Follicle-stimulating Hormone Surge Research Articles

2,3,7,8-Tetrachlorodibenzo-p-dioxin Decreases Responsiveness of the Hypothalamus to Estradiol as a Feedback Inducer of Preovulatory Gonadotropin Secretion in the Immature Gonadotropin-Primed Rat

Sprague–Dawley rats (23-day-old) were dosed with TCDD (32 μg/kg) in corn oil or vehicle alone. Equine chorionic gonadotropin (eCG) was injected (5 IU, sc) 24 h later to induce follicular development. Another 24 h later, half of TCDD- or corn oil-treated rats were injected (sc) with 17β-estradiol-cypionate (ECP, at 0.004 to 0.5 mg/kg). Blood and ovaries were collected on expected proestrous (preovulatory period) at 51, 54, and 58 h after eCG injection as well as in the morning after ovulation (72 h after eCG). Serum concentrations of 17β-estradiol (E), progesterone (P), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were determined by radioimmunoassay. The number of ova shed was measured at 72 h after injection of eCG by irrigating ova from oviducts. During the preovulatory period (∼58 h after eCG injection), a circulating level of 70–100 pg E/ml coincided with LH and FSH surges and later normal ovulation of 10 to 12 ova/rat was observed in controls. However, the same concentration of E was not associated with LH and FSH surges in rats treated with TCDD (32 μg/kg), resulting in reduced ovarian weight gain and reduction of ovulation by 70 to 80% (2–3 ova/rat). Blockage of the gonadotropin surge, reduced ovarian weight gain, and ovulation were all reversed completely by the lowest effective dose of ECP (0.1 mg/kg). At 72 h after eCG, serum P secretion was reduced and serum E levels were significantly increased compared to those of corn oil-treated controls. ECP alone had no effect on serum P levels at any time point, but in rats treated with TCDD and ECP, both the reduction of P (at 58 and 72 h) and the increase in E secretion (72 h) were completely reversed. Further studies confirmed that restoration by ECP of gonadotropin surges and associated ovulation could not be attained until circulating levels of E rose sufficiently high to trigger the LH and FSH surges. The new action threshold of E for inducing gonadotropin surges in rats treated with TCDD (32 μg/kg) was determined to be eight- to 10-fold higher than that in controls. Thus, it is apparent that TCDD decreased the responsiveness of the hypothalamus to E as a feedback inducer of preovulatory gonadotropin secretion.

Contribution of endogenous inhibin to the decline of the secondary surge of follicle-stimulating hormone in the rat.

The involvement of inhibin in the decline of the secondary surge of follicle-stimulating hormone (FSH) was investigated in the rat. After ovariectomy or treatment with inhibin antiserum conducted at 2300 hours during pro-oestrus, plasma concentrations of FSH were maintained at high levels compared with control rats. However, plasma FSH started to decline at 0500 hours during oestrus in both the groups. The same treatments conducted during metoestrus markedly increased plasma FSH after 24 h (twofold compared with the treatments during pro-oestrus), suggesting that the treatments sufficiently depleted circulating inhibin. To examine whether the decline of plasma FSH occurred through a transcriptional mechanism or through a translational mechanism, FSH-beta mRNA expression and the pituitary concentration of FSH were measured. Neither ovariectomy nor inhibin immunization conducted during the night of pro-oestrus, affected the pituitary concentration of FSH after 24 h, whereas a noticeable increase was observed after the treatments conducted during metoestrus. In both stages, both ovariectomy and inhibin immunization significantly increased FSH-beta mRNA expression compared with control rats. In contrast with the pituitary concentration of FSH, the effect of inhibin immunization on FSH-beta mRNA expression was not different between the stages. The present data demonstrate the involvement of inhibin in the decline of the secondary surge of FSH, and suggest that a factor or factors other than inhibin may also be responsible for the fall in FSH. Changes in the pituitary concentration of FSH and FSH-beta mRNA expression suggest that post-transcriptional mechanisms may be involved in the suppression of FSH secretion during oestrus.

