1. The effects of nitric oxide (NO) donors on whole-cell, TTX-sensitive sod
ium currents and single sodium channels in excised patches were examined in
rat hippocampal neurons. The whole-cell sodium current consisted of a larg
e transient component (I-Na,I-t) and a smaller, inactivation-resistant, per
sistent 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-peni
cillamine (SNAP, 100 mu m).
3. The amplitude of I-Na,I-t was not changed significantly by the same conc
entrations 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 ex
cised inside-out patches from cultured hippocampal neurons. SNP at 10-100 m
u 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 ag
ent 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 hig
her than that elicited by SNAP. The time delay before an increase was obser
ved 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 (N
aCN), increased mean current in excised inside-out patches (V-p = +30 mV) f
rom -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 neur
onal persistent Na+ channels, but not transient Na+ channels, through an ox
idizing action directly on the channel protein or on a closely associated r
egulatory protein in the plasma membrane.