A NITRIC-OXIDE SYNTHASE ACTIVITY IS INVOLVED IN THE MODULATION OF ACETYLCHOLINE-RELEASE IN APLYSIA GANGLION NEURONS - A HISTOLOGICAL, VOLTAMMETRIC AND ELECTROPHYSIOLOGICAL STUDY

The role of nitric oxide or related molecules as neuromodulators was i nvestigated in the buccal and the abdominal ganglia of the mollusc Apl ysia californica. In a first step we showed that reduced nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry and specific nitric oxide synthase immunohistochemistry labelled the same neurons a nd fibres in both ganglia, pointing to the presence of a neuronal nitr ic oxide synthase. In a second step, we performed voltammetric detecti on of nitric oxide-related molecules using a microcarbon electrode in a reduction mode. A peak identified as N-nitroso-L-arginine was detect ed at -1.66 V in both ganglia. The identification of this compound as a product of endogenous nitric oxide synthase activity was reinforced by the fact that its peak amplitude was decreased in the presence of N -G-monomethyl-L-arginine, an inhibitor of nitric oxide synthase, and i ncreased with its substrate, L-arginine. An additional proof of a nitr ic oxide synthase activity was the detection of nitrites and nitrates in high concentrations (millimolar range) by capillary electrophoresis . We also showed that these nitric oxide-related molecules modulated a cetylcholine release at two identified synapses in these ganglia. L-Ar ginine decreased acetylcholine release at the inhibitory synapse (bucc al ganglion), whereas it increased acetylcholine release at the excita tory synapse (abdominal ganglion). The nitric oxide synthase inhibitor s, N-omega-nitro-L-arginine and N-G-monomethyl-L-arginine, had opposit e effects. Moreover, the exogenous nitric oxide donor, 3-morpholinosyd nonimine hydrochloride mimicked the effects of L-arginine on both inhi bitory and excitatory cholinergic synapses. The identification of two cholinergic synapses where nitric oxide affects acetylcholine release in opposite ways provides a useful tool to study the cellular mechanis ms through which nitric oxide-related molecules modulate transmitter r elease.