Validation of the mechanisms proposed for the stimulatory and inhibitory effects of progesterone on gonadotropin secretion in the estrogen-primed rat: a possible role for adrenal steroids.

The stimulatory and inhibitory effects of progesterone on luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion were found to be dependent on the length of estrogen exposure in ovariectomized estrogen-primed rats. Progesterone suppressed LH and FSH secretion when administered 16 hours after a single injection of estradiol to ovariectomized rats. If the estradiol treatment was extended over 40 hours by two injections of estradiol 24 hours apart, progesterone administration led to a highly significant elevation of both serum LH and FSH levels 6 hours later. In addition to the direct stimulatory effect on LH and FSH release, progesterone, when injected 1 hour before, was able to antagonize the suppressive effect of a third injection of estradiol on LH and FSH release. In the immature ovariectomized estrogen-primed rat, 10 IU of ACTH brought about a release of progesterone and corticosterone 15 minutes later and LH and FSH 6 hours later. Progesterone, but not corticosterone, appeared to be responsible for the effect of ACTH on gonadotropin release. The synthetic corticosteroid triamcinolone acetonide brought about LH and FSH release in the afternoon, while cortisol, similar to corticosterone, was unable to do so. Nevertheless, triamcinolone acetonide and cortisol brought about increased secretion of FSH the following morning.     

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.     

Stimulatory and inhibitory effects of progesterone on FSH secretion by the anterior pituitary.

The purpose of this study was to investigate whether progesterone exerted progesterone receptor mediated direct effects on the anterior pituitary in the secretion of FSH and whether such effects were mediated through the 5 alpha-reduction of progesterone. Treatment of anterior pituitary dispersed cells for 48 h with 0.5 nM estradiol reduced the ED50 for gonadotropin releasing hormone (GnRH)-stimulated FSH release from 0.58 to 0.36 ng/ml and the ED50 for GnRH-induced LH release from 0.54 to 0.19 ng/ml. When dispersed pituitary cells were treated with 0.5 nM estradiol and exposed to various doses of progesterone for 1 to 6 h, the most consistent rise in basal and GnRH-stimulated FSH release was observed with the 50 nM dose of progesterone with a 3-h exposure period. All three doses of progesterone elevated basal LH and GnRH-stimulated LH was increased by the 50 and 100 nM doses of progesterone during the 3-h period of treatment. Using the 50 nM dose of progesterone, basal and GnRH-stimulated LH was increased after 2, 3 and 6 h of progesterone treatment. When the period of exposure of progesterone was extended to 12, 36 or 48 h, there was a significant inhibition of GnRH-stimulated FSH release. GnRH-stimulated LH release was inhibited at 36 and 48 but not 12 h after progesterone treatment. These studies showed that the effect of progesterone administered for periods of 1 to 6 h enhanced the secretion of LH and FSH whereas progesterone administered for periods beyond 12 h inhibited FSH and LH release by dispersed pituitary cells in culture. These results are similar to those observed in vivo after progesterone treatment. Furthermore estrogen priming of the dispersed pituitary cells was necessary to observe the effects of progesterone. The progesterone antagonist RU486 prevented the progesterone-induced rise in GnRH-stimulated FSH release. Furthermore the 5 alpha-reductase inhibitor N,N-diethyl-4-methyl-3-oxo-4-aza-5 alpha-androstane- 17 beta-carboxamide also prevented the progesterone-induced rise in GnRH-stimulated FSH release in estrogen-treated dispersed pituitary cells. These results indicate that the anterior pituitary is a major site of action of progesterone in the release of FSH and that 5 alpha-reduction of progesterone plays an important role in FSH release.     

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.     

