The changes in expression of three subtypes of TTX sensitive sodium channels in sensory neurons after spinal nerve ligation

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 tetrod otoxin (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 an d that such up-regulation plays an important role in the generation of ecto pic discharges and thus pain behaviors in spinal nerve ligated neuropathic rats. To test this hypothesis, the present study examined the changes in th ree subtypes of TTX-sensitive sodium channels in the DRG after spinal nerve ligation. The changes in the total amount of mRNA for a-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 ligatio n, the expression of type III mRNA was significantly increased at 16-h post operatively (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 da y PO and were reduced to 25-50% of the normal control levels by 7 days afte r nerve ligation. Neurons showing positive immunostaining for type III sodi um channels were rare (similar to3.2% of total population) in the normal DR G but increased after nerve ligation to 21% and 15% of the total neuronal p opulation by 1 day and 7 days PO, respectively. Type III immunoreactivity w as found preferentially in medium to large sized neurons. Thus the majority of neurons with cell bodies having diameters greater than or equal to 40 m um became type III-positive after nerve ligation. The data indicate that th e increased expression of type III sodium channels in axotomized sensory ne urons may be the critical factor for the TTX sensitivity of ectopic dischar ges in injured sensory neurons and thus the generation of ectopic discharge s and neuropathic pain behaviors in spinal nerve ligated rats. (C) 2001 Els evier Science B V All rights reserved.