Abstracts

Rationale: Fenfluramine (FFA), which enhances cerebral serotonin (5-hydroxytryptamine, 5-HT) release, is reported to be effective in improving seizure control in Dravet syndrome patients. Our previous study [1] showed that FFA selectively prevents seizure-induced respiratory arrest (S-IRA) in the DBA/1 mouse model of sudden unexpected death in epilepsy (SUDEP). Apnea is the most common terminal event in witnessed human SUDEP. The present study examined the role of several 5-HT receptor subtypes in mediating this effect of FFA by treating DBA/1 mice with selective 5-HT receptor antagonists prior to FFA administration and subsequent testing for susceptibility to S-IRA and seizure. Methods: Primed DBA/1 mice were subjected to audiogenic seizures (Sz) that resulted in S-IRA using an electrical bell [1], but death was prevented by prompt mechanical support of respiration. At least 24 h after priming, the mice received FFA (15 mg/kg, i.p.). Since this dose of FFA consistently exerted selective blockade of S-IRA in DBA/1 mice at 16 h [1], these mice were tested 16 h after receiving FFA. Thirty min prior to Sz testing a selective 5-HT receptor antagonist or vehicle was administered to evaluate the role of each receptor in the anticonvulsant and S-IRA blocking effects by FFA. The antagonists included: 5-HT1a (WAY100635, 0.1-15 mg/kg), 5-HT1a/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-HT5a (SB699551, 1-20 mg/kg), 5-HT6 (SB271046, 10-20 mg/kg) and 5-HT7 (SB269970, 30-40 mg/kg) receptors. Follow-up studies investigated the effect of 30 min pretreatment with a 5-HT4 receptor agonist, BIMU-8 (20-50 mg/kg, i.p.), on the FFA effects as well as the effect of co-administration of ineffective doses of FFA (10 mg/kg) and BIMU-8 (20 mg/kg) on the incidence of S-IRA in DBA/1 mice. 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 incidence of selective S-IRA blockade and tonic Sz was only observed following treatment with the 5-HT4 antagonist (GR125487, 30 mg/kg). While the antagonists of 5-HT5A and 5-HT7 receptors reversed the anticonvulsant effect of FFA against the severity of Sz, they did not selectively block S-IRA, unlike the 5-HT4 antagonist. This selective effect is proposed to be mediated primarily by actions on the brainstem network that controls respiration (1, 2). Pretreatment with the 5-HT4 receptor agonist, BIMU-8 (30-40 mg/kg), or co-administration of an ineffective dose of BIMU-8 (20 mg/kg) plus an ineffective dose of FFA (10 mg/kg) significantly (p<0.01) reduced the incidence of S-IRA and tonic Sz in DBA/1 mice. Conclusions: These findings suggest that the selective block of seizure-induced sudden death by FFA in DBA/1 mice is mediated by the activation of 5-HT4 receptors, since an antagonist at this receptor blocks this effect. These findings also suggest that fenfluramine and specific 5-HT4 receptor agonists may be of interest for future studies in other SUDEP models and as potential treatments for human SUDEP and other conditions involving respiratory insufficiency. The present results also provide further support for the serotonin theory of SUDEP (1,2).1. Tupal and Faingold, Epilepsia, 2019 Mar;60(3):485-4942. Kommajosyula and Faingold, Epilepsia. May 6, 2019(Support: Zogenix, Inc.) Funding: Zogenix, Inc.