Activating polypeptide Pituitary adenylate cyclase ( PACAP ) and vasoactive intestinal peptide (VIP) belong to a family of polypeptide hormones which include representatives glucagon, secretin and growth hormone releasing hormone . PACAP and VIP are widely distributed in both the central nervous system and in peripheral tissues of various vertebrates . PACAP exists in two forms : parent , 38 - amino acid form ( PACAP38 ) and shorter 27 - amino acid form ( PACAP27 ) . PACAP27 has 68 % sequence identity with VIP consisting of 28 amino acids. Primary structure of PACAP and VIP are evolutionarily well preserved. Both peptides act through a common receptor : VPAC1 and VPAC2 that bind VIP and PACAP with similar affinity . In addition, PACAP may be affected by some type of PAC1 receptors which bind PACAP with much higher affinity than VIP . PACAP and VIP act as neurotransmitters and neuromodulators , as well as regulatory factors , neurotrophic and neuroprotective .