Neuroendocrine changes in the aging reproductive axis of female rhesus macaques (Macaca mulatta)

Femalerhesus macaques show monthly menstrual cycles and eventually enter menopause at approximately 25 yr of age. To help identify early biomarkers of menopause in this nonhuman primate, we monitored reproductive hormones longitudinally from aged female macaques during the transitions from premenopause to perimenopause and postmenopause and found that, indeed, elevated plasma FSH was a better predictive factor of menopause onset than age. In a second experiment, we compared reproductive hormone profiles of young adult macaques (8-10 yr old) with those of regularly cycling old macaques (approximately 24 yr old). Indwelling vascular catheters were used for remote blood collection for at least 100 consecutive days, thereby covering three complete menstrual cycles in each macaque. Plasma levels of estradiol, progesterone, LH, FSH, follicular phase inhibin B, and anti-müllerian hormone (AMH) were determined during each menstrual cycle and were averaged for each animal; group mean differences were analyzed using one-way ANOVA. Old premenopausal macaques showed regular menstrual cycles that were qualitatively indistinguishable from those of young macaques; peak plasma levels of estradiol, progesterone, and LH were not significantly different. In marked contrast, peak plasma FSH concentrations were significantly higher, while inhibin B and AMH levels were generally lower, in the old premenopausal macaques compared with those in the young macaques. These data provide further evidence that rhesus macaques serve as an excellent model to study underlying mechanisms of human menopause. Furthermore, the data suggest that an age-related change in FSH, inhibin B, and AMH secretion may be the first endocrine manifestation of the transition into perimenopause, potentially having value in predicting the onset of the perimenopausal transition.     

Low serum levels of FSH, LH and prolactin in luteal phase inadequacy

In order to elucidate the relationship between prolactin (PRL) levels and corpus luteum function in humans, assessment of temporal relationship between levels of PRL, LH, FSH, estradiol and progesterone was made in eleven normal cycling women and six short luteal women. All hormones were determined by specific radioimmunoassay. The mean PRL level in the luteal phase was higher than that in the follicular phase in normal women. On the other hand, no difference mean was seen between the PRL levels of follicular and luteal phases in short luteal women. In addition, follicular and luteal phase secretion of PRL in the short luteal phase (SLP) was lower than that in the normal control. LH and FSH in the follicular and luteal phases, estradiol secretion in the late follicular and early to mid-luteal phases in SLP were also lower than those in the control. These observations were consistent with the hypothesis that SLP is a sequel to aberrant folliculogenesis. In addition, it is inferred that low PRL levels in the SLP might be due to inadequate augmentation by estrogen, rather than giving PRL any positive controlling role in the maintenance of corpus luteum function.     

Relation between levels of circulating ovarian steroids and pituitary gonadotropin content during the menstrual cycle of the rhesus monkey

Anterior pituitary glands were removed from 27 intact cycling rhesus monkeys sacrificed in the early (Day 2), mid (Days 6--9) and late (Days 11--12) follicular phase, and in the early and late luteal phase (3--5 and 10--15 days after the midcycle luteinizing hormone (LH) surge). Assignment of cycle stage was confirmed by the pattern of circulating steroid and gonadotropin levels seen in the blood samples taken daily throughout the cycle. The anterior pituitary glands were weighed, stored at -30 degrees C and assayed for LH and follicle-stimulating hormone (FSH) content by specific radioimmunoassays. Serum estradiol levels and pituitary LH and FSH contents rose simultaneously during the follicular phase. After the preovulatory gonadotropin surge, pituitary LH content was low and invariant. Pituitary FSH content reached a nadir in the early luteal phase and tended to rise in the late luteal phase. Multiple correlation analyses revealed that there is a positive correlation between rising levels of estradiol in the circulation and pituitary LH (p = 0.003) and FSH (p = 0.017) content, and that there is a significant negative correlation between circulating progesterone levels and pituitary FSH content (p = 0.002). Pituitary LH content is less strongly related to circulating progesterone levels. There was no significant difference in the wet weights of the anterior pituitary glands during the five phases of the menstrual cycle studied.     

