Role of the hippocampus in Nav1.6 (Scn8a) mediated seizure resistance

SCN1A mutations are the main cause of the epilepsy disorders Dravet syndrome (DS) and genetic epilepsy with febrile seizures plus (GEFS+). Mutations that reduce the activity of the mouse Scn8a gene, in contrast, are found to confer seizure resistance and extend the lifespan of mouse models of DS and GEFS+. To investigate the mechanism by which reduced Scn8a expression confers seizure resistance, we induced interictal-like burst discharges in hippocampal slices of heterozygous Scn8a null mice (Scn8a(med/+)) with elevated extracellular potassium. Scn8a(med/+) mutants exhibited reduced epileptiform burst discharge activity after P20, indicating an age-dependent increased threshold for induction of epileptiform discharges. Scn8a deficiency also reduced the occurrence of burst discharges in a GEFS+ mouse model (Scn1a(R1648H/+)). There was no detectable change in the expression levels of Scn1a (Nav1.1) or Scn2a (Nav1.2) in the hippocampus of adult Scn8a(med/+) mutants. To determine whether the increased seizure resistance associated with reduced Scn8a expression was due to alterations that occurred during development, we examined the effect of deleting Scn8a in adult mice. Global Cre-mediated deletion of a heterozygous floxed Scn8a allele in adult mice was found to increase thresholds to chemically and electrically induced seizures. Finally, knockdown of Scn8a gene expression in the adult hippocampus via lentiviral Cre injection resulted in a reduction in the number of EEG-confirmed seizures following the administration of picrotoxin. Our results identify the hippocampus as an important structure in the mediation of Scn8a-dependent seizure protection and suggest that selective targeting of Scn8a activity might be efficacious in patients with epilepsy.