Persistent Nav1.6 current at axon initial segments tunes spike timing of cerebellar granule cells.

Cerebellar granule (CG) cells generate high-frequency action potentials that have been proposed to depend on the unique properties of their voltage-gated ion channels. To address the in vivo function of Nav1.6 channels in developing and mature CG cells, we combined the study of the developmental expression of Nav subunits with recording of acute cerebellar slices from young and adult granule-specific Scn8a KO mice. Nav1.2 accumulated rapidly at early-formed AIS. In contrast, Nav1.6 was absent at early postnatal stages but accumulated at axon initial segments (AIS) of CG cells from P21 to P40. By P40-P65, both Nav1.6 and Nav1.2 co-localized at CG cell AIS. By comparing mature CG cells (P66-P74) Na+ currents properties from wild-type and CG-specific Scn8a KO mice, we found that transient and resurgent Na+ currents were not modified in the absence of Nav1.6 channels whereas persistent Na+ current was strongly reduced. Action potentials in conditional Scn8a KO CG cells showed no alteration in threshold and overshoot, but had a faster repolarization phase and larger post-spike hyperpolarization. In addition, although Scn8a KO CG cells kept their ability to fire action potentials at very high frequency, they displayed increased interspike-interval variability and firing irregularity in response to sustained depolarization. We conclude that Nav1.6 at axon initial segments contributes to persistent current and regulates spike discharge regularity of CG cells.