Abstracts

Rationale: Dravet Syndrome (DS) is an infantile epileptic encephalopathy caused by mutations in the voltage-gated sodium channel Na_v1.1, which leads to hyperexcitable brain activity during development. This altered excitability causes persistent brain circuit dysfunction resulting in convulsive and non-convulsive (absence) seizures, attention deficits, and sleep disruption. Somatosensory corticothalamic (CT) circuits play an important role in regulating attention and sleep. Disrupted somatosensory CT circuit activity contributes to DS phenotypes including hypersynchronous oscillatory activity underlying absence seizures. However, the precise cellular and synaptic mechanisms underlying CT circuit dysfunction remain elusive. We hypothesize that altered thalamic neuron excitability caused by Na_v1.1 haploinsufficiency alters synapse formation during development leading to persistent circuit dysfunction through adulthood.

Methods: To study synaptic and cellular disease mechanisms in DS, we utilized a Nav1.1-haploinsufficiency mouse model of DS at age P13-17 (before high seizure burden), P28-P32 (after high seizure burden), and P58-P63 (adult). We probed synapse and cellular function with acute brain slice electrophysiology and quantified input- and cell-type-specific synapse development utilizing high-resolution fluorescence imaging.

Results: In the period following high seizure burden, we found synaptic-level alterations in the somatosensory thalamus, including an input-specific reduction in glutamatergic synaptic input and an increase in GABAergic input to the ventral posterolateral (VPL) thalamic nucleus in DS mice. Synaptic input to the adjacent ventral posteromedial (VPM) thalamic nucleus remained unaltered. Glutamatergic input to the reticular nucleus of the thalamus (nRT), a GABAergic neuron population providing the primary inhibitory input to the VPL and VPM, exhibited an input-specific reduction glutamatergic input. Furthermore, we discovered that VPL, VPM, and nRT neuron populations exhibited distinct alterations to intrinsic excitability in the disease model. Next, we sought to determine how these synapse- and cell-type-specific changes in the CT circuitry develop in DS mice. Our preliminary data from recording synaptic activity in nRT, VPL, and VPM neurons suggest that GABAergic and glutamatergic synapse activity may be differentially affected throughout development in DS mice. Interestingly, specific thalamic synapse types exhibit distinct developmental time courses that may contribute to synapse-specific deficits in DS.

Conclusions: In summary, a DS mouse model exhibited input- and cell-type-specific impairments in synaptic and intrinsic neuron function. Our ongoing work indicates that synapse-specific developmental regulation may contribute to synaptic impairments in DS mice. Understanding the precise developmental time course of these synaptic and cellular disease mechanisms will help us identify the specific circuit components that may be effective therapeutic targets across disease stages.

Funding: This work was supported by NIH/NINDS, CURE Epilepsy, Dravet Syndrome Foundation, and the Brain Research Foundation.