Steroid regulation of progesterone receptor expression in cultured rat gonadotropes

Male minipuberty involves the gonad-independent activation of preoptic nNOS neurons

BACKGROUND

The maturation of the hypothalamic-pituitary-gonadal (HPG) axis is crucial for the establishment of reproductive function. In female mice, neuronal nitric oxide synthase (nNOS) activity appears to be key for the first postnatal activation of the neural network promoting the release of gonadotropin-releasing hormone (GnRH), i.e. minipuberty. However, in males, the profile of minipuberty as well as the role of nNOS-expressing neurons remain unexplored.

METHODS

nNOS-deficient and wild-type mice were studied during postnatal development. The expression of androgen (AR) and estrogen receptor alpha (ERα) as well as nNOS phosphorylation were evaluated by immunohistochemistry in nNOS neurons in the median preoptic nucleus (MePO), where most GnRH neuronal cell bodies reside, and the hormonal profile of nNOS-deficient male mice was assessed using previously established radioimmunoassay and ELISA methods. Gonadectomy and pharmacological manipulation of ERα were used to elucidate the mechanism of minipubertal nNOS activation and the maturation of the HPG axis.

RESULTS

In male mice, minipubertal FSH release occurred at P23, preceding the LH surge at P30, when balanopreputial separation occurs. Progesterone and testosterone remained low during minipuberty, increasing around puberty, whereas estrogen levels were high throughout postnatal development. nNOS neurons showed a sharp increase in Ser¹⁴¹² phosphorylation of nNOS at P23, a phenomenon that occurred even in the absence of the gonads. In male mice, nNOS neurons did not appear to express AR, but abundantly expressed ERα throughout postnatal development. Selective pharmacological blockade of ERα during the infantile period blunted Ser¹⁴¹² phosphorylation of nNOS at P23.

CONCLUSIONS

Our results show that the timing of minipuberty differs in male mice when compared to females, but as in the latter, nNOS activity in the preoptic region plays a role in this process. Additionally, akin to male non-human primates, the profile of minipuberty in male mice is shaped by sex-independent mechanisms, and possibly involves extragonadal estrogen sources.

Differential responses of progesterone receptor membrane component-1 (Pgrmc1) and the classical progesterone receptor (Pgr) to 17β-estradiol and progesterone in hippocampal subregions that support synaptic remodeling and neurogenesis

Progesterone (P4) and estradiol (E2) modulate neurogenesis and synaptic remodeling in the hippocampus during the rat estrous cycle and in response to deafferenting lesions, but little is known about the steroidal regulation of hippocampal progesterone receptors associated with these processes. We examined the neuronal expression of progesterone receptor membrane component-1 (Pgrmc1) and the classical progesterone receptor (Pgr), by in situ hybridization and immunohistochemistry. Pgr, a transcription factor, has been associated with synaptic remodeling and other major actions of P4, whereas Pgrmc1 is implicated in P4-dependent proliferation of adult neuroprogenitor cells and with rapid P4 effects on membranes. Ovariectomized adult rats were given E2, P4, or E2+P4 on two schedules: a 4-d model of the rodent estrous cycle and a 30-d model of postmenopausal hormone therapy. Pgr was hormonally responsive only in CA1 pyramidal neurons, and the induction of Pgr by E2 was partly antagonized by P4 only on the 30-d schedule. In CA3 pyramidal and dentate gyrus (DG) neurons, Pgr was largely unresponsive to all hormone treatments. In contrast to Pgr, Pgrmc1 was generally induced by E2 and/or P4 throughout the hippocampus in CA1, CA3, and DG neurons. In neuroprogenitor cells of the DG (immunopositive for bromodeoxyuridine and doublecortin), both Pgrmc1 and Pgr were detected. The differential regulation of hippocampal Pgrmc1 and Pgr by E2 and P4 may guide drug development in hormonal therapy for support of neurogenesis and synaptic regeneration.

