Knockdown of sodium channel NaV1.6 blocks mechanical pain and abnormal bursting activity of afferent neurons in inflamed sensory ganglia

Abstract

Inflammatory processes in the sensory ganglia contribute to many forms of chronic pain. We previously showed that local inflammation of the lumbar sensory ganglia rapidly leads to prolonged mechanical pain behaviors and high levels of spontaneous bursting activity in myelinated cells. Abnormal spontaneous activity of sensory neurons occurs early in many preclinical pain models, and initiates many other pathological changes, but its molecular basis is not well understood. The sodium channel isoform NaV1.6 can underlie repetitive firing and excitatory persistent and resurgent currents. We used in vivo knockdown of this channel via local injection of siRNA to examine its role in chronic pain following local inflammation of the rat lumbar sensory ganglia. In normal DRG, quantitative PCR showed that cells capable of firing repetitively had significantly higher relative expression of NaV1.6. In inflamed DRG, spontaneously active bursting cells expressed high levels of NaV1.6′ immunoreactivity. In vivo knockdown of NaV1.6 locally in the lumbar DRG at the time of DRG inflammation completely blocked development of pain behaviors and abnormal spontaneous activity, while having only minor effects on unmyelinated C-cells. Current research on isoform-specific sodium channel blockers for chronic pain is largely focused on NaV1.8, because it is present primarily in unmyelinated C fiber nociceptors, or on NaV1.7, because lack of this channel causes congenital indifference to pain. However, the results suggest that NaV1.6 may be a useful therapeutic target for chronic pain, and that some pain conditions may be primarily mediated by myelinated A-fiber sensory neurons.

A Gain-of-Function Mutation in Nav1.6 in a Case of Trigeminal Neuralgia

“…We have previously shown that Na V 1.6 is widely expressed in TRG neurons, giving rise to myelinated and unmyelinated fibers, and is localized in the nerve endings of unmyelinated fibers in the cornea (38), which are thought to be almost entirely nociceptive (39). Animal studies support a role for Na V 1.6 in nerve injury models (40), toxin-induced pain (41), chemotherapy-induced pain (42) and local DRG inflammatory pain (43,44). Thus, Na V 1.6 is poised to play an important role in the pathophysiology of chronic pain, including in TN.…”

KIF5B Promotes the Forward Transport and Axonal Function of the Voltage-Gated Sodium Channel Nav1.8

“…Previous studies have identified nine other pore-forming sodium channel ␣-subunits in mammals (Catterall et al, 2005); of these, Na v 1.6, Na v 1.7, and Na v 1.9, which are expressed in DRG neurons, are also critical for the sensation and transmission of pain Xie et al, 2013). Our results show that KIF5 interacted with both Na v 1.8 and Na v 1.9, but not with Na v 1.6 or Na v 1.7 in DRGs, suggesting specificity of the interaction between the channels and KIF5.…”

A pharmacological and transcriptomic approach to exploring novel pain targets

“…Myelinated nerves which were affected by inflammation and became spontaneously active expressed more Na V 1.6 channels compared to other myelinated neurons, with the local injection of small interfering Na V 1.6 RNA successfully blocking spontaneous activity and reducing aversive behaviour in the rats 198 . Unmyelinated C-fibres were found to be minimally affected 198 , suggesting Na V 1.6 may only play a major role in myelinated sensory neurons.…”

“…Myelinated nerves which were affected by inflammation and became spontaneously active expressed more Na V 1.6 channels compared to other myelinated neurons, with the local injection of small interfering Na V 1.6 RNA successfully blocking spontaneous activity and reducing aversive behaviour in the rats 198 . Unmyelinated C-fibres were found to be minimally affected 198 , suggesting Na V 1.6 may only play a major role in myelinated sensory neurons. At the same time, a semisection of the rat spinal cord resulted in increased contralateral expression of Na V 1.6 on unmyelinated DRG neurons 199 , suggesting Na V 1.6 could redistribute throughout the organism following neuronal injury.…”

“…While global Na V 1.7 knockout in mice was shown to be non-responsive to a range of neuropathic pain conditions and demonstrated deficiencies in acute pain sensing 175 , it is now evident that Na V 1.7 is not responsible for all that is painful 2,178,186 , dashing the hopes that the Na V 1.7 channel may be a panacea for all pain. The subsequent surging interest in the role of Na V channels in pain have revealed that other Na V isoforms, in particular Na V 1.6 and Na V 1.8, also play a major part in noxious sensing 2,186,197,198,205,207,349,350 , suggesting that further understanding of Na V 1.6 and Na V 1.8 may help us to decipher the pathophysiological mechanisms of pain.…”