Abstracts

Rationale: Fenfluramine (FFA) is effective in Dravet syndrome patients as an add-on drug in controlling their otherwise intractable seizures (Schoonjans et al., 2017). FFA enhances serotonin (5-HT) release in the brain, which can potentially exert its seizure-suppressant effects via activation of the many different 5-HT receptors subtypes. Our initial studies showed a dose- and time-dependent blockade of audiogenic seizures (AGSz) and S-IRA by FFA in the DBA/1 mouse model of SUDEP. The present study examined the role of specific 5-HT receptor subtypes in mediating FFA’s prevention of S-IRA by treating DBA/1 mice with specific 5-HT antagonists and evaluating changes in the effect of FFA on seizure and S-IRA susceptibility. Methods: DBA/1 mice (N=447) were subjected to seizure priming to ensure consistent AGSz and S-IRA susceptibility (Faingold et al., 2010). The seizures were induced using an electrical bell, and death was prevented by resuscitation using a rodent respirator. At least 24 h after priming, the mice received FFA (15 mg/kg, i.p.). Since this dose of FFA exerted a high incidence of selective S-IRA blockade in DBA/1 mice at 16 h, these mice were tested 16 h after receiving FFA. Thirty min prior to AGSz testing a selective 5-HT receptor antagonist or vehicle was administered to evaluate the role of each receptor in the anticonvulsant and S-IRA blockade by FFA. The antagonists included: 5-HT_1a (WAY100635, 0.1-15 mg/kg), 5-HT_1a/1b (SDZ21009, 10-20 mg/kg), 5-HT₂ (ritanserin, 5-20 mg/kg), 5-HT₃ (ondansetron,1-3 mg/kg), 5-HT₄ (GR125487, 20-60 mg/kg), 5-HT_5a (SB699551, 1-20 mg/kg), 5-HT₆ (SB271046, 10-20 mg/kg) and 5-HT₇ (SB269970, 30-40 mg/kg) receptors. Seizure behaviors were recorded on videotape, quantified, and compared statistically (Chi-Square Test; significance level: p<0.05). Results: A significant reversal (p < 0.05) of the FFA-mediated reduction in the incidence of S-IRA was only observed following treatment with the 5-HT₄ antagonist (GR125487, 30 mg/kg). While the antagonists of 5-HT₂ and 5-HT₇ receptors were found to reverse the anticonvulsant effect of FFA against the severity of AGSz but not its S-IRA blocking effect. The 5-HT_1a, 5-HT_1a/1b, 5-HT₃ and 5-HT₆ receptor antagonists were not effective at any dose tested. Conclusions: These findings suggest that the anticonvulsant effect of FFA against S-IRA induced by AGSz in DBA/1 mice is mediated, relatively selectively, by the activation of 5-HT₄ receptors, which is consistent with the normal expression levels of this receptor as compared to several other receptors in DBA/1 mouse brains (Faingold et al., 2011). The results were surprising, since the 5-HT₃ receptor is critical in the ability of a selective serotonin re-uptake inhibitor to reduce seizure severity and block S-IRA (Faingold et al., 2016). These findings also suggest that specific 5-HT receptor agonists may be of interest for future studies in animal models of SUDEP and as a potential preventative treatment for human SUDEP in addition to FFA. Funding: Zogenix, Inc.