The role of estrogen in the feedback regulation of follicle-stimulating hormone secretion in the female rat

Letrozole (CGS 20267) is a non-steroidal aromatase inhibitor which, at its maximally effective dose of 1 mg/kg p.o., elicits endocrine effects equivalent to those seen after ovariectomy. Adult, female cyclic rats were administered letrozole (1 mg/kg p.o.) once daily for 14 days. A control group of animals was ovariectomized on day 1 of treatment and a third group of animals served as untreated controls. During the experiment, vaginal smears were taken daily and at the end of 14 days all animals were sacrificed, trunk blood was taken for serum estradiol, LH and FSH measurements and the uterus and ovaries were removed and weighed. The ovaries were then fixed and prepared for histological examination. Serum hormone measurements showed that after treatment with letrozole, serum estradiol levels were reduced by 76% of untreated controls and serum LH was elevated to 378% of control values. These compared favorably with those seen after ovariectomy, serum estradiol was reduced by 78% and serum LH was elevated to 485% of untreated controls. However, FSH was unchanged after letrozole treatment (125% of control), whereas after ovariectomy FSH rose to 398% of control. Uterine weight was suppressed in the letrozole-treated animals as well as the ovariectomized animals by 60 and 70%, respectively. The histology of the ovaries of animals treated with letrozole were consistent with the serum hormone findings. Except for the effects on serum FSH, these results confirm previous findings that treatment with letrozole elicits endocrine effects similar to those seen after ovariectomy. Furthermore, these results demonstrate that FSH secretion is not under the control of estradiol whereas LH secretion is under feedback control of ovarian estrogen.     

Gonadotropin secretion during prolonged hyperprolactinemia: basal secretion and the stimulatory feedback effect of estrogen

Inoculation of cyclic female rats with the prolactin (Prl)/growth hormone-secreting pituitary tumor, MtT.W15, resulted in a cessation of estrous cyclicity within 5--10 days. Associated with this acyclicity was a persistently low serum concentration of estradiol and marked increases in both circulating Prl and progesterone. At Day 26 of acyclicity, basal serum luteinizing hormone (LH) values measured in samples taken every 20 min from 0900--1100 h were significantly reduced when compared to cyclic, nontumor animals on diestrus Day 2. There was no difference in basal follicle-stimulating hormone (FSH) concentrations. In a separate group of acyclic, tumor-bearing females 42--56 days after transplantation, a single s.c. injection of 20 micrograms estradiol benzoate (EB) at 1030 h elicited significant increases in both serum LH and FSH values between 1700 and 1830 h on the next day. The magnitude of the LH surge was reduced and that of FSH was increased in tumor-bearing animals when compared to cyclic, nontumor females given a similar EB injection on diestrus Day 1. These results demonstrate that chronic hyperprolactinemia is associated with inhibition of basal LH secretion and ovarian estrogen production and an increase in circulating progesterone concentrations. Nevertheless, the stimulatory feedback effects of estrogen on LH and FSH release are still present and functioning in acyclic female rats under chronically hyperprolactinemic conditions. These data suggest that the cessation of regular ovulatory cycles associated with hyperprolactinemia may be due to a deficiency of LH and/or estrogen secretion, but not to a lack of central nervous system response to the stimulatory feedback action of estrogen.     

Gonadotropic and ovarian hormone response in dairy cows treated with norgestomet and estradiol valerate

Injection of estradiol valerate in conjunction with receiving a norgestomet ear implant (Syncro-Mate-B) reduced plasma progesterone (P<.001) and FSH (P<.02) and increased estradiol (P<.01) in postpartum cows during the treatment period. Occurrence of LH and FSH peaks (P<.05) varied between 12 and 48 hr follwing removal of SMB implants and estrous behavior was minimal. FSH was strongly correlated to LH and was inversely related to progesterone in untreated cows and to estradiol in treated cattle. Syncro-Mate-B suppressed luteal activity and peak gonadotropin release during treatment and elicited variable endocrine and behavioral response following treatment.     

