Effects of Nitric Oxide Donors on Ca²⁺-Dependent [¹⁴C]GABA Release from Brain Synaptosomes: the Role of SH-Groups

P. I. Nedvetsky¹, S. V. Konev¹, A. A. Rakovich¹, S. V. Petrenko², and A. A. Mongin^3*

¹Institute of Photobiology, National Academy of Sciences of Belarus, ul. Akademicheskaya 27, Minsk, 220072 Belarus; fax: (375-172) 842-359

²Institute of Radiation Medicine, Minsk, 220600 Belarus

³Albany Medical College, Pharmacology and Neuroscience, MC-60, 47 New Scotland Ave., Albany, NY 12208, USA; fax: (1-518) 262-6178; E-mail: MonginA@mail.AMC.edu

^* To whom correspondence should be addressed.

Received December 10, 1999

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Nitric oxide (NO) modulates processes of synaptic transmission at pre- and postsynaptic levels. In the present work we studied the mechanisms of action of NO on [gamma-¹⁴C]amino-n-butyric acid ([¹⁴C]GABA) release in rat cortical synaptosomes. NO donors--S-nitroso-L-cysteine and hydroxylamine (but not sodium nitroprusside)--inhibited the neurotransmitter efflux in a concentration range from 10 µM to 1 mM. Nitrosocysteine completely and selectively suppressed the Ca²⁺-dependent (vesicular) [¹⁴C]GABA release, while not affecting the Ca²⁺-independent component of the [¹⁴C]GABA transport. The influence of NO donors was not related to activation of guanylyl cyclase, since the membrane-permeable cGMP analog dibutyryl-cGMP did not mimic and the guanylyl cyclase inhibitor methylene blue did not change the NO effects. In contrast, the membrane-permeable SH-reagent N-ethylmaleimide (NEM) resembledthe effects of NO donors on the Ca²⁺-dependent [¹⁴C]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 NO action at the presynaptic level. The NO-induced oxidation of thiols may be involved in physiological and, especially, pathological effects of nitric oxide in the central nervous system.

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KEY WORDS: GABA, nitric oxide, sodium nitroprusside, S-nitroso-L-cysteine, hydroxylamine, SH-groups, DTNB, NEM

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