Innervation of the rat pineal gland by pituitary adenylate cyclase-activating polypeptide (PACAP)-immunoreactive nerve fibres

Pituitary adenylate cyclase-activating polypeptide (PACAP)-immunoreactive n erve fibres were demonstrated in the rat pineal gland. These fibres entered the pineal gland through the conarian nerve at the distal tip of the gland . A high density of the fibres was observed in the capsule of the gland, fr om where the immunoreactive elements penetrated into the pineal perivascula r spaces and parenchyma. The majority of PACAP-immunoreactive nerve fibres also contained calcitonin gene-related peptide (CGRP). Some PACAP-immunorea ctive nerve fibres contained neuropeptide Y (NPY), but only occasionally wa s PACAP colocalized with vasoactive intestinal peptide (VIP). After removal of both superior cervical ganglia, a high number of PACAP-containing nerve fibres were still present in the gland. In the nervous system PACAP is pre sent in two isoforms, PACAP-38 and PACAP-27. The concentration of PACAP-38 in the superficial pineal gland was determined by radioimmunoassay to be 20 .4 pmol/g tissue at midday and 18.9 pmol/g tissue at midnight. The concentr ation of PACAP-27 was only about 3% of the concentration of PACAP-38. In su mmary, this study is the first demonstration of a PACAP-containing innervat ion of the rat pineal gland. The PACAP concentration in the pineal gland do es not exhibit a day-night difference. The colocalization of PACAP with cal citonin gene-related peptide in the pinealopetal nerve fibres indicates tha t the majority of PACAP-immunoreactive nerve fibres might originate from th e trigeminal ganglion.