Retrograde neuronal labelling and double-staining immunohistochemistry of tachykinin- and calcitonin gene-related peptide-immunoreactive pathways in the carotid sinus nerve of the guinea pig.

The origin of tachykinin- and calcitonin gene-related peptide-like immunoreactive (CGRP-LI) nerve fibres in the guinea pig carotid body and carotid sinus was determined by retrograde labelling of the carotid sinus nerve with Fluoro-gold and immunohistochemical double staining with fluorescein- and rhodamine-conjugated second antisera. Fluoro-gold-labelled perikarya with characteristic features of primary sensory neurones were numerous in the glossopharyngeal (petrosal) ganglion and occurred rarely in the closely attached superior vagal (jugular) ganglion. An efferent pathway from the brainstem could not be detected. Co-existence of tachykinin- and CGRP-LI was observed in 25-47% of labelled sensory neurones; less than 1% of Fluoro-gold-containing perikarya were exclusively stained by CGRP antiserum. Co-existence of tachykinin- and CGRP-LI was also demonstrated in nerve fibres of the carotid body and carotid sinus. Somatostatin-, cholecystokinin- and dynorphin-LI did not co-exist with tachykinin-LI in these fibres. Thus, tachykinin/CGRP-LI fibres in the carotid presso- and chemoreceptive areas exhibit a peptide pattern being generally characteristic for sensory fibres supplying great vessels in the guinea pig. In view of the present findings doubt is raised as to a primary involvement of these fibres in presso- or chemoreception, although a modulatory influence on these specific functions appears to be likely.