Expression of estrogen receptors in the hypothalamo-pituitary-ovarian axis in middle-aged rats after re-instatement of estrus cyclicity

During reproductive aging female rats enter an anovulatory state of persistent estrus (PE). In an animal model of re-instatement of estrus cyclicity in middle-aged PE rats we injected the animals with progesterone (0.5 mg progesterone/kg body weight) at 12:00 for 4 days whereas control animals received corn oil injections. After the last injection animals were analyzed at 13:00 and 17:00. Young regular cycling rats served as positive controls and were assessed at 13:00 and 17:00 on proestrus. Progesterone treatment of middle-aged PE rats led to occurrence of luteinizing hormone (LH), follicle stimulating hormone (FSH), and prolactin surges in a subset of animals that were denoted as responders. Responding middle-aged rats displayed a reduction of ER-β mRNA in the preoptic area which was similar to the effect in young rats. Within the mediobasal hypothalamus, only young rats showed a decline of ER-α mRNA expression. A decrease of ER-α mRNA levels in the pituitary was observed in progesterone-responsive rats and in young animals. ER-β mRNA expression was reduced in young regular cycling rats. ER-β mRNA levels in the ovary were reduced following progesterone treatment in PE rats and in young rats. Taken together our data show that cyclic administration of progesterone reinstates ovulatory cycles in intact aging females which have already lost their ability to display spontaneous cyclicity. This treatment leads to the occurrence of preovulatory LH, FSH and prolactin surges which are accompanied by differential modulation of ERs in the hypothalamus, the pituitary and the ovary.

Anterior pituitary gene expression with reproductive aging in the female rat

Although reproductive aging in women is classically attributed to loss of ovarian follicles, recent data have suggested that the entire hypothalamic-pituitary-ovarian axis undergoes functional changes with time. The aim of this study was to characterize age-related changes in pituitary gene expression for factors with known importance for gonadotroph function, including 1) steroid hormone receptors (Esr and Pgr), 2) orphan nuclear receptors [Nr5a1 (steroidogenic factor-1) and Nr5a2 (liver receptor homologue-1)], and 3) pituitary-derived polypeptides (activin, inhibin, and follistatin), as well as 4) gonadotropin subunits and 5) GnRH receptors. We chose to utilize a middle-aged rat model for these studies. Young (Y; 3-mo-old) and middle-aged (MA; 9- to 12-mo-old) rats were ovariectomized, primed with estradiol, and injected with progesterone to induce an LH surge. The mRNA levels for the gonadotropin subunits and GnRH receptors were decreased in middle-aged females relative to young animals. Nr5a1 and follistatin mRNA levels were significantly greater in Y versus MA animals following ovariectomy. Furthermore, steroid-induced regulation of these genes was lost in the MA animals. Regulation of the Nr5a2, Inhba, and Inhbb transcripts was also limited to the young animals. In contrast, there were no significant differences in the mRNA levels of Esr or Pgr family members between age groups at any time point. Although this in vivo model normalizes ovarian steroid levels, it does not control for potential differences in GnRH stimulation with aging. Therefore, in a second set of experiments, we used an in vitro perifusion system to compare the effects of pulsatile GnRH in the two age groups. Nr5a1 mRNA expression was greater in Y than MA animals and was significantly decreased by GnRH pulses in both age groups. Follistatin mRNA levels increased significantly with GnRH treatment in Y animals but were not significantly changed in the MA females. Taken together, these data demonstrate gene-specific blunting of pituitary gene expression postovariectomy and during the steroid-induced surge in middle-aged rats. We propose that age-related changes in pituitary physiology may contribute to reproductive senescence.

Androgens, progestins, and glucocorticoids induce follicle-stimulating hormone beta-subunit gene expression at the level of the gonadotrope

FSH is produced by the pituitary gonadotrope to regulate gametogenesis. Steroid hormones, including androgens, progestins, and glucocorticoids, have all been shown to stimulate expression of the FSHbeta subunit in primary pituitary cells and rodent models. Understanding the molecular mechanisms of steroid induction of FSHbeta has been difficult due to the heterogeneity of the anterior pituitary. Immortalized LbetaT2 cells are a model of a mature gonadotrope cell and express the endogenous steroid receptor for each of the three hormones. Transient transfection of each receptor, along with ligand treatment, stimulates the mouse FSHbeta promoter, but induction is severely diminished using receptors that lack the ability to bind DNA, indicating that induction is likely through direct DNA binding. All three steroid hormones act within the first 500 bp of the FSHbeta promoter where six putative hormone response elements exist. The -381 site is critical for FSHbeta induction by all three steroid hormones, whereas the -197 and -139 sites contribute to maximal induction. Interestingly, the -273 and -230 sites are also necessary for androgen and progestin induction of FSHbeta, but not for glucocorticoid induction. Additionally, we find that all three receptors bind the endogenous FSHbeta promoter, in vivo, and specifically bind the -381 site in vitro, suggesting that the binding of the receptors to this element is critical for the induction of FSHbeta by these 3-keto steroid hormones. Our data indicate that androgens, glucocorticoids, and progestins act via their receptors to directly activate FSHbeta gene expression in the pituitary gonadotrope.

