced pronounced salivary gland vasodilatation. Beta-adrenergic blocking drugs reduced or abolished the hyperaemia of isoproterenol and reduced that of chorda tympani nerve stimulation. The combination of beta-blocking drugs and atropine could abolish or reduce further this nerve induced hyperaemia.3. The above results suggest that stimulation of cholinergic and beta-adrenergic receptors could account for the chorda tympani induced hyperaemia. Conclusive proof of this possibility remains to be determined.     

Neural control of salivary myoepithelial cells

1. The pressures in the ducts of the submaxillary, parotid and sublingual glands were recorded in cats under chloralose anaesthesia. A single stimulus applied to the parasympathetic glandular nerve caused a pressure rise, the size of which increased with the initial pressure. This response was abolished by a small dose of atropine.2. The effect was not due to salivary secretion, since the single stimulus caused salivation only exceptionally. Repetitive stimulation at frequencies too low to evoke secretion could produce summated pressure responses.3. The single stimulus applied to the chorda tympani was found to cause vasodilatation in the submaxillary gland. This was abolished by a small dose of atropine, together with the pressure rise in the duct. However, repetitive stimulation still caused marked vasodilatation but no pressure response. It was therefore inferred that the pressure rise obtained before atropine was not due to vasodilatation in the gland.4. It is concluded that the myoepithelial cells of the salivary glands are supplied with a parasympathetic motor innervation which can cause them to contract.5. Sympathetic stimulation caused no pressure rise when a single stimulus was given but only when repetitive stimulation was used at a frequency approaching that required for secretion.     

Taste placodes are primary targets of geniculate but not trigeminal sensory axons in mouse developing tongue

Tongue embryonic taste buds begin to differentiate before the onset of gustatory papilla formation in murine. In light of this previous finding, we sought to reexamine the developing sensory innervation as it extends toward the lingual epithelium between E 11.5 and 14.5. Nerve tracings with fluorescent lipophilic dyes followed by confocal microscope examination were used to study the terminal branching of chorda tympani and lingual nerves. At E11.5, we confirmed that the chorda tympani nerve provided for most of the nerve branching in the tongue swellings. At E12.5, we show that the lingual nerve contribution to the overall innervation of the lingual swellings increased to the extent that its ramifications matched those of the chorda tympani nerve. At E13.0, the chorda tympani nerve terminal arborizations appeared more complex than those of the lingual nerve. While the chorda tympani nerve terminal branching appeared close to the lingual epithelium that of the trigeminal nerve remained rather confined to the subepithelial mesenchymal tissue. At E13.5, chorda tympani nerve terminals projected specifically to an ordered set of loci on the tongue dorsum corresponding to the epithelial placodes. In contrast, the lingual nerve terminals remained subepithelial with no branches directed towards the placodes. At E14.5, chorda tympani nerve filopodia first entered the apical epithelium of the developing fungiform papilla. The results suggest that there may be no significant delay between the differentiation of embryonic taste buds and their initial innervation.     

Taste bud distribution and regional responsiveness on the anterior tongue of the rat

It has been shown that over 50% of the taste buds in fungiform papillae of the rat are located on the tongue tip. The half on the tip were isolated from the half on the mid-region for chemical stimulation while the summated response of the whole chorda tympani nerve was recorded. The tip grave responses which were 70–90% of the response to stimulation of the entire tongue, while the mid-region elicited responses which were 40–50% of the entire tongue responses. This result was consistent whether the stimulus was NaCl, HCl, or sucrose which shows that responsiveness to these 3 stimulus compounds is found in the same relative proportion within either region of the tongue. The density of papillae per unit of surface area on the tip is over 2.5 times greater than on the mid-region, so the greater responsiveness of the tip may be due to more potent lateral interaction among single taste bud inputs than on the mid-region.     

Effects of isoprenaline and pilocarpine on salivary glands as assessed by gravimetric and image analysis techniques.

Pigs were given 6 daily injections of isoprenaline or pilocarpine. At necropsy the parotid, submaxillary and sublingual salivary glands were removed and weighed. Histological sections were prepared from several regions of each gland and the cross-sectional area of their acini was measured in treated and control animals by means of a Quantimet Image Analysis System. Both isoprenaline and pilocarpine produced a significant increase in the weight of the parotid and submaxillary glands, which was accompanied by an increase in acinar area. There was no change in the weight of the sublingual gland with either drug, but isoprenaline did induce an increase in the area of sublingual acini. A comparison is made of the changes in weight and structure of the salivary glands as shown by these methods.     

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.     

