Gustatory-salivary reflex: neural activity of sympathetic and parasympathetic fibers innervating the submandibular gland of the hamster

Electrophysiological experiments were performed to clarify the neural control mechanisms subserving gustatory-salivary reflex in anesthetized and decerebrate hamsters. Efferent neural activities of postganglionic sympathetic and preganglionic parasympathetic fibers, innervating the submandibular gland, were recorded when taste stimuli were infused into the oral cavity. Neural activities of primary gustatory afferents were also recorded from the chorda tympani (innervating the anterior part of the tongue) and the glossopharyngeal nerve (innervating the posterior part of the tongue). The parasympathetic fibers showed a low rate of spontaneous discharges (about 0.3 Hz), and responded tonically in an excitatory manner to taste stimulation. The magnitude of parasympathetic activity was highly correlated with the magnitude of gustatory afferent responses of the chorda tympani rather than that of the glossopharyngeal nerve. On the other hand, the sympathetic fibers showed irregular burst discharges (1.5 burst/s), and the rate of burst discharges was increased in response to high concentrations of HCl (0.03 M) or NaCl (1 M) solutions. Deafferentation experiments suggest that the parasympathetic activity is mainly influenced by gustatory information via the chorda tympani, while the sympathetic activity can be evoked by both the chorda tympani and glossopharyngeal nerve.     

The reinnervation of the tongue and salivary glands after lingual nerve injuries in cats

The recovery of fibres in the chorda tympani and lingual nerves has been investigated in cats following nerve injury by recording the receptor properties of gustatory, thermosensitive and mechanosensitive units and the return of vasomotor and secretomotor responses. The combined trunk of the chorda tympani and lingual nerves was either crushed (4 animals) or sectioned (3 animals) unilaterally and recovery allowed for 12 weeks. After nerve crush, integrated whole nerve activity recorded from the chorda tympani during stimulation of the tongue with gustatory or thermal stimuli revealed a response profile which was similar to controls. After nerve section little or no activity could be recorded. Recordings made from 52 single units in the chorda tympani after nerve crush revealed that the proportions of gustatory, thermosensitive and mechanosensitive units were similar to those of controls. The units had slower conduction velocities, responded less vigorously and to a narrower range of stimuli. Recordings made from 46 units in the chorda tympani after nerve section revealed very few gustatory or thermosensitive units, the majority were purely mechanosensitive and the decrease in conduction velocity was greater than after nerve crush. Electrical stimulation of efferent vasodilator fibres in both the chorda tympani and lingual nerves, evoked a temperature rise on the dorsal surface of the tongue. This effect was completely restored after nerve crush but was significantly smaller after nerve section. The flow rate of saliva from the submandibular salivary gland was not significantly changed by nerve crush but was significantly smaller after nerve section. There was no evidence for functional reinnervation of gustatory or secretomotor terminals by inappropriate fibre types.     

Inhibitory interactions among rodent taste axons

The left side of the tongue of the Mongolian gerbil, Meriones unguiculatus, was experimentally innervated with both chorda tympani nerves. While this dual innervation did not increase the number or volume of fungiform taste buds on the left side, at least half of the taste buds were dually innervated since they could be neurotrophically maintained by either chorda tympani nerve. Impulse discharges occurred simultaneously in the native (left) and foreign (right) chorda tympani nerves when the taste stimulus was restricted to the left side of the tongue. The marked attenuation of the phasic or tonic portions of some taste responses suggested that dual innervation had enhanced inhibition, especially of foreign chorda tympani responses. This was confirmed when electrical stimulation of the native chorda tympani reduced the peak summated action potential discharges of the foreign chorda tympani to NaCl or sucrose by an average of 52 and 41%, respectively. Inhibition began within seconds and continued with an 11.5-min half-life. The inhibition was unaffected by acutely disconnecting either chorda tympani nerve from the brain. We propose that dual chorda tympani innervation accentuated lateral inhibitory connections that may function normally to reduce spurious sensory signals in taste axons.     

