C-type dorsal root ganglion (DRG) neurons express three types of Na+ currents: fast TTX-sensitive, slow TTX- resistant, and persistent TTX-resistant Na+ currents. The nitric oxide (NO) donors papa-NONOate and S-nitroso-N-acetyl-DL-penicillamine inhibit all three types of Na+ currents. The NO scavenger hemoglobin abolished the effects of papa-NONOate on Na+ currents, indicating that NO or NO-related species inhibit these Na+ currents. NO donor inhibition of all three types of Na+ currents was reversed by washout. Incubation of neurons with 8-bromo cGMP, a membrane-permeable analogue of cGMP, and cG-PKI, an inhibitor of cGMP-dependent protein kinase, had no effect on papa-NONOate-mediated Na+ current block, demonstrating that Na+ current inhibition is independent of cGMP. Alkylation of free thiols with N-ethylmaleimide prevented the actions of papa-NONOate, suggesting that NO, or a related reactive nitrogen species, modifies sulfhydryl groups on Na+ channels or a closely associated protein. Papa-NONOate-mediated block of Na+ currents is not due to a hyperpolarizing shift in steady state voltage-dependent inactivation. The absence of NO-mediated enhancement of slow inactivation in fast and slow Na+ channels indicates that NO does not inhibit fast and slow Na+ channels by facilitating the transition to a slow inactivated state. These results demonstrate that inhibition of Na+ currents is not due to the modulation of fast and slow sodium channel inactivation. Taken together, these results show that NO or NO-related products modify the sulfhydryl groups on Na+ channels and inhibit Na+ currents by blocking the channel conductance.
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