Our previous studies showed that the ectopic discharges in injured sensory neurons and mechanical allodynia that developed after spinal nerve ligation were significantly reduced by application of a low concentration of tetrodotoxin (TTX) to the corresponding dorsal root ganglion (DRG) of the ligated spinal nerve. Based on these data, we hypothesized that expression of TTX-sensitive sodium channels is up-regulated in the injured sensory neurons and that such up-regulation plays an important role in the generation of ectopic discharges and thus pain behaviors in spinal nerve ligated neuropathic rats. To test this hypothesis, the present study examined the changes in three subtypes of TTX-sensitive sodium channels in the DRG after spinal nerve ligation. The changes in the total amount of mRNA for alpha-subunits of sodium channel brain type I (type I), brain type II (type II) and brain type III (type III) were determined by RNase protection assays (RPA). The population of DRG neurons expressing type III sodium channel protein was examined by an immunohistochemical method with antibodies to type III sodium channels. In the normal DRG, the level of mRNA for the type I sodium channel is high while that for type II and type III is very low. After spinal nerve ligation, the expression of type III mRNA was significantly increased at 16-h postoperatively (PO), doubled by 3 days PO and then was maintained at this high level until the end of the experiment (7 days PO). By contrast, the amount of mRNA for type I and type II sodium channels started to decrease at 1 day PO and were reduced to 25-50% of the normal control levels by 7 days after nerve ligation. Neurons showing positive immunostaining for type III sodium channels were rare ( approximately 3.2% of total population) in the normal DRG but increased after nerve ligation to 21% and 15% of the total neuronal population by 1 day and 7 days PO, respectively. Type III immunoreactivity was found preferentially in medium to large sized neurons. Thus the majority of neurons with cell bodies having diameters > or =40 microm became type III-positive after nerve ligation. The data indicate that the increased expression of type III sodium channels in axotomized sensory neurons may be the critical factor for the TTX sensitivity of ectopic discharges in injured sensory neurons and thus the generation of ectopic discharges and neuropathic pain behaviors in spinal nerve ligated rats.