Structural and functional identification of the pituitary adenylate cyclase-activating polypeptide receptor VPAC₂ from the frog Rana tigrina rugulosa

Recently, a frog pituitary adenylate cyclase-activating polypeptide (PACAP)/vasoactive intestinal peptide (VIP) receptor (fPVR) has been characterized, and interestingly, this receptor exhibits characteristics of both mammalian PACAP type II receptors VPAC₁R and VPAC₂R. In order to investigate the receptors responsible for mediating the actions of VIP and PACAP in amphibians, in this report, a frog VPAC₂ receptor (fVPAC₂R) cDNA was isolated. fVPAC₂R shares 47.7, 46.9 and 62.5% amino acid sequence identify with fPVR, human VPAC₁R and human VPAC₂R respectively. Functionally, fVPAC₂R, when expressed in CHO cells, was responsive to both frog peptides including VIP, PACAP38 and PACAP27 where the EC₅₀ values of these peptides in intracellular cAMP production were 0.15, 0.18 and 0.16 μM respectively. The pharmacological profiles of human peptides (VIP, PACAP38 and peptide histidine methionine) to stimulate frog and human VPAC₂Rs were compared, and it was found that these peptides could only activate the frog receptor at micromolar concentrations. fVPAC₂R was found to be widely distributed in various peripheral tissues as well as several regions of the brain. The presence of the receptor transcripts suggests the functional roles of the receptor in mediating the actions of PACAP and/or VIP in these tissues. As VIP and particularly PACAP27 are highly conserved peptides in vertebrate evolution, comparative studies of these peptides and their receptors in non-mammalian vertebrates should provide clues to better understand the physiology of these important peptides in human and other vertebrates.