Gustatory responsiveness of fibers in the hamster glossopharyngeal nerve

1. Mammalian taste receptors are distributed within separate subpopulations, innervated by branches of cranial nerves VII, IX, and X. Most gustatory electrophysiology has focused on input from the fungiform papillae on the anterior portion of the tongue, carried by the chorda tympani branch of the VIIth nerve. However, only a small percentage of the taste buds are located in the fungiform papillae (approximately 18% in the hamster). There have been no studies on the hamster's IXth nerve, which innervates greater than 50% of its taste buds, and most other studies of IXth nerve function have employed only whole-nerve recording. 2. Action potentials were recorded from 83 individual fibers in the IXth nerve of the hamster. Stimuli were five concentrations each of sucrose, NaCl, HCl, and quinine hydrochloride (QHCl), all presented to every fiber at 37 degrees C. Responses were quantified as the number of impulses in 10 s minus the preceding 10 s of spontaneous activity. 3. Across these concentration series, HCl and QHCl were by far the most excitatory stimuli, with mean responses across all cells three to four times greater than those evoked by sucrose or NaCl. The order of effectiveness of the stimuli was H greater than Q much greater than N greater than S. 4. Of the 83 fibers, 56 were stimulated via the foliate papillae and 27 via the single vallate papilla. No fibers responded to both of these fields. There were generally no differences in the sensitivity of these two subpopulations of taste buds, except that QHCl was more effective when applied to the foliates. 5. A "total" response measure was derived by summing the excitatory responses to each stimulus across the entire concentration series. The fibers were then classified according to the best total response, resulting in 52 HCl-, 19 QHCl-, 8 sucrose- and 4 NaCl-best cells. Considering the slope of the concentration-response functions as a criterion for classification produced very similar results. The fiber classification varied somewhat with concentration, with more fibers categorized as HCl- and QHCl-best at the higher concentration levels. 6. Breadth of responsiveness was measured using the equation developed by Smith and Travers. At the concentrations used to examine hamster chorda tympani fibers, IXth nerve fibers were not very responsive and were quite narrowly tuned to the four taste qualities. At higher concentrations the fibers became more broadly responsive across the four stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)     

Taste responsiveness of fungiform taste cells with action potentials

It is known that a subset of taste cells generate action potentials in response to taste stimuli. However, responsiveness of these cells to particular tastants remains unknown. In the present study, by using a newly developed extracellular recording technique, we recorded action potentials from the basolateral membrane of single receptor cells in response to taste stimuli applied apically to taste buds isolated from mouse fungiform papillae. By this method, we examined taste-cell responses to stimuli representing the four basic taste qualities (NaCl, Na saccharin, HCl, and quinine-HCl). Of 72 cells responding to taste stimuli, 48 (67%) responded to one, 22 (30%) to two, and 2 (3%) to three of four taste stimuli. The entropy value presenting the breadth of responsiveness was 0.158 +/- 0.234 (mean +/- SD), which was close to that for the nerve fibers (0.183 +/- 0.262). In addition, the proportion of taste cells predominantly sensitive to each of the four taste stimuli, and the grouping of taste cells based on hierarchical cluster analysis, were comparable with those of chorda tympani (CT) fibers. The occurrence of each class of taste cells with different taste responsiveness to the four taste stimuli was not significantly different from that of CT fibers except for classes with broad taste responsiveness. These results suggest that information derived from taste cells generating action potentials may provide the major component of taste information that is transmitted to gustatory nerve fibers.     

Taste-responsive neurons of the glossopharyngeal nerve of the rat

1. Taste sensibilities of neurons in mammalian glossopharyngeal nerves have been inadequately studied, although they innervate the majority of taste buds and may provide unique taste information. 2. Extracellular responses of glossopharyngeal neural units to taste stimuli infused into foliate or vallate papillae were recorded in anesthetized rats. A 0.3-ml/min infusion of stimuli into papillae resulted in short-latency, 5-s nerve-impulse rates that approached 10 times the response rates observed using less invasive means of stimulation. 3. Sucrose, Na saccharin, NaCl, NH4Cl, KCl, HCl, citric acid, acetic acid, MgSO4, and quinine.HCl were effective stimuli for glossopharyngeal neurons at concentrations that have behavioral significance. 4. Response spectra for individual neural units with either foliate or vallate receptive fields fell into three clusters. Forty-six percent were A units that responded most strongly to acids and chloride salts, NH4Cl being the most effective; neither quinine nor sucrose was effective. Twenty-three percent were S units that responded to sugars and saccharin; quinine, salts, and acids were not effective. Thirty-one percent were Q units that responded to quinine; neither NaCl, HCl, nor sucrose was effective stimulus for these fragile units. 5. Glossopharyngeal A neural units were more sensitive to 1 mM HCl than were electrolyte-sensitive H units of the chorda tympani, although both respond generally to salts and acids. Units relatively specific for sodium salts (N units), which are common in the chorda tympani nerve, were not found in the glossopharyngeal nerve, which explains losses in sodium-specific behavior after cutting only the chorda tympani nerve. 6. Q units were the only glossopharyngeal neural units that responded significantly to quinine, and units with similar response spectra do not occur in the chorda tympani nerve. Q units probably mediate aversive reflexes to quinine that are eliminated by cutting only the glossopharyngeal nerve. Glossopharyngeal S neural units were more sensitive to sucrose and are more common than their counterparts in the chorda tympani, although it is not known how they might compare with sugar-sensitive units in the greater superficial petrosal nerve. 7. These data strongly suggest that posterior taste bud fields innervated by the glossopharyngeal nerve are specialized for functions different from those of anterior taste bud fields innervated by the facial nerve.     

