Molluscan neuropeptides.

Achatin-I, fulicin, fulyal, Mytilus-FFRFamide and Helix CCAP-RP-III are D-amino acid-containing neuropeptides from molluscs. Achatin-I, fulicin and fulyal from Achatina showed excitatory and/or modulatory actions on the penis retractor, radula retractor or ventricular muscles and neurons, though their L isomers were devoid of activity. On the other hand, both Mytilus-FFRFamide and its L isomer showed excitatory effects on the anterior byssus retractor muscle. Moreover, in contrast to Achatina neuropeptides, Helix CCAP-RP-III exhibited no remarkable activities on any of the muscles tested; instead, its L isomer possessed various excitatory effects. The molecular structures of these short peptides would be affected by the L-->D conversion and could influence activity. Molecular biological studies on the fulicin precursor suggest that fulicin, fulyal and related peptides are produced in Achatina ganglia and heart by processing of the ribosomally made precursor, and that L-isomeric fulicin and fulyal further undergo epimerization to yield the D-isomers.