Effects of progesterone on hypothalamic and plasma LHRH

Hirsutism, virilism, polycystic ovarian disease, and the steroid-gonadotropin-feedback system: a career retrospective

This career retrospective describes how the initial work on the mechanism of hormone action provided the tools for the study of hirsutism, virilism, and polycystic ovarian disease. After excessive ovarian and or adrenal androgen secretion in polycystic ovarian disease had been established, the question whether the disease was genetic or acquired, methods to manage hirsutism and methods for the induction of ovulation were addressed. Recognizing that steroid gonadotropin feedback was an important regulatory factor, initial studies were done on the secretion of LH and FSH in the ovulatory cycle. This was followed by the study of basic mechanisms of steroid-gonadotropin feedback system, using castration and steroid replacement and the events surrounding the natural onset of puberty. Studies in ovariectomized rats showed that progesterone was a pivotal enhancer of estrogen-induced gonadotropin release, thus accounting for the preovulatory gonadotropin surge. The effects of progesterone were manifested by depletion of the occupied estrogen receptors of the anterior pituitary, release of hypothalamic LHRH, and inhibition of enzymes that degrade LHRH. Progesterone also promoted the synthesis of FSH in the pituitary. The 3α,5α-reduced metabolite of progesterone brought about selective LH release and acted using the GABA(A) receptor system. The 5α-reduced metabolite of progesterone brought about selective FSH release; the ability of progesterone to bring about FSH release was dependent on its 5α-reduction. The GnRH neuron does not have steroid receptors; the steroid effect was shown to be mediated through the excitatory amino acid glutamate, which in turn stimulated nitric oxide. These observations led to the replacement of the long-accepted belief that ovarian steroids acted directly on the GnRH neuron by the novel concept that the steroid feedback effect was exerted at the glutamatergic neuron, which in turn regulated the GnRH neuron. The neuroprotective effects of estrogens on brain neurons are of considerable interest.

Protein kinase C cross-talk with gonadotrope progesterone receptor is involved in GnRH-induced LH secretion

In the absence of progesterone (P), the anti-P at the receptor RU486 reduces basal and GnRH-stimulated LH secretion both in vivo and in vitro, demonstrating the existence of a ligand-independent activation of progesterone receptor (LIAPR). The aim of the present study was to determine which component of the intracellular LH secretory pathway activated by GnRH is responsible for LIAPR. To do this, anterior pituitary dispersed cells from female rats in proestrus, cultured in the presence of 17beta-estradiol, were incubated with activators or inhibitors of PKC, cAMP-PKA signalling pathways or intracellular calcium (Ca2+) traffic, in the presence or absence of RU486. Results showed that RU486 reduced both GnRH- and the PKC activator PMA-induced LH secretion. In GnRH-stimulated cells incubated with the PKC inhibitor BIS-I or treated with PMA "overnight", RU486 had no effect on reduced LH secretion, nor on stimulated LH secretion elicited by the Ca2+ ionophore ionomycin. Moreover, when GnRH- or PMA-treated cells were co-incubated with 1 microM of the L-type Ca2+ channel blocker nifedipine or the intracellular Ca2+ chelator BAPTA-AM, RU486 potentiated the expected inhibition of these drugs on LH secretion. Activation (forskolin, 8-Br-cAMP) or inhibition (MDL-12,330A) of the cAMP-PKA signalling cascade affected neither the GnRH- and PMA-induced increase of LH secretion nor the reduction of LH secretion due to RU486. Taken together, the data point to the existence of a Ca2+ -independent PKC-PR cross-talk mechanism as part of the intracellular signalling of GnRH-stimulated LH secretion.

Effects of progesterone and estradiol on the reproductive axis in immature diploid and triploid rainbow trout

In fish species, many studies demonstrated the crucial role of estradiol (E2) in the development of the reproductive axis, but progesterone (P) has been described mainly as a precursor steroid and no clear role by itself has been reported. Moreover, a cooperative effect of P (or another progestin) and E2 in fish has never been reported to our knowledge. In the present work, we investigated the effects of P, alone or in combination with E2, on the reproductive-axis of immature rainbow trout (Oncorhynchus mykiss). Liver vitellogenin and estradiol receptor (rtER) mRNA levels increased after E2 treatment, but were unchanged by P treatments as a reflection of peripheral action of steroids. In contrast, at the pituitary level, LH contents increased after E2 and/or P treatments. Focusing on the brain level, we confirmed a clear up regulation of rtER expression by E2 in sterile triploid females, and we also demonstrated a similar stimulating effect of P alone but no cooperative effect together with E2. In conclusion, our data demonstrate that in immature trout, prior to the beginning of the first reproductive cycle, unlike E2, P is able to stimulate the reproductive brain-pituitary axis without affecting vitellogenin synthesis in the liver.

