Immunohistochemical Study of Intralaryngeal Ganglia in the Cat

Optimisation and validation of immunohistochemical axonal markers for morphological and functional characterisation of equine peripheral nerves

BACKGROUND

Horses are affected by various peripheral nerve disorders but defining their aetiology and pathophysiology is hampered by limited understanding of associated morphological and pathological changes and involvement of specific axonal types.

OBJECTIVES

To investigate the hypothesis that selected antibody markers, used in conjunction with various tissue processing methods, would enable identification of axons with different functional modalities within a range of equine peripheral nerves.

STUDY DESIGN

Optimisation and validation study.

METHODS

A range of antibodies were evaluated immunohistochemically via fluorescence confocal microscopy in cadaver equine nerve samples of primary motor, mixed or primary sensory functions (recurrent laryngeal, phrenic and plantar digital) within formalin-fixed paraffin-embedded (FFPE) and formalin-fixed frozen (FFF) tissues subjected to different antigen retrieval protocols.

RESULTS

Immunohistochemistry of FFPE-derived nerve samples with selected antibodies and specific antigen retrieval methods enabled identification of myelinated and unmyelinated axons, cholinergic, sympathetic and peptidergic axons. The recurrent laryngeal and phrenic nerves are composed of myelinated cholinergic (motor), myelinated sensory fibres, unmyelinated adrenergic (sympathetic) axons and unmyelinated peptidergic (sensory) axons. In contrast, as expected, the plantar digital nerve had no myelinated motor fibres being mainly composed of myelinated sensory fibres, unmyelinated sympathetic and unmyelinated peptidergic sensory axons.

MAIN LIMITATION

Attempts specifically to label parasympathetic fibres were unsuccessful in any nerve examined in both FFPE and FFF tissues.

CONCLUSIONS

A panel of antibody markers can be used to reveal morphological and functional properties of equine nerves. Future work should enable better characterisation of morphological changes in equine neuropathies at various stages of disease development.

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

Extending the enteric nervous system

The work reviews the evidence suggesting that lingual components of the autonomic system may be considered the most rostral portion of the enteric nervous system (ENS) defining the concept of lingual ENS (LENS). The LENS is not dissimilar from the more distally located portions of the ENS, however, it is characterized by a massive sensory input generated by collaterals of gustatory and trigeminal fibers. The different neuronal subpopulations that compose the LENS operate reflexes involved in regulation of secretion and vasomotility. Systemic reflexes on the digestive and respiratory apparatus are operated by means of neural connections through the pharynx or larynx. The LENS can modulate the activity of distally located organs by means of the annexed glands.The LENS seems therefore to be a "chemical eye" located at the beginning of the digestive apparatus which analyses the foods before their ingestion and diffuses this information distally. The definition of the LENS supports the concept of an elevated degree of autonomy in the ENS and puts in a new light the role of the gustatory system in modulation of the digestive functions. For its characteristics, the LENS appears to be an ideal model to study the elementary connectivity of the ENS.

Nonadrenergic innervation of the rat laryngeal vasculature

In order to gain a better understanding of the central and local control of laryngeal blood flow, the vascular innervation to the rat laryngeal muscles was examined. To visualize the vascular network, the animals were perfused with a gelatin/India ink solution. The larynges were removed and fixed. The superior laryngeal, cricothyroid, and inferior laryngeal arteries (all branch off the superior thyroid artery) were dissected in continuity into their respective muscles. Specimens were reacted in toto using immunohistochemical techniques for the presence of neuropeptide-Y (NPY), vasoactive intestinal peptide (VIP), calcitonin gene related peptide (CGRP), and neuronal nitric oxide synthase (NOS-1). Results show that all of the laryngeal vasculature is richly innervated by fibers containing these peptides. Qualitatively, the most prominent of these is NPY in association with the superior and the inferior laryngeal arteries, followed by VIP and NOS-1, and finally CGRP distributed equally on all the vessels. Immunopositive fibers are found along the entire course of the feeding arteries, beginning with the superior thyroid artery and continuing down to small arterioles into the terminal vascular beds. These peptides can act as vasodilators, vasoconstrictors, and/or neuromodulators and may work synergistically or antagonistically with other transmitters in controlling laryngeal blood flow. Their effects are dependent on the specific vascular bed in question, that is, in some areas they are vasodilators, in others vasoconstrictors, and in other neuromodulators. What effects they have on the laryngeal vasculature and how they interact within the larynx have yet to be determined.

