Three distinct types of GnRH receptor characterized in the bullfrog

It has been proposed recently that two types of GnRH receptors (GnRHR) exis t in a particular species. Here we present data demonstrating that at least three types of GnRHR are expressed in a single diploid species, the bullfr og. Three different cDNAs. encoding distinct types of bullfrog GnRHR (bfGnR HR-1, bfGnRHR-2, and bfGnRHR-3), were isolated from pituitary and hindbrain of the bullfrog. BfGnRHR-1 mRNA was expressed predominantly in pituitary, whereas bfGnRHR-2 and -3 mRNAs were expressed in brain. The bfGnRHR-1, bfGn RHR-2, and bfGnRHR-3 proteins have an amino acid identity of approximate to 30% to approximate to 35% with mammalian GnRHRs and approximate to 40% to approximate to 50% with nonmammalian GnRHRs. Interestingly, bfGnRHR-2 has a n 85% amino acid homology with Xenopus GnRHR. Less than 53% amino acid iden tity was observed among the three bfGnRHRs. All isolated cDNAs encode funct ional receptors because their transient expression in COS-7 cells resulted in a ligand-dependent increase in inositol phosphate production. Notably, a ll three receptors exhibited a differential ligand selectivity. For all rec eptors, cGnRH-II has a higher potency than mGnRH. In addition, salmon GnRH also has a strikingly high potency to stimulate all three receptors. In con clusion, we demonstrated the presence of three GnRHRs in the bullfrog. Thei r expression in pituitary and brain suggests that bfGnRHRs play an importan t role in the regulation of reproductive functions in the bullfrog.