Abstracts

Rationale: Sudden unexpected death in epilepsy (SUDEP) is a devastating epileptic event. Several pathophysiological mechanisms including respiratory/cardiac depression have been proposed to contribute to SUDEP. DBA mice have been established as relevant animal models for studying SUDEP, as they exhibit seizure-induced respiratory arrest (S-IRA) leading to sudden death after generalized audiogenic seizures (AGS). Previous studies demonstrated that systemic administration of several selective serotonin (5-HT) reuptake inhibitors (SSRIs) reduces S-IRA in DBA mice. SSRIs are known to exert their antidepressant effects centrally. However, 5-HT receptors in peripheral structures can also influence respiratory function, and it has not been established if SSRIs prevent S-IRA by acting on 5-HT reuptake in the periphery or in the brain. Therefore, in the current study, we administered the SSRI fluoxetine intracerebroventricularly (ICV) and evaluated changes in the incidence of S-IRA in DBA/1 mice.Methods: Consistent susceptibility to S-IRA in DBA/1 mice was established by applying acoustic stimulation (electrical bell, 96 dB SPL, maximal stimulation duration 60 s) daily for 3-4 days starting at postnatal day 26-28 (“priming”). Adult primed DBA/1 mice were anesthetized using ketamine/xylazine, and a guide cannula was stereotaxically implanted over left ventricle. One week after surgery, each mouse was tested to verify that it remained susceptible to AGS and S-IRA (with resuscitation) 24 hr prior to ICV injection. Vehicle (dimethylsulfoxide) or fluoxetine was injected into left lateral ventricle using a Hamilton syringe and Harvard microinjection pump at a rate of 0.5 μl/min for 2 min. The infusion cannula stayed in place for 1 min. Acoustic stimulation was applied 15 min after injection of vehicle or fluoxetine. The effect of vehicle or drug on the incidence of S-IRA and AGS was examined and digitally recorded for offline analysis.Results: Vehicle treatment had no effect on the incidence of S-IRA and AGS in DBA/1 mice. ICV injection of fluoxetine at 60 (n = 7) and 90 (n = 10) nmol did not significantly alter the incidence of S-IRA. However, fluoxetine at 120 nmol blocked S-IRA in all of the 7 mice tested (p<0.01). Fluoxetine at 120 nmol specifically blocked S-IRA in 29% of mice and blocked both S-IRA and tonic seizures in the remaining animals. All of the mice still exhibited wild running and/or clonic seizures.Conclusions: These studies demonstrated that ICV administration of the SSRI, fluoxetine, significantly suppressed S-IRA in DBA/1 mice, suggesting that systemically-administered SSRIs act on 5-HT reuptake predominantly in the brain to prevent S-IRA. Although fluoxetine blocked S-IRA, it did not affect the susceptibility of DBA/1 mice to AGS.