Production of Resurgent Current in NaV1.6-Null Purkinje Neurons by Slowing Sodium Channel Inactivation with β-Pompilidotoxin

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Abstract
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 NaV1.6 α subunits, because it is greatly reduced in “med”mutant mice that lack NaV1.6. To identify factors that regulate the susceptibility of α subunits to open-channel block, we voltage clamped wild-type andmedPurkinje 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 inmedneurons. Inmedcells, the resurgent component of β-PMTX-modified sodium currents could be selectively abolished by application of intracellular alkaline phosphatase, suggesting that, like in NaV1.6-expressing cells, the open-channel block of NaV1.1 and NaV1.2 subunits is regulated by constitutive phosphorylation. These results indicate that the endogenous blocker exists independently of NaV1.6 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 NaV1.6 appear well adapted to carry resurgent current. Nevertheless, NaV1.6 is not unique in its susceptibility to open-channel block, because under appropriate conditions, the non-NaV1.6 subunits can produce robust resurgent currents.