Innervation of the serotonin-immunoreactive cells distributed in the wall of the common carotid artery and its branches in the chicken

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Abstract
In the chicken, serotonin-immunoreactive cells were widely distributed not only in the carotid body but also in the wall of the common carotid artery and around each artery arising from the common carotid artery. Almost all of the serotonin cells in the wall of the common carotid artery were intensely immunoreactive to the neuropeptide Y, met- and leu-enkephalin antisera, whereas in the carotid body only a few cells were immunoreactive to these antisera. Innervation of the serotonin cells in and around arteries of chickens was investigated by immunohistochemistry and electron microscopy, in comparison with that of the carotid body. The serotonin cell groups in and around arteries, as well as the carotid body, received numerous peptidergic nerve fibers. Calcitonin gene-related peptide (CGRP)-and substance P-immunoreactive varicose nerve fibers were densely distributed, and somatostatin-immunoreactive fibers were moderately distributed in the serotonin cell groups. Galanin-and vasoactive intestinal peptide (VIP)-immunoreactive fibers were sparsely distributed in the cell groups. By electron microscopy, the serotonin cells in and around arteries were characterized by the presence of numerous dense-cored vesicles, 70–220 nm in diameter. The granule-containing cells were in close association with numerous axons. Naked axons regarded axon terminals were frequently apposed on the granular cells. The axon terminals were usually long and often partly invested the granular cells. Numerous synaptic junctions were detected along the contact between the granular cells and axon terminals. Most of the synaptic junctions showed afferent morphology; the secretory granules were accumulated near and attached to the asymmetrical membrane thickenings. Thus, the serotonin cells in and around arteries, like the carotid body, constitute chemoreceptive tissue.