In this study, we examined whether nitric oxide synthase (NOS) is upregulated in small dorsal root ganglion (DRG) neurons after axotomy and, if so, whether the upregulation of NOS modulates Na(+) currents in these cells. We identified axotomized C-type DRG neurons using a fluorescent label, hydroxystilbamine methanesulfonate and found that sciatic nerve transection upregulates NOS activity in 60% of these neurons. Fast-inactivating tetrodotoxin-sensitive (TTX-S) Na(+) ("fast") current and slowly inactivating tetrodotoxin-resistant (TTX-R) Na(+) ("slow") current were present in control noninjured neurons with current densities of 1.08 +/- 0. 09 nA/pF and 1.03 +/- 0.10 nA/pF, respectively (means +/- SE). In some control neurons, a persistent TTX-R Na(+) current was observed with current amplitude as much as approximately 50% of the TTX-S Na(+) current amplitude and 100% of the TTX-R Na(+) current amplitude. Seven to 10 days after axotomy, current density of the fast and slow Na(+) currents was reduced to 0.58 +/- 0.05 nA/pF (P < 0.01) and 0.2 +/- 0.05 nA/pF (P < 0.001), respectively. Persistent TTX-R Na(+) current was not observed in axotomized neurons. Nitric oxide (NO) produced by the upregulation of NOS can block Na(+) currents. To examine the role of NOS upregulation on the reduction of the three types of Na(+) currents in axotomized neurons, axotomized DRG neurons were incubated with 1 mM N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor. The current density of fast and slow Na(+) channels in these neurons increased to 0.82 +/- 0.08 nA/pF (P < 0.01) and 0.34 +/- 0.04 nA/pF (P < 0.05), respectively. However, we did not observe any persistent TTX-R current in axotomized neurons incubated with L-NAME. These results demonstrate that endogenous NO/NO-related species block both fast and slow Na(+) current in DRG neurons and suggest that NO functions as an autocrine regulator of Na(+) currents in injured DRG neurons.