Primary motor neurons fail to up-regulate voltage-gated sodium channel Na(v)1.3/brain type III following axotomy resulting from spinal cord injury.

Epilepsy occurs in a small proportion of patients with spinal cord injury (SCI), but whether it is due to concomitant traumatic head injury or to changes in cortical motor neurons secondary to axotomy within the spinal cord is not known. Na(v)1.3/brain type III sodium channel expression is up-regulated following peripheral axotomy of dorsal root ganglion (DRG) and facial motor neurons, but, to date, Na(v)1.3 expression has not been examined in upper (cortical) motor neurons following axotomy associated with SCI. In the present study, we examine Na(v)1.3 expression in upper motor neurons within rat primary motor cortex following midthoracic (T9) dorsal column transection, which severs the axons of those cells. Axotomized pyramidal cells were identified by retrograde transport of fluorogold. Immunolabeled cells were confined to layer V of the primary motor cortex and exhibited low levels of Na(v)1.3 staining. After axotomy, no significant changes were detected in Na(v)1.3 density or distribution in injured or uninjured cells, compared with control brains, in contrast to up-regulation of Na(v)1.3 in ipsilateral DRG neurons after sciatic nerve transection. These results do not preclude a role for voltage-gated sodium channels in post-SCI epilepsy but suggest that up-regulated expression of Na(v)1.3 channel is not involved.