Upregulation of sodium channel Nav1.3 and functional involvement in neuronal hyperexcitability associated with central neuropathic pain after spinal cord injury

Spinal cord injury (SCI) can result in hyperexcitability of dorsal horn neurons and central neuropathic pain. We hypothesized that these phenomena are consequences, in part, of dysregulated expression of voltage-gated sodium channels. Because the rapidly repriming TTX-sensitive sodium channel Na(v)1.3 has been implicated in peripheral neuropathic pain, we investigated its role in central neuropathic pain after SCI. In this study, adult male Sprague Dawley rats underwent T9 spinal contusion injury. Four weeks after injury when extracellular recordings demonstrated hyperexcitability of L3-L5 dorsal horn multireceptive nociceptive neurons, and when pain-related behaviors were evident, quantitative RT-PCR, in situ hybridization, and immunocytochemistry revealed an upregulation of Na(v)1.3 in dorsal horn nociceptive neurons. Intrathecal administration of antisense oligodeoxynucleotides (ODNs) targeting Na(v)1.3 resulted in decreased expression of Na(v)1.3 mRNA and protein, reduced hyperexcitability of multireceptive dorsal horn neurons, and attenuated mechanical allodynia and thermal hyperalgesia after SCI. Expression of Na(v)1.3 protein and hyperexcitability in dorsal horn neurons as well as pain-related behaviors returned after cessation of antisense delivery. Responses to normally noxious stimuli and motor function were unchanged in SCI animals administered Na(v)1.3 antisense, and administration of mismatch ODNs had no effect. These results demonstrate for the first time that Na(v)1.3 is upregulated in second-order dorsal horn sensory neurons after nervous system injury, showing that SCI can trigger changes in sodium channel expression, and suggest a functional link between Na(v)1.3 expression and neuronal hyperexcitability associated with central neuropathic pain.