Abstracts

Rationale: The missense mutation p.Asn1768Asp (N1768D) in the gene SCN8A, encoding the voltage-gated sodium channel Nav1.6, was identified in a patient with early-infantile epileptic encephalopathy (EIEE). This disorder is characterized by seizures, ataxia, and high risk of sudden unexpected death in epilepsy (SUDEP). The heterozygous Scn8a knock-in mouse model Scn8aN1768D/+ carrying the patient mutation recapitulates several key features of EIEE. Previous studies expressing Nav1.6-N1768D cDNA in a heterologous system showed that this mutation results in increased persistent sodium current and hyperactivity of transfected neurons. However, the in vivo effects of this mutation on sodium current have not been investigated.Methods: We recorded isolated sodium currents from acutely dissociated CA3 hippocampal neurons prepared from postnatal day 21-25 Scn8aN1768D/+ mice and wildtype littermates using the standard whole-cell voltage clamp recording technique.Results: In bipolar neurons, we found that transient sodium current density did not differ between wildtype and mutant. In pyramidal neurons, we observed a reduction in transient sodium current density in response to a test pulse to -30 mV from the wildtype value of -193±39 pA/pF to -148±12 pA/pF in mutant neurons, and this reduction was observed over a range of voltages. In contrast, in pyramidal neurons we observed an increase in persistent current density recorded in response to a test pulse to -30 mV, measured 48-50 ms after the start of the pulse. Persistent current density was elevated more than 2-fold in both bipolar and pyramidal Scn8aN1768D/+ neurons. The voltage-dependence of sodium current activation in mutant bipolar neurons did not differ from wildtype. However, in mutant pyramidal neurons we observed a small but significant depolarizing shift in the voltage-dependence of sodium current activation, together with a significant change in the slope of the activation curve.Conclusions: We predict that these changes in sodium current, especially the more than doubling of persistent current density, lead to hippocampal neuronal hyperactivity that may contribute to seizures in Scn8aN1768D/+ mice and in EIEE patients carrying this mutation. Supported by NIH grants R01-NS-076752 to LLI, U01-NS-090364 to JP and LLI, and R01-NS-034509 to MHM.