Voltage-Gated Sodium Channels as Therapeutic Targets for Treatment of Painful Diabetic Neuropathy.

Painful diabetic neuropathy (PDN) is one of most common complication of diabetes, usually affecting 50% of diabetic patients and remains important cause of morbidity, mortality and deterioration of quality of life. PDN is well characterised by chronic hyperglycemia, alterations in expression and kinetics of voltage-gated sodium channels (VGSCs) and neuro-inflammation which together may result into sensorimotor deficits in peripheral nervous system. Peripheral nociceptive neurons express variety of sodium channel isoforms particularly Nav1.3, Nav1.7, Nav1.8 and Nav1.9, each play a key role in physiology of nociception by undergoing respective dynamic changes in expression and voltage-dependent gating properties. Thus, they are critical determinants of sensory neuronal excitability and associated neuropathic pain signal. Recent preclinical and clinical trial research has shed light on VGSCs as most compelling target in the treatment of PDN, a development that may open up new therapeutic approaches involving subtype selective sodium channel blockers to boost clinical efficacy, cost effectiveness, better tolerability and targeted treatment. In this review, we have summarized structure and functions of VGSCs and their involvement in the pathophysiology of neuropathic pain along with the current status of pharmacological interventions targeted at VGSCs in the treatment of diabetic neuropathy.

Rufinamide Improves Functional and Behavioral Deficits via Blockade of Tetrodotoxin-Resistant Sodium Channels in Diabetic Neuropathy.

Rufinamide is a structurally novel, antiepileptic drug approved for the treatment of Lennox-Gastaut syndrome. Its mechanism of action involves inhibition of voltage-gated Na+ channels (VGSCs) with possible membrane-stabilizing effects. VGSCs play a significant role in the pathogenesis of neuropathic pain. Therefore, we investigated the effects of rufinamide on tetrodotoxin-resistant sodium current (TTX-R I(Na)) in acutely dissociated rat dorsal root ganglion (DRG) neurons isolated from streptozotocin-induced diabetic rats by using whole-cell voltage-clamp configuration. In addition, the functional and behavioural nociceptive parameters were evaluated to assess its potential in diabetic neuropathy. Diabetic rats demonstrated the mechanical allodynia and thermal hyperalgesia with reduced nerve perfusion and conduction velocity as compared to control. Rufinamide treatments (3 and 10 mg/kg) significantly improved these functional and nociceptive deficits. Diabetic rat DRG neurons exhibited increased TTX-R I(Na) density as compared to control. The voltage-dependent activation and steady-state inactivation curves for TTX-R I(Na) in DRG neurons from diabetic rats were shifted negatively as compared to control. Rufinamide treatments significantly blocked the TTX-R Na+ channel activity as evident from significant reduction in I(Na) density and hyperpolarizing shift in activation and inactivation curves as compared to diabetic control. This suggests that rufinamide acts on TTX-R Na+ channels, reduces channel activity and attenuates nerve functional and behavioral parameters in diabetic rats. Altogether, these results indicate therapeutic potential of rufinamide in the treatment of diabetic neuropathy.

Calpain inhibitor, MDL 28170 confer electrophysiological, nociceptive and biochemical improvement in diabetic neuropathy.

Calpain plays an important role in the pathophysiology of neurological and cardiovascular complications, but its functional association in diabetic neuropathy is not yet elucidated. Therefore, we investigated the role of calpain in modulation of tetrodotoxin-resistant sodium channels (TTX-R Na(+) channels) in dorsal root ganglion (DRG) neurons using a pharmacological approach. The effects of a calpain inhibitor, MDL 28170 (3 and 10 mg/kg, i.p.) on TTX-R Na(+) channels in DRG neurons of streptozotocin-induced diabetic rats were assessed by using whole-cell patch-clamp technique. In addition to this biochemical, functional and behavioral deficits were also measured. Diabetic rats demonstrated the mechanical allodynia and thermal hyperalgesia with reduced nerve perfusion and conduction velocity as compared to control. MDL 28170 treatments significantly recovered these functional and nociceptive deficits. Moreover, diabetic rats exhibited increased calpain activation, lipid peroxidation and proinflammatory cytokines as compared to control. Drug treatment significantly improved these biochemical deficits. Additionally, DRG neurons from diabetic rats illustrated a significant increase in TTX-R sodium current (INa) density as compared to control. MDL 28170 treatments in diabetic rats significantly blocked the altered channel kinetics with hyperpolarizing shift in voltage-dependence of steady-state activation and inactivation curves. All together, our study provides evidence that calpain activation is directly associated with alterations in TTX-R Na(+) channels and triggers functional, nociceptive and biochemical deficits in experimental diabetic neuropathy. The calpain inhibitor, MDL 28710 have shown beneficial effects in alleviating diabetic neuropathy via modulation of TTX-R Na(+) channel kinetics and reduction of oxidative stress and neuro-inflammation.