Abstracts

Rationale: Status epilepticus (SE) is a dynamic condition in which EEG patterns and convulsive behavior evolve if seizure activity continues. To understand mechanisms of these dynamic changes we studied expression of RNAs for selected neurotransmitters in rat brains at specific EEG stages of SE (Treiman et al., Epilepsy Res 5:49-60, 1990). Methods: Five days after implantation of epidural screw electrodes SE was induced in 180 gm male Sprague-Dawley rats by injection of 3 mmol/kg LiCl IP, followed 20 hours later by pilocarpine, 30 mg/kg SC. Control rats were given LiCl but saline rather than pilocarpine and did not develop SE. Experimental rats were sacrificed 5 min after onset of SE EEG stage III (continuous spiking) or stage V (continuous periodic epileptiform discharges) by rapid anesthetization with isoflurane and decapitation. Brains were rapidly removed for dissection of hippocampus and cortex samples, which were stored at -80°C until RNA was extracted, amplified by PCR, and analyzed on Qiagen RT² Profiler PCR Rat GABA & Glutamate Arrays. Brain RNA samples from 3 rats were pooled. Three pools were analyzed from right hippocampus and right cortex from each group (control, EEG stage III, EEG stage V). Statistical significance was set at p<0.05. Results: Eighty-four RNAs were analyzed for each pool. Expression of 5 RNAs was significantly decreased relative to controls in cortex at EEG stage III. In hippocampus expression of 12 RNAs was decreased and that of 1 RNA was increased. In rats where SE was stopped at EEG stage V expression of 15 RNAs was decreased in cortex. In hippocampus expression of 4 RNAs was decreased and that of 2 RNAs increased. Changes in RNA expression were more frequent at stage III compared with stage V in hippocampus, but more frequent at stage V in cortex. Of particular note, expression of GABArg2, which encodes the GABA_A subunit γ2, was decreased 12.5 fold in cortex at SE EEG stage V. Expression of GABAra6, which encodes for the GABA_A subunit ά6 was decreased 119.7 fold in hippocampus at stage III. Furthermore, in both cortex and hippocampus expression of a number of genes encoding transporter proteins for GABA was decreased. Data from ongoing RNA expression studies in SE stage I rats and in rats sacrificed 4 hrs after onset of SE stage V will also be presented. Conclusions: Striking changes in gene expression occur during experimental Li/pilocarpine-induced experimental SE, using the EEG pattern as the marker of the severity and degree of progression of the episode of SE. The marked decrease in GABArg2 and GABAra6 expression could help explain the decreased responsiveness to benzodiazepines observed in late stages of SE both clinically and in experimental models. Furthermore, the decrease in expression of GABA transporter proteins may reflect a compensatory response in the brain to impaired GABA_A receptor function in progressive SE by an attempt to increase synaptic GABA. The changes in gene expression we observed at specific EEG stages may help in understanding the underlying pathophysiology of dynamic changes during SE.