No Evidence for Pituitary Priming to Gonadotropin-Releasing Hormone in Relation to Luteinizing Hormone (LH) Secretion Prior to the Preovulatory LH Surge in Ewes1

The purpose of this study was to determine the occurrence of and the regulatory mechanisms involved in priming of the pituitary to GnRH before the preovulatory LH surge in sheep. Experiment 1: Forty-two ewes had progestagen devices removed after 14 days and were assigned to luteal (Lut) or follicular (Foll) groups. Fifteen days later, blood sampling was initiated either immediately or 36 h after induced luteolysis in groups Lut and Foll, respectively. After 4 h, ewes were administered either saline (n = 5) or 250 ng (n = 8) or 10 microg (n = 8) of GnRH. Five ewes per treatment group were killed 1 h later, while remaining animals were blood sampled for a further 7 h. Experiment 2: Eighteen ewes were allocated to Lut and Foll groups (described above). Blood samples were collected from 2 h before GnRH (10 microg) treatment until 7 h after. Despite up-regulated GnRH-R mRNA levels in Foll ewes, pituitary content and plasma levels of LH and LHbeta mRNA levels were similar between groups. Mean FSHbeta mRNA and plasma FSH levels were elevated in Lut ewes but declined after GnRH treatment. Inversely, plasma estradiol and inhibin-A concentrations were higher in Foll ewes and declined after GnRH treatment. Fewer LH(+ve)/secretogranin II(-ve) (SgII(-ve)) granules were present in gonadotropes of Foll ewes, coincident with increased basal LH levels. Fewer smaller sized granules were present after GnRH treatment. In conclusion, there was no evidence of self-priming before onset of the preovulatory LH surge. Constitutive release of LH(+ve)/SgII(-ve) granules may maintain basal LH levels while smaller sized, presumably mature granules may be preferentially released after GnRH stimulation.

Estrogen Induces Neuropeptide Y (NPY) Y1 receptor gene expression and responsiveness to NPY in gonadotrope-enriched pituitary cell cultures

We showed previously that neuropeptide Y1 receptor (Y1R) expression is increased in the hypothalamus on proestrus afternoon and that this up-regulation of Y1R mRNA may permit neuropeptide Y (NPY) to facilitate release of the preovulatory GnRH surge. Because NPY also modulates LH release directly, we examined steroid regulation of Y1R expression in the female rat anterior pituitary. Treatment of female rats with estrogen in vivo decreased the levels of Y1R mRNA in the whole pituitary gland. In lactotrope/somatotrope-enriched pituitary cells separated by unit gravity sedimentation, 17beta-estradiol (E(2)) treatment likewise suppressed Y1R expression. In contrast, E(2) elevated Y1R mRNA in gonadotrope-enriched cell populations, indicating that estrogen regulates Y1R mRNA expression differently in gonadotropes vs. other pituitary cell types. After exposure to E(2), NPY augmented GnRH-induced LH release from gonadotrope-enriched cells in a manner requiring Y1R activation. Without steroid exposure, this augmentation disappeared, and with progesterone alone, NPY reduced GnRH-induced LH release. In addition, NPY inhibited prolactin secretion from primary pituitary cells in a steroid-free environment, but not in the presence of estrogen. These findings demonstrate that E(2) can directly up-regulate gonadotrope responsiveness to NPY and suggest that this action is mediated at least in part by E(2)'s ability to stimulate Y1R gene expression in gonadotropes. Our observations are consistent with the idea that this regulatory mechanism represents a component of E(2)'s positive feedback actions in pituitary gonadotropes. The biological importance of E(2)'s opposite effects on Y1R expression in other pituitary cell types remains to be determined.

