Abstracts

Rationale: Dravet syndrome (DS) is a rare but catastrophic epilepsy that is first diagnosed in early childhood. Treatment of seizures in DS with antiseizure drugs (ASDs) and other therapies often does not adequately control seizures. Patients with DS often present clinically with febrile seizures that may facilitate status epilepticus. Additional seizure types, including afebrile and generalized tonic-clonic seizures, continue throughout life. Approximately 80% of individuals with DS carry a mutation in the SCN1A gene encoding the Na_v1.1 sodium channel. To facilitate the development of novel therapies for the treatment of seizures in DS, the contract site of the NINDS Epilepsy Therapy Screening Program (ETSP) has evaluated the floxed stop Scn1a*A1783V mouse, first developed by the Dravet Syndrome Foundation of Spain and available from Jackson Laboratories. Methods: Mice with a mutation in Scn1A were obtained from Jackson Labs (#026133) and a breeding colony established.  The mutation mini-cassette includes lox P sites and, in the presence of Cre recombinase, results in an amino-acid substitution from alanine to valine at position 1783 (A1783V), resulting in sodium channels that inactivate at more hyperpolarized potentials. Once a breeding colony was established, animals were subjected to single or repeated epochs of hyperthermia in order to determine if seizure threshold was altered in mice heterozygous for the mutation. Hyperthermia was administered using a heat lamp and a rectal temperature probe wherein core body temperature was elevated 0.5°C every 2 min, to a maximum temperature of 42.5°C. We observed that all animals heterozygous for this mutation had seizures (38-39°C, over several cohorts) whereas no wild-type littermate control mice had seizures. Results: We evaluated several prototype ASDs by administering each compound (i.p) at the previously determined time-to-peak effect, prior to the induction of hyperthermia. Clobazam (10, 30 mg/kg) significantly increased seizure threshold whereas carbamazepine (40 mg/kg) significantly reduced seizure threshold. Sodium valproate (300 mg/kg), stiripentol (100 mg/kg), and phenobarbital (15 mg/kg) had no effect on hyperthermia-induced seizures.  In addition, in a separate cohort of animals, we administered two ASDs, carbamazepine and clobazam, once weekly over 4 weeks in order to verify that the effect on the hyperthermia thresholds were consistent. Clobazam (10 mg/kg) consistently elevated seizure thresholds over repeated testing, whereas carbamazepine (40 mg/kg) consistently lowered seizure thresholds over repeated testing. Conclusions: These data suggest that 1) this model of DS is suitable for screening of novel compounds for the ability to block hyperthermia-induced seizures, and 2) heterozygous mice can be evaluated several times over the course of several weeks, allowing for higher throughput screening. Funding: HHSN271201600048C