Building sensory receptors on the tongue

Neurotrophins, neurotrophin receptors and sensory neurons are required for the development of lingual sense organs. For example, neurotrophin 3 sustains lingual somatosensory neurons. In the traditional view, sensory axons will terminate where neurotrophin expression is most pronounced. Yet, lingual somatosensory axons characteristically terminate in each filiform papilla and in each somatosensory prominence within a cluster of cells expressing the p75 neurotrophin receptor (p75NTR), rather than terminating among the adjacent cells that secrete neurotrophin 3. The p75NTR on special specialized clusters of epithelial cells may promote axonal arborization in vivo since its over-expression by fibroblasts enhances neurite outgrowth from overlying somatosensory neurons in vitro. Two classical observations have implicated gustatory neurons in the development and maintenance of mammalian taste buds--the early arrival times of embryonic innervation and the loss of taste buds after their denervation in adults. In the modern era more than a dozen experimental studies have used early denervation or neurotrophin gene mutations to evaluate mammalian gustatory organ development. Necessary for taste organ development, brain-derived neurotrophic factor sustains developing gustatory neurons. The cardinal conclusion is readily summarized: taste buds in the palate and tongue are induced by innervation. Taste buds are unstable: the death and birth of taste receptor cells relentlessly remodels synaptic connections. As receptor cells turn over, the sensory code for taste quality is probably stabilized by selective synapse formation between each type of gustatory axon and its matching taste receptor cell. We anticipate important new discoveries of molecular interactions among the epithelium, the underlying mesenchyme and gustatory innervation that build the gustatory papillae, their specialized epithelial cells, and the resulting taste buds.

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.

Time course of morphological alterations of fungiform papillae and taste buds following chorda tympani transection in neonatal rats

The time course of structural changes in fungiform papillae was analyzed in rats that received unilateral chorda tympani nerve transection at 10 days of age. Morphological differences between intact and denervated sides of the tongue were first observed at 8 days postsection, with an increase in the number of fungiform papillae that did not have a pore. In addition, the first papilla with a filiform-like appearance was noted on the denervated side at 8 days postsectioning. By 11 days after surgery, the total number of papillae and the number of papillae with a pore were significantly lower on the transected side of the tongue as compared to the intact side. At 50 days postsection, there was an average of 70.5 fungiform papillae on the intact side and a mean of only 20.8 fungiform papillae the denervated side. Of those few remaining papillae on the cut side, an average of 13.5 papillae were categorized as filiform-like, while no filiform-like papillae occurred on the intact side. Significant reduction in taste bud volume was noted at 4 days posttransection and further decrements in taste bud volume were noted at 8 and 30 days postsection. Electron microscopy of the lingual branch of the trigeminal nerve from adult rats that received neonatal chorda tympani transection showed normal numbers of both myelinated and unmyelinated fibers. Thus, in addition to the well-characterized dependence of taste bud maintenance on the chorda tympani nerve, the present study shows an additional role of the chorda tympani nerve in papilla maintenance during early postnatal development.

P2X purinoceptor-mediated excitation of trigeminal lingual nerve terminals in an in vitro intra-arterially perfused rat tongue preparation

