CGRP-immunoreactive cells supplying laryngeal sensory nerve fibres in the cat's nodose ganglion

Mucosal afferents mediate laryngeal adductor responses in the cat

Laryngeal adductor responses (LAR) close the airway in response to stimulation of peripheral afferents in the superior laryngeal nerve. Although both mucosal afferents and proprioceptive receptors are present in the larynx, their relative contribution for reflex elicitation is unknown. Our purpose was to determine which receptor types are of importance in eliciting the LAR. A servomotor with displacement feedback was used to deliver punctate displacements to the body of the arytenoid cartilage and overlying mucosa on each side of the larynx in eight anesthetized cats. The same displacements were delivered both before and after surgical excision of the overlying mucosa. With the mucosa intact, early short-latency component R1 LAR responses recorded from the thyroarytenoid muscles were frequent (ipsilateral > 92%, contralateral > 95%). After the mucosa was removed, the LAR became infrequent (<3%) and was reduced in amplitude in both the ipsilateral and contralateral thyroarytenoid muscle recording sites (P < 0.0005). These findings demonstrate that mucosal mechanoreceptors and not proprioceptive afferents contribute to the elicitation of LAR responses in the cat.

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.

Tyramide amplification allows anterograde tracing by horseradish peroxidase-conjugated lectins in conjunction with simultaneous immunohistochemistry

Current protocols for a combined approach of anterograde tracing with carbocyanine dyes or horseradish peroxidase (HRP) conjugates and immunohistochemistry represent a compromise between sensitive detection of the tracer and the immunohistochemical procedure. Therefore, it was investigated whether the use of tyramide amplification allows sensitive anterograde tracing with wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) in conjunction with simultaneous immunohistochemistry. Vagal afferents were anterogradely labeled by injection of WGA-HRP into the nodose ganglion of rats. By use of tyramide-biotin amplification, a dense fiber plexus of vagal afferents was visualized centrally in the nucleus of the solitary tract and in retrogradely labeled neurons in the dorsal vagal nucleus. In the esophagus and duodenum, large- and small-caliber vagal fibers and terminals could be demonstrated comparably to conventional tracing technique using carbocyanine dyes or WGA-HRP and TMB histochemistry. Combination with immunohistochemistry could easily be done, requiring only one more incubation step, and did not result in loss of sensitivity of the tracing. With this method and confocal microscopy, the presence of Ca binding proteins in vagal afferent terminals could be demonstrated. Tyramide amplification allows sensitive anterograde tracing with low background staining in conjunction with immunohistochemistry of a-axonal markers.

Neuropeptide participation in canine laryngeal sensory innervation. Immunohistochemistry and retrograde labeling

We investigated the quantitative participation of calcitonin gene-related peptide (CGRP), substance P (SP), and leu-enkephalin (ENK) in canine laryngeal sensory innervation by immunohistochemistry in combination with retrograde labeling using the recently introduced retrograde tracer cholera toxin subunit B-conjugated gold (CTBG). In the nodose ganglion, neurons labeled from the internal branch of the superior laryngeal nerve with CTBG were investigated immunohistochemically by means of antisera against CGRP, SP, and ENK. The percentages of neurons immunoreactive to each neuropeptide were as follows: CGRP 81.5%, SP 24.5%, and ENK 7.0%. These results suggest that CGRP is the main sensory neurotransmitter in canine laryngeal sensory innervation.