Pituitary adenylate cyclase activating polypeptide (PACAP) in the rat mammary gland

Pituitary adenylate cyclase activating polypeptide (PACAP), a member of the vasoactive intestinal polypeptide (VIP) family of peptides, is present in the brain and in neuronal elements of a number of peripheral organs. Since no information on PACAP in the mammary gland exists, we have investigated, by radioimmunoassay and immunohistochemistry, the occurrence and distributi on of PACAP immunoreactivity in the mammary gland of lactating and non-lact ating rats. A specific monoclonal mouse anti-PACAP antibody has been used t o show that the peptide is located in nerve fibres associated with bundles of circular and longitudinal smooth muscle surrounding the lactiferous duct of the nipple. PACAP-immunoreactive nerve fibres and nerve bundles are pre sent in the subepidermal connective tissue of the nipple and in the mammary parenchyma, some of the fibres being in close contact with blood vessels. Occasionally, a few delicate varicose fibres are associated with secretory alveoli and lactiferous ducts. The majority of PACAP-positive nerve fibres are, however, located in the glabrous skin of the nipple and the hairy skin adjacent to the nipple forming a subepithelial plexus from which delicate varicose nerve fibres enter the overlying epithelium. Double immunostaining for PACAP and a marker for sensory neurons, calcitonin gene-related peptid e, has disclosed that the two peptides are almost completely colocalized. A minor population of the PACAP-immunoreactive nerve fibres shows co-existen ce with VIP. Although no obvious changes at the immunohistochemical level c ould be observed during pregnancy or lactation, elevated concentrations of immunoreactive PACAP-38 in mammary extracts have been found during lactatio n. Our data suggest that PACAP is involved in the nervous control of mammar y gland function, probably in the transmission of suckling stimuli.