G-protein coupled activation of potassium channels by endogenous neuropeptides in snail neurons.

Members of the mytilus inhibitory peptide (MIP) family play an important role in the modulation of many physiological processes in molluscs. The signal transduction pathways affected by the MIP effect have not, however, been elucidated. Application of guanosine 5'-[gamma-thio]triphosphate tetralithium salt (GTPgammaS), guanosine 5'-[beta-thio]diphosphate trilithium salt (GDPbetaS), the G-protein inhibitor suramin and pertussis toxin (PTX) demonstrated the involvement of the PTX-insensitive G-protein in the signal transduction pathway mediating MIP effects. Both G-protein alpha(i) and betagamma subunits were identified in D-neurons of Helix pomatia by immunoblotting. Their role in signal transduction was shown in electrophysiological experiments, which supported the notion that, in addition to the Galpha subunit, the betagamma dimer also participates in the neuropeptide-induced activation of K-channels in snail neurons. Finally, neuropeptide-activated responses were inhibited by the activation of adenylyl cyclase and by blockers of the phospholipase pathway. We suggest that bifurcation of the signal transduction takes place at the level of G-protein subunits. The alpha subunit may have a direct effect on adenylyl cyclase, while the betagamma subunit may have a direct effect on phospholipase enzymes.