In vitro effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on ovarian, pituitary, and pineal function in pigs

The aims of the study were: (1) to examine 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and/or prolactin (PRL) effects on in vitro secretion of progesterone (P₄) and estradiol (E₂) by luteinized granulosa and theca cells from porcine preovulatory follicles; and (2) to determine the effects of TCDD on PRL, luteinizing hormone (LH), and melatonin luteal phase in pigs. We found that TCDD itself did not affect progesterone secretion, but it abolished the stimulatory effect of PRL in the follicular cells. TCDD stimulated PRL secretion during the luteal phase and inhibited during the follicular phase. Moreover, TCDD increased luteinizing hormone secretion by pituitary cells during the follicular phase. In contrast to protein and steroid hormones, melatonin secretion in vitro was not affected by TCDD. In conclusion, it was found that the pituitary-ovarian axis in pigs is sensitive to TCDD, and the dioxin exhibited a profound ability to disrupt the ovarian actions of prolactin.     

Biphasic stimulatory effects of estrogen on gonadotropin surges induced by continuous administration of gonadotropin-releasing hormone in women

The present experiments were performed to study the effects of preovulatory levels of estrogen on GnRH-induced gonadotropin release. Twelve female volunteers in various phases of the menstrual cycle received estradiol infusion for 66 h at a constant rate of 500 micrograms/24 h which is grossly equivalent to its production rate during the preovulatory follicular phase. In 8 of the women, GnRH was administered concomitantly from 6 h after the initiation of estradiol infusion. The administered doses of GnRH were 2.5 and 5 micrograms/h. Blood samples obtained throughout the infusion were analysed for LH, FSH, estradiol and progesterone. The sole administration of estradiol failed to induce the positive feedback effect on gonadotropin release within the experimental period in the early follicular phase (days 3-7) in 4 women. In 5 women treated during the follicular phase, remarkable LH releases were induced after a lag period by the infusion of both GnRH and estradiol. The induced LH surge formed a prolonged biphasic pattern. Although a similar pattern of FSH was observed in some cases, its response was minimal compared with that of LH. In 3 women during the luteal phase, however, a combined administration of estradiol and GnRH induced only a short term release of LH which was terminated in only 12 h. The present data indicate that 1) Preovulatory levels of estrogen affect the late part of the LH surge which is induced by constant administration of low doses of GnRH resulting in a prolonged biphasic release of LH, and 2) These effects of both hormones are not manifest in the presence of high levels of progesterone. These results indicate the possibility of a role of GnRH and estrogen in the mechanism of the prolonged elevation of a gonadotropin surge at mid-cycle.     

Immunoreactive inhibin levels in peripheral blood during the breeding season in the female Japanese monkey

Serum levels of immunoreactive inhibin as well as FSH, LH, estradiol-17 beta, and progesterone were measured by RIA in four mature female Japanese monkeys (Macaca fuscata fuscata) during the breeding season and subsequent transition into the nonbreeding season. During the breeding season, each monkey showed 2-6 ovulations, which were inferred from underlying endocrine events. The concentrations of serum inhibin increased during the luteal phase, but were low during the follicular phase. Such changes in serum inhibin levels correlated positively with those in serum progesterone levels. Basal levels of serum inhibin also increased during the breeding season, decreased during transition from the breeding season, and were low during the nonbreeding season. The parallel change in serum levels of inhibin and progesterone together with the increased basal levels of serum inhibin during this period suggests that both the CL and antral follicles are sources of circulating inhibin. Decreases in serum FSH levels during the luteal phase suggest that secretion of FSH is controlled by an inhibitory action of ovarian inhibin in addition to steroid hormones.     

The endocrinology of the menopauseProceedings of Xth International Congress on Hormonal Steroids, Quebec, Canada, 17–21 June 1998.