Variations in Plasma Levels of Substance P and Effects of a Specific Substance P Antagonist of the NK1 Receptor on Preovulatory LH and FSH Surges and Progesterone Secretion in the Cycling Cynomolgus Monkey

These studies investigated the role of substance P (SP) in the regulation of the hypothalamic-pituitary-ovarian axis in cynomolgus monkeys with normal menstrual cycles. Plasma concentrations of SP were determined in blood samples taken every morning in normally menstruating cynomolgus monkeys throughout the menstrual cycle. There was a significant decreasing linear trend of SP during the follicular phase (cycle day –13 to day 0) and a significant inverse relationship between SP plasma values and plasma 17β-estradiol (E₂) values from day –13 to day 0 of the adjusted cycle. Correspondingly, SP area under the curve was significantly greater during the follicular phase than the luteal phase. In a second experiment, plasma concentrations of E₂, luteinizing hormone (LH), follicle-stimulating hormone (FSH) and progesterone and length of cycles were measured after five daily intragastric administrations (10 mg/kg) of an NK₁ receptor (SP receptor) antagonist (RPR 100893; 10 mg/kg) initiated after serum E₂ concentrations had exceeded 125 pg/ml. There was a statistically significant reduction in the amplitude (41% of control) and the area under the curve (37% of control) of the preovulatory LH surge. In addition, there was a reduction of the duration of the LH surge (3 ± 0.1 days in controls vs. 2.1 ± 0.2 days in treated animals). The present results show for the first time that there are significant variations in plasma levels of SP, with a strong negative correlation with serum levels of E₂ during the follicular phase of the cynomolgus monkey, and that endogenous SP has a potentiating role in the interactive hypothalamo-anterior-pituitary mechanisms which lead to the preovulatory LH and FSH surges during the menstrual cycle in the monkey.

Plasma Gonadotropin Concentrations in the Cyclic Female Brushtail Possum (Trichosurus vulpecula)

Changes in plasma concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), and their relationship to antral follicle development and ovulation, were determined in female brushtail possums (Trichosurus vulpecula) in experiments in which pouch young were removed (RPY) from lactating females to promote ovarian activity. In Experiment 1 (n=8), the development of preovulatory follicles and ovulation was monitored by laparoscopy. In Experiment 2 (n=15) estrus and mating were monitored by cytology of urine. Ovulation occurred in 4/8 (Experiment 1) and 9/16 (Experiment 2) possums, and in these animals, plasma FSH concentrations fell progressively over the period of preovulatory follicle development and returned to pretreatment levels after ovulation. With the exception of samples taken at the time of the preovulatory gonadotropin surge, mean plasma LH levels remained basal. In those possums that failed to ovulate, plasma FSH concentrations were elevated while plasma LH concentrations were low; these patterns remained unchanged throughout the sampling period. It was not possible to distinguish between animals that would ovulate and those that would not ovulate after RPY on the basis of gonadotropin profiles at the time of RPY. A further group of possums (Experiment 3, n=10) were blood-sampled at hourly intervals for 48 h to characterize preovulatory gonadotropin surges, using laparoscopy to monitor preovulatory follicular development and predict ovulation. A preovulatory LH surge (max. conc. 10.2–43.5 ng/ml, duration 7–9 h) was recorded in 4 animals, with a coincident preovulatory FSH surge (max. conc. 1.4–21.4 ng/ml, duration 3–11 h) observed in 3 of these possums. The patterns of gonadotropin secretion in the cycling brushtail possum conform to those reported for eutherians that ovulate spontaneously and appear to be regulated by similar mechanisms.

Secretion of inhibin A, inhibin B and inhibin pro-alphaC during the oestrous cycle of the golden hamster (Mesocricetus auratus).

Plasma concentrations of inhibin pro-alphaC, inhibin A and inhibin B were determined by enzyme-linked immunosorbent assay at 6 h intervals throughout the 4-day oestrous cycle of the golden hamster. Plasma concentrations of follicle-stimulating hormone (FSH) and oestradiol-17beta were also measured by radioimmunoassay during the oestrous cycle. Plasma concentrations of inhibin A increased from the early morning of day 1 (day 1=day of ovulation) and reached plateau levels at 0500 h on day 2. An abrupt increase in plasma concentrations of inhibin A was found at 1700 h on day 4, when the preovulatory FSH surge was observed. An increase in plasma concentrations of inhibin B occurred on day 1 and reached plateau levels at 1700 h on day 1. The levels remained elevated until 0500 h on day 4 and declined gradually by 2300 h on day 4. Plasma concentrations of inhibin pro-alphaC gradually increased with some fluctuation from day 1 to 1700 h on day 4 and then declined. Significant negative relationships were noted between plasma FSH and both dimeric forms of inhibin from day 1 to day 3. Significant positive relationships were found between plasma oestradiol-17beta and inhibin A or inhibin pro-alphaC throughout the oestrous cycle. In contrast, no significant relationship was found between plasma oestradiol-17beta and inhibin B. These findings suggest that both dimeric forms of inhibin play a role in the regulation of FSH secretion during follicular development. These findings also suggest that inhibin pro-alphaC could be secreted primarily by large follicles, and early atretic follicles could also be responsible for inhibin pro-alphaC secretion. On the other hand, the secretory pattern of dimeric inhibins might shift from inhibin B to inhibin A with follicular development.

