Nitric oxide increases persistent sodium current in rat hippocampal neurons

1. The effects of nitric oxide (NO) donors on whole-cell, TTX-sensitive sodium currents and single sodium chanels in excised patches were examined in rat hippocampal neurons. The whole-cell sodium current consisted of a large transient component (I(Na,t)) and a smaller, inactivation-resistant, persistent component (I(Na,p)). 2. In acutely dissociated neurons, the amplitude of the whole-cell I(Na,p), increased by 60-80% within a few minutes of exposure to either of two NO donors, sodium nitroprusside (SNP, 100 μM) or S-nitroso-N-acetyl-DL-penicillamine (SNAP, 100 μM). 3. The amplitude of I(Na,t) was not changed significantly by the same concentrations of SNP and SNAP, indicating that NO had a selective effect on I(Na,p). 4. Both NO donors significantly increased the mean persistent current in excised inside-out patches from cultured hippocampal neurons. SNP at 10-100 μM increased average mean persistent current at a pipette potential (V(p)) of +30 mV from -0.010 ± 0.014 pA (control) to -2.91 ± 1.41 pA (n = 10). SNAP at 3-100 μM increased the average mean inward current in six inside-out patches from -0.07 ± 0.02 to -0.30 ± 0.08 pA (V(p) = +30 mV). 5. The increase in persistent Na⁺ channel activity recorded in inside-out patches in the presence of SNP or SNAP could be reversed by the reducing agent dithiothreitol (DTT, 2-5 mM) or by lidocaine (1-10 μM). 6. The average mean current recorded in the presence of SNP was 10-fold higher than that elicited by SNP. The time delay before an increase was observed was shorter with SNP (4.0 ± 0.8 min, n = 8) than with SNP (8.4 ± 1.6 min, n = 7). 7. A component of the SNP molecule added on its own, 5 mM sodium cyanide (NaCN), increased mean current in excised inside-out patches (V(p) = +30 mV) from -0.06 ± 0.04 to -0.58 ± 0.21 pA (n = 19). This increase in channel activity could be blocked by 10 μM lidocaine and 2-5 mM DTT. 8. These results suggest that NO may directly increase the activity of neuronal persistent Na⁺ channels, but not transient Na⁺ channels, through an oxidizing action directly on the channel protein or on a closely associated regulatory protein in the plasma membrane.