Abstracts

Rationale: Epilepsy can be very difficult to treat effectively. Intractable epilepsies are associated with high morbidity and mortality. Serotonin (5-HT) has been shown to be protective against epilepsy. Anti-serotonergic drugs tend to worsen seizure control. Dysfunction in the 5-HT system has been implicated in the pathophysiology of sudden unexplained death in epilepsy (SUDEP). Animal studies utilizing genetic and pharmacological blockade of 5-HT receptors support that disruption of 5-HT signaling leads to cortical hyperexcitability. Methods: Here we employ a mouse model in which nearly all 5-HT neurons have been genetically deleted. EEG, EMG, body temperature, ambient temperature, locomotor activity, relative humidity, air flow and breathing plethysmography were recorded in wildtype (WT) mice and mice lacking 5-HT neurons (Lmx1b^f/f/p) before and after treatment with pilocarpine (50 mg/kg i.p. every 20 min until category 5 seizures attained). First pilocarpine dose was preceded 15 min earlier by scopolamine (1 mg/kg i.p.). Results: Lmx1b^f/f/p mice experience seizures after lower doses of pilocarpine compared to WT mice (50 mg/kg i.p. Lmx1b^f/f/p; 200 mg/kg i.p. WT). Lmx1b^f/f/p mice also experienced each Racine category of seizure at a shorter latency than WT. Category 1-2 seizures occurred <5 min from first injection in Lmx1b^f/f/p and 30 min after first injection in WT. Category 3 seizures occurred 15 min from first injection in Lmx1b^f/f/p and 50 min after first injection in Lmx1b^f/f/p. Category 4 seizures occurred 20 min from first injection in Lmx1b^f/f/p and 120 min from first injection in WT. Category 5 seizures occurred 30 min from first injection in Lmx1b^f/f/p and 140 min form first injection in WT. Invariably, Category 5 seizures progressed to motor status epilepticus (SE) in all animals of both genotypes. In the post-ictal period following prolonged seizures Lmx1b^f/f/p mice display profound reduction of respiratory rate and irregularity of respiratory rhythm (i.e. ataxic or apneustic respiration). All Lmx1b^f/f/p mice and 50% WT further progressed to death. The mean duration of convulsive status epilepticus was 10 min in Lmx1b^f/f/p mice and 60 min in WT. Only WT mice in which SE resulted in death were included in this analysis. In 2 WT mice, SE lasted >3 hrs and the animals were euthanized with an overdose of sodium pentobarbital. The latency to death from time of first injection was shorter for Lmx1b^f/f/p compared to WT. Following attenuation of EEG signal and respiratory arrest, cardiac potentials could be recorded for up to 4 min in both genotypes. Conclusions: These results suggest that mice lacking 5-HT neurons are more sensitive to seizure induction and more likely to die following prolonged seizure than WT. These data further suggest that cerebral and/or respiratory mechanisms may be more directly responsible for death than cardiac arrest. These findings may have important implications for SUDEP.