Branched chorda tympani neurons and interactions among taste receptors

The preterminal branching pattern of nerve fibers from the chorda tympani nerve was examined in the tongues of rats using a normal silver stain. The lingual branch of the trigeminal nerve was sectioned unilaterally proximal to where it is joined by the chorda tympani and these fibers were allowed to degenerate from eight to ten days prior to sacrifice of the animals. Termination of the chorda tympani fibers in the anterior tongue was found to be limited to the taste bud region of the fungiform papillae. The control side of the tongue showed ubiquitous fiber terminations in the basal layers of the common epithelium of the filiform papillae as well as in the lateral walls of the fungiform papillae. These fibers were assumed to be of trigeminal origin. Seventy-nine percent of the fungiform papillae on the experimental side of the tongue received fibers from a branched nerve bundle. Branch points within the nerve bundles were located deep within the tongue 300–500 μ below the taste bud in 43% of the papillae, at the base of 36% of the fungiform papillae about 125 μ below the taste bud, and within the papilla. These data corroborate electrophysiological observations that single chorda tympani fibers receive input from more than one taste bud. Lateral inhibition observed among adjacent taste buds has been postulated to result from interaction of fiber inputs at branch points in the afferent fiber. From a theoretical consideration of the morphology of chorda tympani nerve fibers it is concluded that modification of the neural response may be feasible at branch points.     

The effect of brain-derived neurotrophic factor on sensory and autonomic function after lingual nerve repair

Brain-derived neurotrophic factor (BDNF) is important in the response to peripheral nerve injury and may enhance regeneration. We have assessed its role in the functional recovery of sensory afferents and autonomic efferents after repair of the chorda tympani and lingual nerves in the cat. Six months after entubulation repair, with or without the incorporation of BDNF at the repair site, the recovery of secretomotor and vasomotor efferents was determined by recording salivary flow from the submandibular gland and temperature changes on the tongue surface, each evoked by stimulation of the repaired nerve. Electrophysiological recordings from the lingual and chorda tympani nerves proximal to the repair were undertaken to characterise mechanosensitive, thermosensitive, and gustatory afferents. When compared with data from uninjured control animals, both repair groups showed changes in receptor sensitivity and spontaneous discharge, and persistent reductions in conduction velocity, proportion of gustatory and thermosensitive units, rate of salivary secretion, and vasomotor responses. Comparisons between the outcome of repair with or without BDNF revealed few differences. In the BDNF group, fewer units in the chorda tympani responded to gustatory or thermal stimuli and the sensitivity of the gustatory units was lower. The conduction velocity of afferents in the lingual nerve was also lower, but the mechanoreceptive field size was higher. Thus, despite its known trophic role in the gustatory system, BDNF had not enhanced recovery of these or other fibre populations. We conclude that the application of BDNF to a site of lingual nerve repair has a negative effect on the long-term outcome.     

Coupling between chorda tympani nerve fibres in the cat

Evidence for coupling between a small proportion of chorda tympani nerve fibres has been obtained in cats. Single chorda tympani fibres from which a response was recorded following electrical stimulation of another chorda tympani fibre were taken as being coupled to that fibre. The coupled fibres were associated with between 1 and 4 fungiform papillae on the tongue and they showed responses to gustatory, mechanical and electrical stimulation of those papillae.     

Recovery of Chorda Tympani Nerve Function Following Injury

The chorda tympani (CT) nerve carries taste information from the anterior tongue to the brain stem. Injury to the chorda tympani may result in loss or distortion of taste information. This study examined changes occurring in the hamster peripheral taste system during recovery from injury. The hamster chorda tympani nerve was crushed in the middle ear and the animals were allowed to survive from 2 to 16 weeks. At 2 weeks, CT fibers had degenerated distal to the crush site. Up to 16 weeks after crush, there were 67% fewer myelinated fibers in regenerated nerves than in controls. The mean area of the Ca²⁺-ATPase-stained core of the fungiform taste buds was significantly smaller than in controls 2 weeks after injury, but recovered to control values by 4 weeks. Electrophysiological responses to taste stimuli were recorded from the chorda tympani distal to the injury. No responses were seen after 2 weeks; weak and unstable responses were seen after 3 weeks. By 4–8 weeks, relative responses to taste stimuli were similar to control responses, but the variability of the responses to sucrose was significantly greater than that in controls. The frequency of responses to the water rinse following taste stimuli, particularly sucrose, was also greater in the regenerated nerves. The abnormal electrophysiological responses to sucrose may be the result of the differential rate of return of fiber types and/or the transduction mechanisms. In some ways, recovery of the peripheral gustatory system after damage to the chorda tympani nerve recapitulates the later stages of taste bud development.     

