Navβ4 regulates fast resurgent sodium currents and excitability in sensory neurons

Background: Increased electrical activity in peripheral sensory neurons including dorsal root ganglia (DRG) and trigeminal ganglia neurons is an important mechanism underlying pain. Voltage gated sodium channels (VGSC) contribute to the excitability of sensory neurons and are essential for the upstroke of action potentials. A unique type of VGSC current, resurgent current (INaR), generates an inward current at repolarizing voltages through an alternate mechanism of inactivation referred to as open-channel block. INaRs are proposed to enable high frequency firing and increased INaRs in sensory neurons are associated with pain pathologies. While Nav1.6 has been identified as the main carrier of fast INaR, our understanding of the mechanisms that contribute to INaR generation is limited. Specifically, the open-channel blocker in sensory neurons has not been identified. Previous studies suggest Nav beta 4 subunit mediates INaR in central nervous system neurons. The goal of this study was to determine whether Nav beta 4 regulates INaR in DRG sensory neurons. Results: Our immunocytochemistry studies show that Nav beta 4 expression is highly correlated with Nav1.6 expression predominantly in medium-large diameter rat DRG neurons. Nav beta 4 knockdown decreased endogenous fast INaR in medium-large diameter neurons as measured with whole-cell voltage clamp. Using a reduced expression system in DRG neurons, we isolated recombinant human Nav1.6 sodium currents in rat DRG neurons and found that overexpression of Nav beta 4 enhanced Nav1.6 INaR generation. By contrast neither overexpression of Nav beta 2 nor overexpression of a Nav beta 4-mutant, predicted to be an inactive form of Nav beta 4, enhanced Nav1.6 INaR generation. DRG neurons transfected with wild-type Nav beta 4 exhibited increased excitability with increases in both spontaneous activity and evoked activity. Thus, Nav beta 4 overexpression enhanced INaR and excitability, whereas knockdown or expression of mutant Nav beta 4 decreased INaR generation. Conclusion: INaRs are associated with inherited and acquired pain disorders. However, our ability to selectively target and study this current has been hindered due to limited understanding of how it is generated in sensory neurons. This study identified Nav beta 4 as an important regulator of INaR and excitability in sensory neurons. As such, Nav beta 4 is a potential target for the manipulation of pain sensations.