A novel in vitro intra-arterially perfused adult rat tongue-nerve preparation was used to explore the possible actions of P2X purinoceptor agonists (ATP and alpha,beta-methylene ATP (alpha, beta-meATP)) on sensory nerve terminals innervating the rat tongue. We made whole-nerve recordings of the trigeminal branch of the lingual nerve (LN), which conducts general sensory information (pain, temperature, touch, etc.), and the chorda tympani (CT), which conducts taste information. Changes in LN and CT activity following intra-arterial application of P2X agonists were compared. In seven preparations, bolus close-arterial injection of ATP (30-3000 microM, 0.1 ml) or alpha,beta-meATP (10-300 microM, 0.1 ml) induced a rapid (< 1 s after injection), dose-related increase in LN activity that decayed within a few seconds. The minimal concentration of ATP (100 microM) required to elicit a response was about 10-fold higher than that of alpha,beta-meATP (10 microM). Bolus injection of ATP or alpha,beta-meATP induced a moderate decrease in firing frequency in three of seven CT preparations. LN responses to P2X agonists showed signs of rapid desensitisation with the peak frequency of discharge being smaller when the agonists were applied at short intervals. Suramin (200 microM) or PPADS (200 microM) applied by intra-arterial perfusion each antagonised the rapid increase in LN activity following application of alpha,beta-meATP (100 microM). Capsaicin (10 microM, 0.1 ml, n = 5 preparations) was injected intra-arterially to desensitise nociceptive fibres. This was found to block (n = 2) or greatly reduce (n = 3) the excitatory effects of alpha,beta-meATP (100 microM, 0.1 ml) on LN activity, implying that only capsaicin-sensitive nociceptive fibres in LN were responsive to P2X agonists. In contrast to the consistent excitatory responses in LN activity following fast application of P2X agonists as bolus, a variable and moderate change in discharge rate of LN and no change in CT activity (n = 5) was observed after applying ATP (100-300 microM, n = 21) or alpha,beta-meATP (100-300 microM, n = 14) by intra-arterial perfusion. The variable responses in LN activity to slow perfusion in contrast to close-arterial bolus injection are consistent with activation of the rapidly desensitising P2X3 receptors. In summary, ATP and alpha,beta-meATP preferentially activate general sensory afferent fibres (LN) but not taste fibres (CT). We suggest that the increase in whole-nerve activity of LN following application of P2X agonists represents activation of nociceptive fibres which possess P2X3 receptors. Our data indicate that ATP and P2X3 receptors may play a role in nociception, rather than taste sensation in the tongue.

Sensory changes in the territory of the lingual and inferior alveolar nerves following lower third molar extraction

Post-injury inflammation activates nociceptive systems and recruits normally non-nociceptive afferents into a pain processing role. During inflammation, Abeta low threshold mechanoreceptor afferents that usually mediate tactile sensation acquire properties of nociceptors, allowing them to participate in post-injury spontaneous pain and evoked abnormalities such as tenderness and pain to light touch. This study assessed the sensory consequences of post-injury inflammation following extraction of a single, lower third molar tooth. Extensive bilateral evaluations were performed in the territory of nerves assumed to be exposed to both inflammation and mechanical trauma, inflammation alone, or only the central consequences of peripheral inflammation. Testing at the distal termination of nerves assumed to be exposed to local inflammation (mental and lingual nerve territory) revealed decreased detection thresholds (P < 0.05) to electrical stimulation and to mechanical stimulation by sensitive, disposable filaments developed and validated for this application. Testing at sites of assumed inflammation and mechanical trauma (mental nerve territory) showed reduced pain thresholds to electrical stimulation. Thermal detection and pain thresholds were not altered at any location in patients, and no effects were observed in control subjects receiving only local anesthetic injections. These results in humans are consistent with recent experimental evidence that inflammatory processes alter the central consequence of activity in large-diameter Abeta touch primary afferents evoked under natural conditions by gentle mechanical stimulation. These effects result in hyperesthesia, increased sensitivity to light touch, and mechanical allodynia, pain evoked by normally innocuous stimulation of Abeta primary afferents.

Neural activity of the superior salivatory nucleus in rats

We recorded the neural activity of the superior salivatory (SS) neurons in brain slice preparations from neonatal rats in vitro and in decerebrate anesthetized rats in vivo. In the in vitro experiment, the SS neurons were retrogradely labeled by the injection of Rhodamine into the chorda-lingual nerve (labeling SS neurons innervating the submandibular and intra-lingual ganglia) or into the anterior part of the tongue. The SS neurons labeled from the nerve were classifiable into two types: Type-I, tonic firing at a frequency of up to 30 Hz; and Type-II, phasic firing at a higher frequency of up to 70 Hz followed by tonic firing at 30-50 Hz. All of the SS neurons labeled from the tongue were Type-II. Since the anterior tongue is a non-glandular area, the type of cells may be involved in vasodilatation. Type-I neurons, which did not innervate the tongue, may be responsible for salivation. In the in vivo experiment, the reflex activity evoked by taste or mechanical stimulation was recorded from the axons of the SS neurons innervating the submandibular ganglia. These fibers also displayed two firing patterns. One was a tonic firing pattern discharging at 5-30 Hz. The other consisted of a transient firing (about 80 Hz) at the beginning of stimulation and then a prolonged firing at 5-40 Hz. The latter firing pattern was similar to that of the Type-II neurons. These findings suggest that the parasympathetic nerves of the salivary glands contain both the secretory- and vasodilator-type of SS neuron.

