Abstracts

Although there is evidence that astrocytes support neuronal function, the contribution of astrocytes to seizure onset and termination is unknown. The goal of this study was to determine whether there are changes in seizure susceptibility or neuronal damage when the ability of astrocytes to generate ATP is reduced by inhibition of the Krebs[apos] cycle enzyme, aconitase. Adult male Sprague-Dawley rats were anesthetized with ketamine cocktail and 0.5 nmol of fluorocitrate (FC) was injected into the right ventricle. Rats were assigned to one of following groups (6-8 animals each): 1) FC alone at 0.05 or 0.075 mM; 2) kainic acid (7mg/kg) or pilocarpine (240mg/kg) alone; 3) FC at 0.05 or 0.075 mM followed by kainic acid (7mg/kg) or pilocarpine (240mg/kg); 4) isocitrate (15mM or 30 mM) and FC followed by kainic acid (7mg/kg) or pilocarpine (240mg/kg); 5) kainic acid (10mg/kg) or pilocarpine (320mg/kg); 6) isocitrate alone (15 or 30 mM) followed by kainic acid (10mg/kg) or pilocarpine (320mg/kg). The behavior of all animals was observed for at least 4 hours. In some animals, intrahippocampal recording of electrical activity was carried out. Histological analysis for neuronal damage was carried out 24 hours after injection of the convulsant. Measurements of ATP levels were carried out in a second set of animals sacrificed at 4 hours. Animals that received injections of FC, either 0.5 or 0.75 nmol, into the right ventricle had no observable seizure activity or neuronal damage, as measured with silver stain and immunohistochemistry for HSP32 or HSP72, or on basal ATP levels. In addition, no electrographic seizures were recorded in the hippocampus after the ventricular injection of 0.75 nmol FC (n=5). In animals pretreated with FC, administration of kainic acid, at a dose that does not initiate seizures in control animals (7 mg/kg), caused wet dog shakes and neuronal damage in the hilus. Wet dog shakes did not cause any neuronal damage in control animals. If the dose of FC was increased to 0.75 nmol, then subsequent administration of the same dose of kainic acid (7 mg/kg) caused stage 3-5 seizures and damage throughout the hippocampus and cortex. Injection of FC also reduced the dose of pilocarpine needed to produce seizures. Given simultaneously with FC, isocitrate, which bypasses the inhibition of aconitase, blocked the effects of FC in both kainic acid and pilocarpine treated animals. To test whether isocitrate has a direct neuroprotective role, the effect of isocitrate on the neuronal damage induced by kainic acid and pilocarpine was determined. There was no difference in the pattern of HSP72 expression in animals treated with convulsant alone (who had stage 3-5 seizures) and those pretreated with isocitrate. The results demonstrate that a decrease in astrocytic function in the adult brain increases the susceptibility to kainic acid and pilocarpine. The results suggest that some function of normal astrocytes can delay the onset of seizures or raise the seizure threshold. (Supported by NS39941)