Relationship of estradiol to luteal function in the cycling baboon.

Since exogenous 17 beta-estradiol (E2) is luteolytic in some primates, it is reasonable to conclude that endogenous E2 is the naturally occurring luteolytic substance. However, a link between endogenous E2 and spontaneous luteolysis has not been demonstrated. In an attempt to understand the function of E2 in spontaneous luteolysis, we employed the antiestrogen clomiphene. We administered E2, clomiphene, or E2 plus clomiphene to baboons during the luteal phase and measured systemic levels of estrone, E2, progesterone (P), and LH, E2 alone depressed the concentrations of P and bioassayable LH prematurely. The length of the luteal phase was significantly shortened (12.8 +/- 0.9 vs. 15.8 +/- 0.4 days for sham controls; P less than 0.01). Clomiphene blocked the luteolytic effect of exogenous E2, resulting in P levels which were not significantly different (P greater than 0.01) from control and a luteal phase of normal length (15.0 +/- 1.0 days). Treatment with E2 alone caused serum LH concentrations to decrease. The administration of clomiphene alone maintained circulating LH at early to midluteal phase levels but did not alter luteal phase P concentrations or prolong the length of the luteal phase (15.6 +/- 0.4 days). The results suggest that E2-induced luteolysis is a result of the withdrawal of luteotropic support from the (corpus) luteum, while spontaneous luteolysis is a result of some other mechanism which may not involve endogenous E2.

The effect of luteal phase estrogen antagonism on endometrial development and luteal function in women.

Previous studies of the role of estrogen in primate luteolysis, designed to investigate the effects of estrogen antagonism or selective inhibition of luteal phase estrogen production on luteal function, have ignored the impact of such treatments on secretory endometrial development. We examined the effect of luteal phase estrogen antagonism on endometrial maturation and luteal function in six women. In each of two menstrual cycles in each woman, blood samples were obtained on alternate days from cycle days 3-9, daily until 1 day after the urinary LH surge (day 0), and again on alternate days until the onset of menses. In the second of each pair of cycles, clomiphene citrate (100 mg) was administered daily from 2 days after the LH surge until menses. Endometrial biopsy was performed 13 days after the LH surge in each cycle. Serum FSH, LH, estradiol, and progesterone (P) were measured by RIA. The endometrial histological date and concentration of cytosolic (C) and nuclear (N) estrogen (ER) and P (PR) receptors were determined. We found significant (P less than 0.05) increases in luteal phase serum FSH, LH, estradiol, and P levels in the clomiphene cycle compared to those in the control cycle. Endometrial histology was significantly (P less than 0.002) different during estrogen antagonism; a maturation delay of more than 2 days was found in all six women during the clomiphene cycle. Luteal phase duration was unchanged by clomiphene (P = 0.29). Endometrial ER-C [7.38 +/- 2.52 (+/- SEM) vs. 38.75 +/- 10.17 fmol/mg protein], ER-N (248 +/- 84 vs. 685 +/- 80 fmol/mg DNA), and PR-C (97 +/- 38 vs. 189 +/- 38 fmol/mg protein) were significantly lower (P less than 0.03) in the clomiphene cycle than in the control cycle, whereas PR-N was not different (P greater than 0.10). These data suggest that luteal phase estrogen 1) modulates endometrial PR and 2) plays an important role in secretory endometrial development.

Decreased testosterone levels and accelerated extinction of a conditioned taste aversion in fluid-deprived male rats.

The hypothesis that fluid deprivation accelerates extinction of a conditioned taste aversion in male Sprague-Dawley-derived rats by reducing serum testosterone levels was tested. Serum testosterone levels were found to be lower in fluid-deprived males than in nondeprived males (Experiments 1 and 2). Exogenous testosterone treatment that results in high physiological levels of serum testosterone slowed the extinction of fluid-deprived gonadectomized males to rates comparable with those of nondeprived sham males (Experiment 3). It was noted, however, that testosterone treatment was less effective in slowing extinction in fluid-deprived gonadectomized males than in nondeprived gonadectomized males even though the serum testosterone levels were the same (Experiments 3 and 4). These results provide strong support for the original hypothesis, but they suggest that fluid deprivation also reduces sensitivity to testosterone.

Circulating sex steroids after induction of luteinized unruptured follicles in adult guinea pigs.

Luteinized unruptured follicles (LUFs) typically occur when exogenous gonadotropins are administered to adult female guinea pigs prior to the onset of ovulation. In this study, LUFs were induced with human chorionic gonadotropin (hCG) and serum steroids were measured for the next eight days. Serum progesterone was elevated following the hCG injection, but values did not reach the peak levels observed during untreated estrous cycles and declined prematurely. Serum estradiol was similar during untreated cycles and after hCG-treatment. Androstenedione and testosterone were elevated two days after the hCG injection but returned to baseline thereafter. The presence of LUFs was verified by ovarian histology of similarly treated animals. These results demonstrate that hCG-induced LUFs in adult guinea pigs are functionally deficient as compared to corpora lutea arising from spontaneous ovulation.

Enclomiphene induces luteolysis in the nonpregnant guinea pig.

The effect of two antiestrogens, enclomiphene and tamoxifen, on luteal function in the guinea pig was compared to that of estradiol, a known luteolysin. Enclomiphene caused premature luteolysis when administered during the early or mid-luteal phase of the cycle, but was not as potent as estradiol. Tamoxifen had no effect. The luteolytic effect of enclomiphene was mediated by the uterus, as has been shown for estradiol.

Comparison of luteinized unruptured follicles and corpora lutea: steroid hormone production and response to luteolytic and luteotropic agents.

