Abstracts

Rationale: Heterozygous loss-of-function mutations in the SCN1A gene, which encodes the pore-forming subunit of the voltage-gated sodium channel Nav1.1, cause Dravet Syndrome (DS). In addition to intractable seizures, DS patients develop psychomotor delay, cognitive impairment, and social interaction deficits. Specific heterozygous deletion of Nav1.1 in forebrain GABAergic interneurons leads to seizures, premature death, cognitive deficits and autistic features, confirming that disinhibition caused by failure of firing of GABAergic interneurons is the underlying cause of epilepsy in DS mice. However, the contribution of specific interneuron subtypes to DS symptoms remained largely unknown. Here we investigated the specific contributions of Parvalbumin (PV)- and Somatostatin (SST)-expressing interneurons to the physiological and behavioral consequences of DS. Methods: The excitability of interneurons of mice with selective deletion of Na_V1.1 was tested using electrophysiology in acute brain slices. Susceptibility to thermally induced seizures was tested on postnatal day (P) 21 and P35. Locomotor activity in a novel environment was tested in an open field test. Social abilities were assessed in the three-chamber social interaction test, and spatial learning was evaluated in the contextual fear-conditioning test. Results: In slices of cerebral cortex, haploinsufficiency of Nav1.1 in PV-expressing neurons (PV-DS) or SST-expressing neurons (SST-DS), or both (PV&SST-DS), resulted in increased threshold and rheobase for action potential generation and impaired action potential firing in trains. Selective heterozygous deletion of Nav1.1 in either PV or SST interneurons was sufficient to confer sensitivity to thermally induced seizures at postnatal day (P) 35. However, PV&SST-DS mice were susceptible to thermally induced seizures as early as P21. Moreover, at P35, their seizure duration was ~6-fold longer than in PV-DS mice and ~3-fold longer than in SST-DS mice. Homozygous deletion of Nav1.1 in PV neurons resulted in spontaneous seizures and reduced survival. Homozygous deletion of Nav1.1 in SST interneurons resulted in mild epilepsy, with thermally induced seizures only at high temperatures and no premature death. In contrast, mice with homozygous deletion in both PV&SST interneurons had frequent spontaneous seizures and did not survive beyond P27. Our behavioral data demonstrate that different types of interneurons are differentially involved in the behavioral deficits observed in DS mice. PV-DS mice displayed autistic-like behaviors, but not hyperactivity, whereas SST-DS mice were hyperactive, but without autistic-like behaviors. Surprisingly, heterozygous deletion of Nav1.1 in both interneuron types failed to cause cognitive impairment in context-dependent fear conditioning. Conclusions: We show here that Nav1.1 is important for normal firing of both PV- and SST-expressing interneurons and that reduced excitability of these major classes of interneurons contributes synergistically to the epilepsy and premature death and differentially to the comorbidities associated with DS.