Occurrence, ontogeny, ultrastructure and some plasticity of CGRP (calcitonin gene-related peptide)-immunoreactive nerves in the carotid body of rats.

Numerous calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibers are present in the carotid body of adult rats. The immunoreactive nerve fibers first appear in the carotid body at postnatal day 3 and thereafter they increase in number gradually. Nerve section with or without ganglionectomy and experiments involving the neonatal administration of capsaicin indicate that the CGRP-immunoreactive fibers in the carotid body come from the glossopharyngeal, vagal and spinal sensory ganglia, although those from the glossopharyngeal ganglion are predominant. CGRP-immunoreactive nerve fibers are unmyelinated and most of them are located in the interstitial space, and only a few are enclosed by sustentacular cells and directly apposed to the chief cells where no membrane specializations are formed. One to 4 weeks after the severance of either the glossopharyngeal or vagal nerves together with superior cervical ganglionectomy, CGRP-immunoreactive nerve fibers in the carotid bodies are more numerous than the non-operated control. In these specimens thin CGRP-immunoreactive nerve fibers are often seen to be partially enclosed together with immunonegative fibers by thin Schwann cell processes without mesaxons. In specimens 1-4 weeks after the single severance of the glossopharyngeal or the vagal nerve, CGRP-immunoreactive nerve fibers were as numerous as in the non-operated control while immunoreactive fibers remained absent in specimens one to four weeks after the simultaneous severance of the glossopharyngeal, vagal and cervical sympathetic trunks. These findings suggest that the enhanced appearance of neuronal CGRP-immunoreactivity in the carotid bodies one to four weeks after the partial denervation is at least partially due to the regenerative proliferation of new collaterals from persistent immunoreactive nerve fibers and that the sympathectomy markedly enhances the proliferative sprouting.