PACAP (pituitary adenylyl cyclase-activating peptide) is a peptide that exists in 2 forms, 27 or 38 amino acids, and is related to vasoactive intestinal peptide (VIP). Three related class B GPCRs, PAC1, VPAC1 and VPAC2, bind to PACAP; however, VPAC1 and VPAC2 have a much higher affinity for VIP than does PAC1. Several splice variants of PAC1 result in proteins that differ at the N-terminus and third intracellular loop; these variants differ in their affinities for PACAP and abilities to activate Gq and Gs. High expression of PAC1 is observed in the CNS and the adrenal medulla. Studies with PAC1-null mice indicate that PAC1 plays important roles in regulation of circadian rhythms, neutrophil migration, and pulmonary vascular tone.