Impaired motor function in mice with cell-specific knockout of sodium channel Scn8a (NaV1.6) in cerebellar purkinje neurons and granule cells

Abstract

The Scn8a gene encodes the voltage-gated Na channel alpha subunit Na(V)1.6, which is widely expressed throughout the nervous system. Global null mutations that eliminate Scn8a in all cells result in severe motor dysfunction and premature death, precluding analysis of the physiological role of Na(V)1.6 in different neuronal types. To test the effect of cerebellar Na(V)1.6 on motor coordination in mice, we used the Cre-lox system to eliminate Scn8a expression exclusively in Purkinje neurons (Purkinje KO) and/or granule neurons (granule KO). Whereas granule KO mice had only minor behavioral defects, adult Purkinje KO mice exhibited ataxia, tremor, and impaired coordination. These disorders were exacerbated in double mutants lacking Scn8a in both Purkinje and granule cells (double KO). In Purkinje cells isolated from adult Purkinje KO and double KO but not granule KO mice, the ratio of resurgent-to-transient tetrodotoxin- (TTX)-sensitive Na current amplitudes decreased from approximately 15 to approximately 5%. In cerebellar slices, Purkinje cell spontaneous and maximal firing rates were reduced 10-fold and twofold relative to control in Purkinje KO and double KO but not granule KO mice. Additionally, short-term plasticity of high-frequency parallel fiber EPSCs was altered relative to control in Purkinje KO and double KO but not granule KO mice. These data suggest that the specialized kinetics of Purkinje Na channels depend directly on Scn8a expression. The loss of these channels leads to a decrease in Purkinje cell firing rates as well as a modification of the synaptic properties of afferent parallel fibers, with the ultimate consequence of disrupting motor behavior.