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.     

Rapid recovery of estrogen-induced pituitary gonadotropin surges after luteectomy in rhesus monkeys

In the presence of a functional corpus luteum, positive estrogen feedback on the surge modes of gonadotropin secretion is blocked in rhesus monkeys. We investigated the effects of luteectomy (Lx) on the time required for recovery of pituitary responsiveness (LH/FSH surges) to positive estrogen feedback. Estradiol-17 beta-3- benzoate (EB, 50 microgram/kg sc) was given: 1) 24th prior to, 2) the day of, or 3) 24 h after luteal ablation. Daily measurements of serum follicle stimulating hormone (FSH), luteinizing hormone (LH), estradiol-17 beta (e2) and progesterone (P) were made on each monkey for 5 days. Serum P fell to undetectable levels within 24 h after Lx, whereas E2 levels in circulation peaked within 24h after injection of EB. Among early follicular phase monkeys, this EB treatment results in typical midcycle type LH/FSH surges within 48h. Lx alone was not soon followed by significant changes in pituitary gonadotropin secretion. When circulating P levels were undetectable the pituitary responded fully to EB; that is, typical midcycle type FSH/LH surges occurred. When serum P was in the midst of declining after Lx, gonadotropin surges were present, but attenuated. However, when P levels remained elevated for more than 24 h after EB injection, the surge modes of FSH/LH secretion remained fully blocked. These results demonstrate that the suppressive influence of luteal secretions (principally progesterone) on positive estrogen feedback regulation of the surge modes of pituitary gonadotropin secretion is quite transient in these primates.     

The acute stimulatory effect of estrogen on gonadotropin release in gonadotropin-releasing hormone-primed women

In order to prove the acute stimulatory effects of estrogen on pituitary gonadotropin release, we have performed the present experiments in 8 women with a hypergonadotropic state due to surgical castration or primary ovarian failure. They received gonadotropin releasing hormone (Gn-RH) for 12-21 h at the constant rate of 20 micrograms/h. In 5 of the women, estradiol-17 beta was concomitantly administered at the rate of 20 micrograms/h from 6 h after the start of Gn-RH infusion. Blood samples were collected frequently throughout the experiments for the analysis of LH, FSH and estradiol. In response to the sole stimulation of Gn-RH, remarkable and prompt rises in LH (313.5%), but to a lesser degree in FSH (194.2%), were observed within the initial 3 h, and their high levels were maintained throughout the experimental period. However, the additional administration of estradiol brought on a further sudden rise in both gonadotropins levels: 178.3% for LH and 163.5% for FSH within 2 h. These high levels were sustained during estradiol infusions. In 2 of them, blood samples were obtained for several hours after cessation of estradiol infusion. The circulating gonadotropin level dropped precipitously close to the baseline level within 3 h after estradiol infusions. Our data indicate that estrogen has an acute and strong augmentative effect on Gn-RH induced gonadotropin release in addition to its conventional negative and positive feedback effects.     

Endocrine alterations that underlie endotoxin-induced disruption of the follicular phase in ewes

Two experiments were conducted to investigate endocrine mechanisms by which the immune/inflammatory stimulus endotoxin disrupts the follicular phase of the estrous cycle of the ewe. In both studies, endotoxin was infused i.v. (300 ng/kg per hour) for 26 h beginning 12 h after withdrawal of progesterone to initiate the follicular phase. Experiment 1 sought to pinpoint which endocrine step or steps in the preovulatory sequence are compromised by endotoxin. In sham-infused controls, estradiol rose progressively from the time of progesterone withdrawal until the LH/FSH surges and estrous behavior, which began approximately 48 h after progesterone withdrawal. Endotoxin interrupted the preovulatory estradiol rise and delayed or blocked the LH/FSH surges and estrus. Experiment 2 tested the hypothesis that endotoxin suppresses the high-frequency LH pulses necessary to stimulate the preovulatory estradiol rise. All 6 controls exhibited high-frequency LH pulses typically associated with the preovulatory estradiol rise. As in the first experiment, endotoxin interrupted the estradiol rise and delayed or blocked the LH/FSH surges and estrus. LH pulse patterns, however, differed among the six endotoxin-treated ewes. Three showed markedly disrupted LH pulses compared to those of controls. The three remaining experimental ewes expressed LH pulses similar to those of controls; yet the estradiol rise and preovulatory LH surge were still disrupted. Our results demonstrate that endotoxin invariably interrupts the preovulatory estradiol rise and delays or blocks the subsequent LH and FSH surges in the ewe. Mechanistically, endotoxin can interfere with the preovulatory sequence of endocrine events via suppression of LH pulsatility, although other processes such as ovarian responsiveness to gonadotropin stimulation appear to be disrupted as well.     

