Sleep impairment and reduced interneuron excitability in a mouse model of Dravet Syndrome

Highlights

• DS mice have abnormal sleep with increased brief wakes and reduced sleep spindles.

• Delta wave power is reduced in Non-Rapid-Eye-Movement sleep.

• Homeostatic sleep rebound after sleep deprivation is impaired.

• Na_V current and rebound action potentials are reduced in inhibitory RNT neurons.

• Selective gene deletion in inhibitory neurons causes the same sleep impairment.

Abstract

Dravet Syndrome (DS) is caused by heterozygous loss-of-function mutations in voltage-gated sodium channel Na_V1.1. Our mouse genetic model of DS recapitulates its severe seizures and premature death. Sleep disturbance is common in DS, but its mechanism is unknown. Electroencephalographic studies revealed abnormal sleep in DS mice, including reduced delta wave power, reduced sleep spindles, increased brief wakes, and numerous interictal spikes in Non-Rapid-Eye-Movement sleep. Theta power was reduced in Rapid-Eye-Movement sleep. Mice with Na_V1.1 deleted specifically in forebrain interneurons exhibited similar sleep pathology to DS mice, but without changes in circadian rhythm. Sleep architecture depends on oscillatory activity in the thalamocortical network generated by excitatory neurons in the ventrobasal nucleus (VBN) of the thalamus and inhibitory GABAergic neurons in the reticular nucleus of the thalamus (RNT). Whole-cell Na_V current was reduced in GABAergic RNT neurons but not in VBN neurons. Rebound firing of action potentials following hyperpolarization, the signature firing pattern of RNT neurons during sleep, was also reduced. These results demonstrate imbalance of excitatory vs. inhibitory neurons in this circuit. As predicted from this functional impairment, we found substantial deficit in homeostatic rebound of slow wave activity following sleep deprivation. Although sleep disorders in epilepsies have been attributed to anti-epileptic drugs, our results show that sleep disorder in DS mice arises from loss of Na_V1.1 channels in forebrain GABAergic interneurons without drug treatment. Impairment of Na_V currents and excitability of GABAergic RNT neurons are correlated with impaired sleep quality and homeostasis in these mice.

Introduction

Dravet Syndrome (DS) is a debilitating, drug-resistant, and life-threatening childhood-onset epilepsy syndrome. Its manifestations begin with seizures induced by fever or hyperthermia at six–nine months, which progress to spontaneous myoclonic, tonic–clonic, absence, and partial seizures (Dravet et al., 2005, Oguni et al., 2001). During this period of frequent polymorphic seizures, children with DS develop several co-morbid conditions including psychomotor regression, ataxia, sleep disturbance, cognitive impairments, and many die prematurely (Dravet et al., 2005, Oguni et al., 2001). DS is caused by loss-of-function mutations in one allele of the SCN1A gene encoding the Na_V1.1 sodium channel (Claes et al., 2003, Claes et al., 2001). Mouse models of DS develop its key phenotypic features including epilepsy with early (P21) onset, high susceptibility to hyperthermia-induced seizures, ataxia, spontaneous seizures, autistic-like behaviors, and premature death (Catterall et al., 2010, Han et al., 2012a, Kalume et al., 2013, Kalume et al., 2007, Oakley et al., 2009, Ogiwara et al., 2007, Yu et al., 2006). Deletion of Na_V1.1 channels in DS mice preferentially reduces sodium current in inhibitory neurons in the hippocampus but not in excitatory neurons (Yu et al., 2006), suggesting that selective loss of excitability of inhibitory neurons is responsible for hyperexcitability in DS. Reduced Na_V current and excitability in cerebellar Purkinje neurons, which are GABAergic inhibitory neurons, may cause ataxia (Kalume et al., 2007). These findings led to the unified hypothesis that reduced Na_V current in GABAergic neurons in different brain regions may be responsible for the multiple, seemingly unrelated co-morbidities of DS, such as sleep disturbance and cognitive impairment (Catterall et al., 2010, Yu et al., 2006). In support of this hypothesis, specific heterozygous deletion of Na_V1.1 channels in forebrain GABAergic neurons reproduced seizures, comorbidities, and premature deaths analogous to those in DS mice (Cheah et al., 2012).

Sleep disturbances are common in epilepsies and are associated with poor seizure control and poor quality of life (Bazil, 2003, Steriade, 2005). Clinical evaluation of DS patients has revealed an abnormal sleep–wake cycle, with sleep–onset insomnia and difficulty maintaining sleep (Dravet et al., 2005, Kimura et al., 2005, Nolan et al., 2006). In a previous study, we examined sleep–wake cycle and found abnormal circadian rhythms in DS mice (Han et al., 2012b). In the studies reported here, we have examined sleep physiology in DS mice, uncovered abnormal sleep architecture, and correlated it with reduced sodium currents and action potential firing in the GABAergic neurons of the reticular nucleus of the thalamus (RNT). Our results show that, although sleep disorders in epilepsies are often attributed to side effects of antiepileptic drugs, sleep impairment in DS mice arises from mutation of Na_V1.1 channels in forebrain GABAergic interneurons without involvement of drug treatment. This sleep impairment is correlated with cell-specific loss of sodium current and excitability of RNT GABAergic interneurons. Furthermore, our results suggest that both epilepsy and sleep impairment in DS may arise from impaired firing of GABAergic interneurons and therefore may be treatable by appropriate enhancement of GABAergic neurotransmission.