Menopause is defined as the permanent cessation of menstruation resulting from the loss of ovarian follicular activity. Ovarian primordial follicle numbers decrease with increasing age up to about age 38 following which there is a much steeper decline in the last 12 or so years of reproductive life. At the time of the menopause itself, few follicles remain within the ovary. The recent availability of assays specific for the dimeric inhibins A and B has permitted clarification of the endocrine events leading up to and occurring around the time of final menses. Those women who show clear elevations in serum FSH above age 40, while continuing to cycle regularly have significantly lower inhibin B levels than those whose FSH levels remain in the range seen earlier in reproductive life. Early in the menopause transition, when cycle irregularity is first observed, the initial event is a decline in circulating inhibin B levels in the early follicular phase. In the late perimenopause, levels of estradiol and inhibin A also fall, inhibin B levels remain low and FSH is markedly elevated. The variability of hormone levels in women in their 40s, even in those who are continuing to cycle regularly makes FSH and estradiol unreliable markers of menopausal status. Serum androgen levels appear to fall with age rather than having any clear cut relationship to the menopause transition or menopause. The endocrine changes which occur during the menopausal transition and early postmenopausal period have clinical consequences in terms of symptoms and changes in bone mass.     

The effects of melatonin and hypothyroidism on estradiol and gonadotropin levels in female Syrian hamsters

Since melatonin injections administered near the end of the daily photoperiod influence both gonadal and thyroid hormones in the female hamster, the present study was designed to compare the effects of melatonin and hypothyroidism on the reproductive system and to determine whether thyroid status influenced the action of melatonin on the regulation of the hormones of reproduction. The effects of daily melatonin injections were determined in control hamsters, in hamsters rendered hypothyroid with thiourea, and in hypothyroid hamsters receiving thyroxin (T4) hormone replacement. As previously reported, melatonin injections disrupted estrous cyclicity, disrupted the normal pattern of gonadotropin secretion, and resulted in atrophy of the uterus and vagina. These changes coincided with depressed serum and pituitary prolactin (PRL), and depressed levels of estradiol. The effects of melatonin on uterus, vagina, ovary, and on gonadotropin levels were not prevented by T4 replacement, with the exception of a melatonin-induced increase in serum follicle-stimulating hormone (FSH). This suggested that the cessation of estrous cyclicity was not primarily a result of thyroid deficiency. Hypothyroidism, however, like melatonin, resulted in a reduced number of developing and mature follicles and corpora lutea in the ovaries, and in reduced uterine weight. It also produced follicular atresia, reduced the circulating levels of estradiol, and resulted in reduced incidence of estrus smears. T4 replacement, for 2 weeks, prevented the decline in mature follicles and corpora lutea, reduced the extent of follicular atresia, increased circulating levels of estradiol, and increased uterine weight. PRL and luteinizing hormone (LH) data also provided evidence for antagonistic effects of melatonin and T4 in female hamsters. These data raise the question whether melatonin-induced changes in circulating levels of T4 play a role in the seasonal cycles of reproductive competence in the female hamster.     

Hyperprolactinemia and luteal insufficiency in infertile patients with mild and minimal endometriosis.

The objective of the present paper was to assess the presence of hormonal alterations in infertile women with stage I or II endometriosis (Group III, n = 20) compared to fertile women without endometriosis (Group I, n = 14) and to fertile women with endometriosis (Group II, n = 7). Serum levels of FSH, LH, estradiol, TSH, and PRL were measured between days 1 and 5 of the early follicular phase; in the luteal phase, three serum samples were collected for progesterone measurement, and endometrial biopsies were performed. Serum estradiol levels were lower (p = 0.035) in infertile patients with endometriosis than in fertile patients without endometriosis. Six infertile patients with endometriosis presented prolactin levels above 20 ng/ml. This was not observed in the other groups. Luteal insufficiency was more frequent in infertile patients with endometriosis (78.9%) than in fertile patients with (42.9%) or without endometriosis (0%). In a multiple logistic regression analysis, only the presence of endometriosis and infertility was significantly associated with luteal insufficiency. The serum levels of LH, FSH, and TSH were not significantly different among the groups. Luteal insufficiency and altered prolactin secretion were associated with endometriosis, and could be important mechanisms causing infertility in this group of patients.     

