NO DECREASES EVOKED QUANTAL ACH RELEASE AT A SYNAPSE OF APLYSIA BY A MECHANISM INDEPENDENT OF CA2-KINASE-G( INFLUX AND PROTEIN)

1. The exogenous nitric oxide (NO) donor, SIN-1, decreased the postsyn aptic response evoked by a presynaptic spike at an identified choliner gic neuro-neuronal synapse in the buccal ganglion of Aplysia californi ca. 2. The statistical analysis of long duration postsynaptic response s evoked by square depolarizations of the voltage-clamped presynaptic neurone showed, that the number of evoked acetylcholine (ACh) quanta r eleased was decreased by SIN-1, pointing to presynaptic action of the drug. 3. Vitamin E, a scavenger of free radicals, prevented the effect s of SIN-1 on ACh release. SIN-1 still decreased ACh release in the pr esence of superoxide dismutase, a whereas haemoglobin suppressed the e ffects of SIN-1. These results showed that NO is: the active compound. 4. 8-Bromoguanosine 3',5' cyclic monophosphate (8-Br-cGMP) mimicked t he inhibitory effect of NO on ACh release suggesting the involvement o f a NO-sensitive guanylate cyclase. This was reinforced by the reversi bility of the effects of SIN-1 by inhibitors of guanylate cyclase, Met hylene Blue, cystamine or LY83583. Methylene Blue partially reduced th e inhibitory effect of NO. In addition, in the presence of superoxide dismutase, Methylene Blue blocked and cystamine significantly reduced the NO-induced inhibition of ACh release. 5. In the presence of KT5823 or R-p-8-pCPT-cGMPS, two inhibitors of protein kinase G, the reductio n of ACh release by SIN-1 still took place indicating; that the effect s of NO most probably did not involve protein kinase G-dependent phosp horylation. 6. Presynaptic voltage-dependent Ca2+ (L-, N- and P-types) and K+ (I-A and late outward rectifier) currents were unmodified by S IN-1. 7. The modulation of ACh release in opposite ways by L-arginine and N-omega-nitro-L-arginine points to the involvement of an endogenou s NO synthase-dependent regulation of transmitter release.