Neural regulation of blood flow in the rat submandibular gland

Blood flow in salivary glands is regulated mainly by sympathetic and parasympathetic nerve activity. This study was carried out to determine the relative contributions of cholinergic, adrenergic and peptidergic neurotransmitters to the control of submandibular blood flow in the rat using laser-Doppler flowmetry. Parasympathetic impulses caused a rapid atropine-sensitive vasodilation followed by a maintained increase in blood flow, a portion of which remained in the presence of both atropine and L-NAME. In contrast, continuous sympathetic stimulation caused an intense vasoconstriction that was followed by a prolonged after-vasodilation. The same number of impulses delivered in bursts resulted in a cyclic vasoconstriction followed by a rapid vasodilation. Alpha-adrenoceptor blockade largely abolished the vasoconstriction, and the duration and magnitude of the after-vasodilation were reduced. Inhibition of nitric oxide (NO) synthase by L-NAME reduced the vasodilation. The addition of a beta-adrenoceptor antagonist eliminated the sympathetic vasodilator response, but in the presence of complete alpha- and beta-adrenoceptor blockade and L-NAME a small vasoconstriction remained. We conclude that the vasoconstrictor effects of sympathetic stimulation of the rat submandibular gland are due to alpha-adrenergic receptor activation and probably also NPY, and the vasodilator effects are due to NO and beta-adrenergic activity. Parasympathetic vasodilation was due to NO-independent mechanisms mediated by acetylcholine and substance P, and NO-dependent mechanisms mediated by VIP.

Responses of single lingual nerve fibers to thermal and chemical stimulation

The goals of this study were to characterize the responses of: (1) thermally-sensitive fibers of the lingual branch of the trigeminal nerve to cooling from 35 degrees to 10 degrees C at a rate of 1 degrees C/s; and (2) these neurons to a mid-range concentration of NaCl (150 mM), glucose (150 mM), citric acid (0.3 mM), and quinine-HCl (3 mM) at 35 degrees and 25 degrees C. A cluster analysis of 47 neurons' responses to cooling revealed two major groups and one minor group. Group 1 neurons (n=19) had a shorter latency, exhibited faster time-to-peak activity, and responded over a smaller range of temperature compared to Group 2 neurons (n=22). Group 3 neurons (n=6) exhibited the longest response latency and responded over a wider cooler range of temperature. Twenty-five out of thirty-one thermally-sensitive, non-tactile lingual neurons responded weakly to at least one chemical stimulus, with some neurons responding to 2, 3, or all 4 chemical stimuli. Group 1 neurons responded to more chemical stimuli at 35 degrees C, while Group 2 neurons responded more at 25 degrees C. Under their optimal temperature conditions, Group 1 and Group 2 neurons responded most often to citric acid and least often to glucose, with NaCl and Q-HCl eliciting an intermediate number of responses. As a whole, the responses of thermally-sensitive fibers to chemical stimulation were modest at best with an absence of chemical specificity. There was no evidence of a 'best' stimulus, although there was a suggestion of temporal coding.

