Rationale: Sudden unexpected death in epilepsy (SUDEP) is the most common cause of death in refractory epilepsy including in patients with Dravet syndrome (DS). Convulsive seizures in
Scn1aR1407X/+ (DS) mice induce a blunted hypercapnic ventilatory response (HCVR) and hypothermia (Teran et al., 2023), which both occur when there is serotonergic neuron dysfunction. Thus, seizure-induced serotonergic dysregulation could play a key role in cases of SUDEP mediated by respiratory dysfunction. Fenfluramine (FFA), a serotonin agonist, has previously been shown to decrease seizure frequency in DS patients and reduce mortality in DS mice.
This study examined the effect of FFA on mortality and breathing dysfunction induced by seizures in DS mice.
Methods: On postnatal day 21, DS mice were weaned and implanted intraperitoneally with a temperature telemeter and an osmotic minipump (0.25 mL per hour over 14 days). Five doses of FFA were used (n):
0 (16), 1 (12), 3 (11), 10 (9), and 30(10) ug/kg per day. Mice were housed under standard conditions and monitored via video surveillance 24/7.
Ten days following implantation, body temperature of those mice that had survived was increased via a heat lamp until a Racine scale 5 (R5) seizure occurred, or until body temperature reached 42.5°C. A whole-body plethysmographic chamber was used to record breathing while chamber temperature, airflow, and humidity were controlled. Threshold of body temperature for seizure induction, seizure intensity, and mortality were determined. 20 days following implantation hyperthermia-inducing experiments were repeated to determine FFAs impact after wash-out.
Results: Chronic FFA protected mice against spontaneous seizure-induced death during the first 10 days of FFA exposure with 20% mortality at 30 ug/kg (n=10) compared to 37.5% at 0 ug/kg (n=16). FFA also protected against heat-induced seizures in a dose-dependent manner. When seizures were induced by hyperthermia, control mice (n=10) had an 80% mortality, while mice treated with 10 (n=5) and 30 μg/kg (n=8) FFA had 20% and 13% mortality, respectively. In response to seizures of the same severity (R5), mice on the higher doses of FFA had shorter apneas and were more likely to re-initiate breathing, often first with gasping. Additionally, higher doses of FFA, 10 and 30 ug/kg, may have protected against mortality from heat-induced seizures even six days after washout.
Conclusions: Chronic administration of FFA decreased mortality from spontaneous seizures, increased the threshold for heat-induced seizures and prevented ventilatory arrest when severe seizures did occur in DS mice. Future work will investigate the impact of chronic fenfluramine on spontaneous seizures, apneas, and mortality in a larger number of DS mice.
Funding: NIH/NINDS U01-NS0904143 SUDEP Research Alliance
R01NS123155 (GBR)
F31NS110333 NRSA (FAT)