Abstracts

Rationale: Voltage-gated sodium channels (VGSC) are the primary molecules responsible for the initiation and propagation of action potentials in neurons. There are nine different genes encoding VGSC in mammals, and mutations in one of these genes (SCN1A) have been identified as the main cause of Genetic Epilepsy with Febrile Seizures Plus (GEFS+) and Dravet Syndrome (DS). Previous studies in mice with VGSC mutations causing GEFS+ or DS have demonstrated loss-of-function in inhibitory interneurons with only minor effects in pyramidal neurons. In this study, we examined the effects in mice of the D1866Y SCN1A mutation that causes GEFS+ in human. Previous studies demonstrated that this mutation increases persistent sodium channel current in Xenopus oocytes (Spampanato et al., 2004, J. Neuroscience 24:10022), but the effects in oocytes may not reflect those in neurons. To understand the physiological effects of the Scn1a D1866Y mutation, we studied the sodium channel properties in a specific population of interneurons from D1866Y knock-in mice. Methods: We isolated and recorded sodium current from dissociated interneurons from the hippocampus of knock-in mice expressing the D1866Y mutation. To specifically examine parvalbumin-positive (PV+) interneurons, D1866Y mice were crossed to transgenic mice in which PV+ neurons were fluorescently labeled. The properties of the sodium channels in this specific neuronal type were determined by voltage-clamping the fluorescently labeled neurons. Results: Voltage clamp of PV+ interneurons from Scn1aD1866Y/D1866Y P16-P17 mice showed slower kinetics of inactivation compared to wild-type mice. In addition, there was a reduction in use-dependent inactivation at 20 Hz compared to that in neurons from wild-type mice, and there was an increased in the level of persistent current. There was a small positive shift in the voltage-dependence of activation, and no significant difference in the voltage-dependence of inactivation. Conclusions: These data suggest that the Scn1a D1866Y mutation causes a gain-of-function in hippocampal PV+ interneurons, which may be the cause of the increased seizure susceptibility in these mice. Because other Scn1a sodium channel mutations causing GEFS+ have been shown to cause a loss-of-function in interneurons from knock-in mice, these results suggest that different mutations in the same gene can cause seizures by different mechanisms.