Altered GABAergic effects on kainic acid-induced seizures in the presence of hippocampal sclerosis in rats

Abstract

Although deficient inhibitory action of GABAergic neurons is frequently implicated in the pathogenesis of epileptic seizures, their exact contribution to the epileptogenicity is still controversial. In the present study, we investigated the effects of GABAergic action on kainic acid (KA)-induced hippocampal seizure in rats with or without hippocampal sclerosis (HS). HS was produced by pretreatment of KA (12 mg/kg i.p.) 3 weeks prior to induction of acute KA seizure (8 mg/kg i.p.). After development of epileptiform activity in the hippocampus, either muscimol (50 ng/μl, 1.0 μl) or vehicle (phosphate buffer solution, 1.0 μl) was applied locally in the left dorsal hippocampus through a cannula and electrobehavioral observation was performed continuously for 6 h. The seizures were divided into four stages according to their severity. 7 days after the induction of acute seizure, the rats were sacrificed and subjected to histological examinations. In the rats without HS, muscimol reduced the seizure severity as well as neuronal damage, whereas muscimol facilitated the severity of both indicators in the presence of HS. Muscimol accelerated the propagation of epileptiform activity and the onset of more advanced seizure stages regardless of presence or absence of HS. Our study suggest that the GABAa function has dual effects on the final severity of KA-induced seizure depending on the presence or absence of HS and that it accelerates the rate of seizure development in either condition. The altered GABAa function in the presence of HS would probably modify seizure activity.