The role of ovarian steroid hormones in the regulation of basal and stress induced absence seizures.

UNLABELLED Ovarian hormones play an important role in the regulation of absence seizures in patients as well as in animal models. The present study examined whether chronic progesterone exposure would induce tolerance for the occurrence of absence seizures and whether reduction in gonadal steroids (via ovariectomy) would alter the number of basal and stress induced absence seizures in WAG/Rij rats, a genetic model for absence epilepsy. METHODS In Experiment 1, female WAG/Rij rats equipped with EEG electrodes received progesterone (P) (20 mg/kg) or cyclodextrin (CD, solvent) i.p. injections once a day for 3 days while a third group received CD injections on Days 1 and 2 and P on Day 3. The EEG was recorded on the day preceding the injections and at each day after injections. In Experiment 2, female WAG/Rij rats equipped with EEG electrodes, were ovariectomized (OVX) or sham operated. EEG recordings were made before and at the 4th, 8th, 10th, 20th, and 35th day after surgery. Rats were then exposed to three series of 10 foot-shocks (FS, 1.5 mA, 1 s) over 3 days. The EEG was recorded 1 h before and 2 h after each FS series. RESULTS Tolerance developed after a single P injection and the effect of P on SWDs was facilitated by two preceding control injections. No differences were found between OVX and sham-operated females in the occurrence of SWDs either in resting conditions or after acute FS exposure. However, OVX females showed a more prominent day-to-day aggravation in SWDs after repeated FS administration. CONCLUSIONS The data suggest an important interaction between hormones of the hypothalamo-pituitary-adrenal and hypothalamo-pituitary-gonadal axes in seizure control. On the one hand, stress interferes with and facilitates the acute effects of progesterone on the occurrence of SWDs and, on the other hand, rats with an intact hypothalamo-pituitary-gonadal axis can better regulate the stress response and develop tolerance to the stressor.