Regulation of gonadal androgen secretion

The various mechanisms regulating testicular and ovarian androgen secretion are reviewed. Testicular androgen secretion is controlled by luteinizing hormone (LH) and follicle stimulating hormone (FSH), which influence the Leydig cell response to the LH. The contribution of prolactin, growth hormone and thyroid hormones to the Leydig cell function is discussed. The ovarian androgen secretion is regulated in a very similar fashion as the Leydig cell of testis. Prolactin, however, has an inhibitory effect on androgen secretion in the ovary. The intratesticular action of androgens is linked to spermatogenesis. Sertoli cells, by producing the androgen-binding protein, contribute to the intratubular androgen concentration. Inhibin production of the Sertoli cell is stimulated by androgens. In the ovary, androgens produced by the theca interna are used as precursors for the aromatization of estradiol, which stimulates together with FSH the mitosis of granulosa cells. The feedback control of androgen secretion is complicated, as the direct feedback mechanisms are joined by indirect feedback regulations like the peptide inhibin, which can be stimulated by androgens. Intragonadal mechanisms regulating androgen production are the cybernins for testicles and ovaries. In the testicle, estrogens from the Sertoli cells regulate the Leydig cell testosterone biosynthesis. In the ovary, nonaromatizable androgens are potent inhibitors of the aromatization activity in the granulosa cell. A peptide with a FSH receptor binding inhibiting activity is found in male and female gonads. Finally, LH-RH-like peptides have been found in the testicle, which are capable of inhibiting steroidogenesis. These gonadocrinins are similarly produced in granulosa cells of the ovary.     

Multifarious effects of estrogen on the pituitary gonadotrope with special emphasis on studies in the ovine species

The gonadotrope is a complex cell that expresses receptors for gonadotropin releasing hormone (GnRH) and estrogen. It has synthetic machinery for the production of 3 gonadotropin subunits which are assembled into two gonadotropins, luteinising hormone (LH) and follicle stimulating hormone (FSH). The production and secretion of LH and FSH are differentially regulated by GnRH and estrogen. Patterns of secretion of LH are dictated by the pulsatile release of GnRH from the median eminence as well as the feedback effects of estrogen. The means by which estrogen plays such an important role in the regulation of LH and FSH is reviewed in this chapter, with emphasis on work that has been done in the sheep. Estrogen regulates the second messenger systems in the gonadotrope as well as the number of GnRH receptors and the function of ion channels in the plasma membrane. Estrogen also regulates gene expression in these cells. Additionally, GnRH appears to regulate the level of estrogen receptor in the ovine gonadotrope, so there is substantial cross-talk between the signalling pathways for GnRH and estrogen. No clear picture has emerged as to how estrogen exerts a positive feedback effect on the gonadotrope and it is suggested that this might be forthcoming from more definitive studies on the way that estrogen regulates the second messenger systems and the trafficking of secretory vesicles.     

Evidence that gonadotropin-releasing hormone (GnRH) II stimulates luteinizing hormone and follicle-stimulating hormone secretion from monkey pituitary cultures by activating the GnRH I receptor

Mammalian gonadotropin-releasing hormone (GnRH) I is the neuropeptide that regulates reproduction. In recent years, a second isoform of GnRH, GnRH II, and its highly selective type II GnRH receptor were cloned and identified in monkey brain, but its physiological function remains unknown. We sought to determine whether GnRH II stimulates LH and FSH secretion by activating specific receptors in primary pituitary cultures from male monkeys. Dispersed pituitary cells were maintained in steroid-depleted media and stimulated with GnRH I and/or GnRH II for 6 h. Cells were also treated with Antide (Bachem, King of Prussia, PA), a GnRH I antagonist, to block gonadotropin secretion. In monkey as well as rat pituitary cultures, GnRH II was a less effective stimulator of LH and FSH secretion than was GnRH I. In both cell preparations, Antide completely blocked LH and FSH release provoked by GnRH II as well as GnRH I. Furthermore, the combination of GnRH I and GnRH II was no more effective than either agonist alone. These results indicate that GnRH II stimulates FSH and LH secretion, but they also imply that this action occurs through the GnRH I receptor. The GnRH II receptors may have a unique function in the monkey brain and pituitary other than regulation of gonadotropin secretion.     

The role of amino acid neurotransmitters in the regulation of pituitary gonadotropin release in fish.

Both glutamate and gamma-aminobutyric acid (GABA) are involved in pituitary hormone release in fish. Glutamate serves 2 purposes, both as a neurotransmitter and as a precursor for GABA synthesis. Glutamate can be catabolized to GABA by the actions of 2 distinct but related enzymes, glutamate decarboxylase 65 (GAD65) and GAD67. They derive from 2 different genes that likely arose from an early gene duplication prior to the emergence of teleosts more than 400 million years ago. There is good evidence for the involvement of GABA in luteinizing hormone (LH) release in fish. The mechanism of GABA action to stimulate LH release appears to be a combination of effects on GnRH release, potentiation of gonadotropin hormone-releasing hormone (GnRH) action, and in some cases directly at the LH cell. These actions appear to be dependent on such factors as sex or sex steroid levels, and there may also be species differences. Nevertheless, the stimulatory effects of GABA on LH are present in at least 4 fish species. In contrast, convincing data for the inhibitory effects of GABA on LH release have only been observed in 1 fish species. The sites and mechanisms of action of amino acid neurotransmitters on LH release have yet to be fully characterized. Both 130N-methyl-D-aspartic acid (NMDA) and S-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors are likely to have important roles. We suggest that it is a receptor similar to the GABA(A) type which mediates the effects of GABA on LH release in fish, at least partially acting on the GnRH neuron, but likely directly acting at the gonadotroph as well. GABA may also be involved in regulating the release of other pituitary hormones in fish, namely follicle stimulating hormone (FSH = GTH-I), prolactin, and growth hormone. Based on the findings described in this review, a working model for the involvement of glutamate and GABA in the regulation of LH release in teleost fish is proposed.     