Estrogen and leptin have differential effects on FSH and LH release in female rats

Prior experiments have shown that the adipocyte hormone leptin can advance puberty in mice. We hypothesized that it would also stimulate gonadotrophin secretion in adults. Since the secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) is drastically affected by estrogen, we hypothesized that leptin might have different actions dependent on the dose of estrogen. Consequently in these experiments, we tested the effect of injection of leptin into the third cerebral ventricle of ovariectomized animals injected with either the oil diluent, 10 microg or 50 microg of estradiol benzoate 72 hr prior to the experiment. The animals were ovariectomized 3-4 weeks prior to implantation of a cannula into the third ventricle 1 week before the experiments. The day after implantation of an external jugular catheter, blood samples (0. 3 ml) were collected just before and every 10 min for 2 hr after 3V injection of 5 microl of diluent or 10 microg of leptin. Both doses of estradiol benzoate equally decreased plasma LH concentrations and pulse amplitude, but there was a graded decrease in pulse frequency. In contrast, only the 50-microg dose of estradiol benzoate significantly decreased mean plasma FSH concentrations without significantly changing other parameters of FSH release. The number of LH pulses alone and pulses of both hormones together decreased as the dose of estrogen was increased, whereas the number of pulses of FSH alone significantly increased with the higher dose of estradiol benzoate, demonstrating differential control of LH and FSH secretion by estrogen, consistent with alterations in release of luteinizing hormone releasing hormone (LHRH) and the putative FSH-releasing factor (FSHRF), respectively. The effects of intraventricularly injected leptin were drastically altered by increasing doses of estradiol benzoate. There was no significant effect of intraventricular injection of leptin (10 microg) on the various parameters of either FSH or LH secretion in ovariectomized, oil-injected rats, whereas in those injected with 10 microg of estradiol benzoate there was an increase in the first hr in mean plasma concentration, area under the curve, pulse amplitude, and maximum increase of LH above the starting value (Deltamax) on comparison with the results in the diluent-injected animals in which there was no alteration of these parameters during the 2 hr following injection. The pattern of FSH release was opposite to that of LH and had a different time-course. In the diluent-injected animals, probably because of the stress of injection and frequent blood sampling, there was an initial significant decline in plasma FSH at 20 min after injection, followed by a progressive increase with a significant elevation above the control values at 110 and 120 min. In the leptin-injected animals, mean plasma FSH was nearly constant during the entire experiment, coupled with a significant decrease below values in diluent-injected rats, beginning at 30 min after injection and progressing to a maximal difference at 120 min. Area under the curve, pulse amplitude, and Deltamax of FSH was also decreased in the second hour compared to values in diluent-injected rats. In contrast to the stimulatory effects of intraventricular injection of leptin on pulsatile LH release manifest during the first hour after injection, there was a diametrically opposite, delayed significant decrease in pulsatile FSH release. This differential effect of leptin on FSH and LH release was consistent with differential effects of leptin on LHRH and FSHRF release. Finally, the higher dose of E2 (50 microg) suppressed release of both FSH and LH, but there was little effect of leptin under these conditions, the only effect being a slight (P < 0.04) increase in pulse amplitude of LH in this group of rats. The results indicate that the central effects of leptin on gonadotropin release are strongly dependent on plasma estradiol levels. These effects are consistent w     

Steroid feedback inhibition of pulsatile secretion of gonadotropin-releasing hormone in the ewe

The long-term negative feedback effects of sustained elevations in circulating estradiol and progesterone on the pulsatile secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) were evaluated in the ewe following ovariectomy during the mid-late anestrous and early breeding seasons. GnRH secretion was monitored in serial samples of hypophyseal portal blood. Steroids were administered from the time of ovariectomy by s.c. Silastic implants, which maintained plasma concentrations of estradiol and progesterone at levels resembling those that circulate during the mid-luteal phase of the estrous cycle; control ewes did not receive steroidal replacement. Analysis of hormonal pulse patterns in serial samples during 6-h periods on Days 8-10 after ovariectomy disclosed discrete, concurrent pulses of GnRH in hypothalamo-hypophyseal portal blood and LH in peripheral blood of untreated ovariectomized ewes. These pulses occurred every 97 min on the average. Treatment with either estradiol or progesterone greatly diminished or abolished detectable pulsatile secretion of GnRH and LH, infrequent pulses being evident in only 3 of 19 steroid-treated ewes. No major seasonal difference was observed in GnRH or LH pulse patterns in any group of ewes. Our findings in the ovariectomized ewe provide direct support for the conclusion that the negative-feedback effects of estradiol and progesterone on gonadotropin secretion in the ewe include an action on the brain and a consequent inhibition of pulsatile GnRH secretion.     

