Small interfering RNA-mediated selective knockdown of Nav1.8 tetrodotoxin-resistent sodium channel reverses mechanical allodynia in neuropathic rats

The biophysical properties of a tetrodotoxin resistant (TTXr) sodium channel, Na(v)1.8, and its restricted expression to the peripheral sensory neurons suggest that blocking this channel might have therapeutic potential in various pain states and may offer improved tolerability compared with existing sodium channel blockers. However, the role of Na(v)1.8 in nociception cannot be tested using a traditional pharmacological approach with small molecules because currently available sodium channel blockers do not distinguish between sodium channel subtypes. We sought to determine whether small interfering RNAs (siRNAs) might be capable of achieving the desired selectivity. Using Northern blot analysis and membrane potential measurement, several siRNAs were identified that were capable of a highly-selective attenuation of Na(v)1.8 message as well as functional expression in clonal ND7/23 cells which were stably transfected with the rat Na(v)1.8 gene. Functional knockdown of the channel was confirmed using whole-cell voltage-clamp electrophysiology. One of the siRNA probes showing a robust knockdown of Na(v)1.8 current was evaluated for in vivo efficacy in reversing an established tactile allodynia in the rat chronic constriction nerve-injury (CCl) model. The siRNA, which was delivered to lumbar dorsal root ganglia (DRG) via an indwelling epidural cannula, caused a significant reduction of Na(v)1.8 mRNA expression in lumbar 4 and 5 (L4-L5) DRG neurons and consequently reversed mechanical allodynia in CCl rats. We conclude that silencing of Na(v)1.8 channel using a siRNA approach is capable of producing pain relief in the CCI model and further support a role for Na(v)1.8 in pathological sensory dysfunction. (C) 2007 IBRO. Published by Elsevier Ltd. All rights reserved.