Luteinizing hormone secretion elicited in a ligand-independent activation of progesterone receptor manner at pituitary level in the rat: differential effect of two selective estrogen receptor modulators

In the absence of progesterone, RU486 reduced basal and luteinizing hormone-releasing hormone (LHRH)-stimulated LH secretion in pituitaries from proestrous rats, a fact which evidences a ligand-independent activation of progesterone receptors (LIAPR) at pituitary level. This was also observed in pituitaries from rats treated with tamoxifen, and absent in glands from either ovariectomized or raloxifene-treated animals. Both ovariectomy or raloxifene treatment reduced the stimulatory effect of LHRH on LH secretion, while tamoxifen induced an even higher response. Prolactin (PRL) secretion was unaffected by either RU486 or LHRH, nor it was influenced by ovariectomy or raloxifene treatment. However, treatment with tamoxifen elevated PRL in all groups. These findings indicate that LIAPR is an estrogen-dependent phenomenon at the anterior pituitary of the female rat, and that tamoxifen and raloxifene present agonist and antagonist estrogen activity, respectively, at this level.

Glucocorticoids inhibit gonadotropin-releasing hormone by acting directly at the hypothalamic level

Glucocorticoids, the end-product of the hypothalamic-pituitary-adrenal (HPA) axis, suppress gonadotropin release by acting at the level of the pituitary gland. However, experimental evidence suggests that they may also act at the hypothalamic level to suppress gonadotropin-releasing hormone (GnRH) release. The lack of a direct demonstration of this assumption, prompted us to evaluate the effects of glucocorticoids on hypothalamic GnRH release from individually-incubated hemi-hypothalami explanted from male rats. Since testosterone (T), dihydrotestosterone (DHT), and progesterone suppress GnRH release and androgens potentiate the effects of glucocorticoids on GnRH release, we studied also the interaction of these steroids with glucocorticoids on GnRH release. Corticosterone (B), the main glucocorticoid of the rodents with greater affinity for the type I glucocorticoid receptor, and dexamethasone (DEX), a synthetic type II glucocorticoid receptor agonist, were able to suppress basal GnRH release in a concentration-dependent fashion. DEX induced a more profound suppression of GnRH release. Neither T (0.1 nM) nor DHT (0.01 nM) modulated the suppressive effects of low (10 nM) or high (100 nM) concentrations of B on GnRH release. On the other hand, progesterone counteracted the suppressive effect of low concentrations of B (10 nM) on GnRH release, but had no effect on the suppression caused by a higher concentration of B (100 nM). The ability of glucocorticoids to inhibit directly GnRH release suggests that these stress-responsive hormones act also at the hypothalamic level to suppress the reproductive function. The suppressive effect of B was not modulated by androgens, but it was neutralized by progesterone, at least when B was used at low concentrations. We speculate that this steroid "protects" the GnRH-secreting neuron only during basal, but not stress-induced, HPA axis activity when the concentrations of glucocorticoids are more elevated.

Regulation of the preovulatory gonadotropin surge by endogenous steroids

Estradiol secreted by growing ovarian follicle(s) has been considered classically to be the neural trigger for the preovulatory surge of gonadotropins. The observation that the estradiol-induced gonadotropin surge in ovariectomized rats is of lesser magnitude and duration than that found in the cycling rat at proestrus has resulted in a search for other steroid regulators. Progesterone is a major regulator of the preovulatory gonadotropin surge. It can only act in the presence of an estrogen background, which is necessary for the synthesis of progesterone receptors. In the estrogen-primed ovariectomized rat, progesterone is able to initiate and enhance the gonadotropin surge to the magnitude observed on the day of proestrus and limit it to 1 day. The physiological role of progresterone in the induction of the preovulatory gonadotropin surge has been demonstrated by the attenuation of the progesterone-induced surge and the endogenous proestrus surge by progesterone receptor antagonist RU486 and the progesterone synthesis inhibitor trilostane. The promoter region of the follicle-stimulating hormone (FHS)-beta gene contains multiple progesterone response elements and progesterone brings about FSH release as well. The reduction of progesterone in the 5 alpha-position appears to be important for the regulation of progesterone secretion. Corticosteroids appear to play a significant role in the secondary FSH surge on late proestrus and early estrus.

