Morphologic study of the laryngeal taste buds in the cat

Age-Related Expression of α-Gustducin in the Rat Larynx

Objectives:

The age-related changes in distribution of α-gustducin–immunoreactive structures in the larynx of Sprague-Dawley rats were studied.

Methods:

For this purpose, tissues obtained from 12 male rats ranging in age from 5 to 21 weeks were compared with respect to the distribution and morphology of laryngeal taste buds immunoreactive for α-gustducin, the α-subunit of a taste cell–specific G protein.

Results:

Three different morphological types of α-gustducin–immunoreactive structures were seen: typical gemmal forms, clusters composed of 2 or 3 cells, and isolated immunoreactive cells not associated with taste buds. α-Gustducin–immunoreactive structures were most abundant in the epiglottis in all age groups. α-Gustducin–immunoreactive cells in rats seem to be located along the lateral food channels, in which they may come in contact with food. The total number of these α-gustducin–immunoreactive structures did not show any age-related changes, but the percentage of solitary immunoreactive cells in 5-week-old rats was significantly larger than the percentages in 8-, 14-, and 21-week-old animals.

Conclusions:

Solitary α-gustducin–immunoreactive cells, which are abundant in 5-week-old rats but are found in fewer numbers along the base of the epiglottis in mature rats, may be nociceptic in function, whereas the chemosensory clusters or buds that contain α-gustducin–positive cells and are distributed along the lateral food channels on the pharyngeal side of the larynx may have a role in gustatory reception.

The distribution of galanin-immunoreactive nerve fibers in the rat pharynx

Galanin (GAL) consists of a chain of 29/30 amino acids which is widely distributed in the central and peripheral nervous systems. In this study, the distribution of GAL-immunoreactive (-IR) nerve fibers was examined in the rat pharynx and its adjacent regions. GAL-IR nerve fibers were located beneath the epithelium and taste bud-like structure of the pharynx, epiglottis, soft palate and larynx. These nerve fibers were abundant in the laryngeal part of the pharynx, and were rare in other regions. Mucous glands were mostly devoid of GAL-IR nerve fibers. In the musculature of pharyngeal constrictor muscles, many GAL-IR nerve fibers were also located around small blood vessels. However, intrinsic laryngeal muscles contained only a few GAL-IR nerve fibers. The double immunofluorescence method demonstrated that the distribution pattern of GAL-IR nerve fibers was partly similar to that of calcitonin gene-related peptide-IR nerve fibers in the pharyngeal mucosa and muscles. The present findings suggest that the pharynx is one of main targets of GAL-containing nerves in the upper digestive and respiratory systems. These nerves may have sensory and autonomic origins.

The distribution of TRPV1 and TRPV2 in the rat pharynx

Immunohistochemistry for two nociceptive transducers, the transient receptor potential cation channel subfamily V members 1 (TRPV1) and 2 (TRPV2), was performed on the pharynx and its adjacent regions. TRPV1-immunoreactivity (IR) was detected in nerve fibers beneath and within the epithelium and/or taste bud-like structure. In the pharynx, these nerve fibers were abundant in the naso-oral part and at the border region of naso-oral and laryngeal parts. They were also numerous on the laryngeal side of the epiglottis and in the soft palate. TRPV2-IR was expressed by dendritic cells in the pharynx and epiglottis, as well as in the root of the tongue and soft palate. These cells were located in the epithelium and lamina propria. TRPV2-immunoreactive (IR) dendritic cells were numerous in the naso-oral part of the pharynx, epiglottis, and tongue. Abundance of TRPV2-IR dendritic processes usually obscured the presence of TRPV2-IR nerve fibers in these portions. However, some TRPV2-IR nerve fibers could be observed in the epithelium of the soft palate. Retrograde tracing method also revealed that sensory neurons which innervate the pharynx or soft palate were abundant in the jugular-petrosal ganglion complex and relatively rare in the nodose ganglion. In the jugular-petrosal ganglion complex, TRPV1- and TRPV2-IR were expressed by one-third of pharyngeal and soft palate neurons. TRPV2-IR was also detected in 11.5 % pharyngeal and 30.9 % soft palate neurons in the complex. Coexpression of TRPV1 and CGRP was frequent among pharyngeal and soft palate neurons. The present study suggests that TRPV1- and TRPV2-IR jugular-petrosal neurons may be associated with the regulation of the swallowing reflex.

