A cloned frog vasoactive intestinal polypeptide pituitary adenylate cyclase-activating polypeptide receptor exhibits pharmacological and tissue distribution characteristics of both VPAC1 and VPAC2 receptors in mammals

Three receptor subtypes for the neuropeptides vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) have been identified in mammals: the PAC(1) receptor (PAC(1)-R) which is selectively activated by PACAP, and two VPAC receptors (VPAC(1)-R and VPAC(2)-R), which are equally stimulated by PACAP and VIP. The structures of PACAP and VIP have been well conserved during evolution, but little is known about VIP/PACAP receptors in nonmammalian species. An amphibian VTP/PACAP receptor complementary DNA (cDNA) has been cloned and characterized from a frog (Rana ridibunda) pituitary cDNA library. The predicted protein contains seven putative transmembrane domains and exhibits the highest sequence identity (65%) with the human VPAC(1)-R. The cloned cDNA was transiently expressed in LLC-PK1 cells, and its pharmacological profile was determined in comparison with the human VPAC(1)-R. Both PACAP and VIP stimulated cAMP accumulation through the cloned receptor with an EC50 of about 30 nM. In contrast, secretin, at concentrations that stimulate the human VPAC(1)-R, had no effect on cAMP production. RT-PCR analysis revealed the widespread distribution of this frog VIP/PACAP receptor in peripheral tissues. In situ hybridization histochemistry using a complementary RNA probe showed that the receptor gene is highly expressed in several hypothalamic and thalamic nuclei and to a lesser extent, in the pallium and striatum. In the pituitary, the highest messenger RNA levels were detected in the distal lobe. Taken together, these data show that the cloned frog receptor shares several common features with both the VPAC(1)-R and VPAC(2)-R of mammals; the frog receptor exhibits the highest sequence identity with mammalian VPAC(1)-R, but the lack of effect of secretin and the brain distribution of the receptor are reminiscent of the characteristics of the mammalian VPAC(2)-R. The sequence of the frog receptor should thus prove useful to decipher the structure-activity relationships of the VIP/PACAP receptor family.