The effect of delayed nerve repair on the properties of regenerated fibres in the chorda tympani

The characteristics of regenerated fibres in the chorda tympani have been investigated in cats after nerve section without repair or after section followed by nerve repair twelve weeks later. In the unrepaired group the animals were allowed to recover for twenty four weeks and after delayed repair there was a further recovery period of twelve or twenty four weeks. The properties of gustatory, thermosensitive and mechanosensitive units and the return of vasomotor and secretomotor responses were then investigated and data compared with that from normal controls and from animals which had undergone immediate nerve repair. After nerve section, integrated whole-nerve activity recorded from the chorda tympani during gustatory or thermal stimulation of the tongue was reduced when compared to controls, but there were only small differences between the repaired and unrepaired groups. Recordings made from single units in the chorda tympani revealed that more units were spontaneously active after repair (P < 0.05) and the gustatory units produced more impulses when stimulated (P < 0.005). Twelve weeks after delayed repair the units had slower conduction velocities than those in the unrepaired nerves (P < 0.001), but by twenty four weeks after repair they were significantly faster (P < 0.05). There was little difference in the level of recovery twelve weeks after immediate or delayed repair. We conclude that delayed nerve repair results in better recovery than leaving the nerve unrepaired and that a twelve week delay before repair has little effect.     

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

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

Specificity of amiloride inhibition of hamster taste responses

Amiloride, a blocker of epithelial sodium channels, was found to have significant effects on electrophysiological and behavioral taste responses in the golden hamster (Mesocricetus auratus). Recordings from the whole chorda tympani nerve showed that amiloride rapidly, reversibly, and competitively inhibited responses to NaCl applied to the anterior tongue. The apparent dissociation constant for amiloride binding, extrapolated to zero NaCl concentration, was 10 nM, a value comparable to estimates for various transporting tight epithelia. Recordings from single chorda tympani nerve fibers showed that 10 microM amiloride completely inhibited responses of Na-selective N fibers but had minimal effect on responses of electrolyte-sensitive H fibers, even though both types of fibers responded well to 0.1 M NaCl. Sucrose responses were not affected by amiloride. Addition of 100 microM amiloride to 0.1 M NaCl consistently increased consumption of NaCl in two-bottle drinking tests. These data suggest that one mechanism by which the taste of NaCl is sensed, which does not require adsorption or a second messenger, involves entry of Na+ into taste bud cells through an amiloride-blockable sodium channel. Taste bud cells utilizing this mechanism exclusively activate N fibers, which are involved in the control of NaCl intake. A different mechanism for the detection of NaCl and other electrolytes is utilized by taste bud cells that activate H fibers.     

The effects of glossopharyngeal and chorda tympani nerve cuts on the ingestion and rejection of sapid stimuli: an electromyographic analysis in the rat