The distribution of transient receptor potential melastatin-8 in the rat soft palate, epiglottis, and pharynx

Immunohistochemistry for transient receptor potential melastatin-8 (TRPM8), the cold and menthol receptor, was performed on the rat soft palate, epiglottis and pharynx. TRPM8-immunoreactive (IR) nerve fibers were located beneath the mucous epithelium, and occasionally penetrated the epithelium. These nerve fibers were abundant in the posterior portion of the soft palate and at the border region of naso-oral and laryngeal parts of the pharynx. The epiglottis was free from such nerve fibers. The double immunofluorescence method demonstrated that TRPM8-IR nerve fibers in the pharynx and soft palate were mostly devoid of calcitonin gene-related peptide-immunoreactivity (CGRP-IR). The retrograde tracing method also demonstrated that 30.1 and 8.7 % of sensory neurons in the jugular and petrosal ganglia innervating the pharynx contained TRPM8-IR, respectively. Among these neurons, the co-expression of TRPM8 and CGRP-IR was very rare. In the nodose ganglion, however, pharyngeal neurons were devoid of TRPM8-IR. Taste bud-like structures in the soft palate and pharynx contained 4-9 TRPM8-IR cells. In the epiglottis, the mucous epithelium on the laryngeal side had numerous TRPM8-IR cells. The present study suggests that TRPM8 can respond to cold stimulation when food and drinks pass through oral and pharyngeal cavities.

Neurochemical characterization of sea lamprey taste buds and afferent gustatory fibers: presence of serotonin, calretinin, and CGRP immunoreactivity in taste bud bi-ciliated cells of the earliest vertebrates

Neuroactive substances such as serotonin and other monoamines have been suggested to be involved in the transmission of gustatory signals from taste bud cells to afferent fibers. Lampreys are the earliest vertebrates that possess taste buds, although these differ in structure from taste buds in jawed vertebrates, and their neurochemistry remains unknown. We used immunofluorescence methods with antibodies raised against serotonin, tyrosine hydroxylase (TH), gamma-aminobutyric acid (GABA), glutamate, calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), calretinin, and acetylated alpha-tubulin to characterize the neurochemistry and innervation of taste buds in the sea lamprey, Petromyzon marinus L. For localization of proliferative cells in taste buds we used bromodeoxyuridine labeling and proliferating cell nuclear antigen immunohistochemistry. Results with both markers indicate that proliferating cells are restricted to a few basal cells and that almost all cells in taste buds are nonproliferating. A large number of serotonin-, calretinin-, and CGRP-immunoreactive bi-ciliated cells were revealed in lamprey taste buds. This suggests that serotonin participates in the transmission of gustatory signals and indicates that this substance appeared early on in vertebrate evolution. The basal surface of the bi-ciliated taste bud cells was contacted by tubulin-immunoreactive fibers. Some of the fibers surrounding the taste bud were calretinin immunoreactive. Lamprey taste bud cells or afferent fibers did not exhibit TH, GABA, glutamate, or NPY immunoreactivity, which suggests that expression of these substances evolved in taste buds of some gnathostomes lines after the separation of gnathostomes and lampreys.

Calretinin immunoreactivity in taste buds and afferent fibers of the grey mullet Chelon labrosus

The presence of the calcium-binding protein calretinin in taste buds of a teleost, the thick-lipped grey mullet, was investigated using immunohistochemical techniques. Taste bud sensory cells had calretinin immunoreactivity. The nerve fiber plexus innervating taste buds, the ganglia and the viscerosensory roots projecting to the vagal lobe, also showed calretinin immunoreactivity. These results demonstrate for the first time the occurrence of calretinin in the taste buds and the taste afferent system of a teleost.