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)     

Changes in plasma progesterone, estradiol, follicle-stimulating hormone and luteinizing hormone during diestrus and ovulation in rats with 5-day estrous cycles: effect of antibody against progesterone

Progesterone secretion remained significantly higher during diestrus in the 5-day cyclic rat than in the 4-day cyclic animal. Injection of a sufficient amount of antiprogesterone serum (APS) at 2300 h on metestrus in a 5-day cycle advances ovulation and completion of the cycle by 1 day in the majority of animals (75 and 80%, respectively). Progesterone (250 micrograms) administered with APS eliminated the effect of the antiserum. Within 2 h after administration of APS, levels of both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) elevated significantly, while a significant elevation of plasma estradiol above the control value followed as late as 36 h after the treatment. None of the 5-day cyclic rats treated with APS showed ovulatory increases of FSH and LH at 1700 h on the second day of diestrus, although 3 of the 4 animals receiving the same treatment ovulated by 1100 h on the following day. The onset of ovulatory release of gonadotropins might have been delayed for several hours in these animals. These results indicate that recurrence of the 5-day cycle is due to an elevated progesterone secretion on the morning of diestrus, and suggest that a prolongation of luteal progesterone secretion in an estrous cycle suppresses gonadotropin secretion. Rather than directly blocking the estrogen triggering of ovulatory LH surge, the prolonged secretion of luteal progesterone may delay the estrogen secretion itself, which decreases the threshold of the neural and/or hypophyseal structures for ovulatory LH release.     

The stimulatory roles of catecholamines and acetylcholine in the regulation of gonadotropin release in ovariectomized estrogen-primed rats.

Injections of 2 mg of progesterone into ovariectomized estrogen-primed rats significantly increased serum LH and FSH concentrations 3, 5 and 8 hr later. Receptor blockers of noradrenaline (NA), dopamine (DA) or acetylcholine (ACH), phenoxybenzamine (20 mg/kg body weight), pimozide (1mg/kg body weight) or atropine (700 mg/kg body weight), respectively, prevented the progesterone-induced gonadotropin release. On the other hand, none of them blocked the gonadotropin release following unilateral electrochemical stimulation (100 microA for 60 sec) of the medial preoptic area which occurred 0.5 and 1.5 hr later, although pimozide or atropine reduced serum LH concentrations at 4.0 hr after stimulation. Furthermore, the sites of action of NA, DA and ACH with respect to LH release were examined by intracerebral implantation in ovariectomized estrogen-primed rats DA or ACH, when implanted unilaterally into the medial preoptic urea, induced a significant increase in serum LH 5 hr later, whereas NA decreased LH levels. Implantations of NA or ACH into the bed nucleus of the stria terminalis or the medial amygdala increased serum LH although the effect of NA into the latter was not statistically significant. Only implantations of NA among the three substances into the lateral septum induced LH release. These results suggest that all of NA, DA and ACH play stimulatory roles in the regulation of gonadotropin secretion, and that there are regional differences of their effectivenesses in releasing LH within the limbic-preoptic area.     

Inhibitory effects of the antiprogestin, RU 486, on progesterone actions and luteinizing hormone secretion in pituitary gonadotrophs

