Inhibitory effects of nitric oxide on the uptake of [3H]dopamine and [3H]glutamate by striatal synaptosomes.

Ten-minute incubation with 1 mM S-nitroso-L-cysteine (NO-CYS), a nitric oxide (NO) generator, induced a 3.5-fold increase in the basal dopamine (DA) efflux from rat striatal slices. Reduced hemoglobin (100 microM), a NO scavenger, blocked this response. Nomifensine (10 microM), an inhibitor of high-affinity DA transport, reduced the NO-CYS effect by 39%. In a synaptosomal preparation, NO-CYS induced a concentration-dependent efflux of [3H]DA that was also significantly inhibited by 10 microM nomifensine. Because these findings indicated that NO could alter the activity of the striatal DA transporter, we tested the effect of NO and NO-CYS on the uptake of [3H]DA into crude striatal synaptosomes. Although both compounds inhibited [3H]DA uptake with similar dose-response characteristics (IC50 approximately 300 microM and approximately 400 microM, respectively), the effect of NO was quicker in onset. The presence of superoxide dismutase (30 U/ml) and catalase (50 U/ml) had a small impact on the NO-CYS inhibition of [3H]DA uptake (1.8-fold increase in IC50). NO (1 mM) inhibited striatal [3H]glutamate uptake by 23%, but lower concentrations had no significant effect. The duration of the effect of NO gas on [3H]DA uptake varied from < 5 min to > 10 min, depending on the concentration used. All concentrations of NO-CYS tested produced an inhibition of [3H]DA uptake that lasted at least 10 min. However, only concentrations < or = 1 mM NO-CYS (or NO) were washable. The effect of 3 mM NO-CYS lasted > 20 min and could not be reversed by washing. Reduced hemoglobin (300 microM) prevented the action of 3 mM NO-CYS, whereas methemoglobin had no effect. The action of 3 mM NO-CYS was temperature dependent and took about 2 min to fully develop. Scatchard analysis revealed that NO-CYS diminished the capacity of the DA transporter without having a significant effect on the affinity. NO-CYS had no effect on striatal [3H]-mazindol binding, suggesting that NO-CYS did not interact with the DA recognition site of the transporter. These data suggest that NO may play a role in the regulation of DA and, to a lesser extent, glutamate transport in the striatum.