Identification and characterization of a specific sensory epithelium in the rat larynx

A specific laryngeal sensory epithelium (SLSE), which includes arrays of solitary chemoreceptor cells, is described in the supraglottic region of the rat. Two plates of SLSE were found, one on each side of the larynx. The first plate was located in the ventrolateral wall of the larynx, and the second was located in the interarytenoidal region. In SLSE, immunoblotting showed the presence of alpha-gustducin and phospholipase C beta2 (PLCbeta2), which are two markers of chemoreceptor cells. At immunocytochemistry, laryngeal immunoreactivity for alpha-gustducin was localized mainly in solitary chemosensory cells. Double-label immunocytochemistry using confocal microscopy demonstrated that alpha-gustducin-expressing cells in large part colocalize type III IP3 receptor (IP3R3), another key molecule in bitter taste perception. However, some IP3R3-expressing cells do not colocalize alpha-gustducin. At ultrastructural immunocytochemistry, these cells showed packed apical microvilli, clear cytoplasmic vesicles, and cytoneural junctions. SLSE was characterized by high permeability to a tracer due to poorly developed junctional contacts between superficial cells. Junctions were short in length and showed little contact with the terminal web. Ultrastructural analysis showed deep pits among the superficial cells. In SLSE, high density of intraepithelial nerve fibers was found. The lamina propria of the SLSE appeared thicker than that in other supraglottic regions. It was characterized by the presence of a well-developed subepithelial nerve plexus. The immunocytochemical and ultrastructural data suggested that SLSE is a chemoreceptor located in an optimal position for detecting substances entering the larynx from the pharynx or the trachea.

Contribution of free nerve endings in the laryngeal epithelium to CO2 reception in rats

The superior laryngeal nerve (SLN) contains CO2-sensitive fibers. In the laryngeal epithelium, two candidates for CO2 reception have been identified, namely the intraepithelial free nerve endings and the taste buds. To elucidate the contribution of free nerve endings to CO2 reception, electrophysiological activities were recorded during various stages of regeneration of nerve endings following SLN-crush in rats. The left SLN was crushed surgically and maintained from 4 to 40 days for regeneration of nerve endings. Laryngeal sections were processed for immunohistochemical staining of protein gene product 9.5 to observe regeneration of free nerve endings and taste buds in the epithelium. By day 4 after SLN-crush, both the free nerve endings and taste buds had disappeared. Regeneration of the free nerve endings was recognized from day 8, while that of the taste buds started at day 16. On day 40, the number of taste buds on SLN-crush side was similar to that on the untreated side. Electrophysiological recording of SLN throughout the regeneration period (excluding day 4), showed response to intralaryngeal 9% CO2 (stimulation or inhibition) whether or not taste buds were present. Our results showed intralaryngeal CO2 reception without taste bud involvement, indicating that the free nerve endings in the laryngeal epithelium are receptive to intralaryngeal CO2.

Taste buds and nerve fibers in the rat larynx: an ultrastructural and immunohistochemical study

We investigated the rat laryngeal taste buds and their innervation by electron microscopy and immunohistochemical methods. Taste buds were densely arranged in the surface facing the laryngeal cavity of the epiglottis, the aryepiglottic fold, and the cuneiform process of the arytenoid cartilages. The cells of the buds were classified into types I, II, III, and basal cells, the ultrastucture of which was almost the same as that previously reported in lingual taste buds. The type III cells that had synaptic contacts with nerve fibers were considered to be sensory cells. Immunohistochemical analysis revealed thick calbindin D28k-immunoreactive fibers and thin varicose fibers immunoreactive for calcitonin gene-related peptide or substance P in and around the taste bud. Serotonin-immunoreactive cells were also observed here. The results revealed the innervation pattern of laryngeal taste buds to be the same as that in lingual taste buds. Carbonic anhydrase (CA) is known to catalyze the hydration of CO2 and dehydration of H2CO3, and seems to be essential in CO2 reception. Immunoreactivity for CAI was detected in slender cells and that for CAIII was observed in barrel-like cells in the laryngeal taste buds. The pH-sensitive inward rectifier K+ (Kir) channel in the cell membrane may be involved in CO2 reception as well. CAII-reactive cells were also reactive to Kir4.1, PGP 9.5 and serotonin. Our results indicated that CAII and Kir4.1 are located in type III cells of the laryngeal taste buds, and supported the idea that the buds may be involved in the recognition of CO2.