Seasonality and the melatonin signal in relation to age as correlated to the sexual cycle of the one-humped male camel (Camelus dromedarius)

Heparinized blood samples were taken from male immature and mature camels of the Sha'alah breed, housed at the University Animal farm, during the rutting and non-rutting period. Other blood samples were also collected from camels slaughtered at defined seasons (summer, autumn, winter and spring) and from the Buraydah slaughter-house. In addition, specimens from the testes were also taken to confirm the difference between the immature and the mature animals during the non-rutting and rutting seasons. The plasma obtained from the collected blood samples was used for estimation of the following hormones, Melatonin (MLT), Follicle Stimulating Hormone (FSH), Leutinizing Hormone (LH), Testosterone and Prolactin (PRL) using the radioimmunoassay technique. Specimens of testes tissue were fixed in calcium formol, processed for histological examination using standard procedures and stained with H&E. The results clearly differentiated the samples as immature and mature during the non-rutting and rutting seasons. Commercially available human radioimmunoassay (RIA) kits for MLT, FSH, LH, testosterone and PRL were adapted for quantitation of these hormones in serum from the one-humped camel (Camelus dromedarius). Serum samples from 40 camels were assayed in order to determine possible differences between various groups in the concentrations of MLT, FSH, LH, testosterone and PRL in these animals. Among the camels, serum concentrations of melatonin, FSH, LH, testosterone and prolactin reflected age and seasonal differences. Immature camels had overall significantly lower levels in MLT, FSH, LH, testosterone and PRL. Mean FSH and LH levels from confirmed non-rutting (sexually inactive) camels were 0.22 ± 0.08 and 0.37 ± 0.18 ng/mL, respectively. Although rutting (sexually active) camels had higher FSH and LH levels, the differences were not statistically significant (P less than 0.07). Our observations indicate that these RIAs can reliably detect serum MLT, FSH, LH, testosterone and PLT from camels and represent the first quantitation of melatonin in Camilidae in correlation with FSH, LH, testosterone and prolactin.     

Circadian profiles of progesterone,gonadotropins, cortisol and corticotropin in cycling and postmenopausal women

Little is known about the regulation of temporal variations of progesterone over the 24-hr span in young cycling women as well as in postmenopausal women. The purpose of the present study was to investigate the relationships between diurnal variations of progesterone and diurnal variations of hormones of the gonadotropic and corticotropic axes, and to provide further information on the source of progesterone secretion under physiological conditions. Twenty-four-hour hormonal profiles were explored under well-controlled laboratory conditions in 10 healthy women (21–36 yr old) with normal ovulatory cycles during early-mid follicular and late luteal phases, and in 8 healthy postmenopausal women (48–74 yr old). In young cycling women, significant positive relationships were found between progesterone and follicle-stimulating hormone (FSH) – but not luteinizing hormone (LH) – profiles during late luteal phase. Conversely, during follicular phase, significant positive relationships were evidenced between progesterone and cortisol profiles, but not between progesterone and FSH or LH. In postmenopausal women, strong positive correlations were found between progesterone and corticotropin (ACTH) or cortisol profiles. The present results indicate that during late luteal phase, temporal progesterone profiles are associated with FSH rather than with LH profiles. They also provide evidence that adrenal cortex is a major – or possibly the only – source of progesterone production during the follicular phase of the normal ovulatory cycle, and probably the only source after menopause.     

Patterns of circulating gonadotropins and ovarian steroids during the first periovulatory period in the developing sheep.

This report provides evidence that an increment in serum gonadotropin levels occurs at puberty in the sheep and that this reflects the critical hormonal event culminating in first ovulation in this species. Blood samples were collected from 6 female lambs at 4-h intervals for a period of approximately 2 mo around the expected time of puberty (32 wk of age) until behavioral estrus was observed and ovulation was verified by assay of serum progesterone. Patterns of circulating LH, FSH, progesterone, and estradiol concentrations were characterized during the peripubertal period for each lamb. A rise in serum levels of both LH and FSH began approximately 7-10 days before the first preovulatory surge of gonadotropins. Although the increase in gonadotropin levels occurred gradually over several days, serum estradiol levels rose only during the final 40-60 h prior to the preovulatory surge of gonadotropin. Serum progesterone profiles revealed, however, that normal (14-16-day) luteal phases were induced in only 2 of 6 females as a result of the first surge. In four lambs, a short luteal phase of 2.5 days' duration occurred, which was followed by another estradiol rise and a preovulatory surge that then resulted in a full luteal phase of 14 days' duration. These data demonstrate clearly that the precipitating event at puberty in the female sheep is an increase in circulating gonadotropin levels and that the estradiol secreted from the newly stimulated follicle provides the signal for the first preovulatory surge.     

