THE FMRFamide-LIKE NEUROPEPTIDE OF APLYSIA IS FMRFamide

The head ganglia from 350 Aplysia brasiliana were extracted and purified by gel (Sephadex G-15) and cation exchange (CM-Sephadex) chromatography; the fractions were examined with radioimmunoassays (RIA) for the molluscan neuropeptides, FMRFamide and SCP_B. Immunoreactive (ir-) FMRFamide (but not ir-SCP_B) coeluted with authentic FMRFamide from both chromatographic columns. The amino acid composition of the purified peptide was: Phe 2: Arg 1: Met 1. Digestion of purified ir-FMRFamide with carboxypeptidase Y indicated that the four residues were in the same sequence as occurs in FMRFamide. The dose-response curves for purified and synthetic FMRFamide on the radula protractor muscle of Busycon contrarium were coincident, as were their inhibition binding curves in the FMRFamide RIA. The highest concentrations of ir-FMRFamide were in the pedal and pleural ganglia; but SCP_B was concentrated in the buccal ganglion. Synthetic SCP_B has no effect on the radula protractor muscle of Busycon or the isolated heart of Mercenaria. In conclusion, the FMRFamide-like peptide in the gastropod Aplysia is FMRFamide, so this peptide has now been identified in two molluscan classes. Moreover, the proposed structural relationship between FMRFamide and SCP_B is fortuitous, and these two peptides should have different physiological functions in Aplysia.