Circadian and Estral Changes in the Hypothalamic Prostaglandin E2 Content and [3H]Prostaglandin E2 Binding in Female Rats

Prostaglandin E₂, (PGE₂) is involved in the luteinizing hormone-releasing hormone-stimulated luteinizing hormone surge in female rats and may act via specific membrane receptors. The following studies were performed to determine whether there were any changes in the hypothalamic PGE₂ binding and/or PGE₂ content which were specific to proestrus and not to the rest of the estrous cycle. Groups of female Wistar rats were sacrificed at 3-h intervals throughout the estrous cycle to determine both the circadian and circaestral changes in the hypothalamic PGE₂ content and [³H]PGE₂ binding. The hypothalamic PGE₂ content was maximal at 1700 h on each of the 4 consecutive days of the estrous cycle but was independent of the stage of the cycle. [³H]PGE₂ binding also displayed a circadian rhythm; the lowest binding occurred near the circadian peak of PGE₂, suggesting that the PGE₂ binding sites were occupied by endogenous PGE₂. Since such circadian rhythms were not observed in the hypothalamus of male rats, they may be under the control of ovarian steroids. Also, since PGE₂ binding and the PGE₂ content both exhibit a diurnal pattern independent of the day of the cycle, there may be changes in the PGE₂ receptor-mediated process coupled to an adenylyl cyclase which could explain the luteinizing hormone surge in proestrus.