Stimulation with estrogen and progesterone of luteinizing hormone (LH)-releasing hormone release from perifused adult female rat hypothalami: correlation with the LH surge.

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The effects of estradiol benzoate (EB) and progesterone (P) in vivo and P in vitro on LHRH release from perifused preoptic area-medial basal hypothalamus (POA-MBH) tissue fragments were assessed. In the first series of experiments, ovariectomized female rats were given either 10 or 30 micrograms EB on day 0, followed by either oil or 5 mg P at 1000 h on day 2. Rats were killed at 2-h intervals after P or oil injection, and the POA-MBH from these rats were incubated in a perifusion system. LHRH release from the POA-MBH of rats primed with either 10 or 30 micrograms EB displayed similar patterns; from a basal rate at 1200 and 1400 h, LHRH secretion rose to significantly higher levels at 1600 and 1800 h. This increase in LHRH release in vitro was evident from the POA-MBH obtained just before and during the afternoon LH rise in vivo. On the other hand, the LHRH secretion pattern in vitro was different in two ways when rats were treated additionally with P. First, LHRH output was significantly higher at 1200 and 1400 h than that in rats treated with 10 or 30 micrograms EB only; this pattern was maintained later at 1600 and 1800 h from the POA-MBH of rats primed with 30 micrograms EB. Second, within the EB/P-treated (EBP) group, significant elevations in the LHRH output occurred from the POA-MBH obtained at 1800 h when rats were in the midst of the LH surge. In the other series of experiments, the POA-MBH of the EB-primed rats (30 micrograms/rat) were perifused with P (10 ng/ml) or vehicle for 6 h. The mean LHRH release during the 6-h interval was significantly higher after P perifusion; the bulk of this rise was evident at 4 h. These studies show that in vitro, the LHRH output of the POA-MBH from the EB- or EBP-treated rats varies significantly during the day and that the POA-MBH of EB-treated rats secretes large amounts of LHRH in vitro, which correlate well with the afternoon LH rise. In addition, treatment with P in vivo changes the in vitro pattern of LHRH output from the POA-MBH of these EB-primed rats, whereas superfusion of P in vitro induces only a marginal stimulation of LHRH release.