Sulfhydryl groups of choline acetylase

Abstract

Recent reports proposed that nitric oxide was a modulator of cholinergic transmission. Here, we examined the role of NO on cholinergic metabolism in a model of the peripheral cholinergic nervous synapse: synaptosomes fromTorpedoelectric organ. The presence of NO synthase was immunodetected in the cell bodies, in the nerve ending area of nerve-electroplate tissue and in the electroplates. Exogenous source of NO was provided from SIN1, a donor of NO and O·2−, and an end-derivative peroxynitrite (ONOO−). SIN1 increased calcium-dependent acetylcholine (ACh) release induced by KCl depolarization or a calcium ionophore A23187. The formation of ONOO−was continuously followed by a new chemiluminescent assay. The addition of superoxide dismutase, that decreases the formation of ONOO−, did not impair the stimulation of ACh release, suggesting that NO itself was the main stimulating agent. When the endogenous source of NO was blocked by proadifen, an inhibitor of cytochrome P450 activity of NO synthase, both KCl- and A23187-induced ACh release were abolished; nevertheless, the inhibitorNg-monomethyl-l-arginine did not modify ACh release when applied in a short time duration of action. Both NO synthase inhibitors reduced the synthesis of ACh from the radioactive precursor acetate and its incorporation into synaptic vesicles as did ONOO−chemically synthesized or formed from SIN1. In addition, choline acetyltransferase activity was strongly inhibited by ONOO−and SIN1 but not by the NO donors SNAP and SNP or, by NO synthase inhibitors. Altogether these results indicate that NO and ONOO−modulate presynaptic cholinergic metabolism in the micromolar range, NO (up to 100 μM) being a stimulating agent of ACh release and ONOO−being an inhibitor of ACh synthesis and choline acetyltransferase activity.