The effects of RU 486 on the modulation of LH release by progesterone were investigated in cultured anterior pituitary cells from ovariectomized adult female rats. The inhibitory effect of progesterone on LH secretion was demonstrable in estrogen-treated pituitary cells, in which addition of 10(-6) M progesterone to cells cultured in the presence of 10(-9) M estradiol for 52 h reduced the LH response to GnRH (10(-11) to 10(-7) M). When RU 486 was superimposed upon such combined treatment with estradiol and progesterone, the suppressive effect of progesterone on GnRH-induced LH release was completely abolished. The converse (facilitatory) effect of progesterone on LH secretion was observed in pituitary cells pretreated with 10(-9) M estradiol for 48 h and then with 10(-6) M progesterone for 4 h. When RU 486 was added together with progesterone during the 4 h treatment period, the facilitatory effect of progesterone was blocked and LH release fell to below the corresponding control value. The direct effect of RU 486 on LH secretion in the absence of exogenous progesterone was evaluated in cells cultured in the absence or presence of 10(-9) M estradiol and then treated for 4 to 24 h with increasing concentrations of RU 486 (10(-12) to 10(-5) M) and stimulated with GnRH (10(-9) M) during the last 3 h of incubation. In estrogen-deficient cultures, 4 h exposure to RU 486 concentrations of 10(-6) M and above decreased the LH response to GnRH by up to 50%. In cultures pretreated with 10(-9) M estradiol, GnRH-stimulated LH responses was inhibited by much lower RU 486 concentrations, of 10(-9) M and above. After 24 h of incubation the effects of RU 486 were similar in control and estradiol-pretreated pituitary cell cultures. Thus, RU 486 alone has a significant inhibitory effect on LH secretion that is enhanced in the presence of estrogen. The antiprogestin is also a potent antagonist of both the inhibitory and the facilitatory actions of progesterone upon pituitary gonadotropin release in vitro.     

Effects of acute ovariectomy on anterior pituitary gland follicle-stimulating hormone and luteinizing hormone secretion in the metestrous rat

Changes at the anterior pituitary gland level which result in follicle-stimulating hormone (FSH) release after ovariectomy in metestrous rats were investigated. Experimental rats were ovariectomized at 0900 h of metestrus and decapitated at 1000, 1100, 1300, 1500, 1700 or 1900 h of metestrus. Controls consisted of untreated rats killed at 0900 or 1700 h and rats sham ovariectomized at 0900 h and killed at 1700 h. Trunk blood was collected and the serum assayed for FSH and luteinizing hormone (LH) concentrations. The anterior pituitary gland was bisected. One-half was used to assay for FSH concentration. The other half was placed in culture medium for a 30-min preincubation and then placed in fresh medium for a 2-h incubation (basal FSH and LH release rates). The basal FSH release rate and the serum FSH concentration rose significantly by 4 h postovariectomy and remained high for an additional 6 h. The basal FSH release rate and the serum FSH concentration correlated positively (r=0.71 with 72 degrees of freedom) and did not change between 0900 and 1700 h in untreated or sham-ovariectomized rats. In contrast, the serum LH concentration and the basal LH release rate did not increase after ovariectomy. Ovariectomy had no significant effect on anterior pituitary gland FSH concentration. The results suggest that the postovariectomy rise in serum FSH concentration is the result, at least in part, of changes which cause an increase in the basal FSH secretion rate (secretion independent of the immediate presence of any hormones of nonanterior pituitary gland origin). The similarities between the selective rises in the basal FSH release rate and the serum FSH concentration in the ovariectomized metestrous rat and in the cyclic rat during late proestrus and estrus raise the possibility that an increase in the basal FSH release rate may be involved in many or all situations in which serum FSH concentration rises independently of LH.     

Progesterone directly inhibits pituitary luteinizing hormone secretion in an estradiol-dependent manner.

We recently demonstrated that progesterone and estradiol inhibit pituitary LH secretion in a synergistic fashion. This study examines the direct feedback of progesterone on the estradiol-primed pituitary. Nine ovariectomized (OVX) ewes underwent hypothalamic-pituitary disconnection (HPD) and were infused with 400 ng GnRH every 2 h throughout the experiment. After 7 days of infusion, estradiol was implanted s.c. Four days later, estradiol implants were exchanged for blank implants in 4 ewes and for progesterone implants in 5 ewes. These implants remained in place for another 4 days. Blood samples were collected around exogenous GnRH pulses before and 0.5 to 96 h after implant insertion and exchange. Serum LH and progesterone concentrations were determined through RIA. One month later, 4 of the HPD-OVX ewes previously implanted with steroids were reinfused with GnRH and the implantation protocol was repeated using blank implants only. In estradiol-primed ewes, progesterone significantly lowered LH secretion after 12 h of implantation and LH secretion remained inhibited while progesterone implants were in place (p less than 0.05). Removing estradiol transiently lowered LH secretion, and this effect was significant only 24 h after estradiol withdrawal (p less than 0.05). These data suggest that progesterone has a direct, estradiol-dependent inhibitory effect on pituitary LH release and that estradiol may sustain pituitary gonadotrope response to GnRH.     