Relationship between follicular development and the decline in the follicle-stimulating hormone surge in heifers.

Experiment 1 was conducted to determine whether progesterone affects the pattern of the FSH surge or follicular development associated with a follicular wave in heifers. On Day 7 (Day 0 = ovulation), heifers were allocated into a group receiving prostaglandin F2alpha (PGF2alpha; n = 6) or a control group (n = 5). Twenty-four hours later, all detectable follicles (>/= 2 mm) were ablated (Hour 0). Follicular development was monitored Hours 0, 3, 6, 9, 12, and 16, at 8-h intervals thereafter until Hour 112. To monitor FSH concentrations, blood was sampled at Hours -24, 0, 3, 6, 9, 12, and 16, and at 8-h intervals thereafter until Hour 104. There were no differences (p > 0.05) between the PGF2alpha-treated group and controls in the patterns of the FSH surge or follicular development. Experiment 2 tested the hypothesis that 3-mm follicles do not have FSH-suppressing capacity and that suppression increases as follicles grow beyond 5 mm. Twenty-four hours after an injection of PGF2alpha (Days 6-8), heifers were subjected to either ablation of follicles >/= 2 mm or ovariectomy. Intact heifers were allocated into four groups (n = 5) in which all follicles of the new wave were ablated upon reaching either 3, 5, or 7 mm or were not ablated (controls). Blood was sampled at 8-h intervals to monitor FSH and estradiol-17beta. Averaged over Hours 8-120, FSH concentrations (ng/ml) were higher (p < 0.05) in the ovariectomized (2.02 +/- 0.05) and the 3-mm groups (1.91 +/- 0.05) than in the 5-mm (1.52 +/- 0.05), 7-mm (1.35 +/- 0.04), and control groups (1.33 +/- 0.05); and estradiol concentrations (pg/ml) were lower (p < 0.05) in the ovariectomized group (0.19 +/- 0.03) than in the 3-mm (1.48 +/- 0. 16), 5-mm (1.56 +/- 0.15), 7-mm (2.22 +/- 0.27), and control groups (2.55 +/- 0.49). In conclusion, the presence of endogenous progesterone did not affect FSH patterns or follicular development. Follicles </= 3 mm had no detectable capacity to suppress FSH. As follicles grew from 3 to 5 mm, they gained the capacity to suppress FSH; however, as follicles grew beyond 5 mm, FSH-suppressing capacity did not increase. The FSH decline was not attributable to an increase in circulating estradiol.

Hypokalemia alters sex hormone and gonadotropin levels: evidence that FSH may be required for luteinization.

Hypokalemia produced different effects on steroid sex hormone concentrations in plasma and ovary in the mouse. Estradiol levels were slightly increased, whereas circulating progesterone was markedly decreased in all estrous periods. The preovulatory surge of gonadotropins and the secondary surge of follicle-stimulating hormone (FSH) at estrus were also decreased, but basal levels of both gonadotropins were unaffected. Supplementation with luteinizing hormone (LH), FSH, or gonadotropin-releasing hormone (GnRH) at proestrus rapidly normalized plasma and ovarian progesterone levels at this stage of the estrous cycle. Plasma progesterone levels at diestrus were restored only by combined treatment, at the periovulatory stage, with LH and FSH or GnRH but not by LH or FSH alone. The results demonstrate a lack of steroidogenic activity in the corpus luteum of the potassium-deficient mice and, furthermore, that FSH plays an important role in luteinization in the hypokalemic mice. We conclude that alteration of the transcellular potassium gradient may affect the regulation of the periovulatory surge of gonadotropins and progesterone secretion, probably by altering the release of GnRH from the hypothalamus. In addition, the results suggest that FSH may play a certain role as a luteotropic hormone in mice.

Immunoneutralization of follicle stimulating hormone does not affect gonadotrophin surge-inhibiting factor/attenuating factor bioactivity during the rat ovarian cycle.

