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 channels in excised patches were examined in rat hippocampal neurons. The whole-cell sodium current consisted of a large transient component (I-Na,I-t) and a smaller, inactivation-resistant, persistent component (I-Na,I-p). 2. In acutely dissociated neurons, the amplitude of the whole-cell I-Na,I-p increased by: 60-80 % within a few minutes of exposure to either of two NO donors, sodium nitroprusside (SNP, 100 mu M) or S-nitroso-N-acetyl-DL-penicillamine (SNAP, 100 mu m). 3. The amplitude of I-Na,I-t was not changed significantly by the same concentrations of SNP and SNAP, indicating that NO had a selective effect on I-Na,I-p 4. Both NO donors significantly increased the mean persistent current in excised inside-out patches from cultured hippocampal neurons. SNP at 10-100 mu M 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 mu M 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 mu M). 6. The average mean current recorded in the presence of SNP was 10-fold higher than that elicited by SNAP. The time delay before an increase was observed was shorter with SNP (4.0 +/- 0.8 min, n = 8) than with SNAP (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 mu M 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.