Actions of a small cardioactive peptide from Mytilus, APNFLAYPRLamide, on central neurones of Helix aspersa.

1. Mytilus small cardioactive peptide (SCP) was originally isolated from the anterior byssus retractor muscle of Mytilus edulis, with a primary structure, APNFLAYPRLamide. The mechanisms of action of this peptide were examined on identified central neurones of the snail, Helix aspersa, using intracellular recordings and a two electrode voltage clamp. 2. 50 microM APNFLAYPRLamide could elicit a long term excitation on F2 neurones, in normal saline, Na(+)-free saline, Ca(2+)-free saline or in 10 mM Co2+ saline. This indicates that the excitatory effect of APNFLAYPRLamide involved an increase in membrane conductance to both Na+ and Ca2+. 3. 10 microM APNFLAYPRLamide potentiated the Ca2+ inward current of F2 neurones while reversibly reducing the ACh-induced membrane current of these neurones. Forskolin had identical effects to those of APNFLAYPRLamide. This indicates that the action of APNFLAYPRLamide may be through a second messenger. In contrast APNFLAYPRLamide potentiated a cholinergic EPSP evoked in F2 neurones. 4. YPRLamide was inactive even at higher concentrations, for example, 100 microM, while LAYPRLamide exhibited activity but with a lower potency. This indicates that LAYPRLamide is the minimum structure required for activating this class of SCP-like peptide receptor. However, the mode of action of APNFLAYPRLamide on the Na+ current of F2 neurones requires further investigation.