The present study tested the effects of bilateral section of either the chorda tympani or glossopharyngeal nerves on the production of oro-pharyngeal electromyographic (EMG) responses to intra-oral sapid stimulation. The responses of adult rats fitted with intra-oral cannulas and fine-wire electrodes in the anterior digastric (jaw opening) and thyropharyngeus (swallowing) muscles were examined following direct oral stimulation with water and 5 concentrations of sucrose, NaCl, and quinine monohydrochloride (QHCl). One group of rats was tested both before and after bilateral removal of the chorda tympani. A second group of rats was tested subsequent to bilateral removal of the glossopharyngeal nerves. A normal EMG response pattern to suprathreshold QHCl consisted of several intra-oral licks followed by a series of large amplitude mouth openings (gapes). In addition, there was a longer latency to the first swallow following QHCl stimulation compared to water stimulation. Cutting either nerve affected this rejection response to QHCl, but produced little change in the ingestive response to the other stimuli. Following chorda tympani nerve cuts, rats showed an increased latency to the first gape and a small reduction in the number of gapes across the 5 concentrations of QHCl (16%). In contrast, bilateral section of the glossopharyngeal nerves produced a much larger reduction in the number of gapes (54%), but had no effect on the latency to the first gape. In addition, the latency to swallow suprathreshold QHCl was shorter following glossopharyngeal nerve cuts. These observations suggest that gustatory receptors on the anterior tongue, innervated by the chorda tympani, initiate a rejection response, but that receptors on the posterior tongue, innervated by the glossopharyngeal nerve, are necessary for a sustained rejection sequence.     

Hypoglossal motor nerve activity elicited by taste and thermal stimuli applied to the tongue in rats

Single unit activity of hypoglossal motor nerve fibers which innervate the tongue muscles was recorded in lightly anesthetized non-decerebrate and acute decerebrate rats. The pattern of responses to taste and thermal stimuli applied to the tongue surface was classified into 4 types. The type 1 response is characterized by short-lasting rhythmic burst discharges, the type 2 consists of both the rhythmic burst and tonic discharges, the type 3 is long-lasting tonic discharges and the type 4 shows short-lasting burst or short-lasting tonic discharges. In non-decerebrate rats, most of the fibers (93%) showed no or a few spontaneous firings. Sucrose and NaCl were the most effective stimulants, and 70–80% of the fibers showed the type 1 response to these stimuli. Calculating the correlations between response patterns of the fiber to a pair of the stimuli, sucrose and NaCl, and HCl and quinine produced a similar response profile, respectively. In decerebrate rats, however, about 21% of fibers showed a highly regular spontaneous firing (about 30 Hz). Rhythmic burst responses (types 1 and 2) were not induced, and thermal (especially cold) stimulation elicited much better responses than the taste stimuli. HCl and quinine, but not sucrose and NaCl, produced a similar response profile. These characteristic properties of the response in acute decerebrate rats may in part be attributed to inactivation of a ‘rhythmic center’ in the brain stem.     

Cortical responses to electrical and gustatory stimuli in the rabbit.

The distribution of surface positive cortical potentials evoked by electrical stimulation of the chorda tympani, glossopharyngeal and lingual nerves which innervate the tongue was mapped in rabbits. All projections were bilateral. Judging from the extent of the cortical response area and the amplitude and latency of the responses, the major projection of the chorda tympani was ipsilateral, whereas that of the lingual and the glossopharyngeal nerves was contralateral. Both the chorda tympani and the glossopharyngeal nerve project to a confined area in the insular cortex and the lingual nerve projects to the appropriate part of the somatotopic pattern of somatic sensory area I. Further, a single unit study was undertaken to characterize the response of units in the cerebral cortex which was induced by gustatory stimulation of the anterior tongue, Twenty-four gustatory units were found in the insular cortex and the claustrum. The gustatory units were divided into an early response type (21 units) and a late response type (3 units) based on latency measurements. Gustatory units were also classified according to discharge patterns into excitation type (21 units) and inhibition type (4 units). Eleven units responded to 1 or 2 kinds of conventional taste stimuli, and 13 units responded to more than 3 different taste stimuli. Sensitivities of cortical units to the 4 conventional taste stimuli were found to be mutually independent and randomly distributed among cortical units. The frequency of discharges increased in the excitation type units and decreased in the inhibition type units monotonically with the excitation type units and decreased in the inhibition type units monotonically with an increse of NaCl concentration exfept at the highest concentrations.     