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.     

The modes of the anterior hypothalamic area as the regulatory center for the gonadotropin release

The effects of the anterior hypothalamic area (AHA) implants of gonadal steroid estrogen and progesterone as well as the effects of electrical stimulation and electrolytic lesion confined in this area on the gonadotropin secretion were investigated in ovariectomized estradiol (20 microgram sc)-primed adult Wistar rats housed in light and temperature controlled room. Progesterone implants evoked the rise of serum LH by 6 hr whereas estradiol implants suppressed serum FSH by 24 hr after implantation. Electrical stimulation effectively depleted both gonadotropins with a latency not shorter than 6 hr. The lesion significantly prevented FSH elevation investigated at 72 hr post ovariectomy and potentiated FSH secretion in response to estradiol treatment at 3 week post ovariectomy. The result revealed the involvment of the AHA in LH release mechanism which required progesterone activation while its involvement in FSH regulatory mechanism depended upon estrogen. The area was elucidated as the inhibitory as well as the stimulatory loci for the feedback action of estrogen on FSH release.     

Interaction between ovarian and adrenal steroids in the regulation of gonadotropin secretion

Recent work from our laboratory suggests that a complex interaction exists between ovarian and adrenal steroids in the regulation of preovulatory gonadotropin secretion. Ovarian estradiol serves to set the neutral trigger for the preovulatory gonadotropin surge, while progesterone from both the adrenal and the ovary serves to (1) initiate, (2) synchronize, (3) potentiate and (4) limit the preovulatory LH surge to a single day. Administration of RU486 or the progesterone synthesis inhibitor, trilostane, on proestrous morning attenuated the preovulatory LH surge. Adrenal progesterone appears to play a role in potentiating the LH surge since RU486 still effectively decreased the LH surge even in animals ovariectomized at 0800 h on proestrus. The administration of ACTH to estrogen-primed ovariectomized (ovx) immature rats caused a LH and FSH surge 6 h later, demonstrating that upon proper stimulation, the adrenal can induce gonadotropin surges. The effect was specific for ACTH, required estrogen priming, and was blocked by adrenalectomy or RU486, but not by ovariectomy. Certain corticosteroids, most notably deoxycorticosterone and triamcinolone acetonide, were found to possess "progestin-like" activity in the induction of LH and FSH surges in estrogen-primed ovx rats. In contrast, corticosterone and dexamethasone caused a preferential release of FSH, but not LH. Progesterone-induced surges of LH and FSH appear to require an intact N-methyl-D-aspartate (NMDA) neurotransmission line, since administration of the NMDA receptor antagonist, MK801, blocked the ability of progesterone to induce LH and FSH surges. Similarly, NMDA neurotransmission appears to be a critical component in the expression of the preovulatory gonadotropin surge since administration of MK801 during the critical period significantly diminished the LH and PRL surge in the cycling adult rat. FSH levels were lowered by MK801 treatment, but the effect was not statistically significant. The progesterone-induced gonadotropin surge appears to also involve mediation through NPY and catecholamine systems. Immediately preceding the onset of the LH and FSH surge in progesterone-treated estrogen-primed ovx. rats, there was a significant elevation of MBH and POA GnRH and NPY levels, which was followed by a significant fall at the onset of the LH surge. The effect of progesterone on inducing LH and FSH surges also appears to involve alpha 1 and alpha 2 adrenergic neuron activation since prazosin and yohimbine (alpha 1 and 2 blockers, respectively) but not propranolol (a beta-blocker) abolished the ability of progesterone to induce LH and FSH surges. Progesterone also caused a dose-dependent decrease in occupied nuclear estradiol receptors in the pituitary.(ABSTRACT TRUNCATED AT 400 WORDS)     