The effect of smoking on serum progesterone, estradiol, and luteinizing hormone levels over a menstrual cycle in normal women

Since smoking has been shown to affect serum progesterone and estradiol levels in postmenopausal women, we evaluated the levels of these hormones and luteinizing hormone (LH) over an entire menstrual cycle (17 points) in eight healthy nonsmokers and eight healthy smokers. The total length of the cycle and the lengths of the follicular and luteal phases did not differ between the groups. There was no difference in estradiol, progesterone, or LH levels during the periovulatory and luteal phases. Follicular-phase serum progesterone, which had a level 37% higher in smokers, showed a plateau in both groups (28.3 +/- 5.7 ng/dl versus 20.7 +/- 5.7; P less than 0.0001). Follicular-phase serum estradiol showed a rising curve in both groups. The mean value in smokers was slightly higher than that in nonsmokers (107 pg/ml versus 95; P approximately 0.05); during the early part of the follicular phase, prior to the rapid preovulatory increase, the difference was greater (23%) and of higher statistical significance (80 pg/ml versus 65; P less than 0.001). The follicular-phase LH levels of smokers were skewed downward from the levels in nonsmokers, presumably by negative feedback from the elevated estradiol and progesterone levels; the difference was significant (P less than 0.001). The elevations of serum progesterone and estradiol in smokers probably represent activation of adrenocortical secretion by smoking. The greater and more clear-cut rise of progesterone than of estradiol is probably due to the fact that essentially all of the follicular-phase serum progesterone is secreted by the adrenal, while only part of the follicular-phase serum estradiol comes from the adrenal (via androstenedione and estrone).     

Relationships between luteinizing hormone, follicle-stimulating hormone and prolactin secretion and ovarian follicular development in the weaned sow

Folliculogenesis was studied by assessing development of the largest 10 follicles obtained from 10 sows 48 h after weaning and by analyzing changes in plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL) for 24 h before weaning until 48 h after weaning. Follicular diameter, follicular fluid volume, and concentrations of estradiol and testosterone and granulosa cell numbers were determined in all follicles, and 125I-hCG binding to theca and granulosa and maximal aromatase activity in vitro was determined in five follicles/sow. Overall, a significant rise in LH, but not in FSH, occurred at weaning, although in individual sows an increase in LH was not necessarily related to subsequent estrogenic activity of follicles. In 9/10 sows, PRL fell precipitously after weaning. In lactation, LH was negatively, and after weaning, positively, correlated with FSH and PRL. Marked variability in follicular development existed within and between sows. Overall, most follicular characteristics were positively correlated to follicular diameter; however, in larger follicles the number of granulosa cells was variable and unrelated to estrogenic activity, which--together with theca and granulosa binding of hCG--increased abruptly at particular stages of follicular development. Differences in maturation of similarly sized follicles from different sows were related to estrogenic activity of the dominant follicles but not to consistent differences in LH, FSH or PRL secretion. Both the dynamics and the control of folliculogenesis in the sow, therefore, appear to be complex.     

Midcycle and luteal elevations of follicle stimulating hormone in squirrel monkeys (Saimiri boliviensis) during the estrous cycle

Follicle stimulating hormone (FSH) has fundamental importance in reproductive function, but its cyclic pattern has not previously been described in the squirrel monkey, due primarily to the lack of a suitable assay. An homologous radioimmunoassay (RIA) based on recombinant cynomolgus FSH measured changes in serum FSH relative to patterns of bioactive luteinizing hormone (LH), estradiol, and progesterone during the estrous cycle. FSH was observed to have a sharp peak during the late follicular phase coincident with the LH surge and then rose again during the luteal phase. Estradiol was low except for the midcycle rise, suggesting an inhibitory relationship. The rat granulosa cell in vitro FSH bioassay confirmed high levels of this hormone. Measurement of FSH in the squirrel monkey has found a pattern different from Old World primates in the luteal phase, which may provide insight into the reproductive mechanisms of this species.     