Convergence of afferent inputs from the chorda tympani, lingual-tonsillar and pharyngeal branches of the glossopharyngeal nerve, and superior laryngeal nerve on the neurons in the insular cortex in rats

The responses of single neurons in the insular cortex to electrical stimulation of the chorda tympani (CT), lingual-tonsillar branch of the glossopharyngeal (LT-IXth) nerve, pharyngeal branch of the glossopharyngeal (PH-IXth) nerve, and superior laryngeal (SL) nerve were recorded in anaesthetized and paralyzed rats. Ninety-four neurons responding to stimulation of at least one of the four nerves were identified from the insular cortex. Most of the neurons were located in the posterior portion of the insular cortex; the mean location was 0.8 mm anterior to the anterior edge of the joining of the anterior commissure (AC) and was 1.4 mm dorsal to the rhinal fissure (RF). Of the 94 neurons, 84 (89%) received convergent inputs from two or more nerves, and the remaining 10 (11%) received inputs from one nerve. The neurons responding to the CT stimulation were distributed more anteriorly than those responding to other three nerves in the anterior-posterior dimension. Our results indicate that the neurons recorded mainly from the posterior portion of the insular cortex receive convergent inputs from the oropharyngolaryngeal regions.

Development of fungiform papillae, taste buds, and their innervation in the hamster

Fungiform taste buds in mature hamsters are less subject to neurotrophic influences than those of other species. This study evaluates taste-bud neurotrophism during development in hamsters by examining the relation between growing nerves and differentiating fungiform papillae. Chorda tympani (CT) or lingual (trigeminal) nerve (LN) fibers were labelled with Lucifer Yellow as they grew into (CT fibers) or around (LN fibers) developing taste buds. Developing fungiform papillae and taste pores were counted with the aid of a topical tongue stain. The tongue forms on embryonic days (E) 10.5-11 and contains deeply placed CT and LN fibers but no papillae. By E12, the tongue epithelium develops scattered elevations. These "eminences" selectively become innervated by LN fibers that grow to the epithelium earlier and in larger numbers than CT fibers. Definitive fungiform papillae form rapidly during E13-14 and become heavily innervated by LN fibers. Intraepithelial CT fibers, rare at E13, invariably innervate fungiform papillae containing nascent taste buds at E14. During E14-15 (birth = E15-16), most papillae contain taste buds with pores, extensive perigemmal LN innervation, and extensive intragemmal CT innervation. At birth, numbers of fungiform papillae and taste pores are adultlike. The results show that fungiform eminences begin forming in the absence of innervation. The subsequent differentiation of definitive fungiform papillae and their innervation by LN fibers occur synchronously, prior to the differentiation of taste buds and their CT innervation. The hamster is precocious (e.g., compared to rat) in terms of LN development and the structural maturity of the anterior tongue at birth.

Selectivity of lingual nerve fibers to chemical stimuli

The cell bodies of the lingual branch of the trigeminal nerve were localized in the trigeminal ganglion using extracellular recordings together with horseradish peroxidase labeling from the tongue. Individual lingual nerve fibers were characterized with regard to their conduction velocities, receptive fields, and response to thermal, mechanical, and chemical stimuli. Fibers were classified as C, A delta, A beta, cold, and warm. The chemical stimuli included NaCl, KCl, NH4Cl, CaCl2, menthol, nicotine, hexanol, and capsaicin. With increasing salt concentration the latency of the response decreased and the activity increased. The responses elicited by salts (to 2.5 M), but not nonpolar stimuli such as menthol, were reversibly inhibited by 3.5 mM of the tight junction blocker, LaCl3. These data suggest that salts diffuse into stratified squamous epithelia through tight junctions in the stratum corneum and stratum granulosum, whereupon they enter the extracellular space. 11 C fibers were identified and 5 were characterized as polymodal nociceptors. All of the C fibers were activated by one or more of the salts NaCl, KCl, or NH4Cl. Three C fibers were activated by nicotine (1 mM), but none were affected by CaCl2 (1 M), menthol (1 mM), or hexanol (50 mM). However, not all C fibers or even the subpopulation of polymodals were activated by the same salts or by nicotine. Thus, it appears that C fibers display differential responsiveness to chemical stimuli. A delta fibers also showed differential sensitivity to chemicals. Of the 35 characterized A delta mechanoreceptors, 8 responded to NaCl, 9 to KCl, 9 to NH4Cl, 0 to CaCl2, menthol, or hexanol, and 2 to nicotine. 8 of 9 of the cold fibers (characterized as A delta's) responded to menthol, none responded to nicotine, 8 of 16 were inhibited by hexanol, 9 of 19 responded to 2.5 M NH4Cl, 5 of 19 responded to 2.5 M KCl, and 1 of 19 responded to 2.5 M NaCl. In summary, lingual nerve fibers exhibit responsiveness to chemicals introduced onto the tongue. The differential responses of these fibers are potentially capable of transmitting information regarding the quality and quantity of chemical stimuli from the tongue to the central nervous system.