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.     

Plasma gonadotropins and prolactin in pseudopregnancy in the rat

Radioimmunoassay presented a method of measuring plasma levels of FSH,LH and prolactin in pseudopregnant rats. Plasma prolactin levels doubled 15 minutes following cervical probing (p .01) on the day of estrus. Plasma LH levels were not significantly elevated. Due to the use of ether anesthesia at blood collecting 3 hr before and 15 minutes after stimulation, only 1 of 16 rats developed pseudopregnancy. On Day 4 of pseudopregnancy in rats mated with vasectomized males; plasma LH was lower (p .05) than in normal rats on the first day of diestrus, perhaps due to the suppressive action of ovarian progesterone and some estrogen. FSH was higher than in normal rats (p .05) perhaps due to the lesser sensitivity of FSH to the inhibitory effect of progesterone. Large decidoumata developed by Day 9 in uterine horns traumatized on Day 4 (153 plus or minus 8 mg uterus weight compared to 1510 plus or minus 204 mg). Thus, the corpora remain functional after LH and prolactin are at normal and subnormal levels. On Day 9 plasma prolactin was lower than at Day 1 of diestrus (p .001). Plasma FSH was elevated (p .01). Plasma LH was unchanged. The degree of rise of LH levels 5 days following ovariectomy on Day 4 of psuedopregnancy or on the first day of diestrus was greater in the former group (p .02), perhaps due to rebound of LH from suppression by ovarian steroids. FSH rose equally in both groups. Prolactin remain about the same. Increased prolactin release by the adenohypophysis was briefer than expected.     

Blood LH after PGF2alpha in diestrous and ovariectomized cattle.

Two experiments were conducted to determine whether the increased serum LH which occurs within 12 hr after a luteolytic dose of PGF2alpha is dependent upon changes in progesterone or estradiol secretion. In the first experiment, exogenous progesterone abolished the increase in serum LH caused by a subcutaneous injection of 25 mg PGF2alpha in diestrous heifers, but not in ovariectomized heifers. In the second experiment, progesterone pessaries were removed at 6 hr after a subcutaneous injection of 25 mg PGF2alpha. LH remained at pre-PGF2alpha values while the pessaries were in place, but began to increase within 1 hr after they were removed. Blood estradiol also remained at pre-PGF2alpha values until the pessaries were removed, and began to increase at 2 hr after pessary removal. We conclude that the increase in serum LH within 12 hr after PGF2alpha treatment in diestrous cattle is dependent upon withdrawal of progesterone; it is not due to increased serum estradiol.     

The effect of photoperiod on serum concentrations of luteinizing and follicle stimulating hormones in prepubertal heifers following ovariectomy and estradiol injection

Eleven heifers, between 63 and 197 days of age, were exposed to 18 hr light/day (L) or natural photoperiods (N), beginning October 19, 1979. They were ovariectomized 8 weeks later. LH concentrations after ovariectomy were not affected by photoperiod, but the rate of increase of FSH after ovariectomy was greater (P<0.10) for group L than for group N. Three weeks after ovariectomy, heiters were injected, IV, with 0.1 mug/kg estradiol-17beta. LH concentrations initially decreased after injection. This was followed by a series of pulses larger than those prior to injection. FSH concentrations declined after injection and remained low throughout the sampling period. The net response of LH concentrations to estradiol (mean post-injection concentration minus mean pre-injection concentration) was greater (P=0.05) for group L (4.7 +/- 0.49 ng/ml) than for group N (2.9 +/- 0.37 ng/ml). Photoperiod did not affect the net response of FSH concentrations to estradiol. We concluded that exposing prepubertal heifers to 18 hr light/day during the winter resulted in a greater rate of increase of FSH after ovariectomy and greater estrogen-induced LH release. Because the response of LH to estradiol-17beta differed from the response of FSH, these hormones may be regulated differently.