Diverse modes of action of progesterone and its metabolites

Progesterone and its metabolites have a variety of diverse effects in the brain, uterus, smooth muscle, sperm and the oocyte. The effects include changes in electrophysiological excitability, induction of anesthesia, regulation of gonadotropin secretion, regulation of estrogen receptors, modulation of uterine contractility and induction of acrosome reaction and oocyte maturation. The latency of the effects vary from several seconds to several hours. Thus, it is not surprising that multiple mechanisms of action are involved. The classical mechanism of steroid hormone action of intracellular receptor binding has been supplemented by the possibility of the steroid acting as a transcription factor after the binding of the receptor protein to DNA. Other mechanisms include influence of the steroids on membrane fluidity and acting through other cell signalling systems, membrane receptors and GABA(A) receptors. Of particular interest are multiple mechanisms for the same types of action. For example the effect of progesterone on gonadotropin release is largely exerted via the classical intracellular receptor as well as membrane receptors, whereas 3(alpha),5(alpha)-tetrahydroprogesterone-induced LH release occurs via the GABA(A) receptor system. The inhibition of uterine contractility by progesterone is regulated by progesterone receptors while the action of 3(alpha),5(alpha)-tetrahydroprogesterone on uterine contractility is regulated by GABA(A) receptors. The regulation of the differences in the pattern of progesterone effects on estrogen receptor dynamics in the anterior pituitary and the uterus in the same animal are also of considerable interest.

A progesterone metabolite stimulates the release of gonadotropin-releasing hormone from GT1-1 hypothalamic neurons via the gamma-aminobutyric acid type A receptor

The reduced progesterone metabolite tetrahydroprogesterone (3 alpha-hydroxy-5 alpha-pregnan-20-one; 3 alpha,5 alpha-THP) is a positive modulator of the gamma-aminobutyric acid type A (GABAA) receptor. Experiments performed in vitro with hypothalamic fragments have previously shown that GABA could modulate the release of gonadotropin-releasing hormone (GnRH). Using GT1-1 immortalized GnRH neurons, we investigated the role of GABAA receptor ligands, including 3 alpha,5 alpha-THP, on the release of GnRH. We first characterized the GABAA receptors expressed by these neurons. [3H]Muscimol, but not [3H]flunitrazepam, bound with high affinity to GT1-1 cell membranes (Kd = 10.9 +/- 0.3 nM; Bmax = 979 +/- 12 fmol/mg of protein), and [3H]muscimol binding was enhanced by 3 alpha,5 alpha-THP. mRNAs encoding the alpha 1 and beta 3 subunits of the GABAA receptor were detected by the reverse transcriptase polymerase chain reaction. In agreement with binding data, the benzodiazepine-binding gamma subunit mRNA was absent. GnRH release studies showed a dose-related stimulating action of muscimol. 3 alpha,5 alpha-THP not only modulated muscimol-induced secretion but also stimulated GnRH release when administered alone. Bicuculline and picrotoxin blocked the effects of 3 alpha,5 alpha-THP and muscimol. Finally, we observed that GT1-1 neurons convert progesterone to 3 alpha,5 alpha-THP. We propose that progesterone may increase the release of GnRH by a membrane mechanism, via its reduced metabolite 3 alpha,5 alpha-THP acting at the GABAA receptor.

Role of 5 alpha-reduction in progesterone's ability to release FSH in estrogen-primed ovariectomized rats

