A new perspective on the inhibitory role of nitric oxide in sympathetic neurotransmission.

By using, as a model of sympathetic neurons, immortalized rat pheochromocytoma (PC12) cells differentiated by nerve growth factor (NGF), the effect of nitric oxide on sympathetic neurotransmission was examined. The NO donor sodium nitroprusside (SNP; 10(-4)-3 x 10(-4) M) caused an apparent inhibition of dopamine release from PC12 cells, as measured by HPLC. Studies, in the absence of cells, involving the incubation of dopamine (20 ng/ml) or norepinephrine (15 ng/ml) with SNP (10(-6)-3 x 10(-4) M) or authentic NO (6 x 10(-6)-3 x 10(-5) M) revealed a similar reduction in the detection of the catecholamines. In addition, absorption spectroscopy studies showed dopamine and norepinephrine to be oxidized by NO resulting in the formation of their respective quinone products. These observations, coupled with the finding that the ability of dopamine to raise cAMP levels within PC12 cells was reduced after incubation with SNP, reveal that NO inhibits the biological activity rather than the release of catecholamines.