Activity Mapping of Seizures in a Mouse Model of Dravet Syndrome

Dravet syndrome is an infant‐onset epileptic encephalopathy most often caused by heterozygous de novo mutations in SCN1A. SCN1A encodes Nav1.1, a pore‐forming voltage‐gated sodium channel α subunit important for action potential initiation and propagation in neurons. Dravet syndrome patients have pleiomorphic seizure types, are at risk of sudden unexplained death due to epilepsy, and display comorbidities including developmental delay and intellectual disability. Missense and truncation SCN1A variants identified in patients with Dravet syndrome result in loss of Nav1.1 protein function. This suggests patients retain only a single functional SCN1A allele, resulting in haploinsufficiency. Mouse models with heterozygous deletion of Scn1a recapitulate many features of Dravet syndrome, including susceptibility to seizures elicited by body temperature elevation. Experiments in Dravet mouse models suggest hippocampal networks exhibit hyperexcitability, which is thought to be critical for seizure activity. Despite this prediction, little work has been done to map neuronal activity following naturalistic seizures in Scn1a+/− Dravet mice. We examined c‐Fos expression, a biomarker of neuronal activity, in the hippocampus of heterozygous null Scn1a+/− mice at multiple timepoints after seizures to map activity of neurons in the hippocampal network. Brains were collected from heterozygous Scn1a+/− null mice 1–4 hours after an induced or spontaneous seizure. Hyperthermia‐induced seizures were elicited by elevating the core body temperature until a seizure was observed, followed by rapid cooling to basal temperature to terminate the seizure. To control for stress‐induced c‐Fos expression, wild‐type sham controls underwent similar body temperature elevation but did not experience seizures. Spontaneous seizures were identified during continuous video monitoring of home cages. Scn1a+/− null mice confirmed on video to have not experienced a behavioral seizure in at least 4–7 hours prior to tissue collection served as controls for basal c‐Fos expression for the spontaneous seizure cohort. Brains were sectioned and immunolabeled to detect c‐Fos expression. Regional c‐Fos expression was qualitatively scored by an observer blinded to treatment. We observed robust expression of c‐Fos expression in hippocampal neurons following seizure activity. In the hyperthermia model, c‐Fos expressing neurons were mostly restricted to the dentate gyrus at 1 hour post seizure. At 2‐ and 3‐hours post‐seizure, c‐Fos expressing neurons were observed in multiple regions of the hippocampus, including dentate gyrus, hilus, CA3, and CA1. In the spontaneous seizure model, c‐Fos expression patterns in the hippocampus were similar to those observed in the hyperthermia‐induced seizure paradigm, although the timecourse appeared to be more rapid for spontaneous seizures. For both seizure types, c‐Fos expression returned to baseline approximately four hours post‐seizure. This study demonstrates that hippocampal neurons are activated following both hyperthermia‐induced and spontaneous seizures, in agreement with the hypothesis that the hippocampal circuit is engaged during seizures in the Scn1a+/− Dravet syndrome mouse model.