Persistent Na⁺current couples spreading depolarization to seizures inScn8again of function mice

Abstract

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 (IKM) by ablation ofKcnq2in 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 inScn8a(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 component, which occurs predominantly during the light phase and were detected in the mutant mice across ages between P40-100. Laser speckle contrast imaging of cerebral blood flow dynamics resolved SD as bilateral wave of hypoperfusion and subsequent hour-lasting hypoperfusion inScn8aD/+cortex in awake head-restrained mice subjected to a subconvulsive PTZ. Subcortical recordings in freely moving mice revealed that approximately half of the spontaneous cortical seizure-SD complexes arose with 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 incidencein vivo, cortical slices fromScn8aD/+mice showed a raised SD susceptibility, and pharmacological inhibition of persistent Na+current (INaP), which is enhanced inScn8aD/+neurons, inhibited SD generation in cortical slicesex vivo, indicating that INaP contributes to SD susceptibility.Ex vivoCa2+imaging studies using acute brain slices expressing genetic Ca2+sensor (Thy1-GCAMP6s) demonstrated that pharmacological activation of IKM suppressed Ca2+spikes and SD, whereas IKM inhibitor drastically increased the frequency of Ca2+spikes in the hippocampus ofScn8aD/+mice, but not in WT, suggesting that IKM restrains the hyperexcitability resulting fromScn8aGOF mutation. Together, our study identifies a cortical SD phenotype inScn8aGOF mice shared with theKcnq2-cKO model of developmental epileptic encephalopathy and reveals that an imbalance of non-inactivating inward and outward membrane currents bidirectionally modulates spatiotemporal SD susceptibility.