ИСТИНА

Endogenous Helix heptapeptide SEPYLRFamide (local application) acts as an inhibitory modulator of acetylcholine (ACh) receptors in Helix neurons. SEPYLRFamide reduced reversibly the inward current in Helix lucorum identified neurons to local ACh application onto the soma. Ouabain, a specific inhibitor of Na,K-pump, (0.1 mM, bath application) decreased the ACh-induced inward current (ACh-current) also and increased the leak current. Moreover ouabain decreased the modulatory SEPYLRFamide effect on the ACh-current. An inhibitor of Na/Ca-exchange, benzamil (25 microM, bath application), prevented the effect of ouabain on the SEPYLRFamide modulatory effect (peptide-mediated reduction of the ACh-current). Benzamil blocked the ouabain-induced reduction of the ACh-current but did not change the increase in the leak current by ouabain. These results allow us to propose that Na/Ca-exchange may be involved in the Na,K-ATPase pathway for the SEPYLRFamide-mediated reduction cholinesensitivity of nonsynaptic zones in Helix neurons. Taking into account our results and those from the literature we propose the following intracellular mechanism for the SEPYLRFamide-mediated inhibitory modulatory effect on Helix neurone cholinosensitivity with Na,K-pump and Na/Ca-exchange. SEPYLRFamide – activation of membrane receptor – activation of G-protein – activation of neuronal signal molecule – inhibition of membrane Na,K-pump – elevation of intracellular content of Na+ – inhibition of Na/Ca-exchange in normal mode (Na+ - influx, Ca2+ - efflux) and activation of Na/Ca-exchange in reverse mode (Na+ – efflux, Ca2+ – influx) – elevation of intracellular free Ca2+ – activation of Ca2+-sensitive receptors (channels) in endoplasmic reticulum (IP3 receptors and ryanodine receptors) – Ca2+ release from internal stores – elevation of intracellular free Ca2+ – inhibition of neuron cholinosensitivity. Inhibition of the Na,K-ATPase must stop the hydrolysis of ATP and evoke corresponding elevation of intracellular ATP level that can activate ryanodine-sensitive Ca2+ release channel [Sitsapesan et al., 1994]. So it is not excluded that peptide-mediated inhibition of Na,K-ATPase evokes Ca2+ release via elevated ATP and following activation of ryanodine receptors. This process is limited by negative feedback: high intracellular Ca2+ concentration inhibits Ca2+ release via the receptors in Ca2+-depots [Berridge, 1993, 2002]. The above mechanism may be responsible for the reduction in cholinergic synaptic transmission to molluscan neurons by FMRFamide-related peptides.