Actions of the molluscan neuropeptide FMRF-amide on neurones in the suboesophageal ganglia of the snail Helix aspersa.

The effects of the molluscan neuropeptide FMRF-amide were tested on several neurones in the suboesophageal ganglia of the snail Helix aspersa. Almost all neurones tested responded to the peptide, some being hyperpolarized (H response) and others depolarized (D response). The H response is due primarily to an inward potassium current and may be blocked in 20 microM 4-aminopyridine. The hyperpolarizing actions of FMRF-amide and dopamine may be separated by ergometrine which blocks the response to dopamine but not to FMRF-amide. The D response is due mainly to an inward sodium current but this is not blocked by d-tubocurarine, morphine or TTX. It appears to be mediated by a distinct receptor/ionophore as excitation by ACh and 5-HT are both antagonized by d-tubocurarine. The Leu2-substituted analogue FLRF-amide was found to produce similar H responses to FMRF-amide, but was much less potent at producing D responses. It did, however, produce cross-desensitization of the D response to FMRF-amide, suggesting that it does bind to the FMRF-amide receptor.