The physiological role of follicle stimulating hormone (FSH) in the regulation of the release of the putative ovarian factor gonadotrophin surge-inhibiting or -attenuating factor (GnSIF/AF) was investigated. Blood FSH concentrations were immunoneutralized in female rats during different days of the ovarian cycle. FSH antiserum was injected in different protocols to neutralize the FSH surge(s) and/or basal FSH concentrations. All animals were killed on the morning of the day of pro-oestrus of the control rats. Hemi-pituitaries were incubated and gonadotrophin-releasing hormone (GnRH) stimulated and basal luteinizing hormone (LH) and FSH releases and pituitary contents were measured. The biological effectiveness of the FSH antiserum was established by monitoring the histology of vaginal smears and inhibin concentrations in trunk-blood. The predominance of small leukocytes in the smears (low oestradiol bioactivity) and the decreased (bioactive) inhibin secretion imply that anti-FSH neutralizes FSH bioactivity. Simultaneously, the results showed that all anti-FSH injections did not affect the biphasic LH response to GnRH in vitro, which shows an unaltered presence of GnSIF/AF bioactivity in the blood circulation. The results suggest that under physiological conditions GnSIF/AF bioactivity, which keeps the LH responsiveness of the pituitary gland suppressed to the action of GnRH, is not supported by inhibin and not controlled by FSH.

Chronic underfeeding increases the positive feedback efficacy of estrogen on gonadotropin secretion.

Reduced caloric intake has been shown to inhibit reproductive cycles in females of several mammalian species. Previous studies have shown that increased negative feedback efficacy of estrogen on gonadotropin secretion may be responsible. The present study was designed to test the alternate hypothesis that caloric restriction alters the positive feedback efficacy of estrogen on gonadotropin secretion. Adult, cycling female rats were placed on reduced food intake (R) equal to 50% of that consumed by ad/libitum-fed controls (C). When R rats stopped cycling, both R and C rats were ovariectomized (OVX) and immediately implanted subcutaneously with a Silastic capsule containing 100 microg 17beta-estradiol (E2). Blood samples were obtained at 0900-1000 hr and 1600-1730 hr on Days 2, 4, 6, 8, 10, 12, and 14 after OVX and implantation. Follicle-stimulating hormone (FSH), luteinizing hormone (LH), and E2 were measured by radioimmunoassay in duplicate aliquots. Results indicate that underfed female rats retain the ability to respond to elevated estrogen levels with an afternoon surge of gonadotropin which is present for at least 14 days for LH. By contrast, FSH surges in R rats became progressively smaller and were no longer significant after Day 10. The present results also demonstrate that the response of R rats to elevated estrogen levels is significantly greater than that of C rats on Days 2-4 for FSH and 2-14 for LH. It is concluded that an inability to respond to elevated estrogen levels with an afternoon LH surge is not the cause of the cessation of normal estrous cycles. The progressive decrease in the afternoon surge of FSH may be, at least partly, responsible for the decreased follicular development observed in underfed rats. Possible explanations of the enhanced LH response to the positive feedback of estrogen are discussed.

Functional interrelationships between follicles greater than 4 mm and the follicle-stimulating hormone surge in heifers.

The interrelationships between the FSH surge that initiates a follicular wave and the follicles in the wave were examined in heifers. In experiment 1, > or = 5-mm follicles were ablated 5 days after ovulation and heifers (n = 6/group) received a total dosage of 0, 37.5, 75, or 150 units of porcine FSH. Half of the FSH dosage was administered 24 h after ablation followed by the other half 12 h later. Blood samples were taken after the initial FSH injection for FSH assay, and ovaries were examined daily with ultrasound to monitor follicle growth. There were progressively higher FSH concentrations at the mean peak (8 h after initial injection in all groups) as the dosage increased (interaction of dose and time; p < 0.001). Compared to values in controls, the highest dosage (150 units) approximately doubled the number of 5- and 6-mm follicles; this then progressed into a 4- to 7-fold increase in the number of 7- and 8-mm follicles. In experiment 2, either all (controls; n = 6), two (n = 11), one (n = 6), or zero (n = 6) follicles of the first wave of an estrous cycle were retained and the remaining were ablated upon reaching 5 mm. Scanning and blood sampling were performed every 8 h for 72 h after the initial ablation. Mean FSH concentrations during 0 to 72 h decreased (p < 0.004) as the number of retained follicles increased. In heifers in the one-follicle group, the randomly chosen 5-mm follicle developed the characteristics of a dominant follicle. The following conclusions were made: 1) the number of follicles that advanced into a follicular wave was increased by exaggerating the height of the FSH surge, 2) all > or = 5-mm follicles of a wave contributed to the declining portion of the FSH surge, and 3) any 5-mm follicles at the emergence of a wave were capable of becoming the dominant follicle.

