Upregulation of the sodium channel NaVβ4 subunit and its contributions to mechanical hypersensitivity and neuronal hyperexcitability in a rat model of radicular pain induced by local DRG inflammation

High frequency spontaneous firing in myelinated sensory neurons plays a key role in initiating pain behaviors in several different models, including the radicular pain model in which the rat lumbar dorsal root ganglia (DRG) are locally inflamed. The sodium channel isoform Na V 1.6 contributes to pain behaviors and spontaneous activity in this model. Among all the isoforms in adult DRG, Na V 1.6 is the main carrier of TTX-sensitive resurgent Na currents that allow high-frequency firing. Resurgent currents flow after a depolarization or action potential, as a blocking particle exits the pore. In most neurons the regulatory β4 subunit is potentially the endogenous blocker. We used in vivo siRNA mediated knockdown of Na V β4 to examine its role in the DRG inflammation model. Na V β4 but not control siRNA almost completely blocked mechanical hypersensitivity induced by DRG inflammation. Microelectrode recordings in isolated whole DRGs showed that Na V β4 siRNA blocked the inflammation-induced increase in spontaneous activity of Aβ neurons, and reduced repetitive firing and other measures of excitability. Na V β4 was preferentially expressed in larger diameter cells; DRG inflammation increased its expression and this was reversed by Na V β4 siRNA, based on immunohistochemistry and Western blotting. Na V β4 siRNA also reduced immunohistochemical Na V 1.6 expression. Patch clamp recordings of TTX-sensitive Na currents in acutely cultured medium diameter DRG neurons showed that DRG inflammation increased transient and especially resurgent current; effects blocked by Na V β4 siRNA. Na V β4 may represent a more specific target for pain conditions that depend on myelinated neurons expressing Na V 1.6.