Effects of nitric oxide donors on Ca2+-dependent [C-14]GABA release from brain synaptosomes: The role of SH-groups

Nitric oxide (NO) modulates processes of synaptic transmission at pre- and postsynaptic levels. In the present work we studied the mechanisms of actio n of NO on [gamma-C-14]amino-n-butyric acid ([C-14] GABA) release in rat co rtical synaptosomes. NO donors-S-nitroso-l-cysteine and hydroxylamine (but not sodium nitroprusside)-inhibited the neurotransmitter efflux in a concen tration range from 10 mu M to 1 mM. Nitrosocysteine completely and selectiv ely suppressed the Ca2+-dependent (vesicular) [14C]GABA release, while not affecting the Ca2+-independent component of the [C-14]GABA transport. The i nfluence of NO donors was not related to activation of guanylyl cyclase, si nce the membrane-permeable cGMP analog dibutyryl-cGMP did not mimic and the guanylyl cyclase inhibitor methylene blue did not change the NO effects. I n contrast, the membrane-permeable SH-reagent N-ethylmaleimide (NEM) resemb led the effects of NO donors on the Ca2+-dependent [C-14]GABA release. The degree of inhibition of the release by nitrosocysteine, hydroxylamine, and NEM correlated with their ability to oxidize intra-synaptosomal SH-groups. These data suggest that synaptosomal sulfhydryl groups are the target for N O action at the presynaptic level. The NO-induced oxidation of thiols may b e involved in physiological and, especially, pathological effects of nitric oxide in the central nervous system.