Abstracts

RATIONALE: Despite recent advances in the treatment of epilepsy, a substantial patient population remains refractory to pharmacotherapy. Therefore, the objective of this study was to determine if electrophysiological recordings from combined entorhinal cortex (EC)-hippocampal (HC) [/italic] brain slices obtained from KA-treated animals would prove useful as a model system for the routine screening of novel anticonvulsant therapies for pharmacoresistant temporal lobe epilepsy (TLE).
METHODS: Male, Sprague-Dawley rats were injected with either saline or multiple doses of KA (5 mg/kg, i.p., repeated to status epilepticus (SE) (Hellier et al, 1998)). Behavioral seizure activity was scored (Racine scale) for subsequent correlation with histological and electrophysiological results in control and KA-treated rats. Neuronal cell loss and mossy fiber sprouting were quantified 24 hours, 1, 4, or 10 weeks post-injections by cell counting and densitometric analysis of standard cresyl violet and Timm[scquote]s stained horizontal sections (40 [mu]m). Extracellular recordings from combined EC-HC horizontal brain slices (400 [mu]m) were performed within 1 week of KA-induced SE. Differences in 1) baseline electrophysiological properties in normal Ringer, 2) latency and extent of seizure-like activity in [dsquote]hyperexcitable[dsquote] Ringer (6mM K⁺), and 3) responsiveness to traditional (e.g., phenytoin, carbamazepine) and non-traditional (e.g., retigabine, levetiracetam) anticonvulsants were compared in slices from KA- and saline-treated rats.
RESULTS: All KA-lesioned rats demonstrated repeated stage 4/5 behavioral seizures at the time of KA-administration. Significant cell loss was detected in layer III of mEC, the hilar region of the dentate gyrus and the CA3 cell body region 24 hours following KA-induced SE. This cell loss was not progressive, despite the gradual onset of spontaneous seizures in these animals. Analysis of extracellular field potential recordings demonstrated that in normal Ringer solution, multiple stimulus-linked population spikes and spontaneous burst activity could be observed in layer II of mEC, CA1, and CA3 of slices from KA-treated rats. Slices from saline-treated controls did not demonstrate any signs of baseline hyperexcitability. Increased extracellular [K⁺] resulted in a shortened latency to onset of bursting and faster spontaneous burst rates in slices from KA-lesioned rats versus controls. Spontaneous bursting in CA1 and CA3 regions was attenuated by 50 [mu]M phenytoin and completely blocked by 10 [mu]M retigabine in slices from both KA- and saline-treated rats. However, neither carbamazepine nor levetiracetam significantly altered spontaneous bursting.
CONCLUSIONS: These experiments suggest that KA-induced selective cell loss primes the ventral EC-HC circuit by markedly increasing slice hyperexcitability. Such KA-induced hyperexcitability may provide a fast and reliable model for the routine screening of novel anticonvulsant treatments and may provide insight into the mechanisms underlying pharmacoresistant TLE.
[Supported by: NIH contract N01-NS-4-2311 (HSW)]