Persistent Na⁺ current couples spreading depolarization to seizures in Scn8a 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. While SD typically arises as an isolated hemispheric event, we previously reported that reducing M-type potassium current (I_KM) 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 due to gain-of-function (GOF) mutations in Scn8a (N1768D/+, hereafter D/+) produces a similar compound cortical excitability phenotype. Chronic 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 P33-100. Laser speckle contrast imaging of cerebral blood flow dynamics resolved SD as a bilateral wave of hypoperfusion and subsequent hour-lasting hypoperfusion in Scn8a ^D/+ cortex in awake head-restrained mice evoked by a PTZ injection. 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 Scn8a ^D/+ mice showed a raised SD susceptibility, and pharmacological inhibition of persistent Na⁺ current (I_NaP), which is enhanced in Scn8a ^D/+ neurons, inhibited SD generation in cortical slices ex vivo as well as in head-fixed mice in vivo, indicating that I_NaP contributes to SD susceptibility. Ex vivo Ca²⁺ imaging studies using acute brain slices expressing genetic Ca²⁺ sensor (Thy1-GCAMP6s) demonstrated that pharmacological activation of I_KM suppressed Ca²⁺ spikes and SD, whereas an I_KM inhibitor strongly increased the frequency of hippocampal Ca²⁺ spikes in Scn8a ^D/+, but not WT slices, suggesting that I_KM restrains the Scn8a GOF hyperexcitability. Together, our study identifies a cortical SD phenotype in Scn8a GOF mice shared with the Kcnq2-cKO model of developmental epileptic encephalopathy, and reveals that an imbalance of non-inactivating inward and outward tonic membrane currents bidirectionally modulates spatiotemporal SD susceptibility.