Cyclic and diurnal alterations in the response of luteinizing hormone and prolactin to prostaglandin E2 during the rat oestrous cycle

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Abstract
Two .mu.g of prostaglandin E2 (PGE2) were infused into a lateral ventricle of the brain of female rats at 0900 or 1300 h on the different days of the estrous cycle and the effect on luteinizing hormone (LH) and prolactin (Prl) release was determined. At 0900 h PGE2 caused a pronounced release of LH in proestrous, estrous and metestrous rats whereas the LH response in diestrous rats was moderate. The secretion of Prl was only stimulated in rats from the proestrous phase. When infused at 1300 h PGE2 had marked stimulatory effect on the release of LH in all groups of rats. The response was almost the same in estrous, metestrous and diestrous rats but in proestrous rats a 2-fold higher LH response was observed. On each day of the estrous cycle the LH-releasing activity of PGE2 was greater at 1300 h than at 0900 h. The overall greatest responsiveness of LH to PGE2 was noted at 1300 h on proestrus, i.e., at a time which was prior to the onset of the spontaneous LH surge. At 1300 h, as at 0900 h, PGE2 was only capable of stimulating Prl release in proestrous rats. PGE2-induced Prl release was greater at 1300 h than at 0900 h. In adult male rats the stimulatory effect of PGE2 on LH and Prl release was independent of the time of administration. Apparently the neuroendocrine elements of the hypothalamopituitary unit in mature female rats exhibit cyclic as well as diurnal alterations in the responsiveness to PGE2.