On a neural mechanism for cortical processing of taste quality in the rat

The responses of 31 chorda tympani fibers and 47 cortical neurons were recorded in response to 6 concentrations of NaCl, and single concentrations of sucrose, HCL, and quinine hydrochloride solutions applied to the anterior portion of the tongue in rats. The neural responses were analyzed in terms of the two hypotheses of quality coding: across-neuron response pattern and across-region response pattern notions. In a behavioral experiment, animals were given a conditioned taste aversion to one of 5 concentrations of NaCl solution by pairing it with a gastrointestinal illness caused by i.p. injection of 0.15 M LiCl. Behavioral taste profiles were constructed for each stimulus from the suppression of drinking, which indicates the extent of generalization of aversion to each of the 4 basic taste stimuli. Among the two neural analyses employed for the chorda tympani and cortical units, across-region correlation coefficients for cortical neurons that were derived from the across-region response pattern theory showed the highest correlation (r= 0.89) with the behavioral suppression rates. Across-neuron correlation coefficients in the chorda tympani fibers also showed a good correlation (r= 0.81) with the behavioral data. However, the taste profile for 1.0 M NaCl in chorda tympani fibers was very similar to that for the lower concentrations of NaCl, in spite of the suggestion from the behavioral experiment and the neural analyses of cortical responses that 1.0 M NaCl has HCl and quinine components besides the NaCl component. The present result confirmed the idea that processing of taste information in the cortex involves differences in both response magnitude across neurons and the spatial localization of those neurons.     

The role of substance P and calcitonin gene-related peptide containing nerve fibers in maintaining fungiform taste buds in the rat after a chronic chorda tympani nerve injury.

Taste buds in the anterior part of the tongue of adult rats were denervated by unilateral resection of the chorda tympani nerve in the middle ear. Three months later one group of animals was perfused and their tongues were processed for demonstration of substance P (SP) and calcitonin gene-related peptide (CGRP) immunoreactivity. Fungiform taste buds found on the denervated side showed increased numbers of intragemmal SP- and CGRP-immunoreactive (IR) fibers compared to the normal side. Compared to the normal side, the number of taste buds appeared to be fewer on the denervated side. Moreover, taste buds on this side seemed to be only partially restored. Another group of animals was given the neurotoxin capsaicin which causes a depletion of SP and CGRP from sensory axons. The animals were perfused 2 or 3 weeks after the capsaicin treatment, and their tongues prepared for SP and CGRP immunohistochemistry or for histological examination of taste buds. Very few SP- and CGRP-IR fibers were present in capsaicin-treated animals. In these animals almost all fungiform taste buds and papillae on the chorda tympani-injured side disappeared. In contrast, normal numbers of taste buds were still present on the contralateral side where the chorda tympani innervation remained intact. It is conceivable that taste buds on the chorda tympani-innervated part of the tongue, deprived of the normal chorda tympani-innervation, can regenerate and become reinnervated by SP- and CGRP-containing fibers, and that these are essential for partially restoring and maintaining the structure of the denervated taste buds and the fungiform papillae.     

Taste fiber responses during reinnervation of fungiform papillae

Crushing or transecting the chorda tympani nerve of the gerbil (Meriones unguiculatus) caused ipsilateral degeneration of taste buds in the fungiform papillae. In less than two weeks some taste fibers regenerated into the tongue and formed new taste buds and receptor cells. The recovery process was evaluated electrophysiologically in 53 gerbils by acute recording proximal to the nerve injury site. Initially the chorda tympani was electrically silent. In gerbils tested at later times spontaneous activity appeared. This was followed by responses to pressure on the tongue. Taste responses returned as early as dasy 11. The receptive field of regenerated taste fibers was limited to a small number of fungiform papillae. Taste responses were always associasted with the presence of one or more taste buds in the receptive field. Taste buds identified as responsive to chemicals contained some fusiform cells. We found thast the taste responses of single fiber, few-fiber and multi-unit preparations reflected the diversity of responses found in normal taste axons as determined by recording from 26 normal single fibers and 27 normal whole nerves. The early emergence of a variety of fiber types and responses to many chemicals in regeneration is inconsistent with the proposition that the relative chemical responsiveness of a receptor cell is strictly a function of its age; the response of a given young taste receptor is not necessarily limited to a few of the standard taste stimulants.     