Improved model for the induction of proestrus-like gonadotropin surges in long-term ovariectomized rats

In long-term (greater than 4 wk) ovariectomized rats the positive response of the gonadotropin release apparatus to a priming dose of estradiol is moderate as compared with that of proestrous rats exposed to endogenous estradiol. In the present study, high sensitivity to estrogen was restored in long-term ovariectomized rats by pretreatment with estradiol benzoate (EB, 20 micrograms, day 0) and progesterone (P, 2.5 mg, day 3). Estradiol benzoate (20 micrograms) given on day 5 induced proestrus-like surges of LH and FSH in the afternoon on day 6. Additional administration of P (2.5 mg at noon on day 6) had a facilitatory effect. Stimulation of LH release could be evoked in rats by the described regimen 1, 6 or 50 wk after ovariectomy. The long-term ovariectomized rat injected with EB and P as described might provide a useful model for neuroendocrinological investigations on the gonadotropin surge mechanism.     

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.     

Dissociation of LH and FSH Responses to LHRH during estrogen therapy of patients with ovarian failure

This study examines the effect of oral estrogen treatment on gonadotropin secretion in three young women with gonadal failure. Each subject was treated with 0.1 mg BID of ethinyl estradiol for four weeks, and the LH and FSH responses to 200 microgram of intravenously administered LHRH were measured basally and weekly during therapy. Significant reduction of basal levels of FSH occurred within one week of treatment, with obliteration of LHRH-mediated FSH responsiveness within two weeks. By contrast, basal levels of LH were significantly reduced by the end of the second week of treatment, and LHRH-mediated LH levels were sustained for three weeks. In one subject an LHRH test was performed every other day for two weeks after cessation of therapy. Return of FSH responsiveness was delayed one week beyond that of LH, which occurred within three days of discontinuation of estrogen. These results indicate that during the early phase of oral estrogen replacement therapy, FSH secretion may be selectively blunted; after discontinuation of treatment, recovery of FSH secretion lags behind recovery of LH.     

The effect of the presence and pattern of luteinizing hormone stimulation on ovulatory follicle development in sheep

The aim of this study was to examine the role of LH on the growth of the large preovulatory follicle and its secretion of hormones in sheep. Ewes with ovarian autotransplants were treated with GnRH-antagonist at the time of luteal regression and different LH regimes applied for 60-66 h before administration of an ovulatory stimulus (hCG). In Experiment 1 (N = 24; n = 8), ewes received either no LH or constant or pulsatile infusion of LH at the same dose (1.25 microg/h). In Experiment 2 (N = 12, n = 6), LH was constantly infused at a rate of 1.25 microg or 2.5 microg oLH/h. In Experiment 1, animals receiving either pulsatile or constant LH exhibited increases in estradiol and inhibin A secretion (P < 0.001) and a depression in FSH (P < 0.001) that resembled the normal follicular phase. Similarly in Experiment 2, doubling the dose of LH resulted in a two-fold increase in ovarian estradiol secretion (P < 0.05) but no other changes. All animals receiving LH, regardless of the pattern of stimulation, ovulated and established a normal luteal phase. In contrast, no LH treatment resulted in constant immuno-active LH without pulses, unchanged FSH and inhibin A concentrations (P < 0.05), and basal estradiol secretion (P < 0.001). Morphologically normal large antral follicles were observed in this group and although corpora lutea formed in response to hCG, progesterone profiles were abnormal. In conclusion, these results suggest that LH is an essential requirement for normal ovulatory follicle development and subsequent luteal function and show that a pulsatile mode of LH stimulation is not required by ovulatory follicles.     

