Na_V Channels and Pain

The role of Na_V Channels in several types of pain opens a gateway for the development of specific sodium-channel inhibiting agents for the treatment of chronic pain. In this short piece we demonstrate the use of Alomone Labs Na_V-related products in pain research. Right: DRGs play a pivotal role in nociception.

Introduction

Pain sensation is an extremely common pathophysiological condition. The voltage-gated sodium (Na_V) channel isoforms Na_V1.7, 1.8 and 1.9 are selectively expressed in pain-conducting pathways of the peripheral nervous system (PNS). These isoforms are mostly localized to peripheral sensory ganglia, including dorsal root ganglia (DRG) and trigeminal ganglia (TG). Na_V1.7 is specifically associated with pain pathways since it is highly expressed in both unmyelinated C-fibers that contain the neuropeptide substance P and in free nerve endings in nociceptive receptor fields.

The association between Na_V channels and pain is further demonstrated by individuals who carry Na_V1.7 mutations. Some mutations, completely abolish nociception and these individuals do not experience pain.

Na_V channels have also been implicated in inflammatory pain states. Multiple studies have shown changes in sodium channels in experimental animals with inflammatory lesions. Inflammation typically causes increased expression of the Na_V1.3, 1.7, 1.8, and 1.9 isoforms in sensory neuronal cell bodies.

The role of Na_V channels in several types of pain opens a gateway for the development of specific sodium-channel inhibiting agents for the treatment of chronic pain².

Na_V Channels and Pain

Resurgent Na⁺ currents are currents activated during repolarization, and are the driving force for the generation of additional action potentials and thus contribute to repetitive firing of neurons. TTX-sensitive (TTX-s) resurgent currents are detected in cerebellar and dorsal root ganglia (DRG) neurons, and are achieved in a Na_V1.6-dependent manner . A novel TTX-resistant (TTX-r) resurgent Na⁺ current was isolated in rat DRGs . In many aspects this current is similar to the TTX-s resurgent current in that it requires the presence of Na_Vβ4 subunit. However, TTX-r resurgent currents exhibit slower kinetics and occur at more depolarized voltages. They are also sensitive to a Na_V1.8 specific blocker. The involvement of Na_V1.8 in resurgent currents is further strengthened by immunoprecipitation studies using Anti-Na_V1.8 (SCN10A) Antibody (#ASC-016) in rat DRG lysates. Na_V1.8 was shown to interact with Na_Vβ4. These TTX-r resurgent currents may contribute to the membrane excitability of nociceptive DRG neurons under normal conditions. The increase in both types of resurgent currents by inflammatory mediators could contribute to neuronal hyperexcitability associated with inflammatory pain⁸.

Pain-related bone cancer is caused in part by increased excitability of DRG neurons. Western blot analysis and electrophysiological recordings of rat DRGs show that Na_V1.8 channel expression and activity are upregulated in DRG neurons, and contribute to the development of cancer-induced bone pain³. Similar data is observed for Na_V1.9, for which changes in protein levels are significant, as observed in western blot analysis and immunohistochemistry using Anti-SCN11A (Na_V1.9) Antibody (#ASC-017) (Figure 1)⁴.

Painful diabetic neuropathy is a complication from which diabetic patients can suffer from. It is believed to originate in the peripheral system. A study showed that Na_V1.7 and Na_V1.8 channels are both upregulated in small DRGs and in peripheral nociceptive C-fibers in both immunohistochemistry and western blot analyses using Anti-Na_V1.8 (SCN10A) Antibody and Anti-Na_V1.7 (SCN9A) Antibody (#ASC-008) (Figure 2). The increased excitability in small DRGs from diabetic rats might underlie the decreased conduction in the diabetic high-firing-frequency polymodal C-fibers, thus uncovering a novel mechanism for hyperalgesia associated with diabetes⁷.

In a study, nociceptive sensory neurons were generated from HUES6 embryonic stem cells. Characterization of the cells was achieved in part by immunocytochemical staining of the derived nociceptors using Anti-TRPV1 (VR1) Antibody (#ACC-030) and Anti-SCN11A (Na_V1.9) Antibody. These cells also express cardiac specific Na_V1.5 channels, stained using Anti-Na_V1.5 (SCN5A) (1978-2016) Antibody (#ASC-013), reinforcing its role during development. This work demonstrates that nociceptors can be derived from human pluripotent stem cells (hPSCs) and furthermore establishes a platform for studying developmental processes in nociceptive neurons and the possibility of developing targeted pharmacology¹.

Studies show that inflammatory cytokines are elevated in neuropathic pain. Furthermore, administration of inflammatory cytokines induces neuropathic pain which is also accompanied by an increase in pain-related Na_V channel expression. IL-10, an anti-inflammatory cytokine was shown to reverse the effects of inflammatory cytokines, and furthermore decrease the expression of Na_V1.3, Na_V1.6 and Na_V1.8 channels as shown in immunocytochemistry and western blot analyses of rat DRGs using Alomone Labs Anti-SCN3A (Na_V1.3) Antibody (#ASC-004), Anti-Na_V1.6 (SCN8A) Antibody (#ASC-009) and Anti-Na_V1.8 (SCN10A) Antibody. Results suggest that down-regulating Na_V channels might contribute to the effects of IL-10 in neuropathic pain⁵.

KIF5, a kinesin, is responsible for mediating the plus end-directed, microtubule-dependent transport of cargo proteins. It also plays an important role in transporting ion channels across long distances in axons. Following peripheral inflammation or nerve injury, Na_V1.8 accumulates in peripheral nerves. KIF5 was found to be responsible for transporting Na_V1.8 to the plasma membrane and axons in dorsal root ganglion (DRG) neurons. Na_V1.8 co-immunoprecipitates with KIF5 and both proteins immuno-colocalize in DRG and in the sciatic nerve as shown in immunohistochemical staining using Anti-Na_V1.8 (SCN10A) Antibody (Figure 3). This study provides a mechanism for Na 1.8 accumulation following inflammation⁶.

References

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8. Tan, Z.Y. et al. (2014) J. Neurosci. 34, 7190.