Effects of Estradiol and Progesterone on Plasma Gonadotropins, Prolactin, and LHRH in Specific Brain Areas of Ovariectomized Rats1

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Abstract
We examined the effects of physiological concentrations of estradiol (E2) and progesterone (P) on plasma LH, FSH, and prolactin (PRL) and on LHRH concentrations in several microdissected brain regions in ovariectomized (OVX) rats. One week after ovariectomy (Day 0), rats received Silastic capsules containing only sesame oil or 37.5, 75.0 or 150 µg E2/ml of oil s.c. The E2 capsules produced plasma estrogen concentrations of 7.0, 9.6, or 15.4 pg/ml, respectively, on Day 2 whereas oil-treated controls had 5.4 pg/ml of E2 in plasma. All three E2 -treated groups of rats had LH surges of comparable magnitude during the afternoon of Day 2. Two Silastic capsules of P (50 mg/ml) were implanted at 0900 h on Day 2 into E2 -treated rats. In animals in which E2 levels were 7.0 or 9.6 pg/ml, P only moderately amplified the LH surges. However, when plasma E2 concentrations reached 15 pg/ml, P treatment evoked a massive release of LH and advanced the time of release by 1 h. In contrast, the stress of inserting oil capsules into ether-anesthetized E2-treated rats at 0900 h on Day 2 had no effect on afternoon LH surges in these animals. FSH surges occurred only in rats receiving both E2 and P. PRL surges were induced using the highest concentrations of E2 and were advanced in time by P. When the highest dose of E2 was used, LHRH concentrations in the median eminence (ME) increased prior to and decreased during the E2 P-induced surge. These changes were not paralleled by any changes in the other anterior brain regions we examined (suprachiasmatic preoptic nucleus, suprachiasmatic nucleus, medial preoptic nucleus, anterior hypothalamic nucleus, and retrochiasmatic area). No changes in LHRH concentrations were observed under any other steroid treatment regimen. Higher plasma E2 concentrations induced by the 150µ g/ml E2 capsule were correlated with higher LHRH concentraions in the median eminence concomitant with a parallel trend in some of the anterior brain areas: MPN (P<0.06), SPN (P<0.06), and RCA (P<0.03).