In ovariectomized estrogen-primed rats, progesterone as well as 5 alpha-dihydroprogesterone (5 alpha-DHP) are capable of inducing the release of gonadotropins. This study examined the need of 5 alpha-reduction as a prerequisite for the action of progesterone. The 5 alpha-reductase inhibitor, N,N-diethyl-4-methyl-3-oxo-4-aza-5 alpha-androstane-17 beta-carboxamide was injected at a 1 or 2 mg dose/rat 2 h prior to an injection of 0.4 or 0.8 mg progesterone/kg body weight at 0900 h to immature ovariectomized, estrogen-primed rats and serum was analyzed for LH and FSH at 1500 h. Pituitary and hypothalamic 5 alpha-reductase activity was measured at the time of progesterone administration and at the time of the surge by incubating tissue homogenates with [3H]progesterone. Substrate, ([3H]progesterone) and product ([3H]5 alpha-DHP), were separated by reverse phase HPLC. The pituitary 5 alpha-reductase activity was not blocked at 1500 h. However, both pituitary and hypothalamic 5 alpha-reductase was blocked at the time of progesterone administration. No effect was seen by acute administration of the 5 alpha-reductase inhibitor upon either the 0.4 or 0.8 mg progesterone/kg-induced release of LH and FSH. There was, however, a specific, significant inhibition of progesterone-induced FSH but not LH release when the 5 alpha-reductase inhibition was sustained throughout the afternoon of the gonadotropin surge. These results indicate a biologically significant role for the irreversible 5 alpha-reduction of progesterone in the modulation of the release of FSH.

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.

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)

Effects of estradiol and progesterone on the sensitivity to pain and on morphine-induced antinociception in female rats

Long-term ovariectomized (OVX) rats were exposed to 2- or 14-day replacement with pellets made of cholesterol (CHOL), estradiol (E2), progesterone (P4), or a combination of E2 and P4. Following the treatment with steroids the antinociceptive effect of morphine (5 mg/kg,sc) was measured by a hot-plate method. Pellets of E2 (0.5 and 5%) caused dose- and time-dependent reductions of morphine-induced antinociception as compared with OVX rats treated with CHOL pellets. Moreover, OVX rats pretreated with E2 pellets had decreased basic sensitivity to nociceptive stimulus (hyperalgesia). Treatment for 2 and 14 days with 75% P4 pellets produced significant reduction of MOR antinociception. The low dose of P4 (10% pellet) did not change the effect of MOR on Day 2 but significantly increased the antinociceptive effect of MOR on Day 14. Replacement of OVX rats with one 0.5% E2 pellet plus one 10% P4 pellet resulted in marked inhibition of the antinociceptive effect of MOR on Day 2 as well as on Day 14. Central injection 30 min before MOR of either LHRH antagonist or the antiserum against LHRH into OVX rats pretreated for 14 days with both steroids had no effect on the degree of the antinociception. The results suggest that the effects which ovarian steroids exert on opioid systems vary according to the dose, the duration of treatment, and the type of steroid administered.

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.

Role of 17 beta-hydroxysteroid dehydrogenase in the modulation of nuclear estradiol receptor binding by progesterone in the rat anterior pituitary gland and the uterus

Progesterone has been shown to decrease occupied pituitary and uterine nuclear estradiol receptor (E2R) binding in mature and immature estrogen-primed rats. Progesterone has also been shown to stimulate pituitary but not uterine 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) in the rat. The conversion of estradiol to its less active metabolite estrone by 17 beta-HSD and activation of phosphatase are among mechanisms considered to be involved in the reduction of E2R. To determine if 17 beta-HSD stimulation was a mechanism by which progesterone induced nuclear E2R decrease, the synthetic estrogen ethinylestradiol, which is not oxidized by 17 beta-HSD, was used instead of estradiol to prime adult ovariectomized rats. When ethinylestradiol-primed rats received 0.8, 2.0 or 4.0 mg/kg body wt of progesterone 2 h before sacrifice, the total and occupied nuclear E2R accumulation in the anterior pituitary by a subsequent ethinylestradiol injection 1 h later did not show any decrease. This response was different from that observed previously in estradiol-primed animals in which progesterone showed a multiphasic decrease of occupied form of nuclear E2R. However, in the uterus of ethinylestradiol-primed rats, a partial decrease of total and occupied nuclear E2R accumulation was observed in the presence of the three doses of progesterone used. The decrease of uterine nuclear E2R with the three progesterone doses was different from the dose-dependent effect of progesterone observed in the uterus of estradiol-primed rats. Affinity constants of the interaction between [3H]estradiol and the nuclear E2R were similar among groups treated with ethinylestradiol, estradiol and progesterone. These results demonstrate the involvement of 17 beta-HSD in the reduction of anterior pituitary gland E2R by progesterone in the estradiol-treated animals. Furthermore, the mechanism of decrease of E2R by progesterone in the uterus appears to be different from the pituitary gland.