Sensory innervation of the pharynx and larynx

To shed light on supraesophageal complications of reflux disease, sensory innervation--particularly, distinct distribution, area, and density of sensory fibers--of the feline pharyngolaryngeal mucosa was reported. The investigations were performed by means of histochemistry (tracer techniques) and immunohistochemistry. The pharyngeal mucosa from the Eustachian cushion to the middle level of aryepiglottic fold, except the laryngeal surface of epiglottis, was supplied by the glossopharyngeal sensory fibers, whereas the laryngeal sensory fibers innervated between the apex of epiglottis and the level of the first tracheal ring in the larynx and between the middle level of aryepiglottic fold and the caudal end of piriform sinus in the pharynx. Most areas of the mucosa, except the subglottis, had unilateral innervation. The subglottis, including the caudal aspect of vocal fold and the posterior glottis, had bilateral supply with ipsilateral predominance. The density of sensory fibers in the vestibule of larynx involving the posterolateral aspect of arytenoid eminence was much heavier than the other parts. Sensory nerve fibers around the caudal pole of palatine tonsil, and in the root of tongue and the hypopharyngeal wall also were dense. Regional distribution and density of substance P and calcitonin gene-related peptide immunoreactive fibers showed almost the same pattern as did the sensory fibers.

Immunohistochemical localization of choline acetyltransferase of a peripheral type in the rat larynx

As shown in the accompanying paper, choline acetyltransferase, so far the best histochemical marker for identifying cholinergic structures, has at least one alternative splice variant. The variant, termed pChAT because of its preferential expression in peripheral organs, encouraged us to study peripheral, probably cholinergic, cells and fibers by immunohistochemistry using an antiserum against a peptide specific for pChAT. We chose the larynx of the rat, since cholinergic innervation in this organ has been well established by physiological studies, but not sufficiently by chemical neuroanatomy. Neuronal somata positive for pChAT were found in the intralaryngeal ganglia. Our double staining study indicated that these somata always possessed acetylcholinesterase activity, while the reverse did not hold true. Nerve fibers positive for pChAT were distributed widely in the intrinsic laryngeal muscles, laryngeal glands, blood vessels and laryngeal mucosa. In the intrinsic laryngeal muscles, pChAT-positive terminals were apposed closely to motor end-plates which were stained positively for acetylcholinesterase activity. Denervation experiments revealed that there were three types of pChAT-positive fibers in the larynx: (1) special visceral efferent fibers to the intrinsic laryngeal muscles, which decreased dramatically in number after vagotomy; (2) parasympathetic postganglionic fibers near the laryngeal glands and blood vessels, which appeared unaffected after vagotomy or cervical sympathectomy: and (3) afferent fibers innervating the laryngeal mucosa, which reduced markedly in number after vagotomy performed distal, but not proximal, to the nodose ganglion. Such afferent fibers remained unchanged following the neonatal capsaicin treatment, suggesting their independence from those containing substance P.

Neurotransmitters and neuromodulators involved in laryngeal innervation

The distribution and role of neurotransmitters and neuromodulators in laryngeal innervation are reviewed, and our recent findings regarding the nitrergic innervation of the larynx are demonstrated for the better understanding of the complexity of the laryngeal innervation system. Noradrenergic innervation of the larynx was studied with fluorescence histochemistry and electron microscopy after application of 5-hydroxydopamine. These studies confirmed the existence of noradrenergic innervation for the submucosal glands and blood vessels, and the origin and course of noradrenergic nerve fibers contained in the laryngeal nerves and their destinations in the larynx. Cholinergic innervation of the larynx has not been clarified in detail. Many kinds of neuropeptides have been demonstrated to be involved in laryngeal innervation. Vasoactive intestinal polypeptide originating from intralaryngeal ganglionic neurons participates in laryngeal vasodilation and reduction of laryngeal seromucous secretion. Neuropeptide Y nerve fibers are few in the larynx, and most originate from the superior cervical ganglion. They are distributed around the large or medium-sized blood vessels, especially arteries. They are also associated with excretory structures. Substance P was the first neuropeptide found to be a sensory neurotransmitter in the laryngeal afferent system. It is also involved in regulation of laryngeal blood flow and secretion. Calcitonin gene-related peptide is associated with the sensory, autonomic, and motor innervation of the larynx. The majority of enkephalin nerve fibers are located close to excretory structures, although no information on the physiological significance of enkephalin is available. In addition to the above neuropeptides, the peptides histidine isoleucine, histidine methionine, and helospectin have been shown to exist in the larynx. The nitrergic innervation of the larynx has been recently studied with NADPH-diaphorase histochemistry and immunohistochemistry using antiserum against nitric oxide synthase. Nitric oxide originates from the neurons in the intralaryngeal ganglia and is believed to modulate blood flow and secretion of the larynx. It controls the laryngeal exocrine secretion in cooperation with intrinsic vasoactive intestinal polypeptide and/or extrinsic calcitonin gene-related peptide. Nitric oxide from the nodose ganglion may modulate nociception of the larynx. The existence of nitrergic neurons located in the intrinsic laryngeal muscles has been demonstrated. Many of them are bipolar or pseudounipolar, so they might be sensory in nature. The effect of injury of the recurrent laryngeal nerve on the induction of nitric oxide synthase in the laryngeal motoneurons is also discussed.