The role of estrogen-dependent progesterone receptor in protein kinase C-mediated LH secretion and GnRH self-priming in rat anterior pituitary glands

The aim of the present study was to explore the involvement of pituitary progesterone receptor (PR) in PKC-mediated LH secretion and LHRH self-priming and the role of the estrogen (E) environment. Eight randomly selected hemipituitaries from adult female rats in proestrus or from 2 weeks ovariectomized (OVX) rats were incubated, in the absence of progesterone (P), over 3 h in Dulbecco's modified Eagle's medium (DMEM). In the first experiment, hemipituitaries were incubated continuously with: medium alone, GnRH (10 nM), the PKC stimulator PMA (100 nM), the PKC inhibitor staurosporine (100 nM), the antiprogestin at the receptor RU486 (10 nM), LHRH+staurosporine, GnRH+RU486 or PMA+RU486. In the second experiment, hemipituitaries were incubated, one h apart, with GnRH to determine the GnRH self-priming and this was compared with the priming effect of PMA. Also, the effect of staurosporine and RU486 during the induction period (1st h) on GnRH and PMA priming was evaluated. Medium was aspirated at the end of each h to determine LH accumulation and to evaluate GnRH self-priming. Both GnRH and PMA stimulated LH secretion. Staurosporine and RU486 reduced basal and GnRH-stimulated LH secretion, and RU486 reduced PMA-stimulated LH secretion from proestrus pituitaries. The stimulating effect of GnRH and PMA on LH secretion and the inhibitory action of staurosporine and RU486 on basal or stimulated LH secretion were significantly reduced in OVX-rats. Both GnRH and PMA induced GnRH priming. Staurosporine during the induction h reduced GnRH self-priming while RU486 reduced both GnRH self-potentiation and PMA priming. The magnitude of these inhibitory effects was blunted in OVX-rats. These results showed that PKC signaling pathway in the gonadotrope mediates, at least in part, basal and GnRH-stimulated LH secretion and GnRH self-priming. Also, the results are suggestive of an interaction of PKC signaling pathway with E-dependent PR in a ligand-independent activation manner in the gonadotrope.

Effects of progesterone (P) and antiprogestin RU486 on LH and FSH release by incubated pituitaries from rats treated with the SERM LY11701 8-HCl and/or recombinant human FSH

The aim of the present study was to investigate the role of the estrogen (ES) background on the effects of P or its antagonist RU486 on basal and LHRH-stimulated LH and FSH secretion. To do this, pituitaries collected from: intact rats in proestrus; rats injected with the ES antagonist LY11701 8-HCl; rats injected with recombinant-human FSH (r-hFSH) to stimulate ovarian hormonogenesis; and rats injected with both LY11701 8-HCl and r-hFSH were incubated with or without LHRH (10 nM) in the presence of P (100 nM) or RU486 (10 nM). RU486 decreased basal and LHRH-stimulated release of LH and FSH and LHRH self-priming in pituitaries from control rats, while P increased both pituitary responsiveness and LHRH self-priming. These effects were absent in pituitaries from rats treated either with the ES antagonist or r-hFSH, which, in the absence of P or RU486 in the incubation medium, reduced gonadotropin release. Because r-hFSH did not increase E2 serum concentration significantly, the putative FSH-dependent ovarian non-steroidal gonadotropin surge inhibiting factor (GnSIF) might be the hormonal cause of the reduced secretion of LH and FSH. Combined treatment with LY117018-HCl and r-hFSH had additive inhibitory effects on gonadotropin release. These results indicate that ES-inducible P receptor (PR) in the pituitary can be activated in a ligand-independent manner by intracellular messengers giving rise to enhanced basal and LHRH-stimulated gonadotropin secretion. The results also suggested that the r-hFSH-stimulated ovarian bioactive entity GnSIF and RU486 may share a similar mechanism of action involving pituitary PR.

