Abstracts

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 medial entorhinal cortex (mEC)-hippocampal (HC) brain slices obtained from KA-treated animals would prove effective as a model system for detecting novel anticonvulsant therapies for pharmacoresistant temporal lobe epilepsy (TLE).
Male, Sprague-Dawley rats were injected with either saline or KA (5 mg/kg, i.p.) per hour to the onset of stage 4/5 seizures on the Racine scale (Hellier et al., 1998). Extracellular recordings from combined mEC-HC horizontal brain slices (400 [mu]M) were performed 1 week later. Baseline electrophysiological responses from Layer II medial EC were recorded in normal, oxygenated Ringer solution (3mM KCl). The extracellular solution was then switched to one containing 6mM KCl and 50 [mu]M picrotoxin in order to compare differences in the latency to onset of spontaneous burst (SB) activity, SB rates, and SB durations in slices from control and KA-treated rats. Further, the ability of [ldquo]traditional[rdquo] (e.g., phenytoin, carbamazepine) and [ldquo]non-traditional[rdquo] (e.g., retigabine) anticonvulsants to block SBs were compared in slices from KA- and saline-treated rats. All experiments were recorded at 31[plusmn]1[deg]C.
Analysis of extracellular field potential recordings from Layer II of the mEC demonstrated that in normal Ringer solution, slices from KA-treated rats demonstrated an average of 1.50 bursts/minute, while [italic]no[/italic] bursting in control slices was observed. Increasing extracellular [K⁺] in the presence of picrotoxin resulted in bursting in all slices recorded. Under these conditions, slices from KA-treated rats demonstrated significantly shortened latency to onset of spontaneous bursting (4.40 [plusmn] 1.1 minute versus 15.0 [plusmn] 0.9 minute) and faster SB rates (14.0 [plusmn] 0.9 bursts/minute vs. 10.0 [plusmn] 0.1 bursts/minute) than controls (p[lt]0.05, n=18-22 rats). In KA-treated slices, superfusion of the slice with phenytoin (10, 30, 50 [mu]M) was unable to reduce the frequency of spontaneous bursting below 10 bursts per minute, whereas control slices demonstrated a dose-dependent reduction in burst frequency at the concentrations used. In contrast, superfusion of the slices with the novel anticonvulsant retigabine (10 [mu]M, within the range of therapeutic plasma levels) dramatically reduced spontaneous burst rates in slices from both control and KA-treated rats.
In slices from the KA-treated animals, the marked hyperexcitability, short latency to onset of bursting, inability of phenytoin to dramatically reduce SB rates and the consistent SBs observed even in the absence of picrotoxin may prove invaluable as a paradigm for detecting novel anticonvulsant therapies for pharmacoresistant TLE.
[Supported by: NO1-NS-4-2311 (HSW), R01- NS-44210 (KSW).]