Abstracts

Rationale: Sudden Unexpected Death in Epilepsy (SUDEP) is a leading cause of premature mortality in patients with Dravet Syndrome (DS). DS is a genetic epilepsy syndrome caused primarily by a mutation in the SCN1A gene. The mechanism of SUDEP is not fully understood, but respiratory dysfunction is implicated. Seizures can cause respiratory challenges and blood gas abnormalities. Reactive glia and pro-inflammatory cytokine upregulation have been observed in the brains of human epilepsy patients and animal epilepsy models. The morphological and functional changes that reactive glia undergo, as well as the presence of inflammatory cytokines, may alter surrounding neuron function. We hypothesize that persistent glial reactivity and excessive glial-mediated inflammation within cardiorespiratory brainstem nuclei such as the nucleus of the solitary tract (NTS) in DS prevents adequate response to seizure-induced respiratory insult and contributes to SUDEP.

Methods: A DS mouse model expressing a heterozygous mutation in the sodium channel gene Scn1a^A1783V/WT (Het) was evaluated and compared with wild-type (WT) littermate control. FosB expression in the nucleus of the solitary tract (NTS), ventrolateral medulla (VLM), and retrotrapezoid nucleus (RTN) was evaluated by immunohistochemistry to determine long-term neural activity in these brainstem respiratory nuclei in P20-P30 animals. Microglia and astrocyte reactivity within the NTS was also evaluated using immunohistochemistry. The mRNA expression of cytokines TNFa, IL-6, and IL-1B were determined in the NTS using qPCR in P20-P30 and P45-P55 Het and WT mice to evaluate inflammation in this important cardiorespiratory nucleus. To determine intrinsic NTS neuronal excitability of Het mice, electrophysiological recordings were made from acute brain slices. The tractus solitarius (TS), a fiber bundle that innervates the NTS, was stimulated and extracellular field excitatory post-synaptic potentials (fEPSP) were recorded in the NTS. Input/output relationships were assessed and compared for Het and WT mice.

Results: FosB results revealed that Hets have greater activity in respiratory nuclei (NTS, VLM, and RTN) than WT at the age when most experience sudden death. Astrocyte and microglial reactivity did not differ between Het and WT in the NTS. There was no difference in pro-inflammatory cytokine mRNA levels in the NTS between Het and WT, however, there was greater TNF a mRNA in P20-P30 Het compared to P45-P55 Het. There were no differences in the intrinsic neuronal excitability of the NTS in Hets compared to WT.

Conclusions: The FosB data may support respiratory nuclei involvement in the sudden death that occurs so frequently in Hets of this age group. Glial reactivity and intrinsic excitability differences within the NTS may not be implicated. Further studies are needed to determine the significance of TNFa in NTS in the high mortality of this DS model.

Funding: This project was supported by the Dravet Syndrome Foundation (CSM).