Airway receptors

There are many types of afferent receptor in the airways; at least five in the larynx: pressure, drive, cold, irritant and C-fibre; and at least four in the trachea and bronchi: slowly and rapidly adapting stretch receptors (SARs and RARs), C-fibre receptors, and those in neuroepithelial bodies (NEBs). Histologically enough sensory structures have been identified to account for the various patterns of afferent activity, but most correlations are poor. For the larynx, four or more sensory structures have not definitively been identified with afferent discharges and reflex responses. For the trachea and bronchi, only SARs have been clearly identified morphologically and physiologically. The reflexes and afferent discharges from RARs and C-fibre receptors are fairly clear, some at least of the sensory terminals lie in the epithelium, but receptor complexes have not been mapped out. Nerves in NEBs have been identified, but not their local and central reflex actions.

Morphology of the nerve endings in laryngeal mucosa of the horse

To discuss the significance of laryngeal sensation on various disorders of the horse, we studied the morphological and topographical characteristics of sensory structures in the laryngeal mucosa using immunohistochemistry and immunoelectron microscopy. Various sensory structures, i.e. glomerular endings, taste buds and intraepithelial free nerve endings, were found in the laryngeal mucosa by immunohistochemistry for protein gene product 9.5 (PGP 9.5) and neurofilament 200kD (NF200). Glomerular nerve endings were distributed mainly in the epiglottic mucosa; some endings were also found in the arytenoid region arising from thick nerve fibres running through the subepithelial connective tissue. Some terminals directly contacted the epithelial cells. Taste buds were distributed in the epithelium of the epiglottis and aryepiglottic fold. In the whole mount preparation, the taste buds were supplied by the terminal branching of the thick nerve fibres. In some cases, the taste buds were arranged around the opening of the duct of the epiglottic glands. The intraepithelial free nerve endings were found to be immunoreactive for substance P (SP) and calcitonin gene-related peptide (CGRP). These nerve endings were surrounded by the polygonal stratified epithelial cells in the supraglottic region, and by the ciliated cells in the subglottic region. The density of the intraepithelial free nerve endings was highest in the corniculate process of the arytenoid region and lowest in the vocal cord mucosa. The densities of CGRP- and SP-immunoreactive nerve endings in the arytenoid region were (mean +/- s.d.) 30.6+/-12.0 and 10.0+/-4.9 per unit epithelial length (1 mm), respectively and in the vocal fold mucosa, 1.1+/-0.9 and 0.8+/-0.7, respectively. Approximately one half of the CGRP immunoreactive nerve endings were immunoreactive for SP, and most SP-immunoreactive nerve endings were also immunoreactive for CGRP. Well-developed subepithelial plexus with numerous intraepithelial fibres were observed in flat or round mucosal projections that existed on the corniculate process of the arytenoid region. In conclusion, the laryngeal mucosa of the horse seems to have morphology- and/or location-dependent sensory mechanisms against various endo-and exogenious stimuli.