Influence of the hypothalamic-pituitary-adrenal axis on the menstrual cycle and the pituitary responsiveness to estradiol in the female rhesus monkey (Macaca mulatta)

In order to examine the effect of glucocorticoids on the menstrual cycle of rhesus monkeys, cortisol was injected twice daily during the follicular phase. This cortisol treatment did not alter basal gonadotropin secretion but blocked the normal follicular rise of estrogens, the gonadotropin surge and the luteal rise of progesterone, and delayed the onset of the next cycle. In a second study, estradiol benzoate (E2B) was injected on the sixth day following the start of menstrual bleeding either with or without concurrent adrenocorticotropic hormone (ACTH) treatment. E2B injection was able to stimulate surges of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) whether or not the animals had been treated with ACTH. These data suggest that, the action of cortisol, the final mediating step in the hypothalamic-pituitary-adrenal axis, occurs at the level of the gonads versus the pituitary in the rhesus monkey. While the pituitary response to endogenous gonadotropin-releasing hormone or exogenous E2B stimulation appears to remain unaffected, normal folliculogenesis is disrupted, preventing the follicular secretion of estrogens and the subsequent gonadotropin surges. The effects of corticosteroids are temporary, with normal cycling returning when plasma corticosteroids return to basal concentrations, albeit after a delay.     

Neonatal castration of male and female rats affects luteinizing hormone responses to estrogen during adulthood

We examined the positive and negative feedback effects of estradiol (E2) on luteinizing hormone (LH) and prolactin (Prl) secretion in adult male and female rats which were gonadectomized within 24 h after birth (long-term castrates) and compared these responses to those elicited by E2 in short-term castrated (7 days) adult males and females. The high serum E2 did not reduce the elevated serum LH concentrations in long-term castrates until 4 days of treatment. Also, only after negative feedback was established were the positive feedback actions of E2 observed. In contrast, Prl surges were observed after 2 days of E2, and baseline Prl serum levels were elevated by Day 3 of E2 in long-term castrated male and female rats. Some long-term castrates lacked both LH and Prl surges, and E2 was ineffective in altering basal gonadotropin secretion in these animals. Short-term castrated males had elevated serum Prl levels but no Prl surges. Seemingly, when the hypothalamus is deprived of estrogen or androgen from birth to adulthood, an equal percentage of males and females become refractory to the positive feedback effects of estrogen during adulthood. Thus, it is difficult to separate castration effects from those which may be produced by the endogenous androgen secreted during the first 26 h of life.     

Steroid control of gonadotropin secretion

Current knowledge about the mechanism and site of action of estradiol (E2) and progesterone (P) during the menstrual cycle and the physiological role of androgens is reviewed. In normal women, the positive feedback effect of E2 at the pituitary level is the principal event of the follicular phase inducing the LH surge. P, by its negative feedback at the hypothalamic level and by its positive feedback at the pituitary level regulates GnRH and LH secretion during the luteal phase. Androgens do not directly play a role in gonadotropin regulation.     

Maturation of negative and positive estrogen feedback in the prepubertal female rat

The development of estrogen feedback system on gonadotropin release during sexual maturation in female rats was studied. Animals (Wistar strain rats) were divided into 6 groups according to their ages; 10, 15, 20, 25, 30, and 35 days. Both LH and FSH levels in serum increased significantly in response to ovariectomy in all age-groups studied when measured one week postoperatively, though in the rats aged 10-15 days the increase in FSH following castration was only slight. In rats older than 25 days, the postcastration gonadotropin rise, calculated as a percent increase from the basal figure, decreased gradually with increasing age. Ovariectomized rats injected with estradiol benzoate (EB, 5 micrograms/100 g BW) showed significantly lower levels of both LH and FSH than those in castrated controls. However, the inhibitory action of EB on postcastration gonadotropin output was found to be relatively less effective in rats older than 25 days. Ovariectomized rats primed with EB were again injected with a 2nd dose of EB (5 micrograms/100 g BW) at noon 3 days after priming. The 2nd EB injection induced a significant rise in LH 6 h later in 30- and 35-day-old, though not in younger, animals. On the other hand, the FSH response to EB was markedly enhanced during days 15-25 of age. These results indicate that the estrogen negative feedback action on gonadotropin release is already operating in female rats at a very early age, and that the brain sensitivity to estrogen decreases slightly during the late prepubertal phase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influences of age and ovarian follicular reserve on estrous cycle patterns, ovulation, and hormone secretion in the Long-Evans rat

