Abstracts

Rationale: Temporal lobe epilepsy (TLE) is one of the most common forms of intractable epilepsy. TLE is characterized by an initial injury followed by chronic epileptic state of spontaneous recurrent seizures (SRS) after latent period. Hippocampal sclerosis and aberrant mossy fiber sprouting are the hallmarks of TLE. These features are expressed in pilocarpine-treated rats, which represent a suitable model of TLE for testing the efficacy of novel therapeutic agents. In this study, we evaluated the hippocampal interneuron deficits, the frequency of SRS, electrographic activity, and the aberrant mossy fiber sprouting in the lithium-pilocarpine-treated rats. To evaluate the efficacy of GABAergic neurosteroids, we tested the effect of allopregnanolone in the rat pilocarpine model of TLE. Methods: Chronic epilepsy was induced by pilocarpine injections in rats with lithium pretreatment. Pilocarpine was administered in multiple doses of 20 mg/kg, every 30 min for 2 hr, to induce continuous stages III-V seizures and status epilepticus (SE) for >2 hr. Diazepam was given to terminate the SE after 2 hr. Rats were monitored for the occurrence of SRS and EEG events during 2-6 months post pilocarpine. Electrographic seizure activity was verified by recording of EEG from hippocampus and frontal cortex. Neuropeptide-Y (NP-Y) and parvalbumin (PV) immunohistochemistry was carried out to determine the extent of GABAergic interneuron loss, while Timm staining was performed to confirm the aberrant mossy fiber sprouting. The efficacy of allopregnanolone (5 mg/kg, sc for 5 days) was evaluated in rats with frequent SRS and electrographic seizures. Results: Repeated low dose pilocarpine injections were associated with progressively increased seizures, high-amplitude EEG spikes, and SE. SRS were evident after latency of 60 days post pilocarpine. Rats showed an average of 2 seizures per day or 15 years per week. At 5 months post pilocarpine, majority of rats exhibited bilateral neurodegeneration and a robust aberrant mossy fiber sprouting. Absolute neuron counting by Stereoinvestigator technique revealed that NP-Y and PV-immunoreactive GABAergic interneurons were extensively reduced in epilepsy rats. Electrographic seizure discharges were well correlated with behavioral seizures. However, about 15% of pilocarpine-treated rats failed to show SRS partly because of inadequate SE induction. There was no widespread loss of interneurons or mossy fiber sprouting in these rats. Allopregnanolone therapy markedly decreased the frequency and severity of SRS and intensity of electrographic seizure activity in epilepsy rats. Conclusions: These results show that pilocarpine model of epilepsy in rats is associated with widespread hippocampal interneuron loss, frequent spontaneous and electrographic seizures and robust mossy fiber sprouting, which are clinical features of TLE in humans. Allopregnanolone treatment diminished spontaneous seizures in the pilocarpine model, confirming the efficacy of neurosteroid therapy. ** Supported by NIH grant NS052158 **