Distribution of NADPH-diaphorase activity in the feline laryngeal mucosa

We studied nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase activity in the feline laryngeal mucosa using a histochemical technique in an effort to clarify the role of nitric oxide (NO) in the larynx. Many NADPH-diaphorase-positive nerve fibres were distributed around the blood vessels and the laryngeal glands. The majority of neuronal cells in the intralaryngeal ganglia were NADPH-diaphorase-positive. It is likely that NADPH-diaphorase-positive nerve fibres around the blood vessels and glands in the laryngeal mucosa originate from the intralaryngeal ganglia, and that NO regulates circulation and secretion in the larynx.

Neurochemical substances in neurons of the canine intrinsic laryngeal muscles

The localization of vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP), catecholamines (CA) and carbon monoxide (CO) in neurons of the intrinsic laryngeal muscles of the dog was investigated using immunohistochemistry. Cells with VIP-like immunoreactivity were found between the striated muscle fibres of the cricothyroid and posterior cricoarytenoid muscles, some aggregated into a ganglion and some solitary. Neurons with immunoreactivity to haem oxygenase-2 (HO-2), an immunohistochemical marker for CO, were also seen. VIP-or HO-2-negative cells also existed in various numbers in the intramuscular ganglia. No CGRP-like immunoreactive neurons were found although CGRP-like immunoreactive varicose fibres were seen in the intramuscular ganglia. No neurons or fibres immunoreactive to tyrosine hydroxylase (TH), the key enzyme in synthesis of CA, were detected. The exact function of these intramuscular ganglia is yet to be clarified, but they may be sensory neurons.

Beaded nerve terminals in feline laryngeal mucosa

To investigate the mechanism of airway defense reflex, beaded nerve terminals were studied by immunohistochemical techniques. In the supraglottic region the density of PGP 9.5-immunoreactive nerve fibers was the highest at the base of the glottic surface in the epiglottis, and in the glottic region it was the highest in the arytenoid region. In the subglottic region the number of positive nerve fibers was less than the number at the base of the glottic surface in the epiglottis, and when the laryngeal mucosa was processed with NaOH to dissolve the epithelium, it was possible to observe beaded nerve terminals more clearly. These beaded nerve terminals were found just beneath, in the epithelial basement membrane. Electron microscopic examination of beaded nerve terminals revealed a large quantity of secretory granules and mitochondria, suggesting that their structure is similar to that of nerve terminals. Thus these beaded nerve terminals may function as mechanoreceptors.

Relationship of neuropeptides to nitrergic innervation of the canine laryngeal glands

The participation of vasoactive intestinal polypeptide (VIP) or calcitonin gene-related peptide (CGRP) in the nitrergic innervation of the canine laryngeal glands was investigated using a double-staining technique of NADPH-diaphorase (NADPHd) histochemistry and VIP or CGRP immunohistochemistry. NADPHd-positive nerve fibers with varicosities appeared to terminate in some acinar cells. Double staining revealed that NADPHd reactivity and VIP- or CGRP-like immunoreactivity were colocalized in some nerve fibers distributed around the acini. A cluster of NADPHd-positive cells were occasionally found in the larynx. Many NADPHd-positive cells had VIP-like immunoreactivity and no NADPHd-positive cells were CGRP-like immunoreactive. These findings suggest that nitric oxide participates in the neural control of the laryngeal exocrine secretion in cooperation with intrinsic VIP and/or extrinsic CGRP.

