Nitric oxide ((NO)-N-.) can act as an antioxidant by directly scavenging reactive free radicals, inhibiting the oxidative chemistry of iron, and signaling the up-regulation of antioxidant enzymes. However, the cellular utility of (NO)-N-. as an antioxidant requires that constitutive nitric oxide synthase (NOS) be activated rapidly by a signal(s) for oxidant formation, We report here that superoxide (O-2(.-)), added directly as potassium superoxide (KO2), produced a superoxide dismutase-sensitive and hydrogen peroxide-independent stimulation of NOS activity, measured by the conversion of [H-3]arginine to [H-3]citrulline and nitrite formation, in a synaptic particulate fraction from rat brain cerebral cortex. O-2(.-) produced maximal activation of NOS in the presence of the antioxidant urate and ATP. Stimulation of NOS activity by O-2(.-) was abolished by N-monomethyl-L-arginine and by the Ca2+ chelator EGTA but not by 7-nitroindazole, which would be expected to inhibit neuronal NOS. We propose that limited activation of NOS by O-2(.-) may be an important contributor to brain oxidant defenses and, more generally, a signal for cellular adaptation and survival, although excessive generation of nitrogen oxides would be expected to produce neurotoxicity. (C) 2002 Elsevier Science (USA). All rights reserved.