Abstracts

Rationale: Genetic studies have identified a growing collection of disease-causing genes providing a fundamental molecular foundation on which to build our understanding of the mechanisms of human familial epilepsy. A GABRG2(R43Q) mutation is found in an Australian family with generalised epilepsy. GABRG2 encodes the GABA-A γ2 subunit, required for the action of therapeutically diverse benzodiazepine receptor drugs. Using the knock-in Gabrg2(R43Q) mouse model we test the impact of the mutation on sedative, anticonvulsant and anxiolytic actions of the benzodiazepine, diazepam. Methods: Experiments were performed on heterozygous Gabrg2(R43Q) mutant and wild type (WT) mice between the ages of P35 and P55. Epidural EEG recording were made for one hour prior and one hour following i. p. injection of diazepam. In each treatment group three measures of efficacy were determined. The numbers of spontaneous spike-and wave discharges (SWDs), diagnostic of absence seizures in the mutant mice, were counted. As a measure of sedation, the effect of diazepam and saline treatment on locomotor activity was examined using an automated movement detector for a test period of 10 min. To measure anxiolytic effects, mice position in the open and closed arms of the elevated plus maze was measured using an automated system for a test period of 5 min. Results: Both the mutant and WT mice had significantly and similarly reduced levels of locomotion at 5mg/kg dose of diazepam (n=4 and 7, p<0.05) while 1mg/kg diazepam was without significant effect in either groups (n=4 and 6). The lower dose was used for subsequent experiments to eliminate sedation as an experimental confound. EEG recordings revealed a significant reduction in the number of SWDs (52±11 to 9±3 n=4, p=0.03) in the Gabrg2(R43Q) mice suggesting a robust antiepileptic action of 1 mg/kg of diazepam. For the anxiety assay, diazepam 1mg/kg had no impact on the relative time spent in open arm for the Gabrg2(R43Q) mice (35±10% (saline) vs 26±16% (diazepam), n= 10 and 6, p=0.6) although a significant increase in relative time spent in the open arms was observed in WT mice following diazepam treatment (20±9% (saline) vs 63±12% (diazepam), n= 9 and 8, p=0.01).