Diverse actions of ovarian steroids in the serotonin neural system

All of the serotonin-producing neurons of the mammalian brain are located in 10 nuclei in the mid- and hindbrain regions. The cells of the rostal nuclei project to almost every area of the forebrain and regulate diverse neural processes from higher order functions in the prefrontal cortex such as integrative cognition and memory, to limbic system control of arousal and mood, to diencephalic functions such as pituitary hormone secretion, satiety, and sexual behavior. The more caudal serotonin neurons project to the spinal cord and interact with numerous autonomic and sensory systems. All of these neural functions are sensitive to the presence or absence of the ovarian hormones, estrogen and progesterone. We have shown that serotonin neurons in nonhuman primates contain estrogen receptor beta and progestin receptors. Thus, they are targets for ovarian steroids which in turn modify gene expression. Any change in serotoninergic neural function could be manifested by a change in any of the projection target systems and in this manner, serotonin neurons integrate steroid hormone information and partially transduce their action in the CNS. This article reviews the work conducted in this laboratory on the actions of estrogens and progestins in the serotonin neural system of nonhuman primates. Comparisons to results obtained in other laboratory animal models are made when available and limited clinical data are referenced. The ability of estrogens and progestins to alter the function of the serotonin neural system at various levels provides a cellular mechanism whereby ovarian hormones can impact cognition, mood or arousal, hormone secretion, pain, and other neural circuits.

Luteinizing hormone secretion from wild-type and progesterone receptor knockout mouse anterior pituitary cells

The progesterone receptor (PR) has a central role in the hypothalamo-pituitary events culminating in the preovulatory LH surge, and mice with genetically ablated PR provide a model for dissecting cellular pathways subserving this role. The aims of this study were to determine 1) whether the GnRH self-priming response and acute progesterone augmentation of secretagogue-stimulated LH secretion are present in cultured wild-type (WT) mouse pituitary cells, and 2) whether the PR is essential for self-priming by comparing the responses in PR knockout (PRKO) cells. Pituitary cells from ovariectomized WT or PRKO mice cultured +/- 17beta-estradiol (E(2)) for 3 days were challenged with hourly pulses of 1 nM GnRH or 54 mM K(+). A background of E(2) had no effect on the initial LH secretory response for either WT or PRKO cells. However, for subsequent GnRH pulses, E(2) was permissive for the GnRH self-priming response in WT cells. PRKO cells exhibited a blunted GnRH self-priming response. Exposure to progesterone for 90 min before secretagogue stimulation resulted in a modest (1.5-fold) augmentation of the LH response to GnRH but not K(+) pulses in WT cells; progesterone had no effect in PRKO cells. Unlike in the rat, the PR antagonists RU486 or ZK98299 failed to prevent potentiation of LH secretory responses to multiple GnRH pulses in WT cells. Although RU486 blocked progesterone augmentation of the initial GnRH pulse, it was ineffective in blocking progesterone's action after multiple GnRH pulses. In WT cells, 8- bromo-cAMP (8-Br-cAMP) was able to substitute for the GnRH priming pulse; 8-Br-cAMP also augmented GnRH-stimulated secretion in PRKO cells but less effectively. 8-Br-cAMP augmented K(+)-stimulated LH secretion in WT and PRKO cells equally. These results suggest that, although mouse gonadotropes show GnRH self-priming, they have adapted strategies different than rat cells for amplifying the GnRH signal as shown by the residual self-priming in PRKO cells, the modest or absent augmentation by acute progesterone of GnRH- or K(+)-stimulated secretion in WT cells, and the reduced ability of PR antagonists to interfere with GnRH self-priming and progesterone augmentation. We speculate that the adaptations could involve, at least in part, differences in the ratio of PR isoforms.

Progesterone receptors as neuroendocrine integrators

Intracellular progesterone receptors (PRs) are ligand-inducible transcription factors that mediate the majority of the effects of progesterone (P) on neuroendocrine functions. During the past decade, evidence has accumulated which suggest that PRs can also be activated independently of P, by signals propagated through membrane-bound receptors to the interior of cells. The activation of PRs by this type of "cross-talk" mechanism has been implicated in the physiological regulation of several important neuroendocrine processes, including estrous behavior and periovulatory hormone secretions. We review evidence that both ligand-dependent and ligand-independent activation of PRs occurs in central neurons and in anterior pituitary cells and that the convergence and summation of these signals at the PR serves to integrate neural and endocrine signals which direct several critically important neuroendocrine processes. An integrative function for PRs is reviewed in several physiological contexts, including the display of lordosis behavior in female rodents, the neurosecretion of gonadotropin-releasing hormone surges, secretion of preovulatory gonadotropin surges, and release of periovulatory follicle stimulating hormone surges. The weight of evidence indicates that cross talk at the intracellular PR is an essential component of the integrative mechanisms that direct each of these neuroendocrine events. The recurrence of PR's integrative actions in several different physiological contexts suggests that other intracellular steroid receptors similarly function as integrators of neural and endocrine signals in other neuroendocrine processes.