Axon populations in cat lingual and chorda tympani nerves

The lingual and chorda tympani nerves from five cats were examined so that normal axonal populations could be determined. After perfusion fixation, the chorda tympani and lingual nerves were removed and processed, and sections were taken from individual and combined nerves for both light and electron microscopy. The chorda tympani remained as a distinct group of smaller axons for at least 4 mm distal to its junction with the lingual nerve. The mean number +/- S.D. of myelinated axons in the chorda tympani central to the junction was 1322 (+/- 268) and in the lingual nerve central to the junction, 3227 (+/- 510). The counts were not significantly different distal to the junction, and there were no side-to-side differences. Mean myelinated axon circumferences were significantly smaller in the chorda tympani (12.86 +/- 0.87) than in the lingual nerve (22.79 +/- 1.99; p less than 0.01). The mean size of axons in the chorda tympani was slightly but consistently larger on the left (13.1 +/- 0.73) than on the right side (12.61 +/- 1.01; p less than 0.05). Distal to the junction, the average proportion of non-myelinated axons was 44% in both chorda tympani and lingual nerves.

Direct amygdaloid projections to the superior salivatory nucleus: a light and electron microscopic study in the cat

Amygdaloid projections to the superior salivatory nucleus (SSN) were investigated in the cat by using the anterograde and retrograde tracing techniques of horseradish peroxidase (HRP). After HRP injections were made into the lingual nerve, retrogradely labeled SSN neurons were located in the lateral tegmental field medial to the spinal trigeminal nucleus from the middle level of the superior olivary nucleus to the caudal level of the facial nucleus. These labeled neurons, triangular, oval or polygonal in shape, were small to medium-sized (12-29 microns) and formed loosely packed clusters. In further HRP studies, HRP injections were made into the amygdala and in the reticular formation containing the SSN neurons. The results suggested that the SSN receives direct afferents from the central nucleus of the amygdala with ipsilateral predominance. Final proof of such direct connections from amygdala to the SSN can be obtained only by electron microscopic study. Therefore, HRP injections were made into the lingual nerve and in the amygdala in the same animal and electron microscopic observations were carried out on the SSN. It appeared that anterogradely labeled amygdalo-tegmental fibers formed axosomatic and axodendritic synaptic contacts with retrogradely labeled SSN neurons.

Microanatomic analysis of the trigeminal nerve and potential nerve graft donor sites

A histologic study was undertaken to define the microanatomic characteristics of two commonly injured peripheral trigeminal nerve branches (lingual and mandibular nerves) and the two nerves most frequently procured for use in their interpositional graft repair (sural and greater auricular nerves). Nerves, obtained from fresh human cadavers, were evaluated for total fascicular area, fascicle number, axon number, axon size, and axon density. The peripheral branches of the trigeminal nerve (third division) were morphometrically similar, with only a slight decrease in axon density in the lingual nerve. Comparisons between the donor nerves, however, showed numerous discrepancies at the axonal level. While the fascicular area of the sural nerve was only slightly smaller, axon numbers and densities were only one-half that of the trigeminal nerves. Although the greater auricular nerve was appreciably smaller in overall size, a much better correlation existed with the trigeminal nerve in axonal qualities. These microanatomic findings raise questions about the potential capability of these graft choices to optimally restore axonal connections between nerve ends in trigeminal nerve repair.