Inhibitory effect of progesterone on occupied estrogen receptors of anterior pituitary and uterus in adult rats

Abstract The acute effects of a single progesterone injection on estradiol receptor (E(2)R) binding in the anterior pituitary and uterus were examined in the mature ovariectomized rat. Adult ovariectomized female rats receiving estradiol were injected with various doses of progesterone followed 1 h later by a final injection of 2 mug of estradiol. All animals were sacrificed 1 h after estradiol and the E(2)R binding in the nuclei and cytosol was determined. In the anterior pituitary, progesterone decreased total nuclear E(2)R in a biphasic inhibitory pattern, with maximal effects at 0.8 and 4 mg/kg body wt doses of progesterone and a smaller decrease with the 2 mg/kg body wt dose. This progesterone-induced decrease of nuclear E(2)R was due to a decrease in the occupied form of nuclear E(2)R. The unoccupied nuclear E(2)R and cytosol E(2)R binding were not altered by progesterone administration. In the uterus, progesterone caused a dose-dependent decrease in total nuclear E(2)R binding. This effect also occurred in the occupied form of E(2)R. The uterine unoccupied nuclear E(2)R and cytosol E(2)R were not altered by progesterone administration. These studies emphasize differences in the tissue specificity of progesterone action on occupied nuclear E(2)R which are presumably responsible for the mediation of estradiol action, and provide a biochemical basis for our previous observations of multiphasic effect of progesterone on gonadotropin secretion.

Effect of progesterone on the activity of occupied nuclear estrogen receptor in vitro

In previous studies, we have demonstrated that progesterone administration in vivo can selectively alter estrogen receptor levels and distribution in the rat anterior pituitary. The present study represents an attempt to extend these observations to an in vitro model. Cytosolic and nuclear preparations of uterine homogenates from ovariectomized adult rats were shown to be capable of temperature-dependent estrogen-mediated receptor activation and translocation from cytosol to nuclei upon recombination. Addition of progesterone to isolated cytosol did not diminish estrogen receptor binding capacity over at least a 2 h period at 22 degrees C. Preincubation of the subcellular fractions with progesterone, followed by removal of free progesterone prior to cytosol-nuclear recombination, resulted in dramatic reduction in nuclear estrogen receptor activity. This action was equally apparent whether progesterone was introduced to the cytosolic or nuclear fraction, and was confined to the steroid-occupied subpopulation of nuclear receptors. The ability of this in vitro system to mimic the estrogen receptor-suppressive effect of progesterone provides a good model in which to analyze the biochemical basis for a direct estrogen-inhibitory effect of progesterone on estrogen action.

Positive and negative feed-back effect of progesterone on luteinizing hormone secretion in post-menopausal women

Progesterone is known to exert a biphasic feedback effect on luteinizing hormone (LH) secretion in animals and it has been demonstrated that this effect is dependent upon both duration of exposure to progesterone and the dose administered. In this paper we sought to determine whether a similar biphasic effect exists in humans. The pattern of LH secretion was assessed in six healthy oestrogen treated post-menopausal women before and after they were given progesterone (50 mg/day) for 1 and 7 days. Progesterone treatment for 1 day resulted in a significant elevation in the basal serum LH concentration and in individual LH pulse amplitude with no change in LH pulse frequency. In contrast, progesterone treatment for 7 days increased LH pulse amplitude with no change in basal serum LH concentrations and a significant reduction in LH pulse frequency. We concluded that firstly, progesterone does exert a biphasic feedback effect on LH secretion and that the nature of this effect is determined by the duration of exposure to the progesterone stimulus. Secondly, as LH pulsatility has been shown to be an accurate indicator of GnRH pulsatility, that the reduction in LH pulse frequency after a long exposure to progesterone is due to a hypothalamic effect of progesterone whereas the positive feedback effect may be the result of a pituitary or hypothalamic action.

A possible role for progesterone in the preovulatory gonadotropin surge through modulation of LHRH degrading activity

