Abstracts

Rationale: Patients with epilepsy are at risk of dying due to sudden unexpected death in epilepsy, or SUDEP. While the underlying pathophysiological mechanisms that lead to death are unknown, previous studies have shown that SUDEP occurs more often during the night. Historically, this nighttime death has been attributed to sleep. However, our lab has demonstrated that the nocturnal tendency of seizure-associated death persists in mice housed in constant darkness, suggesting an independent role for circadian rhythms. Additionally, multiple mouse models of spontaneous seizure-associated death are more likely to die during the dark phase of the twenty-four-hour day. This conserved nocturnal phenotype between diurnal and nocturnal mammals suggests some underlying circadian rhythm shared between humans and mice may mediate this nighttime risk of death. One compelling target is the neurotransmitter serotonin, which has been shown to oscillate throughout the day and is lowest during the night.

Methods: Adult (3-7 month) Lmx1b^f/f (homozygous for floxed Lmx1b alleles) and Lmx1b^f/f/p (homozygous floxed Lmx1b alleles and hemizygous ePet1 Cre recombinase) were used for this study (N = 12-16 per group, male and female mice used). Conditional knockout of the Lmx1b gene in ePet1-containing neurons eliminates >99% of serotonin neurons in the central nervous system. Seizure naïve Lmx1b^f/f/p and Lmx1b^f/f wild-type littermates were housed in a 12:12 light-dark cycle and subjected to a single maximal electroshock (MES) seizure (30 mA, 60 Hz, 200 ms) at six evenly spaced time points (Zeitgeber time (ZT) 2, 6, 10, 14, 18, and 22).

Results: Consistent with findings from C57BL/6J mice, wild-type Lmx1b^f/f mice are more likely to die if seizures are induced during the dark phase of the twenty-four-hour day (58.3% vs. 21.4% mortality at ZT 18 and 6, respectively). On the other hand, Lmx1b^f/f/p mice demonstrate high mortality regardless of time of day (45.5% and 60.0% mortality at ZT 18 and 6, respectively).

Conclusions: Based on human SUDEP cases and accumulating evidence from mouse models of seizure-associated death, seizures that occur during the night appear to carry a higher risk of mortality. How time-of-day and circadian mechanisms contribute to this shared nocturnal phenotype is poorly understood. Elimination of serotonin neurons in the central nervous system eliminates this nocturnal tendency of death. Future studies that explore how serotonin-related physiology is influenced by seizures at different times of day will clarify this pathological mechanism. Understanding how daily fluctuations in serotonin contribute to this nocturnal death phenotype will help us better understand how serotonergic nuclei and circadian rhythms contribute to the pathophysiology of SUDEP.

Funding: University of Iowa Graduate College Post-Comprehensive Exam Research Fellowship (BLK), NIH/NIGMS T32 GM007337 (University of Iowa MSTP), NIH/NINDS R01 NS095842 (GFB), Beth L. Tross Epilepsy Professorship (GFB)