Stimulatory effect of a specific substance P antagonist (RPR 100893) of the human NK1 receptor on the estradiol-induced LH and FSH surges in the ovariectomized cynomolgus monkey.

Utilizing a human NK1 receptor antagonist (RPR 100893), the present in vivo study was designed to test the hypothesis that endogenous substance P (SP) modulates the action of 17beta-estradiol in inducing luteinizing hormone (LH) and follicle stimulating hormone (FSH) surges in ovariectomized cynomolgus monkey. Plasma concentrations of LH and FSH as well as NK1 receptor antagonist and SP were measured during the development of the negative and positive feedback phases which follow a single administration of estradiol benzoate (50 microg/kg) to long-term ovariectomized monkeys. Daily administration by gastric intubation of 1 mg/kg or 10 mg/kg of the NK1 receptor antagonist (RPR 100893) leads to detectable levels of the antagonist in the blood of treated animals for at least 6 hr after its administration. These levels are in agreement with the experimentally determined IC50 value of the antagonist. The most striking finding of this study is that LH and FSH releases are enhanced during the descending arm of the estradiol benzoate-induced LH and FSH surges, which suggests that endogenous SP normally has an inhibitory role during this time. The enhancement of LH release is approximately 50%, regardless of the amount of the NK1 antagonist used. However, the enhanced FSH release is more important. Furthermore, blockade of the NK1 receptor with the smaller dose of the antagonist leads to a small, but significant, increase in plasma levels of SP, indicating that blockade of SP receptors leads to an increased release of SP. Collectively, these results further substantiate the link which exists between the ovarian steroid 17beta-estradiol and SP systems. Also, for the first time, these results demonstrate an inhibitory involvement of the human NK1 receptor in the 17beta-estradiol-induced pseudo-ovulatory gonadotropin surges in the ovariectomized monkey.

Polycystic ovarian condition in the dehydroepiandrosterone-treated rat model: hyperandrogenism and the resumption of meiosis are major initial events associated with cystogenesis of antral follicles.

The purpose of this study was to elucidate the early effects of dehydroepiandrosterone (DHEA) in the polycystic rat model by charting cytological changes in the early antral follicle of the ovary and constructing a serum hormonal profile. Histological examinations of ovaries from DHEA-treated rats for ten consecutive days revealed that the oocyte of antral follicles, ranging from 1.5 mm to 3.4 mm in diameter, had become activated, i.e., had resumed meiosis. Tabulation and statistical analysis revealed a highly significant difference in the percentage of oocyte activation between the ovaries of DHEA-treated and control rats. Granulosa cells associated with those antral follicles included in our statistical analysis showed no evidence of atresia. A few follicles not included in our analysis contained oocytes that had resumed meiosis and whose associated granulosa cells were atretic. The observed resumption of meiosis occurred in the absence of surges of follicle stimulating hormone (FSH) and luteinizing hormone (LH). During meiosis, a period when many oocytes become activated, levels of serum androgens (DHEA, testosterone, and androstenedione) were high, while FSH, LH, and prolactin (PRL) levels did not differ significantly from those in the controls. Follicles that resume meiosis may be members of a group of follicles that produces a signal(s) when the oocyte becomes uncoupled from the granulosa cell. This signal(s) permit(s) a reprogramming of the accompanying granulosa cells of the follicle to engage in certain developmental processes of cystogenesis. Just what cascade of signals is necessary to achieve this selection remains elusive at this time and is the subject of our continuing investigations.

Locus coeruleus lesions decrease norepinephrine input into the medial preoptic area and medial basal hypothalamus and block the LH, FSH and prolactin preovulatory surge

