Abstracts

Rationale: Reduced GABA inhibition appears to be sufficient to cause seizures in human epilepsy as well as in animal models of genetic epilepsy. The presence of AntiGAD antibody, a loss of subsets of GABA neurons, or a reduction in GABAergic synaptic transmission results in hyperexcitable epileptogenic brain. GABA insufficiency also results in a lack of experience dependent plasticity. Clinically, it is often difficult to determine whether recurrent seizures and chronic anticonvulsant therapy cause behavioral deficit, or underlying brain substrate cause both behavioral deficit and epilepsy. We used seizure-prone GAD65(GAD2) deficient mice that exhibit stress-induced seizures to show that deficit in exploratory behavior precedes and is predictive of seizure occurrence. Methods: NOD.129X1-Gad2tmBae/J mice were obtained from Jackson Laboratory. We mated three pairs of GAD2 (+/-) to obtain five GAD2 knock-out mice (-/-) and their littermates (10 heterogygotes (+/-) and 7 wild type (+/+). Each successive week after wean, from 4 weeks to 8 weeks, the mice were placed in the center square of an open field arena (152.5 cm x 152.5 cm) for 10 minutes both to induce seizures and to test their exploratory behavior. Their path was traced and total number of squares crossed by an animal as well as latency and duration of seizures were recorded for a total of 10 minutes each week. Results: The stress of being placed in a novel, open-filed induced seizures in 100% of GAD2 deficient mice as early as 4 weeks of age, while open field failed to induce seizures (0% seizure induction) in any of the heterozygotes or wild type mice. There was significant reduction in latency to seizure at 5 weeks of age (29 ± 4sec) after they have experienced their first seizure at 4 weeks of age (428 ± 83 sec)p<0.05). Latency to seizures remained low in subsequent weeks (94-122 sec). Seizures were characterized by falls followed by head jerks or limb jerks and tonic stiffness that lasted 30 sec to 1 min followed by recovery in 1 -4 minutes. Exploratory behavior of GAD2 knock-out mice was markedly impaired and significantly different from their wild type or heterozygous littermates(#line crossed: 76 ± 47 versus 302 ± 84, p=0.0001); they tended to stay near the walls and were fearful and hesitant in exploring the open field. Their behavioral deficits were observed prior to seizures and predictive of subsequent seizures. Conclusions: The occurrence of markedly reduce exploratory behavior and stress-induced epilepsy in this seizure-prone mice provides a clear example of coexistence of behavioral deficit and epilepsy in an animal due to an underlying genetic defect. Our result support the concept of “essential” comorbidity in epilepsy. supported by NIH K02 NS48237 and Illinois State Health Department