Seizure frequency in pilocarpine-treated rats is independent of circadian rhythm

A single pilocarpine-induced status epilepticus results in a chronic state of spontaneous recurrent seizures resembling human temporal lobe epilepsy. We asked whether pilocarpine-induced epileptic seizures are more prevalent during day than during night as has been previously suggested (Cavalheiro et al., 1991; Bertram & Cornett, 1994; Quigg et al., 1998; Arida et al., 1999; Hellier & Dudek, 1999).

A prolonged status epilepticus (40 min) was induced by pilocarpine in 30-day-old rats and terminated by diazepam. Control rats received saline instead of pilocarpine. After initial pilocarpine-induced seizures were observed, a telemetric EEG system was used to enable a continuous video-EEG monitoring of pilocarpine-treated and control rats in an environment with a fixed 12-hours light-dark-cycle (lights on from 06.00h to 18.00h).

We collected continuous video-EEG data of a group of 11 pilocarpine-treated and 4 control rats. Seizures were confirmed by typical EEG patterns and motor symptoms and occurred in regular or clustered patterns in chronic epileptic animals. Seizures were evenly distributed (Chi2-value (df=23)=0.17327, P=0.9999; n=11) during the light period (56.86%) and the dark period (43.14%).

We found that seizure frequency in pilocarpine-treated rats was not related to circadian rhythm. Therefore combined video-EEG monitoring can detect spontaneous recurrent seizures more reliably than the video-based monitoring alone. In particular, during dark periods, the seizure rate may be underestimated due to the reduced quality of commonly-used infrared-video systems.

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