This study examined the influences of aging and reduced ovarian follicular reserve on estrous cyclicity, estradiol (E(2)) production, and gonadotropin secretion. Young virgin and middle-aged (MA) retired breeder female rats were unilaterally ovariectomized (ULO) or sham operated (control). Unilateral ovariectomy of young rats reduced the ovarian follicular reserve by one-half, to a level similar to that found in MA controls. Unilateral ovariectomy of MA females reduced the follicular pool further, to one half of MA controls. The incidence of regular cyclicity was significantly lower in MA ULO females than in young controls, with intermediate cycle frequency in young ULO and MA controls. Among cyclic rats, the magnitude of the proestrous LH surge was highest in young controls, intermediate in young ULO rats and MA controls, and lowest in MA ULO females. Similarly, ovulation rates were highest in young controls, intermediate in young ULO rats and MA controls, and lowest in MA ULO females. While young ULO rats exhibited augmented secondary FSH surges on estrous morning, middle-aged ULO females displayed secondary FSH levels comparable to young controls. The effects of age and reduced follicle number on estrous cyclicity and gonadotropin secretion were not due to altered E(2) secretion, as preovulatory E(2) levels were similar among all groups. Thus, experimental reduction in the follicular reserve exerts acute effects on the preovulatory LH surge, ovulation rate, and estrous cyclicity in both young and MA rats. However, decreased follicle number increases FSH levels only in young rats, indicating aging-related alterations in the feedback regulation of FSH.     

Disruption of periovulatory FSH and LH surges during induced anovulation by an inhibitor of prostaglandin synthesis in mares

The role of passage of follicular fluid into the peritoneal cavity during ovulation in the transient disruption in the periovulatory FSH and LH surges was studied in ovulatory mares (n=7) and in mares with blockage of ovulation by treatment with an inhibitor of prostaglandin synthesis (n=8). Mares were pretreated with hCG when the largest follicle was ≥32 mm (Hour 0). Ultrasonic scanning was done at Hours 24 and 30 and every 2h thereafter until ovulation or ultrasonic signs of anovulation. Blood samples were collected at Hours 24, 30, 32, 34, 36, 38, 48, and 60. Ovulation in the ovulatory group occurred at Hours 38 (five mares), 40, and 44. Until Hour 36, diameter of the follicle and concentrations of FSH, LH, and estradiol-17β (estradiol) were similar between groups. Between Hours 34 and 36, a novel transient increase in estradiol occurred in each group, and color-Doppler signals of blood flow in the follicular wall decreased in the ovulatory group and increased in the anovulatory group. In each group, FSH and LH periovulatory surges were disrupted by a decrease or plateau between Hours 38 and 48 and an increase between Hours 48 and 60. The discharge of hormone-laden follicular fluid into the peritoneal cavity at ovulation was not an adequate sole explanation for the temporally associated transient depression in FSH and LH. Other routes from follicle to circulation for gonadotropin inhibitors played a role, based on similar depression in the ovulatory and anovulatory groups.     

