Abstracts

Rationale: Severe Myoclonic Epilepsy in Infancy (SMEI, Dravet syndrome) is associated with loss of function in the Nav1.1 voltage-gated sodium channel, a brain specific channel important for neuronal excitability. Seizures begin at 6-9 months typically associated with elevated body temperature or fever. Later spontaneous seizures and status epilepticus are associated with cognitive and motor decline. Current therapies are ineffective. We developed a mouse model of SMEI (mSMEI) by targeted disruption of Scn1a which recapitulates the spontaneous and thermally induced seizures of SMEI. mSMEI have reduced sodium current in GABAergic inhibitory interneurons but not excitatory cells resulting in selective failure of firing of GABAergic neurons. This dysfunction is the likely cause of seizures and co-morbidities in mSMEI. We hypothesize that combinations of drugs that enhance GABAergic neurotransmission by complementary mecha nisms will be efficacious in compensating for the selective reduction of firing of GABAergic interneurons. . In this study, protection against thermally induced seizures was determined for two drugs that enhance GABAergic neurotransmission, tiagabine (TGB) a presynaptic GABA reuptake inhibitor and clonazepam (CLN), a postsynaptic allosteric modulator of the GABA-A receptor. Methods: 30 minutes after i.p. injection, body core temperature was elevated from 36.5 C to 42.5 C or until a generalized tonic-clonic (GTC) seizure. Myoclonic (MC) seizures were seen preceding GTC seizures. The difference in body core temperature at seizure onset between treated and untreated mSMEI was determined. Dose-effect relationships were determined by Hill fits. Isobolographic analysis was used to determine antagonism, additivity, or synergy when the drugs were given in combination. Results: Untreated mSMEI had MC seizures at an average body temperature of 38.2 C and GTC seizures at 38.4 C. The maximal protective effect was greater for CLN than TGB (3.6 0.2 C vs 1.4 0.3 C respectively for MC seizures and 4.1 0.08 C vs 3.1 0.2 C for GTC seizures). The half maximal protective dose was lower for CLN than TGB (0.1 0.03 mg/kg vs 0.4 0.2 mg/kg for MC seizures and 0.1 0.01 vs 0.5 0.08 mg/kg for GTC seizures). Above 0.3 mg/kg, TGB increased the number of MC seizures seen before GTC from 7.5 1.4 in untreated mSMEI up to 348 29 at 40 mg/kg. MC seizures were not increased with CLN. Combined doses of CLN and TGB tested at 1:1 and 1:3 fixed dose ratios, showed additive efficacy using isobolographic analysis without an increase in MC seizures. Conclusions: GABA-acting antiepileptic drugs CLN and TGB protect against thermally-induced seizures in mSMEI. TGB was associated with a substantial increase in MC seizures at higher doses, a significant toxicity. Combined therapy with CLN and TGB was additive without an increase in MC seizures. Combinations of GABA-acting drugs with complementary mechanisms of action result in additive efficacy with reduced toxicity in a mouse model of SMEI.