The effect of nerve growth factor on functional recovery after injury to the chorda tympani and lingual nerves

Nerve growth factor (NGF) is known to ameliorate central changes and enhance the regeneration of damaged axons in the early stages after peripheral nerve injury. We have assessed the long-term outcome of placing NGF at a nerve repair site by determining the functional characteristics of several groups of sensory afferent and autonomic efferent fibres in the cat lingual nerve. Six months after entubulation repair, with or without the incorporation of NGF, the recovery of secretomotor and vasomotor efferents was determined by recording salivary flow from the submandibular gland and temperature changes on the tongue surface, each evoked by stimulation of the repaired nerve. Electrophysiological recordings from the lingual and chorda tympani nerves proximal to the repair allowed characterisation of mechanosensitive, thermosensitive and gustatory afferents. When compared with data from uninjured control animals, both repair groups showed changes in spontaneous discharge and persistent reductions in conduction velocity, receptor sensitivity, proportion of gustatory units, and rate of salivary secretion. Comparisons between the outcome of repair with or without NGF revealed few differences. In the NGF group the conduction velocity of afferents in the lingual nerve was lower, and the level of spontaneous activity was higher. However, NGF appeared to preferentially enhance the regeneration of thermosensitive afferents, suggesting that it may play a role in determining the phenotypic profile of the regenerating axonal population. This suggests that future therapeutic enhancement of regeneration after peripheral nerve injury may require a combination of factors to encourage regeneration of specific fibre groups.     

Local gustatory functions associated with segmental organization of the anterior portion of cat's tongue

Neural basis for local taste specificity within the anterior two-thirds of the mammalian tongue remains obscure. The present electrophysiological study aimed to clarify the topographical organization of tongue receptive areas in the cat and monkey and, also, to evaluate taste responsiveness of different tongue regions within the field of the cat's lingual nerve. The anterior portion of the cat's tongue was found to consist of three receptive areas, each receiving somatic as well as gustatory fibers from one of three lingual nerve ramifications, namely the anterior, medial, and posterior branches. The organization of tongue receptive areas in the monkey was found to be similar to that in the cat except that the anterior portion of the tongue comprised at least five receptive zones which overlapped more extensively than in the cat. Summed responses of the chorda tympani nerve component of the three lingual nerve branches to gustatory and thermal stimulations of the cat's tongue were expressed relative to the whole chorda tympani response to 1 m NaCl, whereas the response of the trigeminal nerve component to cooling was expressed relative to the whole trigeminal nerve response to a standard temperature. It was found that the anterior branch field responded poorly to all taste qualities but strongly to cooling of the tongue. The medial branch field was highly responsive to NaCl, water and warmed saline, whereas the responsiveness of the posterior branch field to HC1 was stronger than other areas. The findings show localization of specific sensory properties over different tongue regions.     

Gustatory neural response in the mouse

Gustatory responses from the mouse chorda tympani nerve were tested with various chemical solutions. Magnitudes of integrated chorda tympani responses to the 4 basic taste stimuli were greater in the order of HCl, sucrose, NaCl and quinine-HCl. Sucrose was the most effective sugar tested, while NaCl was the least effective salt, but divalent chloride salts were prominently effective stimulants. Many of single chorda tympani fibers responded specifically to one or two of the 4 basic taste stimuli. Single fibers sampled were classified into the following 5 types: sweet-type, Na-type, CaMg-type, acid-type and quinine-type.     

Elucidating coding of taste qualities with the taste modifier miraculin in the common marmoset

To investigate the relationships between the activity in different types of taste fibers and the gustatory behavior in marmosets, we used the taste modifier miraculin, which in humans adds a sweet taste quality to sour stimuli. In behavioral experiments, we measured marmosets' consumption of acids before and after tongue application of miraculin. In electrophysiological experiments responses of single taste fibers in chorda tympani and glossopharyngeal nerves were recorded before and after tongue application of miraculin. We found that after miraculin marmosets consumed acids more readily. Taste nerve recordings showed that after miraculin taste fibers which usually respond only to sweeteners, S fibers, became responsive to acids. These results further support our hypothesis that the activity in S fibers is translated into a hedonically positive behavioral response.