The ovarian cycle in rats: a long term EEG study

Abstract

Rats of the WAG/Rij strain are considered as a genetic model for generalised absence epilepsy. The relationship between the phase of the estrous cycle and the number of spontaneously occurring spike-wave discharges (SWDs) was investigated during the 4 days of the cycle. Vaginal smears were daily made while the EEG was recorded continuously. The phase of the estrous cycle was determined on the basis of cytological examination. There were more spike-wave discharges at pro-estrus than at the other three days, suggesting a role for ovarian hormones. A circadian pattern emerged for the number of SWDs: a minimum during the first few hours of the light and a maximum during the first few hours of the dark period. This maximum occurred at the time at which maximum progesterone plasma levels can be expected. This suggests that progesterone enhances SWDs. There was no difference in the first few hours of the light period in number of SWDs between pro-estrus and the three other days. The lack of differences suggests that estradiol has no effect on SWDs. The circadian pattern of spike-wave discharges is in temporal agreement with data on circadian fluctuations in brain concentrations of progesterone and its metabolites dihydroprogesterone and allopregnanolone. This opens the possibility that the circadian distribution of SWDs is controlled by the circadian brain concentrations of progesterone and its metabolites.