Control of transient, resurgent, and persistent current by open-channel block by Na channel β4 in cultured cerebellar granule neurons

Voltage-gated Na channels in several classes of neurons, including cells of the cerebellum, are subject to an open-channel block and unblock by an endogenous protein. The Na-V beta 4 (Scn4b) subunit is a candidate blocking protein because a free peptide from its cytoplasmic tail, the beta 4 peptide, can block open Na channels and induce resurgent current as channels unblock upon repolarization. In heterologous expression systems, however, Na-V beta 4 fails to produce resurgent current. We therefore tested the necessity of this subunit in generating resurgent current, as well as its influence on Na channel gating and action potential. ring, by studying cultured cerebellar granule neurons treated with siRNA targeted against Scn4b. Knockdown of Scn4b, confirmed with quantitative RT-PCR, led to five electrophysiological phenotypes: a loss of resurgent current, a reduction of persistent current, a hyperpolarized half-inactivation voltage of transient current, a higher rheobase, and a decrease in repetitive. ring. All disruptions of Na currents and. ring were rescued by the beta 4 peptide. The simplest interpretation is that Na-V beta 4 itself blocks Na channels of granule cells, making this subunit the first blocking protein that is responsible for resurgent current. The results also demonstrate that a known open-channel blocking peptide not only permits a rapid recovery from nonconducting states upon repolarization from positive voltages but also increases Na channel availability at negative potentials by antagonizing fast inactivation. Thus, Na-V beta 4 expression determines multiple aspects of Na channel gating, thereby regulating excitability in cultured cerebellar granule cells.