Calbindin D28k-immunoreactive afferent nerve endings in the laryngeal mucosa

The distribution of the calbindin D28k in the laryngeal sensory structures was studied by immunohistochemistry, immunoelectronmicroscopy, and double immunofluorescence with calretinin-immunoreactivity. Moreover, origin of the nerve endings were observed using retrograde tracer, fast blue. Immunoreactivity for calbindin D28k was found in the various types of nerve endings in the larynx, namely, laminar nerve endings, nerve endings associated with the taste buds, intraepithelial nerve endings, and endocrine cells. The laminar endings with calbindin D28k-immunoreactivity were observed in the subepithelial connective tissue. In some endings, terminals were expanded. The laminar endings were also observed in the perichondrium of the epiglottic cartilage. In the epiglottic and arytenoid epithelia, thick nerve fibers with calbindin D28k-immunoreactivity ascending to taste buds and intragemmal nerve fibers were also observed. Within the epithelial layer, intraepithelial free nerve endings with calbindin D28k-immunoreactivity were observed. Furthermore, diffuse endocrine cells were observed within the laryngeal epithelium. By immunoelectron microscopy, immunoreaction products in the endings mentioned above were localized in the cytoplasm of the axon terminals and nerve fibers which contained with numerous mitochondria. Out of the 100 laminar endings, 18 endings were immunopositive for both calbindin D28k and calretinin, 33 were positive for calbindin D28k but negative for calretinin, and 49 were positive for only calretinin in the double immunofluorescence microscopy. The nerve fibers associated with the taste buds and the free nerve endings, which immunostained for calbindin D28k, were not stained with antibody against calretinin. After injection of the fast blue in the laryngeal mucosa, fast blue-labeled cells were mainly observed in the nodose ganglia. Of the total number of labeled cell in the nodose and dorsal root ganglia at the level C1 to Th2, 65.1% occurred in nodose ganglia (572/879, n = 6). In the nodose ganglia, 79.7% of labeled cells (456/572) were immunoreacted for calbindin D28k. The distribution of calbindin D28k-immunoreactivity may be differnt from that of calretinin. It is suggested that calbindin D28k have regulatory role on intracellular calcium concentration in the laryngeal sensory corpuscles.

Taste cell function. Structural and biochemical implications

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

Quantitative analysis of nerve changes in the lateral retinaculum in patients with isolated symptomatic patellofemoral malalignment. A preliminary study

Neural damage in 16 lateral retinacula excised at the time of Insall proximal realignments or isolated lateral retinacular releases performed in patients with symptomatic patellofemoral malalignment was evaluated by means of conventional histology and immunohistochemical and morphometric analyses. A relationship between clinical and histologic findings was found. An increase in the proportion of innervated tissue was correlated with anterior knee pain syndrome. We found a significant relationship between total neural area and pain. The group with moderate pain had the highest number of nerves and the highest neural area. In reference to total neural area and pain, there was a significant difference only between the patients with moderate pain and those with light pain, but not between patients with severe pain and those with moderate pain. The group with severe pain also showed a high neural area, although with a lower number of nerves. The severe-pain group had the largest nerves (24% of nerve fibers surpassing 25 microns diameter) in a zonal disposition, in which there were groups of nerve fibers in some fields and no nerve fibers in others. The group with moderate pain had an increase in medium and small nerve fibers (mean diameter, 18 microns), predominantly of tiny perivascular fibers. Moreover, we believe that instability in patients with patellofemoral malalignment can be explained in part because of loss of proprioception due to neural damage.

Neural cell adhesion molecule, neuron-specific enolase and calcitonin gene-related peptide immunoreactivity in hamster taste buds after chorda tympani/lingual nerve denervation

Hamster fungiform papilla taste buds persist in an atrophic form following sensory denervation. While atrophic and innervated taste buds are morphologically similar, it is not known whether their gemmal cells have similar molecular characteristics. Three neurochemicals, neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide have been implicated in trophic phenomena, synaptogenesis and cell recognition in neurons and sensory neuroepithelia. The present study uses immunocytochemical localization of these molecular markers to characterize normal and denervated fungiform taste buds following unilateral chorda tympani/lingual nerve denervation in hamsters. In normal taste buds, immunoreactivity to neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide was present in a group of cells located centrally in the bud as well as in fungiform nerve fibres and endings. After denervation, gemmal cell immunoreactivity to all three markers was reduced and often confined to a single or a few bud cell(s). Also, fibre staining was absent except for sparse calcitonin gene-related peptide-immunoreactive fibres associated with blood vessels and within the fungiform papillae. These remaining fibres may be autonomic or somatomotor in origin. These results indicate that sensory denervation of hamster taste buds reduces, but does not wholly eliminate the immunoreactivity of surviving gemmal cells to neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide. While the number of taste bud cells expressing the markers appears to be nerve-dependent, immunoreactivity in sensory-denervated bud cells of hamster may reflect the influence of local tissue factors.