The preovulatory surge of gonadotropins is triggered by estradiol and enhanced to its full magnitude by progesterone. Progesterone may exert this effect through several mechanisms. One of the mechanisms is through the ability of progesterone to induce an increase in the hypothalamic content and release of LHRH. The purpose of this study was to determine if progesterone might not act through yet another mechanism and facilitate LHRH release of the proestrous gonadotropin surge through modulation of luteinizing hormone releasing hormone (LHRH) degrading activity. Sixty-day-old Sprague-Dawley rats were ovariectomized; 14 days later, the estradiol-progesterone milieu of proestrous was mimicked in these animals through the use of estradiol containing silastic implants and subcutaneous progesterone injections. The LHRH degrading activity of the hypothalamus, pituitary and serum were monitored subsequently at preselected time points. In the hypothalamus, estradiol alone was capable of inducing significant increase in degrading activity; progesterone alone had no effect; however, progesterone subsequent to estradiol priming suppressed the increase induced by estradiol alone. In the pituitary, neither estradiol alone nor progesterone alone nor progesterone subsequent to estradiol priming had any significant effect on degrading activity. In the serum, estradiol induced a rapid and significant increase in activity; progesterone alone suppressed activity; progesterone subsequent to estradiol priming induced a similar but more rapid suppression. Therefore, the overall tendency was for estradiol to stimulate and progesterone to suppress LHRH degrading activity in the tissues studied. The results of this study indicate that progesterone has the capacity to suppress LHRH degrading activity and may be one of the mechanisms capable of increasing the availability of LHRH to the anterior pituitary gland thereby facilitating the preovulatory gonadotropin surges.

Integration of the effects of estradiol and progesterone in the modulation of gonadotropin secretion

Estradiol secreted by the maturing follicle is the primary trigger for the surge of gonadotropins leading to ovulation. Progesterone has stimulatory or inhibitory actions on this estrogen-induced gonadotropin surge depending upon the time and dose of administration. The administration of progesterone to immature ovariectomized rats primed with a low dose of estradiol induced a well-defined LH surge and prolonged FSH release, a pattern similar to the proestrus surge of gonadotropins. A physiological role of progesterone is indicated in the normal ovulatory process because a single injection of the progesterone antagonist RU 486 on the day of proestrus in the adult cycling rat and on the day of the gonadotropin surge in the pregnant mare's serum gonadotropin stimulated immature rat resulted in an attenuated gonadotropin surge and reduced the number of ova per ovulating rat. Progesterone administration brought about a rapid LHRH release and an decrease in nuclear accumulation of estrogen receptors in the anterior pituitary but not the hypothalamus. The progesterone effect was demonstrated in vitro in the uterus and anterior pituitary and appears to be confined to occupied estradiol nuclear receptors. In in vivo experiments the progesterone effect on estradiol nuclear receptors appeared to be of approximately 2-h duration, which coincided with the time period of progesterone nuclear receptor accumulation after a single injection of progesterone. During the period of progesterone effects on nuclear estrogen receptors, the ability of estrogens to induce progesterone receptors was impaired. Based on the above results, a model is proposed for the stimulatory and inhibitory effects of progesterone on gonadotropin secretion.

Selective modulation of FSH and LH secretion by steroids

Significant divergence between the pattern of FSH and LH secretion has been observed in the ovulatory cycle, after ovariectomy and during puberty. The presence of an FSH-releasing factor, gonadal FSH inhibiting and releasing peptides and changes in the pulsatile pattern of LHRH secretion are among the postulates used to explain the divergent secretion of FSH and LH. Experiments in our laboratory have shown considerable evidence of differential regulation of FSH and LH secretion by steroids in the absence of gonadal regulatory peptides. Natural and synthetic estrogens show significant differences in the suppression of FSH and LH in the ovariectomized rat using a standard uterine response to the estrogen as the end point. In the immature ovariectomized rat treated with a low dose of estradiol that is sufficient for the synthesis of progesterone receptors to ensure progesterone sensitivity, but not large enough to induce estrogen triggered LH surges, progesterone administration resulted in a pattern of LH and FSH secretion similar to that observed on the day of proestrus in the cycling rat. Selective secretion of FSH was induced in the estrogen primed immature rat model by the administration of progesterone metabolite 5 alpha-dihydroporgesterone (5 alpha-DHP) while selective LH secretion was induced by 3 alpha, 5 alpha-tetrahydroprogesterone (3 alpha,5 alpha-THP). The selective secretion of FSH and LH induced by progesterone metabolites was confirmed in the immature female rat primed with PMSG and maintained in constant light. 5 alpha-DHP was also able to induce a greater release of FSH when administered to the adult cycling rat on proestrus. The priming of the pituitary gonadotrope in secreting a high baseline level of FSH or responding to LHRH in releasing a greater amount of FSH appeared to be an important factor in selective FSH release and such priming can be brought about by 5 alpha-DHP in the absence of gonadal regulatory peptides.