Characterization of nitric oxide generator-induced hippocampal [³H]norepinephrine release. I. The role of glutamate

In this study we compared the effects of two nitrogen monoxide (NO) generators, hydroxylamine and S-nitroso-L-cysteine (NO-CYS), on hippocampal [³H]norepinephrine ([³H]NE) release. A 10-min incubation with hydroxylamine (3-3,000 μM) or NO-CYS (30-10,000 μM) induced a concentration-dependent increase in the basal [³H]NE efflux with EC₅₀ values of approximately 100 μM and 1 mM, respectively. Reduced hemoglobin, a NO scavenger, blocked both hydroxylamine- and NO-CYS-evoked [³H]NE release. Long-term exposure (≥25 min) to 100 μM hydroxylamine, or to millimolar concentrations of NO-CYS, evoked a tetrodotoxin-insensitive [³H]NE release. However, a 10-min stimulation with either 100 μM hydroxylamine or 300 μM NO-CYS was sensitive to 0.5 μM tetrodotoxin, a voltage-sensitive sodium channel blocker. This suggested that under these conditions hydroxylamine and NO-CYS induce [³H]NE release indirectly in part, perhaps via releasing an excitatory neurotransmitter. Indeed, kynurenate, a nonselective ionotropic glutamate receptor antagonist, produced an 80% inhibition of the NO generator-evoked [³H]NE release. CGS 19755, a N-methyl-D-aspartate receptor antagonist, had no significant effect, whereas the α-amino-3-hydroxy-5-methylisoxazole-4- propionic acid/kainate receptor antagonists, CNQX and GYKI 52446, inhibited the hydroxylamine response by 50%. In synaptosomes, a preparation in which synaptic interactions are nonsignificant, NO-CYS induced a dose-dependent release of both [³H]NE and [³H]glutamate. These data suggest that, in hippocampal slices, NO generators evoke [³H]NE release both directly from noradrenergic terminals and indirectly via releasing glutamate.