Abstracts

Rationale: The reticular nucleus of the thalamus (RNT) lies between the cerebral cortex and the thalamus. It is populated entirely by GABAergic neurons that modulate the bi-directional flow of information between the thalamus and the cortex and are essential for normal sleep rhythms. Severe Myoclonic Epilepsy of Infancy (SMEI or Dravet Syndrome) is a malignant childhood epilepsy syndrome caused by heterozygous loss-of-function mutations in SCN1A, the gene encoding the Type-I brain Na channel, NaV1.1. GABAergic hippocampal interneurons and cerebellar Purkinje cells of Scn1a heterozygotes, our mouse model of SMEI, have greatly reduced NaV currents (47.0 7.0 % and 60 6.0 % of wild-type (WT), respectively). Firing ability of these neurons is remarkedbly reduced, suggesting that their dysfunction plays a key role in hyperexcitability, epilepsy and ataxia observed in SMEI. In this study, we examined changes in whole cell sodium current and excitability of RNT GABAergic neurons as well as their associated effects on sleep in our mouse model of SMEI. Methods: Whole-cell voltage and current clamp recordings were obtained from acutely dissociated RNT neurons using an Axopatch 200B amplifier and HEKA Pulse software. Referential digital video-EEG and bipolar EMG recordings were obtained from freely moving mice using thin platinum wire EEG and EMG electrodes on a Telefactor system during wake and sleep. Results: GABAergic RNT neurons had moderately reduced whole-cell NaV current (78 6.2 % of WT). Rebound bursts of activity following hyperpolarization are characteristic of RNT GABAergic neurons in WT mice. Current clamp experiments on RNT GABAergic neurons from SMEI mice revealed reduced firing during rebound from hyperpolarization (4.0 1.0 action potentials for SMEI vs. 8.5 1.0 for WT in the first 500 ms of rebound), suggesting that the reduced NaV current causes reduced cell excitability. Burst firing by RNT neurons plays a key role in the generation and regulation of slow-wave EEG patterns in sleep. Sleep disturbances are very common in patients with SMEI and other epilepsies. To test whether impaired excitability of the RNT neurons is accompanied by altered sleep architecture in SMEI mice, wake and sleep Video-EEG-EMG activity was recorded from WT and SMEI mice. SMEI mice exhibited normal EEG patterns when awake. However, during sleep, EEG activity was abnormal. Numerous interictal spikes were observed, 100 % occurring during Non-Rapid-Eye-Movement (NREM) sleep. None occurred during Rapid-Eye-Movement (REM) sleep. In addition, SMEI mice exhibited 2.3-fold more brief (< 4s) arousals from sleep than WT. Power spectral analysis revealed that SMEI mice have reduced EEG activity during NREM sleep (50 6 % of WT). Conclusions: Although sleep disorders in epilepsy have been attributed to anti-epileptic drugs, our results suggest that impairment of NaV currents and excitability of RNT GABAergic neurons may lead directly to sleep disturbance in SMEI that is independent of drug treatment.