Effects of nitric oxide donors on basal and K+-evoked release of [3H]noradrenaline from rat cerebral cortex synaptosomes.

We investigated the effects of nitric oxide (NO) donors, S-nitroso-N-acetylpenicillamine and sodium nitroprusside on basal and K+-evoked release of [3H]noradrenaline from superfused synaptosomes from the rat cerebral cortex. Both substances produced concentration-dependent increases in the release of the labeled transmitter under basal and depolarized conditions. The effects of the donors on basal release were Ca2+-independent but were not inhibited by the carrier-uptake blocker, desipramine; the effects were abolished by hemoglobin (an NO scavenger). Thirty-five minutes after stimulation with sodium nitroprusside, the synaptosomes were still responsive to KCl stimulation, indicating that the donor's effects were not caused by damage to the synaptosome membrane. The cGMP analogue, 8-bromo-cGMP, had no effect on basal release, and the enhanced release produced by sodium nitroprusside was not inhibited by the specific inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one, indicating that NO's effects on basal release of the neurotransmitter are guanylate cyclase-independent. Both of the NO donors had more marked effects on release of [3H]noradrenaline during K+-stimulated depolarization. The NO-mediated increase in this case was partially antagonized by 10 microM LH-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one, and 8-Br-cGMP was also capable of producing concentration-dependent increases in the K+-stimulated release of the transmitter. These findings indicate that the effects of the NO donors on [3H]noradrenaline release during depolarization are partially mediated by the activation of guanylate cyclase.