A single receptor transduces both inhibitory and stimulatory signals of FMRFamide-related peptides

In the oviduct of Locusta migratoria, an inhibitory neuropeptide, PDVDHVFLRFamide (SchistoFLRFamide) has separate binding and activation regions. VFLRFamide is the minimum sequence required for binding, which is comparable to the parent peptide, whereas the His residue, which does not contribute to binding, is a critical amino acid for the inhibitory activity of the receptor. In this study, the His residue of HVFLRFamide was substituted by Tyr, Leu, Ile, or Val to yield a group of HVFLRFamide analogues. As revealed by bioassay, all of these hexapeptide analogues exert stimulatory effects on oviduct muscle contraction. However, results from three sets of binding experiments indicate that these stimulatory FMRFamide-related peptides (FaRPs) share the same binding site as PDVDHVFLRFamide and HVFLRFamide, the inhibitory FaRPs. First, unlabeled stimulatory FaRPs competitively displace bound [125I]YDVDHVFLRFamide. Second, two binding sites for the stimulatory peptide YVFLRFamide were identified and both of them have similar binding affinities and maximum binding capacities as the two binding sites for PDVDHVFLRFamide. Third, unlabeled PDVDHVFLRFamide and HVFLRFamide competitively displace the bound [125I]YVFLRFamide in the same manner as unlabeled YVFLRFamide. These findings suggest the presence of a novel ligand-receptor reaction system. In this system, inhibitory peptides and stimulatory peptides share a single receptor by having the same binding sequence VFLRFamide, but are able to produce opposite muscle responses due to differences in activation sites. Correspondingly, this single receptor could be coupled with two different intracellular signaling systems to mediate either inhibitory or stimulatory responses.