Persistent Na+ current couples spreading depolarization to seizures in <i>Scn8a</i> gain-of-function mice

Spreading depolarization (SD) is a slowly propagating wave of massive cellular depolarization that transiently impairs the function of affected brain regions. Although SD typically arises as an isolated hemispheric event, we previously reported that reducing M-type potassium current (IKM) by ablation of Kcnq2 in forebrain excitatory neurons results in tightly coupled spontaneous bilateral seizure-SD complexes in the awake mouse cortex. Here we find that enhanced persistent Na+ current attributable to gain-of-function mutations in Scn8a (N1768D/+, hereafter D/+) produces a similar compound cortical excitability phenotype. Chronic direct-current (DC)-band EEG recording detected spontaneous bilateral seizure-SD complexes accompanied by seizures with a profound tonic motor component, which occur predominantly during the light phase and were detected at ages between postnatal Days 33 and 100. Laser speckle contrast imaging of cerebral blood flow dynamics resolved SD evoked by an injection of pentylenetetrazol as a bilateral wave of hypoperfusion and subsequent hour-long hypoperfusion in Scn8aD/+ cortex in awake head-restrained mice. Subcortical recordings in freely moving mice revealed that approximately half of the spontaneous cortical seizure-SD complexes arose with a concurrent SD-like depolarization in the thalamus and delayed depolarization in the striatum. In contrast, SD-like DC potential shifts were rarely detected in the hippocampus or upper pons. Consistent with the high spontaneous incidence in vivo, cortical slices from Scn8aD/+ mice showed a raised SD susceptibility, and pharmacological inhibition of persistent Na+ current (INaP), which is enhanced in Scn8aD/+ neurons, inhibited SD generation in cortical slices ex vivo and in head-fixed mice in vivo, indicating that INaP contributes to SD susceptibility. Ex vivo Ca2+-imaging studies using acute brain slices expressing a genetic Ca2+ sensor (Thy1-GCAMP6s) demonstrated that pharmacological activation of IKM suppressed Ca2+ spikes and SD, whereas an IKM inhibitor strongly increased the frequency of hippocampal Ca2+ spikes in Scn8aD/+ but not wild-type slices, suggesting that IKM restrains the Scn8a gain-of-function hyperexcitability. Our study identifies a cortical SD phenotype in Scn8a gain-of-function mice shared with the Kcnq2 conditional knockout model of developmental epileptic encephalopathy and reveals that an imbalance of non-inactivating inward and outward tonic membrane currents modulates spatiotemporal SD susceptibility bidirectionally.