Structure–activity and possible mode of action of S-Iamide neuropeptides on identified central neurons of Helix aspersa

Intracellular recordings were made from identified neurons from the suboesophageal ganglia of Helix aspersa. The inhibitory action of nine S-Iamide peptides was investigated. Structure–activity studies suggest that all act through a common receptor, which normally requires FVRIamide at the C terminal, with a preferred length of seven amino acids. Substitution at the N-terminal with alanine (A), threonine (T), proline (P) or leucine (L) results in little change in potency, suggesting the N-terminal requirements are relatively flexible. Ion substitution experiments suggest that potassium is the main ion involved in the inhibitory response to S-Iamide application. Studies using a range of compounds, which modify second messenger systems, would suggest that S-Iamide peptides may interact with adenylate cyclase. No evidence was found for an interaction with either guanylate cyclase or nitric oxide synthase.