The in vitro inhibitory action of antiprogestin RU486 on LH and FSH secretion in the absence of progesterone in rats is estrogen-dependent

Previous in vivo findings show that in the virtual absence of progesterone (P), the antiprogestin RU486 reduces LH and FSH secretion in proestrous rats, indicating that activation of P receptor (PR) can occur in the absence of the cognate ligand. The present study investigates, in vitro, whether or not the inhibitory effect of antiprogestin RU486 on gonadotropin secretion in the absence of P is estrous cycle dependent, and whether its specific expression in proestrus mirrors the high estrogen (E2) background. In the first experiment we investigated the effect of RU486 (10 nM) and/or LHRH (10 nM) on LH and FSH secretion in incubated pituitaries collected on each day of the estrous cycle of the rat. In the second experiment, we determined the effect of RU486 and/or LHRH on preovulatory LH and FSH release by pituitaries from female rats that were ovariectomized (OVX), treated with the antiestrogen LY117018-HCL (Eli Lilly & Co.), or injected with 20 micrograms of estradiol benzoate (EB). The third experiment investigated the effect of RU486 and/or LHRH on LH and FSH release by pituitaries collected from intact or EB-treated (0.1 mg/kg over three consecutive days) male rats. RU486 reduced both basal and LHRH-stimulated LH and FSH secretion in proestrous pituitaries from normal 4-day cyclic rats. By contrast, in diestrous pituitaries, RU486 increased both parameters of LH secretion but was without effect on FSH release. RU486 was also without effect in pituitaries collected from rats in estrus or metestrus, or from OVX or antiestrogen-treated rats. Moreover, EB injection or treatment induced the full inhibitory effect of RU486 in pituitaries from female and male rats, respectively. The above results suggested that P occupancy of the receptor is not required for the formation or function of the active receptor and hence for preovulatory LH and FSH secretion, and that this form of PR activation at pituitary level is E2-dependent and not genetically determined.

Progesterone regulation of the progesterone receptor in rat gonadotropes

For rat pituitary cells, progesterone receptor (PR) protein localizes to gonadotropes and PR messenger RNA is induced by E2 and rapidly but transiently down-regulated by progesterone. Here we quantitatively establish the down-regulatory effect of progesterone on PR protein and evaluate possible mechanisms. Nuclear PR-immunoreactivity (PR-IR) in gonadotropes, identified by dual immunofluorescence, was analyzed by quantitative confocal microscopy. Pituitary cells from female rats were cultured +/- 0.2 nM E2 for 3 days. We confirmed the E2 requirement for PR induction in gonadotropes and determined that the increase in PR-IR required about 24 h. After removal of E2, PR-IR decreases were not found until 24-36 h. Addition of progesterone (40 nM) to E2-treated cells led to a dramatic loss in PR-IR by 9 h (26% of control); by 24 h, PR-IR was barely detectable. Reappearance of nuclear PR-IR required progesterone removal (8-fold increase by 12 h after progesterone removal) and protein synthesis (cycloheximide inhibited the reappearance of PR-IR). Although progesterone decreased PR-IR whether or not E2 was present concurrent with progesterone, the recovery of PR-IR required E2. RU486 completely blocked progesterone-induced PR down-regulation. Because the sustained progesterone-induced loss of PR protein did not correlate with previously reported temporal changes in PR messenger RNA levels, we examined a role for protein degradation. When cells were coincubated with progesterone and the proteasome inhibitor, MG132 (1 microM), the expected decrease in PR protein was abrogated. In summary, progesterone leads to a rapid and extensive reduction in nuclear PR protein in gonadotropes. The progesterone-dependent down-regulation of PR occurs, at least in part, by a proteasome-mediated pathway. Recovery of PR protein requires removal of progesterone, the presence of E2, and protein synthesis. These dynamic changes in nuclear PR levels coincide with the temporal extent of the preovulatory LH surge in rats and could provide a basis for progesterone's biphasic action on LH secretion.

