RFamide neuropeptide actions on the molluscan ventricle: Interactions with primary neurotransmitters

Different RFamide neuropeptides, some of non-molluscan origin, were examined for their effect on the ventricles of Buccinum undatum and Busycon canaliculatum. None of the peptides tested were inhibitory on these ventricles. All the peptides were extremely active, causing excitation of the preparations at low concentrations. The neuropeptides were then tested with the primary neurotransmitters. In the case of serotonin, the excitatory primary neurotransmitter, the RFamide neuropeptides induced a response, which was greatly enhanced by serotonin. Acetylcholine, the inhibitory neurotransmitter, induced relaxation whenever added, following a neuropeptide. The neuropeptides seemed to be independent of external Ca2+, since in Ca2+-free media tension was induced. On the contrary, serotonin was dependent on external Ca2+. These findings indicate that the neuropeptides generated tension via a different receptor to that of the primary neurotransmitters, using a different 2nd messenger and activating different Ca2+ sources. Finally, the parent neuropeptide Phe-Leu-Arg-Phe-NH2, when added following a different RFamide peptide, excited the preparation further, thus indicating the presence of a receptor that has higher affinity for some structures than others. When Phe-Met-Arg-Phe-NH2 followed Phe-Leu-Arg-Phe-NH2, no such response was recorded since the latter is of higher potency than the former.