Relationship of neuropeptides to nitrergic innervation of the canine laryngeal glands

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.

Localization of heme oxygenase-2 in the canine larynx

The localization of heme oxygenase-2 (HO-2) in the larynx of the dog was investigated using immunohistochemistry. HO-2-positive cells were seen among neurons in intralaryngeal ganglia. Nerve fibers positive to HO-2 immunohistochemistry were seen surrounding laryngeal glands and arterioles and also in the adventitia of arterioles. HO-2-positive fibers were also seen running parallel to the mucosa in the lamina propria but no positive fibers were seen in the epithelium. Some of the intramuscular neurons found in the intrinsic laryngeal muscles were HO-2-positive, although no positive motor fibers were seen, and the neuromuscular junctions were also HO-2-negative. The results implicate the participation of HO-2-in the parasympathetic innervation of the larynx.

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.

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.

Coexistence of calcitonin gene-related peptide and NADPH-diaphorase in the canine superior cervical ganglion

By means of double staining technique of NADPH-diaphorase (NADPH-d) histochemistry and calcitonin gene-related peptide (CGRP) immunohistochemistry, we investigated the coexistence of NADPH-d reactivity and CGRP immunoreactivity in the canine superior cervical ganglion (SCG). Most of NADPH-d reactivity and CGRP immunoreactivity were coexisted in the principal postganglionic neurons. These neurons were distributed throughout the ganglion without specific localization. The present findings suggest the intimate role of CGRP and nitric oxide in postganglionic neurons of the canine SCG.