Ultrastructure of calcitonin gene-related peptide- and substance P-like immunoreactive nerve fibres in the carotid body and carotid sinus of the guinea pig

Previous studies have demonstrated that substance P- (SP) and calcitonin gene-related peptide-like immunoreactivities (CGRP-LI) coexist in sensory nerve fibres in the guinea-pig carotid body and carotid sinus. In the present study the ultrastructure of these nerve fibres was investigated by means of single- and double-labelling immunocytochemistry. In both, carotid body and carotid sinus immunoreactive fibres were unmyelinated axons of small diameter (0.12-0.56 microns). At the subcellular level, SP- and CGRP-LI were colocalized in intra-axonal dense core vesicles, suggesting corelease and simultaneous action of these two compounds. SP/CGRP-LI nerve fibres within the carotid body were mainly found in the interparenchymal connective tissue, but also occurred in relationship to blood vessels and nests of glomus cells. Neither in the carotid body not in the carotid sinus, SP/CGRP-LI axons corresponded to the large terminals which are generally considered to represent the main chemoreceptor and baroreceptor endings, respectively. Thus, SP/CGRP-LI fibres either belong to the chemo- and baroreceptors of the C-fibre class or constitute a fibre population not directly involved in conduction of baro- and chemoreflexes.