The aim of this work was to study the role of the dorsal noradrenergic ascending pathway (DNAP), which originates in the locus coeruleus (LC) on the preovulatory surge of luteinizing hormone (LH) follicle-stimulating hormone (FSH) and prolactin (PRL) by producing bilateral electrolytic lesions (cathodal or anodal) in this nucleus. LC lesions were placed at 11.00 h on proestrus in female rats with regular 4-day estrous cycles. Intact rats, sham-operated as well as animals with missed lesions served as controls. In Experiment I, anodal current was applied and hourly blood samples were withdrawn (from 13.00 to 17.00 h) via a jugular catheter from conscious, freely moving rats for determination of plasma LH, FSH and PRL concentrations. In Expt. II, Expt. I was repeated using cathodal current and collecting blood samples hourly from 13.00 to 18.00 h. In both experiments the animals were sacrificed on the next morning when the occurrence of ovulation was checked. The medial septal area (MSA), medial preoptic area (MPOA), and medial basal hypothalamus (MBH) were dissected and assayed for norepinephrine (NE), dopamine (DA) and 5-hydroxyindoleacetic acid (5-HIAA) content. Experiment III was performed in order to test if a hormonal discharge occurred immediately after lesion placement. Blood samples were collected immediately before and 15, 30, 60 and 90 min postoperatively (from 11.00 to 12.30 h). Either anodal or cathodal lesions blocked the proestrous surge of LH, FSH and PRL. The hypothesis that the lesions advanced or delayed these hormonal surges was rejected since we found no increases in the hormonal levels from 11.00 to 12.30 or from 13.00 to 18.00 h, and ovulation was not observed on the following morning in the lesioned animals. Since control, sham-operated and missed-lesion groups exhibited LH, FSH and PRL surges and ovulation, this blockage appears to be caused by the destruction of the LC neurons. Also, this blockade was correlated with a decrease in the NA content in the MPOA and MBH, but not in the MSA, whereas the DA and 5-HIAA content were not changed in all groups examined. The results lead us to suggest that the integrity of noradrenergic afferent input from the LC to luteinizing hormone-releasing hormone neurons in the MPOA and MBH is essential for triggering the preovulatory surge mechanisms for gonadotrophins and PRL.

Effects of progesterone on the secondary surge of follicle-stimulating hormone in the rat.

In the cyclic rat, the secondary surge of FSH on estrus appears to depend on the LH surge-induced fall in serum concentrations of inhibin. To investigate the involvement of progesterone in the regulation of the secondary surge of FSH, 4-day cyclic rats were treated on proestrus with an antagonist of LHRH (LHRHant) and with an ovulatory dose of ovine (o) LH, progesterone, the antiprogestin RU486, or the combination of RU486 and oLH. Serum concentrations of gonadotropins and inhibin at 1830 h on proestrus and at 0030 h on estrus were determined, and the expression of inhibin/activin subunit mRNAs in the ovary at 0030 h on estrus was analyzed by in situ hybridization. Rats receiving saline showed low expression of alpha-, beta(A)-, and beta(B)-subunit mRNAs in the ovary and low serum levels of inhibin in conjunction with the elevated serum concentrations of FSH on estrus. Administration of LHRHant blocked the decrease in the synthesis and secretion of inhibin and abolished the FSH secondary surge, whereas the injection of oLH prevented these effects. Exogenous progesterone, compared with LHRHant injection, increased alpha-, beta(A)-, and beta(B)-subunit mRNA hybridization intensity in the ovary and serum inhibin immunoreactivity, and also restored, in part, the surge of FSH on estrus. The antiprogestin RU486 did not modify the effect of oLH on either inhibin/ activin subunit mRNAs in the ovary or serum levels of inhibin, but blocked the FSH surge. These results indicate that, in the cyclic rat, 1) the secretion of progesterone on proestrous afternoon, induced by the LH surge, is not involved in the fall of ovarian inhibin synthesis and secretion; and 2) in combination with a drop in serum inhibin, a stimulatory action of progesterone on another factor, possibly pituitary activin, could be necessary to elicit a complete secondary surge of FSH.

Low Plasma Estradiol is Required for the Expression of Daily Increase in Plasma Gonadotropins in the Lactating Golden Hamster (Mesocricetus auratus).

Mechanisms responsible for a daily increase in plasma gonadotropins during lactation of golden hamsters (Mesocricetus auratus) were investigated. A daily afternoon (1700 h) increase in plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH) and progesterone was observed until at least day 15 of lactation (day 0 of lactation = day of parturition). On the other hand, plasma estradiol did not show any significant change at 1100, 1700 and 2300 h. Such increases occur in hamsters nursing 8 pups. In hamsters nursing 2 pups, however, only a daily increase in LH was noted on day 5 of lactation and the diurnal rhythm of LH was difficult to see because of the increase in basal levels of LH on day 10 and 15 of lactation. The diurnal increases in plasma LH and FSH disappeared within 2 days after removal of the litter on day 10 of lactation and by day 14 of lactation (4 days after litter removal) most animals had resumed ovulating. Ovariectomy on day 2 of lactation did not affect the profile of plasma LH on subsequent days. In contrast, the basal levels of plasma FSH increased, suggesting the absence of an inhibitory ovarian signal (probably inhibin). A negative relationship existed between plasma estradiol and the daily surges of LH and FSH at 1700 h; the daily rise in gonadotropins occurred when plasma levels of estradiol were low. The afternoon increases in plasma LH and FSH were prevented by treatment with physiological levels of estradiol, comparable to those observed in the cyclic hamster. Antiserum against LHRH injected at 1100 h on day 10 of lactation completely suppressed the daily rise of plasma LH and FSH. These results indicate that during lactation low levels of estradiol are required for the expression of diurnal increase in plasma LH and FSH in the hamster. Moreover, the low levels of estradiol results in the daily release of LHRH.

