Production of Resurgent Current in Na_V1.6-Null Purkinje Neurons by Slowing Sodium Channel Inactivation with β-Pompilidotoxin

Voltage-gated tetrodotoxin-sensitive sodium channels of Purkinje neurons produce "resurgent" current with repolarization, which results from relief of an open-channel block that terminates current flow at positive potentials. The associated recovery of sodium channels from inactivation is thought to facilitate the rapid firing patterns characteristic of Purkinje neurons. Resurgent current appears to depend primarily on Na_V1.6 α subunits, because it is greatly reduced in "med" mutant mice that lack Na_V1.6. To identify factors that regulate the susceptibility of α subunits to open-channel block, we voltage clamped wild-type and med Purkinje neurons before and after slowing conventional inactivation with β-pompilidotoxin (β-PMTX). β-PMTX increased resurgent current in wild-type neurons and induced resurgent current in med neurons. In med cells, the resurgent component of β-PMTX-modified sodium currents could be selectively abolished by application of intracellular alkaline phosphatase, suggesting that, like in Na_V1.6-expressing cells, the open-channel block of Na_V1.1 and Na_V1.2 subunits is regulated by constitutive phosphorylation. These results indicate that the endogenous blocker exists independently of Na_V1.6 expression, and conventional inactivation regulates resurgent current by controlling the extent of open-channel block. In Purkinje cells, therefore, the relatively slow conventional inactivation kinetics of Na_V1.6 appear well adapted to carry resurgent current. Nevertheless, Na_V1.6 is not unique in its susceptibility to open-channel block, because under appropriate conditions, the non-Na_V1.6 subunits can produce robust resurgent currents.