Sodium Channels in Spinal Cord Injury, Nerve Injury and Neuropathic Pain

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Neuropathic pain and inflammatory pain occur frequently after traumatic nerve injury, spinal cord injury (SCI), and limb amputation, and represent unmet medical needs for VA and, more generally, the U.S. population. Currently available treatments are often ineffective or only partially effective, in part because of off-target cardiac and CNS side effects. Inappropriate spontaneous firing and hyper-responsiveness of dorsal root ganglion (DRG) neurons, due to activity of voltage-gated Na channels (Nav), are major contributors to neuropathic and inflammatory pain. Our aim is to identify and characterize Na channel isoforms and functionally-related molecules that drive DRG neuron hyperexcitability, so that we can target them for more effective pain relief. Preferential expression of several Na channel isoforms in DGR neurons including nociceptors, and the up-regulation after nerve injury of another isoform, which is not present at high levels within the CNS or cardiac tissue, makes these Na channel subtypes potentially attractive as therapeutic targets for pain. Highly specific interactions with binding partners may regulate activity of these channels, suggesting that targeting of Na channel partners may expand therapeutic strategies for pain. Our recent progress has included identification of Na channel Nav1.7 as a key player in human pain, and demonstration of accumulation of three Na channel isoforms, Nav1.7, Nav1.8 and Nav1.3 within painful human neuromas. We have also demonstrated the presence of several MAP kinases (activated p38 and ERK1/2) within neuromas. We now plan to build upon our progress, via the following specific aims: 1. Examine the contribution of specific Na channel isoforms, their accessory molecules, and other related channel proteins to mechanosensitivity in experimental neuromas. 2. Analyze the expression of specific Na channel isoforms, their accessory molecules, and other related channel proteins in human painful neuromas. 3. Study FHF2-mediated regulation of Nav1.7 and Nav1.8 Na channels that drive chronic pain. 4. Study MAPK-mediated modulation of Nav1.7 and Nav1.8 Na Channels that drive chronic pain. 5. Investigate whether contusive SCI triggers sensitization of DRG neurons via regulation of Na channel expression or modulation of their functional properties, thereby producing enhanced nociceptor hyperexcitability and pain.