Abstracts

Rationale: Sudden unexpected death in epilepsy (SUDEP) has been associated with disturbances in cardiorespiratory function and possible autonomic nervous system (ANS) imbalance. The vagal complex is the primary central regulator of parasympathetic visceral function, individuals with refractory epilepsy demonstrate cardiac abnormalities indicative of ANS dysfunction, and dysregulation of the nucleus tractus solitarius (NTS) coincides with SUDEP susceptibility in genetic epilepsy models. However, epilepsy-related alterations in the vagal circuitry regulating this response are not well understood. This study investigated neural activity in the vagal complex and cardiac function in the pilocarpine-induced status epilepticus model of temporal lobe epilepsy (TLE).Methods: Pilocarpine (281 mg/kg) was administered to 5-6 week old GIN mice to induce status epilepticus (SE) and eventual development of TLE. A cohort of animals was reserved to monitor seizures and survival and received no further treatment. Another group was implanted with telemetry for electrocardiography (ECG). In this group, ECG data was collected for 24 hours pre- and post-treatment and at 6 wks and 12 wks post-treatment. ECG recordings were analyzed for heart rate (HR), RR interval, heart rate variability (HRV), and circadian rhythms. For electrophysiological recordings in identified GABAergic NTS neurons, coronal brainstem slices from GIN mice were taken at 1-2 wks, 6 wks, and 12 wks post-injection. Spontaneous action potential (AP) firing, as well as spontaneous excitatory postsynaptic current (sEPSC) frequency and amplitude were measured.Results: Pilocarpine-treated mice had a 37% survival rate (versus 100% for vehicle-treated controls) by 150 d post-SE. HR, RR interval, HRV, and circadian cardiac rhythms were not significantly altered in association with TLE development. One week post-SE, GABAergic NTS neurons had significantly increased spontaneous AP frequency compared to control mice (75% increase) and sEPSC frequency was significantly higher (247% increase) than in control mice (p<0.05). Increased synaptic and membrane excitability in pilocarpine-treated mice was maintained at 6 and 12 weeks post-SE.Conclusions: Increased neural excitability in the NTS suggests that GABAergic NTS neurons receive greater excitatory synaptic drive, which is sustained several weeks after pilocarpine induced SE and which may contribute to increased AP firing. Increased AP firing could reflect a sustained increase in glutamate release from the afferent fibers of the vagus nerve onto GABAergic NTS neurons and/or changes in intrinsic membrane properties, making them more susceptible to excessive depolarization and tonically-increased GABA neuron activity in the vagal complex. This suggests a tonic dampening of parasympathetic tone and ANS imbalance associated with TLE development.