Nitrergic neurons in the canine intrinsic laryngeal muscle

Nitrergic ganglionic cells located in the canine intrinsic laryngeal muscle were studied by NADPH-diaphorase (NADPH-d) histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemistry. Cells intensely stained by NADPH-d histochemistry were found between the striated muscle fibers of the intrinsic laryngeal muscle. Most of these cells were bipolar or pseudounipolar in form. Some NADPH-d negative cells were observed to be enveloped in a mesh by varicose NADPH-d positive nerve fibers. The findings obtained by nNOS immunohistochemistry corresponded well with those obtained by NADPH-d histochemistry, indicating that NADPH-d activity in the ganglion in the intrinsic laryngeal muscle is nNOS. The present findings clearly indicate that some of the ganglion cells located in the canine intrinsic laryngeal muscle are nitrergic, and that the ganglionic cells synapse together with the participation of nitric oxide in integrating ganglionic cells.

Arrangement and number of intralaryngeal ganglia and ganglionic neurons: comparative study of five species of mammals

The arrangement and number of intralaryngeal ganglia and their neurons in five mammals (dog, rat, guinea pig, rabbit and cat) were examined morphologically. Intralaryngeal ganglions were situated mainly in branches of the internal branch of superior laryngeal nerve (Int-SLN), dorsal and/or dorsolateral to the posterior cricoarytenoid muscle, and around the inferior laryngeal nerve in dogs, rats, guinea pigs and cats, but they were identified at the branching out point of the Int-SLN exclusively in rabbits. The ganglion of each animal was spindle-shaped, with a surrounding fibrous capsule, and it contained many ganglionic neurons, vessels and connective tissue cells. The ganglionic neuron was oval-shaped and had a round nucleus: the diameter was smaller (20-25 microns) in the rat than in the other mammals (25-30 microns). More than 80 per cent of ganglionic neurons occurred in the supraglottis of all the animals except the rat. In the rat, this value was approximately 40 per cent.

Precise localization of VIP-, NPY-, and TH-immunoreactivities of cat laryngeal glands

The precise distribution of vasoactive intestinal polypeptide (VIP)-, neuropeptide Y (NPY)-, and tyrosine hydroxylase (TH)-immunoreactive (ir) fibers in the cat's laryngeal glands was examined by immunoelectronmicroscopy. A relatively dense population of VIP-ir fibers was recognized close to the basal lamina of the glandular and myoepithelial cells. Some VIP-ir fibers contacted with the basal lamina and some of them pierced it and ran intercellularly in the adjoining glandular cells without making synaptic contacts with them. NPY- and TH-ir fibers were located in the vicinity of the basal lamina, but they were less abundant than VIP-ir fibers at this region. They never terminated or penetrated the basal lamina. Pattern of distribution of TH-ir fibers was similar to that of NPY-ir fibers. The estimated ratio of VIP-, TH-, and NPY-ir fibers was 20:4:1 from the density of fibers in the laryngeal glands. This value was equal between serous and mucous glandular cells, so both types of glandular cells may receive the same pattern of autonomic innervation.

Electron microscopic immunohistochemical study of intra-epithelial nerve fibers in the canine larynx

Substance P (SP)-, calcitonin gene-related polypeptide (CGRP)-, and vasoactive intestinal polypeptide (VIP)-immunoreactive (IR) nerve fibers were observed in the epithelium of the canine subglottic region. To investigate the morphological differences among SP-, CGRP-, and VIP-IR nerve fibers in the epithelium, this study was performed by using immunohistochemical staining and electron microscopy, after which the origins of the nerve fibers were examined by denervation of the bilateral superior and inferior laryngeal nerves. SP- and CGRP-IR nerve fibers entered the epithelium through the basement membrane, ascended among the basal and ciliated cells, and reached under the epithelial junctional complex to terminate with varicosities. Furthermore, subsurface cistern-like structures or bouton en passant type synapse-like structures were observed among some varicosities of these nerve fibers and ciliated cells in the epithelial apical portion. On the other hand, VIP-IR nerve fibers entered through the basement membrane, and terminated with varicosities at the height of the middle portion of the subglottic epithelium. The varicosities of the VIP-IR nerve fibers were larger, and these varicosities had a greater number of synaptic vesicles than the SP- or CGRP-IR nerve fibers. After section of the bilateral superior and inferior laryngeal nerves, the SP- and CGRP-IR nerve fibers disappeared, while the VIP-IR nerve fibers were not affected. The density and distribution pattern of VIP-IR nerve fibers clearly differed from the SP- and CGRP-IR nerve fibers of sensory origin.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of SP- and CGRP-immunoreactivity in the cat's larynx

