Actions of molluscan neuroactive peptides on identified central neurones of Helix aspersa

The identified central neurones Helix aspersa, were used to elucidate the action of molluscan neuroactive peptides, using intracellular recording and two electrode voltage clamp. APGWamide, a RPCH/AKH-related peptide, has been isolated from the ganglia of Fusinus, Lymnaea, Achatina fulica and Helix aspersa. APGWamide and its fragment, GWamide, are equally potent in their actions on both presynaptic and postsynaptic sites. Both peptides had a very weak postsynaptic effect on F1 neurones but presynaptically inhibited the evoked-IPSPs of these neurones. In contrast, they showed a direct inhibition on F2 neurones which was due to an increase in gK. SEEPLY-OH is conserved in the tetrapeptide protein precursor of Aplysia and Lymnaea. At 50 μM, SEEPLY-OH had no direct effect on neurones under tested but potentiated the ACh-induced H-responses on F4a neurones, the FMRFamide-induced H-responses on F5 neurones and the FMRFamide-induced D-responses on E16 neurones. SEEPLY-OH either was inactive or reduced the ACh-induced D-responses on F1 and F2 neurones. SEEPLY-OH also potentiated the evoked-EPSPs of Helix neurones. Mytilus inhibitory peptides, ^2S-MIP and ^2A-MIP, only exhibited an inhibition on E16 neurones which was due to an increase in g_K and g_Cl, but they had no significant effect on ACh-induced responses and evoked-IPSPs of Helix neurones. APNFLAYPRLamide isolated from the ABRM of Mytilus edulis excited F2 neurones through an increase in g_Na and g_Ca, and inhibited the ACh-induced responses on F2 neurones. Both effects may involve a second messenger. Fusinus FMRFamide-related peptides, GSLFRFamide and SSLFRFamide, showed an inhibition on F2 neurones through an increase in g_K. GSLFRFamide was inactive. The sequence of LFRFamide was therefore crucial for the biological effect of Fusinus hexapeptides. Furthermore, a tetrapeptide sequence was essential for the biological action of FMRFamide analogues since tripeptide and dipeptide were inactive.