Administration of hCG prior to spontaneous ovulation induces the formation of luteinized unruptured follicles (LUFs) in guinea pigs. Serum progesterone (P) in animals with LUFs is significantly lower. This study was designed to determine whether follicular maturity affected the incidence of LUFs and P production as well as to compare isolated LUFs with corpora lutea (CL) in relation to hormone production and response to luteotropic and luteolytic agents. Ovarian histology and serum steroids following injection of hCG at various times during the estrous cycle indicated that greater follicular maturity increased the incidence of ovulation and P production. LUFs and CL contained equivalent amounts of P/mg tissue, but LUFs were significantly smaller than CL. Cells from CL and LUFs responded to hCG in vitro with significant increases in P release, but the response was greatest with LUFs. Prostaglandin F2 alpha (PGF) attenuated the response to hCG in vitro by mid-cycle CL and LUFs, but not by CL obtained during the early luteal phase. We conclude that the attenuated luteal-phase P profile following induction of LUFs is not an intrinsic deficiency in hormone production, but may arise from the smaller mass of luteal tissue and from earlier than normal development of responsiveness to the luteolytic effects of PGF.

Changes in serum and ovarian steroids during reproductive development in the female guinea pig.

This study was designed to measure ovarian hormones prior to and during the first estrous cycle in guinea pigs. Blood was obtained from 12 animals throughout the first estrous cycle. Ovaries and peripheral serum were obtained from 25 additional animals at various stages of development prior to and after first ovulation. Estradiol, progesterone, androstenedione, and testosterone were measured in all sera and half of the ovaries. The remaining ovaries were fixed for histology. Serum estradiol was nondetectable until a few days before first ovulation, but was present in the ovary throughout development. Serum progesterone was nondetectable until the day of ovulation, but the luteal phase pattern was similar to that observed in adults. Serum androgens were detectable throughout development, with androstenedione higher than testosterone. The immature ovary contained more testosterone than androstenedione, but this pattern was reversed after ovulation. These results indicate that the immature ovary in the guinea pig contains minimal amounts of estradiol and progesterone, the first estrous cycle is similar to that in adults, and that the pattern of ovarian androgen content changes during the peripubertal period.

Effects of clomiphene on luteal function in the nonpregnant cynomolgus macaque.

Although estradiol-17 beta (E2) induces premature regression of the corpus luteum (CL), its role in spontaneous luteolysis which occurs at the end of the nonfertile cycle has not been demonstrated. We compared the effects of an estrogen antagonist on E2-induced and spontaneous luteolysis by administering clomiphene (10 mg/day) to cynomolgus macaques during the luteal phase in the presence and absence of exogenous E2 (supplied by subcutaneous Silastic implants). Other animals received either vehicle or E2 implants. Luteal function was assessed by progesterone concentrations and luteal phase length. Clomiphene maintained normal luteal function in the presence of luteolytic levels of E2 in five of six monkeys. However, clomiphene alone did not prolong luteal function beyond that observed in monkeys receiving vehicle. To assess the direct effect of clomiphene on the CL, we incubated monkey luteal cells with human chorionic gonadotropin and clomiphene, E2, or clomiphene plus E2. Clomiphene (1500 ng/ml) alone and E2 (1000 ng/ml) alone significantly (P less than 0.05) inhibited progestin production. Clomiphene and E2 together depressed progestin production to an even greater extent. The data suggest that the mechanisms involved in E2-induced and spontaneous luteolysis differ.

Experimental induction of estradiol positive feedback in intact male monkeys: absence of inhibition by physiologic concentrations of testosterone.

Estradiol benzoate (E2B) induces an increase in serum luteinizing hormone (LH) in gonadectomized macaques of both sexes and in intact females but not males. To determine the interaction of testosterone (T) with E2B, we treated 6 intact male cynomolgus macaques as follows: 1) with E2B alone; 2) E2B after pretreatment with estradiol-17 beta (E2) for 2 wk; and 3) Treatment 2 plus exogenous T along with the E2. In Experiment 4, 6 female cynomolgus macaques were treated with large doses of T for 10 days prior to E2B. Serum levels of T and E2 were quantified by radioimmunoassay; LH by bioassay. In Experiment 1, LH decreased from 5.20 +/- 0.95 micrograms/ml (mean +/- SEM) to 0.47 +/- 0.04 micrograms/ml. In Experiment 2, E2 pretreatment depressed LH and T, and these animals responded to E2B with an increase in LH. Replacement of T (Experiment 3) did not inhibit positive feedback. Intact females treated with T responded to E2B with an increase in LH. These results suggest that a testicular product other than T is involved in the inhibition of the positive feedback response to E2B in intact male macaques, and that E2 pretreatment overcomes this effect.

Small doses of human chorionic gonadotropin prevent estradiol-induced luteolysis.

The manner in which exogenous 17 beta-estradiol (E2) induces premature luteolysis in primates is unclear. In an effort to determine whether exogenous luteotropic hormone inhibits E2-induced luteolysis, E2 capsules were implanted subcutaneously in 11 cynomolgus macaques during the early luteal phase; six animals received injections of human chorionic gonadotropin (hCG; 7.5, 10, or 15 IU/day) for 10 days, and the remaining monkeys received saline. Blood was collected once daily for measurement of E2, progesterone, and bioactive luteinizing hormone (LH). Peak progesterone concentrations were between 0.7 and 5.0 ng/ml and declined prematurely in monkeys given E2 plus saline; the luteal phase was 11.5 +/- 0.6 days (mean +/- SE). With E2 plus hCG treatment, serum progesterone continued to increase after E2 capsule placement and reached peak levels of 4.0-13.0 ng/ml; the luteal phase was 15.3 +/- 0.5 days. Therefore, E2-induced luteolysis was overcome by concurrent administration of hCG. These results suggest that exogenous tropic hormone circumvents the inhibitory influence of E2 on luteal function, but the details of the interaction remain unknown.