Rationale:
Sudden unexpected death in epilepsy (SUDEP) is a leading cause of death in patients with epilepsy. There is increasing evidence that apnea during a seizure is the primary cause of death following a seizure in SUDEP. However, the physiological mechanisms that drive breathing failure are completely unknown. The most critical drive to breathe is referred to as the central CO2 chemoreflex: wherein the brainstem relies on the detection of tissue CO2 levels to appropriately regulate breathing. There is evidence in epilepsy patients that seizures caused a prolonged decrease in the hypercapnic ventilatory response (HCVR), a measurement of CO2 chemoreception. This has been replicated in Scn8aN1768D/+ (D/+) mice, a model of SUDEP that mimics the seizure activity of patients with SCN8A epileptic encephalopathy. The most well-documented risk factor for SUDEP is repeated generalized tonic-clonic seizures. Thus, we hypothesize that repeated seizures further suppress CO2 chemoreception, increasing the likelihood of SUDEP. In this study, we explore the HCVR before and after different numbers of induced and spontaneous seizures D/+ mice.
Methods:
8–12-week-old D/+ and wild type littermate control (WT) mice were implanted with telemetric probes to record body temperature and, after recovery from surgery, whole-body plethysmography was used to record breathing parameters. Once mice were habituated to the experimental chamber (respiratory rate < 200 breaths per minute), the HCVR was tested by measuring the respiratory parameters at 0%, 3%, and 6% inspired CO2 (21% O2, balanced N2). Next, one to four audiogenic seizures were evoked (15 KHz pure tone at 80 dB), each five minutes apart, and then the HCVR was tested 5 and 20 minutes postictal. If spontaneous seizures occurred, no audiogenic seizures were induced, and the postictal HCVR was tested. Chamber temperature and humidity were measured throughout the experiment and, along with body temperature, were used to determine tidal volume. Minute ventilation was measured as a product of respiratory frequency and tidal volume, and HCVR was determined by the slope of minute ventilation and inspired CO2.
Results:
Baseline ventilation and response to 3% and 6% inspired CO2 levels of WT and D/+ mice (Fig.1) were not different (2-way ANOVA, p = 0.9725). There is a significant difference between preictal and postictal HCVR for D/+ mice exposed to two (Fig. 2E; 2-way ANOVA, p = 0.009) and four (Fig. ED; 2-way ANOVA, p = 0.0002) audiogenic seizures. No clear difference was observed between the blunted HCVR at 5 or 20 minutes postictal or between 2 and 4 induced seizures. The postictal HCVR impairment appears to be similar after spontaneous seizures, but more data is required.Conclusions:
The data suggests that multiple consecutive seizures can lead to a blunted HCVR; however, it does not appear that more seizures further blunt the HCVR. The data can be used to support future experiments examining mechanisms of impaired HCVR and ultimately develop treatments to prevent SUDEP and improve the quality of life for persons with epilepsy.
Funding:
NINDS R01 NS133139
Harrison Undergraduate Research Award (University of Virginia)