Stimulation of gonadotropin-releasing hormone surges by estrogen. I. Role of hypothalamic progesterone receptors

Estrogen (E2) stimulates GnRH surges by coupling a daily neural signal to neuronal circuitries governing GnRH release. We have hypothesized that E2 promotes this coupling process by inducing expression of neuronal transcription factors, which are subsequently activated by neurotransmitter-mediated mechanisms representing the daily neural signal. These experiments tested the specific hypothesis that the progesterone receptor (PR) functions in this manner, viz. as an E2-induced factor whose activation is necessary for the stimulation of GnRH surges. Two complimentary experiments were performed to determine whether activation of hypothalamic PRs is obligatory for the stimulation of GnRH surges by E2. In the first, the effects of a PR antagonist on GnRH and LH surges were assessed in ovariectomized (OVX), E2-primed rats. Rats were OVX on diestrous day 2, treated with 30 microg estradiol benzoate or oil vehicle, sc, and then administered either oil vehicle or the type I antiprogestin, ZK98299 at 0900 h on proestrus. GnRH release rates and plasma LH levels were determined in each animal by microdialysis of median eminence and atrial blood sampling, respectively. Estrogen, but not oil vehicle, treatment evoked robust and contemporaneous GnRH and LH surges in animals that received no PR antagonist on proestrus. Additional treatment with ZK98299, however, completely blocked both GnRH and LH surges. In a second experiment, specific involvement of anteroventral periventricular (AVPV) PRs in E2-induced GnRH surges was assessed. Additional groups of OVX, E2-primed rats were fitted with intracerebroventricular cannulas, and PR antisense oligonucleotides were infused into the third ventricle adjacent to the AVPV to prevent expression of PR in this periventricular region. Animals infused with PR antisense oligos did not exhibit any LH surges, whereas surges were observed in saline-, missense-, and sense oligo-treated controls. Immunohistochemistry confirmed the effectiveness of PR antisense oligonucleotides in blocking PR expression. These findings provide direct support for the hypothesis that activation of PRs, specifically those in hypothalamic regions including the AVPV, is an obligatory event in the stimulation of GnRH surges by E2.

Progesterone receptor A and B messenger ribonucleic acid levels in the anterior pituitary of rats are regulated by estrogen

In target tissues of most mammalian and avian species, progesterone receptors (PR) are expressed as structurally related, but functionally distinct, isoforms A and B, and they are regulated by estrogen (E) as well as by their cognate ligand, progesterone (P(4)). The objectives of the present work were to identify mRNA expression for the A and B isoforms of PR in the anterior pituitary of the rat, to examine its regulation by gonadal steroids, and to compare this regulation with that in the primary target organ, the uterus. Messenger RNAs for the PR isoforms, determined by two separate reverse transcription-polymerase chain reaction protocols, one that detects PR A and PR B equally and the other specific for PR B, were identified in anterior pituitary of female and male rats. In anterior pituitary of cycling female rats, steady-state mRNA levels for both PR A+B and PR B were highest at 0900 h on proestrus, declined rapidly to nadir values at 0900 h on metestrus (PR A+B) or 0900 h on estrus (PR B), and remained below proestrous values through 2100 h on diestrus. Administration of E to intact proestrous female rats caused significant increases in mRNA for both PR A+B and PR B on estrus and metestrus. Blockade of P(4) action by administration of the antiprogestins RU-486 and ZK-98299 on proestrus had no effect on PR mRNA levels on the morning of estrus. Ovariectomy two and ten days after surgery markedly reduced mRNA levels for both PR A+B and PR B. Whereas treatment of 10-day-ovariectomized rats with E led to marked induction of mRNA for PR A+B and PR B two days later, treatment with P(4) one day after treatment had no effect on basal or E-stimulated PR mRNA. Regulation of PR mRNA expression in the pituitary differed from that in the uterus, in which P(4) treatment of ovariectomized rats antagonized the E-induced rise in mRNA for PR B, and antiprogestins increased mRNA for both isoforms. In addition to induction of PR mRNA in the pituitary of female rats by E in vivo, we also demonstrated induction by E in primary culture of anterior pituitary cells in vitro. We conclude that in the anterior pituitary of female rats, both the A and B isoforms of PR are expressed and regulated by E.