CGRP and substance P in intraepithelial neuronal structures of the human upper respiratory system

The distribution of intraepithelial nerve fibres and neuroendocrine cells within the surface and glandular epithelium of human nasal mucosa and larynx was examined using immunohistochemical techniques. Neuronal structures were immunostained for the general neuroendocrine marker protein gene-product (PGP) 9.5, and the two neuropeptides substance P (SP) and calcitonin gene-related peptide (CGRP) using immunofluorescence and streptavidin-biotin-peroxidase complex (S-ABC) methods. Intraepithelial nerve fibres with free nerve endings contained PGP 9.5 and were found within the respiratory surface epithelium of the nasal mucosa and the squamous epithelium of the larynx. A subpopulation of these nerve fibres showed positive immunoreactivties with antibodies against SP and CGRP. Nerve fibres within the ductal epithelium of subepithelial excretory ducts passing the basal membrane and reaching the luminal part were detected. These nerve fibres showed CGRP-like immunoreactivity but not for SP. A dense network of nerve fibres within the squamous surface epithelium was detected in the subglottic and epiglottic region containing CGRP and SP in a small subpopulation of nerve fibres. Single intraepithelial taste buds in the epiglottic region and neuroendocrine cells within the subglottic epithelium expressed PGP 9.5.

Innervation of taste buds in the canine larynx as revealed by immunohistochemistry for the various neurochemical markers

The distribution and innervation of the canine laryngeal taste buds were observed using immunohistochemistry with antibodies against protein gene product 9.5 (PGP 9.5) and neurofilament protein (NFP). We also observed the immunohistochemical distribution of serotonin, tyrosine hydroxylase (TH) and various neuropeptides including calcitonin gene-related peptide (CGRP), substance P (SP), vasoactive intestinal peptide (VIP), galanin, methionine enkephalin (ENK) and neuropeptide Y (NPY). The taste buds in the canine larynx were densely distributed in the mucosa at the basal portion of the epiglottis and cuneiform process of the arytenoid cartilage. The taste cells were immunoreactive for PGP 9.5 and serotonin. The nerve fibers with immunoreactivity for PGP 9.5 in the taste buds were observed in the perigemmal region and intra- and subgemmal plexuses, and these were classified into two types based on their diameter. The thick nerve fibers corresponded to the fibers immunoreactive for NFP, while the thin nerve fibers corresponded to the fibers immunoreactive for TH and various neuropeptides. Numerous nerve fibers immunoreactive for SP and CGRP were observed in the perigemmal region, and intra- and subgemmal plexuses. A few galanin- and ENK-immunoreactive nerve fibers were also observed in the taste buds, whereas NPY-immunoreactive nerve fibers were noted beneath them. All peptide-containing fibers except for VIP-immunoreactive nerves were situated in the subgemmal regions. In conclusion, the multiple innervation to the laryngeal taste buds were documented. Thick nerve fibers are likely to be irritant receptors, while thin varicose nerve fibers seem to regulate taste buds themselves. The laryngeal taste buds may be among the important structures which are sensitive to exogeneous chemical and/or mechanical stimuli, for the protection of the airway and the regulation of the respiratory function.

Characteristics of laryngeal receptors analyzed by presynaptic recording from the cat medulla oblongata

In order to clarify the neural mechanisms for the protective laryngeal reflex, we conducted physiological analysis of laryngeal sensory receptors. In the present study, presynaptic unit activities, which might accurately reflect characteristics of the laryngeal receptor, were recorded with a glass microelectrode in the nucleus of the tractus solitarius of the medulla oblongata in ketamine-urethane anesthetized cats, and the responses to the mechanical and/or chemical stimuli were analyzed. From the results, it was demonstrated that highly sensitive mechanoreceptors and polymodal receptors exist in the laryngeal mucosa; they are particularly numerous in the laryngeal surface of the epiglottis and arytenoid region, and uncommon in the vocal fold. Mechanoreceptors on the laryngeal mucosa were classified into a rapidly adapting group and a slowly adapting group, while all polymodal receptors adapted rapidly to mechanical stimulation. These results suggest that these non-specific polymodal and rapidly adapting receptors may correspond to more superficial receptors such as free nerve endings and some taste buds, and also monomodal slowly adapting mechanoreceptors may correspond to deeper terminals in the subepithelium. It is also considered possible that the structures and the characteristics of these receptors are appropriate to elicit the protective laryngeal reflexes by non-specifically detecting various kinds of stimuli.