Chorda tympani and lingual nerve responses to astringent compounds in rodents.

A wide variety of compounds in foods and beverages produce astringent sensations when introduced into the oral cavity. There is controversy, however, whether "astringency," with its associated puckering and drying sensations, is a fundamental taste quality or is a tactile sensation. To address this issue, electrophysiological recordings were made from the gerbil chorda tympani nerve and the rat lingual nerve. The chorda tympani nerve transmits taste information from the anterior 2/3 of the tongue, whereas the lingual nerve transmits tactile, thermal and pain sensations from the anterior 2/3 of the tongue. The astringent compounds tested were: tannic acid, tartaric acid, gallic acid, aluminum ammonium sulfate and aluminum potassium sulfate. Tannic acid, tartaric acid, and gallic acids were tested at concentrations up to 120 mM over a pH range from approximately 2 to 6. The aluminum salts were tested at concentrations up to 160 mM only at low pH's. All compounds rapidly (and at lower concentrations, reversibly) stimulate the chorda tympani nerve in a concentration-dependent manner at all pH's tested. The rapidity and reversibility of the chorda tympani responses suggest that astringent-tasting compounds interact directly with taste cells rather than indirectly by precipitating salivary proteins. At pH 6, tannic acid, tartaric acid, and gallic acid all elicit robust chorda tympani responses, implying that the ionized forms of these compounds produce taste sensations. None of these compounds stimulate lingual nerves over the same concentration and pH ranges used in the chorda tympani experiments.(ABSTRACT TRUNCATED AT 250 WORDS)     

Identification and quantification of mRNA for nerve growth factor in histological preparations

Hybridization histochemistry has been used to detect mRNA for nerve growth factor (NGF) in histological preparations of mouse salivary glands and rat iris using a 32P-labelled cDNA probe and autoradiography. Label was visible over the tubular cells of the male mouse submaxillary gland but not the sublingual gland. A much lower label density was found over the tubular cells of the female submaxillary gland, whereas sections of liver and pancreas were negative. Quantitative autoradiography allowed the detection of low levels of mRNA for NGF in the rat iris which was elevated by prior culture of the tissue. The results provide direct histological evidence for the presence of specific NGF-mRNA in the mouse submaxillary gland and rat iris, with increased levels following culture.     

The effect of bilateral sectioning of the chorda tympani and the greater superficial petrosal nerves on the sweet taste in the rat

The effects of bilateral deafferentation of the greater superficial petrosal (GSP) and the chorda tympani (CT) nerves on the ingestion of sucrose solutions were studied in rats. The rats received five daily sequential 30 second exposures for each sucrose concentration, and the average number of licks per exposure was calculated. Sucrose concentrations of 0.01, 0.03, 0.10, 0.32, and 1.00 M were presented in ascending order across days, both before and after bilateral sectioning of both the CT and the GSP nerves, the CT alone, the GSP alone, or a sham surgery. Prior to surgery, mean lick rate increased with increasing concentrations of sucrose. Following surgery, the rats with combined GSP and CT nerve sections showed a significant decrease in mean rate of licking to the sucrose solutions. The rats with GSP sections showed a similar decrease in mean lick rate to the sucrose solutions. Animals with sections of the CT nerve and sham-operated animals showed no significant reduction in mean lick rate to the sucrose solutions. These results indicate that the GSP nerve is important to the rat in reinforcing high lick rates to sucrose.     

The problem of the effect of parasympathetic denervation on the protein metabolism of the submaxillary salivary glands of the cat

Summary The author studied the influence of unilateral and bilateral section of the chorda tympani on the protein metabolism and the secretory function of the submaxillary salivary glands in cats. The reflex and the paralytic secretion in the denervated glands was absent. The nitrogen content of mucin in the tissues of these glands increased.Changes in protein metabolism and in the secretory activity of the glands were more marked with bilateral denervation. Denervation did not stop secretion and synthesis of protein. Administration of pilocarpine caused typical changes in the salivary secretion and in the content of nitrogenous substances in the saliva and in the tissue of the salivary glands: 1) thelatent period decreased; 2) the maximum quantity of the secreted saliva was changed; 3) after denervation the total amount of saliva secreted in 2 hours decreased (the content of the nitrogenous substances, however, was much higher than before denervation; 4) after pilocarpine the nitrogen content of mucin in the tissues of the denervated glands decreased and the content of protein which was not precipitated with mucin and which is typical of an actively secreting gland increased.Thus, the parasympathetic denervation alters protein metabolism and the secretory function of the submaxillary salivary glands in cats.(Presented by Active Member AMN SSSR V. N. Chernigovskii) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 50, No. 10, pp. 67–73, October, 1960