Hypersecretion of follicle-stimulating hormone (FSH) on estrous afternoon in rats treated with RU486 in proestrus

1. Intact or ovariectomized (OVX) cyclic rats injected or not with RU486 (4 mg/0.2 ml oil) from proestrus onwards were bled at 0800 and 1800 h on proestrus, estrus and metestrus. Additional RU486-treated rats were injected with: LHRH antagonist (LHRHa), estradiol benzoate (EB) or bovine follicular fluid (bFF) and sacrificed at 1800 h in estrous afternoon. LH and FSH serum levels were determined by RIA. 2. RU486-treated intact or OVX rats had decreased preovulatory surges of LH and FSH, abolished secondary secretion of FSH and hypersecretion of FSH in estrous afternoon. The latter was decreased by LHRHa and abolished by EB or bFF. In contrast, EB induced an hypersecretion of LH in RU486-treated rats at 1800 h in estrus. 3. It can be concluded that in the absence of the proestrous progesterone actions, the absence of the inhibitory effect of the ovary in estrus evoked a LHRH independent secretion of FSH.

Gonadotropin-releasing hormone regulation of pituitary follistatin gene expression during the primary follicle-stimulating hormone surge.

Follistatin is produced in the gonadotrope and folliculostellate cells of the pituitary gland and is thought to indirectly regulate FSH biosynthesis and secretion through its ability to bind activin. Recent measurements of follistatin gene expression during the rat estrous cycle revealed a marked increase in pituitary follistatin messenger RNA (mRNA) levels at the time of the preovulatory FSH surge. This finding suggests a role for follistatin in the regulation of FSH at this dynamic time of the cycle. The aim of the present study was to identify the hormonal control mechanisms responsible for stimulating follistatin gene expression on proestrus. In particular, the roles of estrogen (E) and GnRH were assessed using an in vivo ovariectomized (OVX) animal model in which steroid priming results in daily gonadotropin surges. Follistatin mRNA and serum FSH levels were unchanged throughout the day in untreated OVX rats. E priming of OVX rats elicited a 2-fold elevation in follistatin mRNA levels between 1600-2000 h coincident with the peak of the E-induced FSH surge. To determine whether this effect of E on follistatin mRNA levels was the result of the direct or indirect effects of E on the pituitary, follistatin mRNA levels were examined in E-primed OVX rats that had been treated with pentobarbital at 1430 h (to block hypothalamic neurosecretion). Pentobarbital treatment prevented the E-induced increase in follistatin mRNA levels, suggesting that the effects of E are mediated via GnRH or other hypothalamic factors. The effects of GnRH on follistatin gene expression were examined further using an in vitro perifusion model. Proestrous or metestrous pituitaries were perifused for 8 h with pulsatile GnRH (one pulse per h), continuous GnRH, or medium only. Continuous GnRH treatment resulted in a significant elevation in follistatin mRNA levels in both proestrous and metestrous pituitaries, whereas pulsatile GnRH had no effect at either cycle stage. These results suggest that the proestrous GnRH surge is responsible at least in part for the elevation in pituitary follistatin mRNA levels that is associated with the primary FSH surge. GnRH-induced follistatin production on proestrus probably plays a role in the dynamic regulation of FSH at this time of the ovulatory cycle.

Gonadotropin-releasing hormone regulation of pituitary follistatin gene expression during the primary follicle-stimulating hormone surge

Gonadotropin-releasing hormone regulation of pituitary follistatin gene expression during the primary follicle-stimulating hormone surge

Mechanism of the inhibitory action of RU486 on the secondary follicle-stimulating hormone surge.

