Effect of nitric oxide and cyclic guanosine monophosphate on the desensitization of choline receptors in snail neurons

Abstract

Desensitization of choline receptors is studied after pharmacological effects on the intracellular level of nitric oxide and cyclic guanosine monophosphate in snail neurons. It is shown that inhibitors of NO synthase do not alter desensitization, whereas an activator of guanylate cyclase as well as intracellular injection of guanosine monophosphate boost it.Key Words: nitric oxide; cyclic 3',5"guanosine monophosphate, desensitization of choline receptors; snail neuronsThe recently discovered second messenger nitric oxide (NO) [4,5] is an intermediate link between the transmitter receptor and soluble guanylate cyclase. Binding to the heme group of the enzyme, NO activates it and stimulates the synthesis of cyclic 3',5'-guanosine monophosphate (cGMP) [4,5].As was previously shown by us, cGMP is involved in desensitization of the choline receptors of the identified RPa3 and LPa3 neurons of the snail when sodium nitroprusside, an activator of guanilate cyclase, is used [1]. Stimulation of soluble guanylate cyclase by endogenous NO points to its role in the activation of cGMP synthesis in neurons and its involvement in the regulation of choline receptor desensitization. In this connection this study was devoted to an investigation of the influence of NO synthase inhibitors, cGMP, and guanylate cyclase modulators on desensitization of choline receptors of snail neurons. MATERIALS AND METHODS Experiments were carried out on the identified RPa3 and LPa3 neurons Of the edible snail (Helix lucorum)in a preparation of isolated ganglia. Transmembrane ion currents were recorded using the technique of the double-electrode voltage-clamp on the membrane.Desensitization of the choline receptors was assessed according to the suppression of the amplitude of the acetylcholine (AC)-induced inward current (AC current) during a series of 11-13 local rhythmic AC applications. To lower the amplitude of the AC current the first 10 stimuli were applied at a constant interval, the duration of which was 45-240 sec in series on different neurons. The next 1-3 stimuli were delivered at 10-rain intervals for estimation of the extent and rate of restoration of the diminished reaction.The following substances were used: inhibitors of NO synthase No-methyl-L-arginine acetate (L-NMMA, Fluka), the hydrochloride of the methyl ether of N-nitro-L-arginine (L-NAME, Sigma); the NO donor S-nitroso-N-acetylpenicillamine (SNAP, RBI); the inhibitor of soluble guanylate cyclase LY-