Elevated ovarian expression and serum concentration of α inhibin in the luteal phase during follicular development in the squirrel monkey (Saimiri boliviensis) compared to the human

The goal of the present investigation was to determine in the squirrel monkey the source and pattern of inhibin, a hormone known to effect reproductive steroid levels via pituitary and ovarian mechanisms. Since this seasonally polyestrous species is known to have elevated serum levels of reproductive steroids compared to other primates, the levels of ovarian alpha subunit mRNA expression and serum total alpha inhibin, estradiol, progesterone, and luteinizing hormone were measured and compared to human levels. Expression of the alpha subunit was robust in monkey luteal tissue compared to expression in human luteal tissue. Squirrel monkey serum inhibin peaked 4 days after the luteinizing hormone surge and correlated with progesterone changes. These luteal serum levels of inhibin were greater than 12 times higher than the human levels yet bio‐LH activities were less than in the human during the luteal phase. Inhibin concentrations during the non‐breeding season were generally half the levels measured in the breeding season and undetectable in ovariectomized animals. However, exogenous FSH stimulation induced a marked rise in inhibin, which correlated with an estradiol rise. In conclusion, abundant alpha inhibin subunit expression in the luteal ovary of the squirrel monkey and loss of serum delectability in ovariectomized animals indicates that the principle source of inhibin in the squirrel monkey is the ovary. Elevated serum inhibin levels during the luteal phase concurrent with ovulatory‐size follicular development is unique among species studied thus far. Possible simultaneous inhibin production from both follicular and luteal tissue may be responsible for the exceptionally high inhibin levels. Am. J. Primatol. 47:165–179, 1999. © 1999 Wiley‐Liss, Inc.     

The influence of estradiole and tibolone administration on leptin levels in women with surgically induced menopause

Background: Several studies suggest that changes in estrogens and androgens during menopause play a role in the regulation of leptin production. Some authors present hypothesis that sex hormone replacement therapy can modulate leptin levels but up to date evidence shows that the influence of endogenous estrogens, androgens levels and sex hormone therapy on leptin concentration remains uncertain. Aim: To evaluate the influence of surgically induced menopause on serum leptin levels and the influence of different types of hormonal therapy on serum leptin concentrations. Methods: 58 women with surgically induced menopause were divided into three groups. Women who did not receive any hormonal substitution (group 1), women who received Estradiol l mg per day (group 2) and women who received Tibolone 2,5 mg per day (group3). The levels of leptin, estradiol, testosterone, testosterone, dehydroepiandrosterone sulfate, FSH, LH and progesterone were measured in all subjects on the 5th day and after 3 months following the surgical procedure. Results: Mean serum leptin concentrations did not differ statistically in any of the studied groups in the begining and in the end of the study. There was no correlations between serum leptin and estradiol, LH, FSH, progesterone, testosterone, free testosterone and DHEAS concentrations in any of groups before and after treatment. Conclusion: Changes in sex hormone concentrations caused by ovariectomy do not influence serum leptin concentrations. Also the short term administration of low dose estrogen therapy or tibolone in postmenopausal subjects does not change serum leptin levels.     

Estradiol induces and progesterone inhibits the preovulatory surges of luteinizing hormone and follicle-stimulating hormone in heifers

Objectives were to determine: 1) whether estradiol, given via implants in amounts to stimulate a proestrus increase, induces preovulatory-like luteinizing hormone (LH) and follicle-stimulating hormone (FSH) surges; and 2) whether progesterone, given via infusion in amounts to simulate concentrations found in blood during the luteal phase of the estrous cycle, inhibits gonadotropin surges. All heifers were in the luteal phase of an estrous cycle when ovariectomized. Replacement therapy with estradiol and progesterone was started immediately after ovariectomy to mimic luteal phase concentrations of these steroids. Average estradiol (pg/ml) and progesterone (ng/ml) resulting from this replacement were 2.5 and 6.2 respectively; these values were similar (P greater than 0.05) to those on the day before ovariectomy (2.3 and 7.2, respectively). Nevertheless, basal concentrations of LH and FSH increased from 0.7 and 43 ng/ml before ovariectomy to 2.6 and 96 ng/ml, respectively, 24 h after ovariectomy. This may indicate that other ovarian factors are required to maintain low baselines of LH and FSH. Beginning 24 h after ovariectomy, replacement of steroids were adjusted as follows: 1) progesterone infusion was terminated and 2 additional estradiol implants were given every 12 h for 36 h (n = 5); 2) progesterone infusion was maintained and 2 additional estradiol implants were given every 12 h for 36 h (n = 3); or 3) progesterone infusion was terminated and 2 additional empty implants were given every 12 h for 36 h (n = 6). When estradiol implants were given every 12 h for 36 h, estradiol levels increased in plasma to 5 to 7 pg/ml, which resembles the increase in estradiol that occurs at proestrus. After ending progesterone infusion, levels of progesterone in plasma decreased to less than 1 ng/ml by 8 h. Preovulatory-like LH and FSH surges were induced only when progesterone infusion was stopped and additional estradiol implants were given. These surges were synchronous, occurring 61.8 +/- 0.4 h (mean +/- SE) after ending infusion of progesterone. We conclude that estradiol, at concentrations which simulate those found during proestrus, induces preovulatory-like LH and FSH surges in heifers and that progesterone, at concentrations found during the luteal phase of the estrous cycle, inhibits estradiol-induced gonadotropin surges. Furthermore, ovarian factors other than estradiol and progesterone may be required to maintain basal concentrations of LH and FSH in heifers.     

