Abstracts

Rationale: A GABAA γ2 gene mutation (R43Q) has been linked to inherited forms of epilepsy in man. Like patients with the mutation, gene targeted knock-in mice display increased seizure susceptibility. Epilepsy develops in this model presumably as a consequence of GABAA receptor dysfunction. GABAA receptors can exert their action “directly” by altering the moment-to-moment excitability of the brain. Alternatively, GABAA receptors play an integral role in the developing brain, dysfunction of which may lead to long-term changes in neuronal network stability. Discerning between direct and development effects is vital to our understanding of epileptogenesis. Methods: We developed a conditional knock-in mouse model harbouring the R43Q mutation to address this question. Comparison of allelic expression of mice containing a loxP flanked Neomycin cassette with a Cre-recombinase excised line revealed a relative reduction in expression of the γ2(Q) neo allele by 90%, forming the basis of our switch. The spatio-temporal resolution of the unsilencing excision event was explored using the Z/EG Cre reporter strain crossed with a forebrain-specific tTA regulated Cre-recombinase line. Results: In the absence of doxycycline, gene activation was confirmed in the hippocampus and cortex within useful timeframes. Increased sensitivity to the proconvulsant, pentylenetetrazol (PTZ) is an established phenotype of mice harbouring the R43Q mutation. Using the conditional approach we ‘silenced’ the mutant allele from inception to post natal day 21. Interestingly, PTZ susceptibility was significantly reduced in these mice. Conclusions: Expression of mutant γ2 GABAA receptors in early development increase seizure susceptibility in adulthood, presumably as a consequence of altered brain development. (This study was in part supported by a Program grant from the Australian National Health and Medical Research Council (NH&MRC)