Abstracts

Rationale: Sudden unexpected death in epilepsy (SUDEP) is a major cause of mortality in people with drug-resistant epilepsy. There is increasing evidence that defects in CO2 homeostasis may play a major role in SUDEP. We recently demonstrated that patients with Dravet Syndrome (DS) commonly have peri-ictal breathing dysfunction, including prolonged depression of CO2 chemoreception. Moreover, postictal death in Scn1aR1407X/+ (DS) mice is due to terminal central apnea. It has also been found that hippocampal seizures induced in rats lead to a decrease in firing rate of 5-HT neurons, which are central CO2 chemoreceptors. Here we test the hypothesis that when seizures in DS mice are not fatal, they can cause a transient decrease in the hypercapnic ventilatory response (HCVR), a measure of CO2 chemoreception. Methods: DS mice and WT littermates were implanted with temperature telemetry probes (IPTT, BMDS) after weaning. To assess baseline breathing and HCVR, unanesthetized DS and WT (Scn1a+/+) mice of ages 21-26d were placed in a whole-body plethysmography chamber. After an acclimatization period, mice were exposed to hyperoxic (50% O2) air to measure baseline breathing, and were exposed to increasing levels of CO2 (3, 5, and 7%; 50% O2; balance N2). Seizures were induced in DS mice by placing them in a separate chamber and increasing body temperature using a heat lamp, until a generalized tonic-clonic seizure (GTCS) occurred or 42.5°C was reached. As a control, WT mice were heated up to the average temperature at which DS mice developed a GTCS. WT and DS mice were immediately transferred back to the plethysmography chamber and the HCVR to 7% CO2 was measured. Data were normalized to body weight and statistical analysis was performed with multiple t-tests and Holms-Sidak multiple comparison test as appropriate (p<0.05). Results: The average temperature at which DS mice developed a GTCS was 41.9+-0.6°C. The baseline (pre-ictal) HCVR in DS mice (n=14) was not statistically different from WT littermates (n=12). When WT mice were heated, the slope of the HCVR was increased, primarily due to an increase in respiratory rate (p<0.0001). However, there was a trend towards a decrease in the slope of the HCVR in DS mice following a heat-induced GTCS (p=0.06), due to a significant decrease in respiratory rate (p=0.006; see figure). The postictal decrease in HCVR could not be explained by a decrease in O2 consumption, which increased on average after seizures. Conclusions: Our results provide the first evidence that seizures impair CO2 chemoreception in a mouse model of epilepsy. This is consistent with the finding that seizures can inhibit serotonin (5-HT) neurons, which would in turn decrease CO2 chemoreception and contribute to postictal hypoventilation. 5-HT has been implicated in SUDEP due to its stimulatory effect on breathing and its anticonvulsant properties. Future experiments will examine whether changes in 5-HT alter the rate of SUDEP, or alter changes in the HCVR or ventilation in response to seizures, using the tryptophan hydroxylase inhibitor para-chlorophenylalanine and 5-hydroxytryptophan to reduce or increase 5-HT, respectively. Overall, our results suggest that postictal ventilatory abnormalities, including blunted CO2 chemoreception, may play a major role in SUDEP in patients with DS and may be a biomarker for those at high risk. Funding: NIH NINDS (U01 NS090414 & U01 NS090407) to GR & F31 NS110333 to FT