The goal of this proposal is to demonstrate the importance of sodium channel Nav1.6 in neuropathic and inflammatory pain, and to identify specific targets and strategies for the development of new and effective pain therapeutics. Dysregulated expression and mutations of sodium channels have been linked to hyperexcitability of dorsal root ganglia (DRG) neurons which underlies peripherally-originated inflammatory and neuropathic pain, both of which constitute the majority of neuropathic pain disorders. Nav1.6, the predominant sodium channel in large DRG neurons, is abundant at axon initial segments and mature nodes of Ranvier in myelinated fibers-the two neuronal compartments essential for conduction of nerve impulses. Our preliminary data shows that Nav1.6 is also expressed in most nociceptive DRG neurons which give rise to unmyelinated fibers, consistent with our previous finding of impaired C-fiber compound action potential conduction in sciatic nerve of Nav1.6-null mice. The continued presence of Nav1.6 in DRG neurons from mice knocked-out of other peripheral-specific sodium channels suggests that Nav1.6 channels support the conduction of pain signals in these mice. Ectopic firing by injured Ab afferents emanating from large DRG neurons suggests an important contribution of Nav1.6, to neuropathic pain. Thus, substantial evidence suggests that Nav1.6 contributes to injury-induced hyperexcitability of DRG neurons in chronic pain. Studies have shown that injury reduces levels of Nav1.6 transcripts in DRG neurons. Paradoxically, our preliminary data show significant Nav1.6 immunostaining at nodes of transected myelinated axons in rat sciatic nerve, consistent with similar published studies of ligated infraorbital nerves. We have also reported that Nav1.6 is a substrate for activated MAP kinase p38, which is activated in DRG neurons after injury, and is modulated by a host of cytosolic proteins including members of the FGF family and calmodulin. Thus, injury- mediated modulation of Nav1.6 channels may also contribute to altered neuronal hyperexcitability. In this proposal, we will carry out studies of injury-mediated effects on pain thresholds and hyperexcitability at the cellular level in DRG neurons following knock-down or knock-out of Nav1.6 channels to test the hypotheses that: i) Loss of Nav1.6 in DRG neurons ameliorates pain behavior;ii) Injury alters Nav1.6 sodium currents in nociceptive and large DRG neurons; iii) Modulation of Nav1.6 channels contributes to signal transduction of pro-nociceptive factors. We will also use molecular and immunological methods, and voltage-clamp and current-clamp recordings to: iv) Assess injury-mediated effects on altered regulation of expression and modulation of Nav1.6;and v) Correlate changes at the molecular and cellular levels in injured and spared DRG neurons to changes in pain thresholds.
Veterans are a vulnerable population for experiencing neuropathic pain both because of the injuries they sustain on the battlefield and also because of diabetes in the aged. Cancer chemotherapy and HIV are also accompanied with pain, and further exacerbate a severe problem. Effective pain management is necessary to facilitate Veterans rehabilitation process and improve quality of life for pain patients. However, first line therapeutics for neuropathic pain, including non-selective sodium channel blockers, are only partially effective and manifest serious side effects which limit their clinical utility. Thus, new targets for the development of well- tolerated pain drugs are urgently needed. We hope that our research on the contribution of sodium channel Nav1.6 to chronic pain will identify specific targets for the development of new pain drugs, and because of the role of Nav1.6 in other disorders, our research may also be relevant for a larger spectrum of neuroexcitability disorders.
|Dib-Hajj, Sulayman D; Geha, Paul; Waxman, Stephen G (2017) Sodium channels in pain disorders: pathophysiology and prospects for treatment. Pain 158 Suppl 1:S97-S107|