PACAP in Avians: Origin, Occurrence, and Receptors - Pharmacological and Functional Considerations

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a novel member of the secretin / glucagon / vasoactive intestinal peptide (VIP) superfamily. In vertebrates, including avians, it occurs in two forms: PACAP38 and PACAP27. PACAP structure is well conserved during evolution, being identical in mammals, and showing one amino acid dfifference in avians (chick, turkey). PACAP is widely distributed in the central nervous system and peripheral tissues and displays a pleiotropic activity, including functions as a hypophysiotropic hormone, neuromodulator, and neurotrophic factor. PACAP exerts its biological actions through three types of receptors designated PAC1, VPAC1 and VPAC2. This review (1) presents the current knowledge on PACAP origin, distribution and function, (2) compares the avian findings with those found in mammals, and (3) describes receptor-linked mechanisms in avians, including recent data on receptor-related signal transduction pathways, with a special emphasis on receptor pharmacology and function.