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

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DESCRIPTION (provided by applicant): This research proposal encompasses four inter-related projects that seek to do the following. Specific Aims I and II: Delineate mechanisms of, and identify effective therapies for, chronic pain resulting from traumatic nerve injury and limb amputation. Neuromas resulting from traumatic nerve injury and traumatic limb amputation can act as sites of abnormal ectopic impulse generation, which are linked to neuropathic pain. In many cases, neuroma-induced pain is not responsive to existing medications. We have shown that sodium channel Nav1.3 accumulates in both painful human neuromas and in experimental neuromas. The biophysical characteristics of Nav1.3 support its contribution to ectopic impulse generation in neuromas. In translational Aim I, we will build upon our viral- mediated shRNA (AAV-shRNA) knockdown of Nav1.3 in rat DRG in vivo and test the hypothesis that AAV- shRNA-Nav1.3 attenuates neuroma-induced pain. Multiple studies have indicated a contribution of channel isoform Nav1.7 to neuropathic pain. Nav1.7 accumulates in injured axon tips within human neuromas and in experimental neuromas, and has been genetically linked to human pain. As an alternative to targeting Nav1.3, Aim II will test AAV-shRNA mediated knockdown of Nav1.7 toward attenuation of neuroma-induced pain. Specific Aim III: Decipher specific sodium channel isoforms responsible for action potential conduction along nociceptive nerve fibers. In this mechanistic aim, we will assess the contribution of Nav1.7, Nav1.8 and Nav1.3 to electrogenesis in small-diameter axons of DRG neurons. We have recently developed the capability to patch-clamp small diameter (<1mm) DRG neurites in vitro and demonstrated that TTX-S and TTX-R sodium conductances are sequentially activated during action potentials. To advance our understanding of axonal electrogenesis, a logical next step is to identify the underlying channel subtypes and assess their contribution to excitability in nociceptive axons, as we propose in this specific aim. In addition, we will assess the functional effect of gain- of-function variants of Nav1.7 and Nav1.8 that are known to be associated with chronic pain syndromes in humans, on electrogenesis within the axonal compartment, utilizing our DRG culture/patch-clamp system. Specific Aim IV: Determine the contribution of aberrant sodium channel expression in SCI-induced spasticity. Spasticity after SCI has been classically thought to be a result of enhanced synaptic transmission along the spinal reflex pathways as well as loss of inhibition. However, studies at the circuit level, and computer simulation studies, suggest a contribution of increased intrinsic excitability of motor neurons and altered expression of sodium channels within motor neurons to spasticity after SCI. We have demonstrated upregulated expression of Nav1.3 within dorsal horn neurons after SCI and several investigators have reported upregulated expression of sodium channels in spinal and facial motor neurons following peripheral axotomy. Additionally, we have shown that Nav1.8 is mis-expressed in human CNS neurons, cerebellar Purkinje neurons) in patients with multiple sclerosis and in mice with EAE. In this mechanistic/translational aim, we will use molecular, patch-clamp, and knockdown methods to test the hypothesis that dysregulated sodium channel expression in motor neurons contributes to spasticity following SCI, and will determine whether knockdown of such dysregulated channels ameliorates spasticity.