Abstracts

Rationale: Voltage-gated sodium channels (VGSCs) are responsible for the initiation and propagation of action potentials in excitable cells. Mutations in Scn1a have been identified as causing Genetic Epilepsy with Febrile Seizures Plus (GEFS+), a disorder characterized by myoclonic seizures, ataxia, absences and atonic seizures. The R1648H mutation in SCN1A has been shown to be responsible for GEFS+ in humans. However, mice with mutations in Scn8a, which encodes the Na_V1.6 VGSC, have shown reduced seizure susceptibility in kainic acid and flurothyl induced seizure models. Altered expression of Scn8a in a GEFS+ mouse model ameliorated seizure severity and extended the lifespan in these double-mutant mice (Scn1a^R1648H/+; Scn8a^Med/+). Our hypothesis is that reduced expression of Na_V1.6 in the hippocampal neuronal circuits that underlie seizure generation reduces network excitability and lowers seizure susceptibility in mice with reduced Scn8a expression. The overall goal is to determine the mechanism by which decreased Scn8a expression leads to reduced seizure susceptibility and amelioration of seizure severity.Methods: To examine hippocampal network excitability of CA3 pyramidal neurons in Scn8a^Med/+ mice, we performed extracellular recordings on hippocampal slices in the presence of elevated potassium. To examine the network properties in double heterozygous mice (Scn1a^R1648H/+; Scn8a^Med/+), synchronous bursts were induced in hippocampal CA3 pyramidal neurons with elevated extracellular potassium. To determine whether intrinsic properties of hippocampal CA3 pyramidal neurons is rescued in double heterozygous mice, we elicited repetitive firing of action potentials and measured for deficiencies in the initiation of action potential firing.Results: We observed a reduction in burst activity in slices from Scn8aMed/+ mice beginning at P20, when Na_V1.6 protein levels are near adult levels. This decrease with age indicates that developmental changes in Scn8a expression are important for neuronal burst activity. Elevated extracellular potassium induced burst activity in Scn1a^R1648H/+ mice but not in wild-type or Scn8a^Med/+ mice, suggesting that the R1648H mutation increases seizure susceptibility. Current clamp recording of CA3 pyramidal neurons from double-mutant mice demonstrated reduced firing of action potentials compared to wild-type.Conclusions: These data indicate that reduced Na_V1.6 expression lowers network excitability in Scn8a^Med/+ mice, and functions as a genetic modifier for a human GEFS+ mutation. They also demonstrate that the hippocampus is an important region for Scn8a-mediated seizure resistance, which suggests that selectively targeting Scn8a could be a promising therapeutic strategy as a treatment for epilepsy.