Abstracts

Rationale: Epilepsy may be driven by an increase in the magnitude of gamma activity (20-80 Hz) and fast oscillations (80-500 Hz), which has been reported in the hippocampus and entorhinal cortecx (EC) of patients with temporal lobe epilepsy and animal models of this disorder. However, determining differences in baseline frequency in vivo can be influenced by behavioral and peripheral factors in awake subjects, and pharmacological factors in anesthetized subjects. Therefore, we addressed whether changes in neuronal activity that have been identified during epileptogenesis in vivo can also be detected in the hippocampus and entorhinal cortex in vitro under stable recording conditions.Methods: Male Sprague Dawley rats (38-44 days) were injected with atropine methylnitrate (2.5 mg/kg s.c.) followed by pilocarpine HCl (380 mg/kg s.c.). Diazepam (5 mg/kg i.p.) was given 1 hr after the onset of status epilepticus (status), or 3 hrs after saline or pilocarpine injection if status did not occur (control). Horizontal hippocampal-EC slices (400 μm thick) were made using standard procedures, 1–3 weeks after status (n = 21 slices in 7 rats), or 2-6 weeks after saline (n = 7 slices in 3 rats). A 96 tip multielectrode array (Cyberkinetics Neurotechnology Inc.; 1 mm electrode depth; 400 μm interelectrode distance) was lowered ~100 μm into slices, maintained in an interface recording chamber. Forty consecutive 50 msec windows were sampled from a 2 kHz sample rate recording, which was made >10 min after the electrode array was lowered, and was absent of any detectable spontaneous field potential. The magnitude of the FFT was calculated for each channel in three different frequency bands (20-80 Hz, 80-200 Hz, 200-500 Hz), and this value was averaged across 3 neighboring channels selected from regions of interest (CA3, CA1, subiculum, dentate gyrus, EC). A t-test was used with a criterion value of 0.05 to determine differences between slices from control rats and slices from rats that had status.Results: When slices from control animals were compared to slices from rats that had status, there was a significant increase in gamma oscillations in CA3, CA1, the subiculum, and the EC, but not the dentate gyrus. When the frequency band 80-200 Hz was examined, there were significant increases in the FFT magnitude in area CA1 and the dentate gyrus, but not in other regions. There were no significant changes in FFT magnitude in any region in the 200-500 Hz frequency band. Conclusions: Pilocarpine-induced status epilepticus leads to an increase in gamma oscillations in all of the regions examined except for the dentate gyrus, and an increase in fast oscillations (80-200 Hz) in area CA1 and the dentate gyrus. These changes were detected in the hippocampus and EC at time points that precede the onset of seizures and in vitro epileptiform activity. These results confirm previous reports that changes in network activity begin early in epileptogenesis, and suggest the in vitro model as a useful system to examine the physiological markers of epileptogenesis.