Recent evidence utilizing RU486 has implicated progesterone (P) and glucocorticoids, in addition to a drop in serum inhibin, in the development of the secondary FSH surge on the morning of estrus. To assess the role of these steroids, we treated proestrous female rats with the antiprogestin/antiglucocorticoid RU486 (6 mg/kg sc) at 1230 h, and with dexamethasone (dex; 8.4 or 16.2 mg/kg sc), or with the steroid biosynthesis inhibitor aminoglutethimide (AG; 150 mg/kg ip) at 1030 h, alone or in combination with RU486. The effects of these treatments on uterine ballooning and intraluminal fluid content (an index of P action), ovulation, and serum levels of P, corticosterone (B), FSH, LH, and inhibin-alpha at 1830 h proestrus and 0900 h estrus were examined. In accord with previous work from our laboratory, RU 486 caused uterine intraluminal fluid retention on the morning of estrus and significantly suppressed the preovulatory surges of both FSH and LH, and the secondary surge of FSH without affecting the fall in inhibin-alpha. Treatment with dex alone raised serum FSH at both 1830 h proestrus and 0900 h estrus, coincident with suppression of serum inhibin-alpha. When administered in combination with RU486, dex partially reversed the increased uterine intraluminal fluid retention at 0900 h estrus, but did not modify the inhibitory effect of RU486 on the primary gonadotropin surges or the secondary surge of FSH. AG alone significantly suppressed serum P, B, and gonadotropins (LH to a greater extent than FSH) at 1830 h proestrus and blocked ovulation and uterine intraluminal fluid release at 0900 h estrus; it did not, however, suppress the secondary FSH surge or prevent the fall in serum inhibin-alpha. When administered 2 h before RU486, AG did not prevent the RU486-induced inhibition of the primary gonadotropin surges or the secondary FSH surge. We conclude from these results that development of the secondary FSH surge does not require P or glucocorticoid action and that RU486 suppression of the secondary FSH surge does not involve blockade of binding of these steroids to their receptors. Our data are compatible with ligand-independent activation of the P receptor, susceptible to blockade by RU486, as the mechanism underlying the enhanced secretion of FSH from the gonadotrope on the morning of estrus.

Oestradiol and the preovulatory surges of luteinising hormone and follicle stimulating hormone in ewes during the breeding season and transition into anoestrus

This study was designed to see if giving exogenous oestradiol, during the follicular phase of the oestrous cycle of intact ewes, during the breeding season or transition into anoestrus, would alter the occurrence, timing or magnitude of the preovulatory surge of secretion of luteinising hormone (LH) or follicle stimulating hormone (FSH). During the breeding season and the time of transition, separate groups of ewes were infused (intravenously) with either saline (30 ml h −1; n = 6) or oestradiol in saline ( n = 6) for 30 h. Infusion started 12 h after removal of progestin-containing intravaginal sponges that had been in place for 12 days. The initial dose of oestradiol was 0.02 μg h −1; this was doubled every 4 h for 20 h, followed by every 5 h up to 30 h, to reach a maximum of 1.5 μg h −1. Following progestin removal during the breeding season, peak serum concentrations of oestradiol in control ewes were 10.31 ± 1.04 pg ml −1, at 49.60 ± 3.40 h after progestin removal. There was no obvious peak during transition, but at a time after progestin removal equivalent to the time of the oestradiol peak in ewes at mid breeding season, oestradiol concentrations were 6.70 ± 1.14 pg ml −1 in ewes in transition ( P < 0.05). In oestradiol treated ewes, peak serum oestradiol concentrations (24.8 ± 2.1 pg ml −1) and time to peak (41.00 ± 0.05 h) did not differ between seasons ( P > 0.05). During the breeding season, all six control ewes and four of six ewes given oestradiol showed oestrus with LH and FSH surges. The two ewes not showing oestrus did not respond to oestrus synchronisation and had persistently high serum concentrations of progesterone. During transition, three of six control ewes showed oestrus but only two had LH and FSH surges; all oestradiol treated ewes showed oestrus and gonadotrophin surges ( P < 0.05). The timing and magnitude of LH and FSH surges did not vary with treatment or season. In blood samples collected every 12 min for 6 h, from 12 h after the start of oestradiol infusion, mean serum concentrations of LH and LH pulse frequency were lower in control ewes during transition than during mid breeding season ( P < 0.05). Oestradiol treatment resulted in lower mean serum concentrations of LH in season and lower LH pulse frequency in transition ( P < 0.05). We concluded that enhancing the height of the preovulatory peak in serum concentrations of oestradiol during the breeding season did not alter the timing or the magnitude of the preovulatory surge of LH and FSH secretion and that at transition into anoestrus, oestradiol can induce oestrus and the surge release of LH and FSH as effectively as during the breeding season.