The distribution of substance P (SP) and calcitonin gene-related peptide (CGRP) immunoreactive (ir) fibres in the cat's larynx was investigated utilizing immunohistochemistry. Many SP- and CGRP-ir fibres with varicosities were found within and below the epithelium and along the basement membrane of the mucosa of all different regions except in the membranous portion of the vocal fold. In the subepithelium, some SP- and CGRP-ir nerve bundles and nerve fibres were recognized around the vessels and glands. In the mucosa, the pattern of distribution and the density of SR-ir fibres were similar to those of CGRP-ir fibres. These reactive fibres were denser in the supraglottic region than in the subglottic region. In the taste bud-like structures, only SP-ir fibres appeared, whereas in the motor endplates, CGRP-reaction was found exclusively. The present findings suggest that the regional distribution of SP- and CGRP-immunoreactivity might be related with sensory and autonomic innervation in the larynx.

Ganglions and ganglionic neurons in the cat's larynx

Localization, projections and role of ganglia and ganglionic neurons in the laryngeal framework were demonstrated in cats. Six to 8 large size ganglia containing 50 to 80 ganglionic neurons per ganglion in the paraglottic space, 4 to 6 small ganglia involving 5 to 25 cells in each ganglion dorsal to the posterior cricoarytenoid muscle, and 1 to 3 small ganglia including 15 to 25 perikarya per ganglion around the inferior laryngeal nerve were observed. Each ganglion showed spindle shaped covering fibrous capsule. Ganglionic neurons totaling 600 to 800 were oval shaped with an average diameter of 25 microns. Projections of the ganglionic neurons to the superior cervical ganglion (SCG) and nodose ganglion (NG) through the ipsilateral internal branch of the superior laryngeal nerve and to periphery were detected. From the dorsal motor nucleus of vagus, SCG and NG, the ganglionic cells received projections ipsilaterally. On immunocytochemistry many vasoactive intestinal polypeptide (VIP)-immunoreactive (ir) neurons, some neuropeptide Y (NPY)- and tyrosine hydroxylase (TH)-ir neurons and a few substance P (SP)-ir cells were recognized in ganglions. VIP-, NPY-, TH-, SP-, and calcitonin gene-related peptide (CGRP)-ir fibers were also observed in and around vessels and glands. Following denervation, VIP-, NPY- and TH-ir neurons and fibers did not change. These results prove that laryngeal ganglionic neurons have endogenic autonomic, especially cholinergic nature and innervate vessels and glands.

Immunoelectron microscopic studies of synaptic organization in the intralaryngeal ganglia of the cat

The synaptic organization of nerve terminals containing calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), substance P (SP) and enkephalin (ENK) in the intralaryngeal local ganglia of the cat was investigated by immunoelectron microscopy. CGRP-immunoreactive (IR) and VIP-IR varicose fibers formed mainly axo-dendritic synapses, whereas SP-IR and ENK-IR varicose fibers made axo-somatic synapses to the principal neurons of the local ganglion. The synaptic specializations of the CGRP-IR varicosities were asymmetrical, or Gray's type I, whereas the other peptide-IR varicosities showed symmetrical, or Gray's type II, synaptic specializations. After denervation of the extrinsic nerves, CGRP-IR varicose fibers disappeared from the ganglion, but VIP-IR, SP-IR and ENK-IR varicose fibers and synapses remained. These results suggest that local ganglia act as an integration center of laryngeal function rather than as a unidirectional parasympathetic relay center.

Ganglionic neurons in vagal and laryngeal nerves projecting to larynx, and their peptidergic features in the cat

The distribution of cell clusters in the cervical vagal nerve (CVN), superior laryngeal nerve (SLN) and recurrent laryngeal nerve (RLN), and the peptidergic features of their ganglionic neurons projecting to the larynx, were investigated in the cat using a combination of retrograde tracing by wheat germ agglutinin (WGA) and immunocytochemistry. In the CVN, a few medium sized cell clusters at a level caudal to the nodose ganglion, and some small cell clusters along the course of the vagus, were found. In the SLN and RLN, some medium sized ganglia were located close to the laryngeal framework and a few small groups of cells occupied more rostral parts. Some neurons of the cell clusters in the CVN and most of the ganglionic cells in the SLN and RLN exhibited WGA-immunoreactive (IR) cells, which projected to the larynx. In these WGA-positive ganglionic neurons, many cells showed vasoactive intestinal polypeptide-IR neurons, some neuropeptide Y-IR, and a few substance P-IR and calcitonin gene-related peptide-IR cells were also identified. The present findings indicate that neurons of the cell clusters in the laryngeal nerves, particularly those in the vicinity of the laryngeal framework, project to the larynx and may be autonomic.