Bombesin is a tetradecapeptide originally isolated from frog skin and demonstrated to have a wide range of actions in mammals. Based on structural homology and similar biological activities, gastrin-releasing peptide (GRP) has been considered the mammalian equivalent of bombesin. We previously reported that frogs have both GRP and bombesin, which therefore are distinct peptides. We now report the cloning of a bombesin receptor subtype (BB4) that has higher affinity for bombesin than GRP. PCR was used to amplify cDNAs related to the known bombesin receptors from frog brain. Bombesin, a tetradecapeptide, was isolated from the skin of the frog Bombina bombina by Anastasi et al. (1) in 1971. Amphibian bombesin was found to have multiple effects in mammals (2, 3) and bombesin-like immunoreactivity was observed in mammalian brain, gastrointestinal (GI) tract, and lung (4-6). In 1979, using gastrin release as a bioassay McDonald and co-workers (7) isolated from porcine stomach a 27-amino acid peptide homologous to the C terminus of bombesin and named it gastrin-releasing peptide (GRP). GRP is widely distributed in mammals; it acts as a neurotransmitter in brain, a paracrine hormone in GI tract, and a growth factor in developing lung (8-10). Because GRP reproduces all of bombesin's biologic effects, GRP was initially considered to be mammalian bombesin (see Fig. 1). In 1992, Nagalla et al. (11) showed that GRP and bombesin are clearly distinct peptides, in that frogs have both GRP and bombesin, and phylogeneticThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.analysis suggests that the peptides separated prior to the vertebrate radiation. This suggests that GRP and bombesin will have different receptors and that mammals, like frogs, may have bombesin-related peptides (BRPs) distinct from GRP. Neuromedin B (NMB) is a second mammalian bombesin-like peptide. NMB was originally isolated from porcine spinal cord (12) and like GRP is widely distributed in brain and GI tract.Making matters more complex, bombesin does not exist in just a single form. In all frogs studied to date, there are multiple forms of bombesin (13, 14). In Bombina orientalis, (Fig. 1) (G protein) coupled, have seven hydrophobic domains, and activate phospholipase C to increase inositol 1,4,5-trisphosphate, diacylglycerol, and intracellular calcium. A second nomenclature system for the bombesin receptors is also in use. In this system, the NMB receptor is known as the BB1 receptor, the GRP receptor is the BB2 receptor, and the BRS-3 receptor is the BB3 receptor (22). Because of the high homology between bombesin-like peptides, there is potential for cross-talk between ligands and receptors.