Concentrations of steroids and gonadotropins in follicular fluid from normal heifers and heifers primed for superovulation

The concentrations of six steroids and of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were measured in follicular fluid from preovulatory and large atretic follicles of normal Holstein heifers and from preovulatory follicles of heifers treated with a hormonal regimen that induces superovulation. Follicular fluid from preovulatory follicles of normal animals obtained prior to the LH surge contained extremely high concentrations of estradiol (1.1 +/- 0.06 micrograms/ml), with estrone concentrations about 20-fold less. Androstenedione was the predominant aromatizable androgen (278 +/- 44 ng/ml; testosterone = 150 +/- 39 ng/ml). Pregnenolone (40 +/- 3 ng/ml) was consistently higher than progesterone (25 +/- 3 ng/ml). In fluid obtained at 15 and 24 h after the onset of estrus, estradiol concentrations had declined 6- and 12-fold, respectively; androgen concentrations had decreased 10- to 20-fold; and progesterone concentrations were increased, whereas pregnenolone concentrations had declined. Concentrations of LH and FSH in these follicles were similar to plasma levels of these hormones before and after the gonadotropin surges. The most striking difference between mean steroid levels in large atretic follicles (greater than 1 cm in diameter) and preovulatory follicles obtained before the LH surge was that estradiol concentrations were about 150 times lower in atretic follicles. Atretic follicles also had much lower concentrations of LH and slightly lower concentrations of FSH than preovulatory follicles. Hormone concentrations in follicles obtained at 12 h after the onset of estrus from heifers primed for superovulation were similar to those observed in normal preovulatory follicles at estrus + 15 h, except that estrogen concentrations were about 6-40 times lower and there was more variability among animals for both steroid and gonadotropin concentrations. Variability in the concentrations of reproductive hormones in fluid from heifers primed for superovulation suggests that the variations in numbers of normal embryos obtained with this treatment may be due, at least in part, to abnormal follicular steroidogenesis.     

Relationship between serum gonadotropins and pituitary immunoreactive gonadotropins and steroid receptors during the first FSH increase of the estrous cycle and following steroid treatment in heifers

The objectives were to determine the effects of (i) time during the first FSH increase of the estrous cycle (time-course study) and (ii) exogenous steroid treatment (steroid feedback study) on the relationship between circulating serum gonadotropins, and the proportions of pituitary cells immunoreactive for gonadotropins and steroid receptors during the estrous cycle in heifers. Pituitaries were collected from heifers (n=40) slaughtered at 13h (n=8), 30h (n=24) and 66h (n=8) after estrous onset, corresponding to before, during and after the first FSH increase of the estrous cycle. Heifers slaughtered during the FSH increase (at 30h) either received no treatment (n=8), or were treated (n=16) with estradiol benzoate and/or progesterone before slaughter. During the time-course study, the proportion of pituitary cells immunoreactive for FSH increased (P<0.05) during the first transient FSH increase reflecting serum concentrations. The proportion of pituitary cells immunoreactive for LH was unaltered, a reflection of serum LH concentrations. The proportion of estrogen receptors (ER)-alpha, but not ER-beta, was decreased (P<0.05) at 30h compared with at either 13 or 66h. During the steroid feedback study, exogenous progesterone with or without estradiol suppressed (P<0.05) the proportions of pituitary cells immunoreactive for gonadotropins, serum FSH concentrations and LH pulse frequency. Steroid treatment did not alter the proportion of pituitary cells positive for estrogen receptors (alpha and beta). While progesterone receptors (PR) were not detected in the anterior pituitary by immunohistochemistry during the early estrous cycle or in response to steroid treatment, quantitative real-time PCR revealed that mRNA for progesterone receptors was expressed at very low levels. The expression of pituitary PR mRNA was decreased (P<0.05) at 30 and 66h compared with 13h, and was suppressed (P<0.05) following steroid treatments. Alterations in pituitary steroid receptors are implicated in the differential regulation of gonadotropin secretion during the first transient FSH rise, but not in response to exogenous steroids. The time-course study and steroid feedback responses support the hypothesis that LH pulse frequency is tightly linked to regulation of GnRH pulse frequency. Serum FSH is regulated by its own synthesis, as reflected by pituitary FSH content and perhaps by alterations in pituitary sensitivity to circulating steroids by changes in steroid receptor content.