Trigeminal nerve responses in the rat elicited by chemical stimulation of the tongue

Epithelial and neural mechanisms underlying trigeminal chemoreception were investigated by recording lingual nerve responses to chemical stimulation of the tongue. The chloride salts, NaCl, KCl, NH4Cl, and CaCl2, each elicited distinctly different, integrated whole-nerve responses (thresholds, 0.5-2.0 M). Incubation of the tongue with lanthanum (2.5 mM), which reduces the permeability of epithelial tight junctions, reversibly attenuated these salts responses. It did not affect neural responses to mechanical and thermal stimuli. Incubation with other established transport inhibitors--amiloride, tetra-ethyl ammonium or tetrodotoxin, had no effect on the salt responses. Acetic and hydrochloric acids (thresholds, 0.1-1.0 M), and MgCl2 and BaCl2 (greater than or equal to 0.5 M), also elicited distinctive responses. Other salts, MgSO4, Na isethionate and LaCl3 (greater than or equal to 1.0 M), and also ethanol (4 M), capsaicin (100 mM), nicotine (29.6 mM) and dextrose (0.5-2.5 M), did not elicit responses. These results indicate that ions of selected salts can diffuse through the tight junctions of the lingual epithelium to activate the trigeminal nerve, and suggest that both the cation and the anion may be important in determining if the nerves are activated and the wave-form of the responses.

The effects of hetero-reinnervation within the rat tongue

This study centred on the effects of hetero-reinnervation within the rat tongue. Lingual, greater auricular and vagus nerves were used. Proximal segments of each of these nerves were joined to the distal hypoglossal stump using 10/0 nylon suture. Histological sections of the tongue were examined. In lingual hypoglossal cross-union, histological evidence indicated that neuromuscular junction (nmj) reinnervation was present to some degree in all intrinsic tongue muscles and within geniohyoid. Lingual nerve fibres were also found within the dorsal keratinised lingual epithelium, reinnervating taste buds and intralingual ganglia. Greater auricular nerve fibres were unable to make contact with nmjs, nor were they found to any appreciable degree within the lingual mucosa. Vagal nerve fibres were found close to, but not innervating, nmjs, but did reinnervate taste buds, lingual vasculature and intralingual ganglia. It was concluded that nodose cells are unable to reinnervate vacated nmjs, but like lingual fibres can reinnervate lingual mucosa, intralingual ganglia and vasculature.

Quantitative diameter analysis of lingual nerve axons in man

Previous studies on axon counts and fiber-diameter spectra in lingual nerves have been carried out only on animal models. This study reports an histological investigation on a series of 20 lingual nerves removed post mortem from human subjects. The results show wide variation in the myelinated fiber counts--a variation which does not appear to be related to the ages of the subjects. When the results are compared with those of a previous study (Heasman and Beynon, 1983), it is seen that the lingual nerve:inferior dental nerve ratio of axon counts is not a consistent index. The fiber-diameter spectrum for the human lingual nerve is characterized by a bimodal curve with the more pronounced peak in the small-diameter fiber range.

Graphic-digitizer analysis of axon spectra in ethmoidal and lingual branches of the trigeminal nerve

Sections were removed from the lingual and ethmoidal nerves of cats and histologically prepared, and the fibers were analyzed under the light microscope. Neural dimensions were measured by a new technique, employing a graphic digitizer and computer. The outline of a neural structure was traced with the digitizer pen, and the total number of axons, their cross-sectional areas, shapes, diameter spectra, and locations within the nerve were calculated. Both nerves had unimodal axon spectra with the peak between 2 and 6 mum diameter. Differences in axon composition occurred over the diameter range of 9 to 20 mum; the lingual nerve had many axons in this range, the ethmoidal nerve only a few. The total number of myelinated axons was near 4000 in the lingual nerve, near 1400 in the ethmoidal nerve; only the latter had many large-sized Remak bundles (containing C-fibers). Most myelinated axons were not perfectly circular but exhibited various degrees of distortion.