Pituitary adenylate cyclase activating peptide is a sensory neuropeptide: Immunocytochemical and immunochemical evidence

Pituitary adenylate cyclase activating peptide (PACAP) is a vasoactive intestinal peptide-like hypothalamic peptide occurring as two variants, PACAP-27 and the C-terminally extended PACAP-38. Immunoreactive PACAP has also been demonstrated in the enteric nervous system and in the innervation of the respiratory tract. We have examined the possibility that PACAP occurs in the sensory nervous system of the rat. Immunocytochemistry revealed PACAP in numerous nerve fibres in the superficial layer of the dorsal horns of the spinal cord, in nerve cell bodies (most of them of small size) of the spinal ganglia and trigeminal ganglia and in nerve fibres running close to and within the surface epithelium in the skin of the nose, the tongue, the larynx-trachea, and the urinary bladder as well as around the ducts of the submandibular gland. In all locations, PACAP co-existed with calcitonin gene-related peptide and substance P, the PACAP-immunoreactive fibres and cell bodies constituting a subpopulation of those storing substance P and/or calcitonin gene-related peptide. Additional PACAP-immunoreactive fibres not associated with epithelia seemed to lack calcitonin gene-related peptide and substance P. Capsaicin treatment reduced the density of PACAP- and calcitonin gene-related peptide/substance P-immuno-reactive fibres in the tissues examined. On the whole, the immunocytochemical results agreed with those obtained by radioimmunoassay for PACAP and CGRP. The data favour a role for PACAP in primary sensory neurons.