OPPOSITE EFFECTS OF NITRIC-OXIDE ON IDENTIFIED INHIBITORY AND EXCITATORY CHOLINERGIC SYNAPSES OF APLYSIA-CALIFORNICA

The effects of nitric oxide on evoked acetylcholine (ACh) release were studied at two identified cholinergic neuro-neuronal synapses of the nervous system of the mollusc Aplysia californica. The NO-donor, 3-mor pholinosydnonimine (SIN-1), decreased the amplitude of evoked inhibito ry postsynaptic currents (buccal ganglion) and potentiated that of evo ked excitatory postsynaptic currents (abdominal ganglion). SIN-1 acted by modulating the number of ACh quanta released. 8Br-cGMP mimicked th e effects of NO on ACh release in both types of synapses thus pointing to the involvement of a NO-sensitive guanylate cyclase. Presynaptic v oltage-dependent Ca2+ and K+ (I-A and late outward rectifier) currents were not modified by SIN-1 suggesting another final target for NO/cGM P. The labelling of a NO-synthase by immunostaining in several neurone s as well as the modulation of ACh release by L-arginine indicate that an endogenous NO-synthase is involved in the modulation of synaptic e fficacy in both buccal and abdominal ganglia.