Structure-activity studies of RFamide analogues on central neurones of Helix aspersa

The effects of FMRFamide were compared with those of FMRFamide analogues, FLRFamide, LFRFamide, FFRFamide, LLRFamide, D-FMRFamide, F-D-MRFamide and FM-D-RFamide, and the fragments, MRFamide and LRFamide, on identified central neurones, F1, F2, F5 and E16, of the snail Helix aspersa, using intracellular recording and two electrode voltage clamp techniques. All FMRFamide analogues showed an inhibitory effect on F1 neurones with an order of potency: FLRFamide > FMRFamide > FFRFamide > LFRFamide >> LLRFamide. FMRFamide and FLRFamide exhibited a biphasic response on F2 neurones. At lower concentrations (< 10 microM), both peptides usually only excited while at higher concentrations (> 30 microM), exhibited an excitation followed by an inhibition. FFRFamide only excited F2 while LFRFamide and LLRFamide only inhibited F2. LRFamide and MRFamide (100 microM) were inactive on both F1 and F2. FLRFamide, LFRFamide, LLRFamide, FFRFamide and D-FMRFamide showed cross-interaction on the outward current induced by FMRFamide in F5. All peptides induced an outward current and also reduced the FMRFamide-induced current reversibly. In contrast, MRFamide, LRFamide and F-D-MRFamide failed to have direct effects on these neurones nor interact with the FMRFamide-induced current. We conclude that on F2 neurones Phe is essential for the activation of the RFamide receptor mediating the excitation and Met or Leu are important to activate the RFamide receptors mediating the inhibition. Removal of the N-terminal Phe, to give LRFamide and MRFamide render the peptides inactive. Therefore a tetrapeptide sequence is essential for the biological activity of FMRFamide analogues on these Helix neurones. FLRFamide, LFRFamide, LLRFamide, FFRFamide and D-FMRFamide exhibit a cross-interaction with FMRFamide. It is possible that these peptides also act on the same class of RFamide receptors as agonists to cause cross desensitization.