Effect of neonatal melatonin administration on sexual development in the rat

In order to study the mechanisms by which melatonin modulates sexual development, 5-day-old female Wistar rats have been treated with a single s.c. injection of melatonin, 3 h before the darkness onset. Criteria for sexual development were the age of vaginal opening and the circulating levels of prolactin, LH, FSH and estradiol. Also, pineal melatonin content was measured. There was a precocious puberty (P less than 0.01) in melatonin-treated rats measured by the age of the vaginal opening. An increase in the number of estrous smears over the whole period studied was observed in melatonin-treated animals as compared to controls. Along with these modifications, there was decrease in pineal melatonin content and serum prolactin levels, on day 21 of life (P less than 0.05), with an increase in both parameters on day 30 of age, in melatonin-treated rats as compared to controls, with no modifications at any other time studied. No differences were detected for serum LH levels considering the whole period studied for both groups. There was a faster decrease in plasma FSH levels with age in melatonin-treated animals than in controls. Serum estradiol levels were decreased in the peripubertal period in melatonin-treated rats as compared to controls. All these data suggest that the modifications induced by neonatal melatonin administration on prolactin, FSH and estradiol could be responsible for the precocious puberty shown in this study.     

Effects of estradiol and progesterone on circulating LH and FSH secretion,and ovarian antral follicle growth in anestrous ewes

In the ewe, ovarian antral follicles emerge or grow in a wave-like pattern and each wave is preceded by a peak in the serum FSH level. The purpose of the current study was to investigate whether in anestrous Western White Face ewes, a combination of progesterone and estradiol affects the circulating FSH peak secretion and the number of small ovarian follicles. Five ewes were treated with subcutaneous silastic rubber implants (10 cm × 0.47 cm), containing 10% estradiol-17β w/w (controls) and 5 ewes were treated with the same estradiol implant, along with subcutaneous implants (11 cm × 0.48 cm) containing 10% progesterone w/w for 12 days. Daily transrectal ovarian ultrasonography and blood sampling was performed from 5 days before, to 9 days after the period of implantation. Blood samples were also taken every 12 min for a 6 h period on day −2, 6 and 13 prior to or after implant insertion (day 0, day of implant insertion). Pulsatility in the serum LH levels was eliminated by the implants (P < 0.05). During the implantation period, the serum FSH peak amplitude was lower in ewes treated with implants releasing estradiol and progesterone, compared to ewes treated with implants releasing only estradiol (P < 0.05). No follicular waves emerged during implant treatment in both groups (P < 0.05) and the number of serum FSH peaks did not differ during implantation, compared to before implantation. During the implantation period, the number of small follicles did not differ in ewes with implants releasing estradiol and progesterone, compared to ewes treated with implants releasing only estradiol. To conclude, supra-physiological concentrations of estradiol completely eliminated the serum LH pulsatality and suppressed the follicular wave emergence, while the FSH secretory peaks that preceded the follicular waves were not affected. Supra-physiological concentrations of estradiol-17β with physiological concentrations of progesterone decreased the serum FSH peak amplitude, eliminated the serum LH pulses, but did not decrease the size of the small follicle pool in anestrous ewes.