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.     

Regulation of testicular function in men: implications for male hormonal contraceptive development

In the adult male, the testes produce both sperm and testosterone. The function of the testicles is directed by the central nervous system and pituitary gland. Precise regulation of testicular function is conferred by an elegant feedback loop in which the secretion of pituitary gonadotropins is stimulated by gonadotropin hormone-releasing hormone (GnRH) from the hypothalamus and modulated by testicular hormones. Testosterone and its metabolites estradiol and dihydrotestosterone (DHT) as well as inhibin B inhibit the secretion of the gonadotropins both directly at the pituitary and centrally at the level of the hypothalamus. In the testes, LH stimulates testosterone synthesis and FSH promotes spermatogenesis, but the exact details of gonadotropin action are incompletely understood. A primary goal of research into understanding the hormonal regulation of testicular function is the development of reversible, safe and effective male hormonal contraceptives. The administration of exogenous testosterone suppresses pituitary gonadotropins and hence spermatogenesis in most, but not all, men. The addition of a second agent such as a progestin or a GnRH antagonist yields more complete gonadotropin suppression; such combination regimens effectively suppress spermatogenesis in almost all men and may soon bring the promise of hormonal male contraception to fruition.     

G proteins and autocrine signaling differentially regulate gonadotropin subunit expression in pituitary gonadotrope

Gonadotropin-releasing hormone (GnRH) acts at gonadotropes to direct the synthesis of the gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH). The frequency of GnRH pulses determines the pattern of gonadotropin synthesis. Several hypotheses for how the gonadotrope decodes GnRH frequency to regulate gonadotropin subunit genes differentially have been proposed. However, key regulators and underlying mechanisms remain uncertain. We investigated the role of individual G proteins by perturbations using siRNA or bacterial toxins. In LβT2 gonadotrope cells, FSHβ gene induction depended predominantly on Gα(q/11), whereas LHβ expression depended on Gα(s). Specifically reducing Gα(s) signaling also disinhibited FSHβ expression, suggesting the presence of a Gα(s)-dependent signal that suppressed FSH biosynthesis. The presence of secreted factors influencing FSHβ expression levels was tested by studying the effects of conditioned media from Gα(s) knockdown and cholera toxin-treated cells on FSHβ expression. These studies and related Transwell culture experiments implicate Gα(s)-dependent secreted factors in regulating both FSHβ and LHβ gene expression. siRNA studies identify inhibinα as a Gα(s)-dependent GnRH-induced autocrine regulatory factor that contributes to feedback suppression of FSHβ expression. These results uncover differential regulation of the gonadotropin genes by Gα(q/11) and by Gα(s) and implicate autocrine and gonadotrope-gonadotrope paracrine regulatory loops in the differential induction of gonadotropin genes.     

Testosterone inhibits 11-ketotestosterone-induced spermatogenesis in African catfish (Clarias gariepinus).

Male fish produce 11-ketotestosterone as a potent androgen in addition to testosterone. Previous experiments with juvenile African catfish (Clarias gariepinus) showed that 11-ketotestosterone, but not testosterone, stimulated spermatogenesis, whereas testosterone, but not 11-ketotestosterone, accelerated pituitary gonadotroph development. Here, we investigated the effects of combined treatment with these two types of androgens on pituitary gonadotroph and testis development. Immature fish were implanted for 2 wk with silastic pellets containing 11-ketotestosterone, testosterone, 5alpha-dihydrotestosterone, or estradiol-17beta; cotreatment groups received 11-ketotestosterone in combination with one of the other steroids. Testicular weight and pituitary LH content were higher (two- and fivefold, respectively) in the end control than in the start control group, reflecting the beginning of normal pubertal development. Treatment with testosterone or estradiol-17beta further increased the pituitary LH content four- to sixfold above the end control levels. This stimulatory effect on the pituitary LH content was not modulated by cotreatment with 11-ketotestosterone. However, the stimulatory effect of 11-ketotestosterone on testis growth and spermatogenesis was abolished by cotreatment with testosterone, but not by cotreatment with estradiol-17beta or 5alpha-dihydrotestosterone. Also, normal pubertal testis development was inhibited by prolonged (4 wk) treatment with testosterone. The inhibitory effect of testosterone may involve feedback effects on pituitary FSH and/or on FSH receptors in the testis. It appears that the balanced production of two types of androgens, and the control of their biological activities, are critical to the regulation of pubertal development in male African catfish.     

Developmental programming: impact of prenatal testosterone excess on pre- and postnatal gonadotropin regulation in sheep

The goal of this study was to explore mechanisms that mediate hypersecretion of LH and progressive loss of cyclicity in female sheep exposed during fetal life to excess testosterone. Our working hypothesis was that prenatal testosterone excess, by its androgenic action, amplifies GnRH-induced LH (but not FSH) secretion and, thus, hypersecretion of LH in adulthood, and that this results from altered developmental gene expression of GnRH and estradiol (E2) receptors, gonadotropin subunits, and paracrine factors that differentially regulate LH and FSH synthesis. We observed that, relative to controls, females exposed during fetal life to excess testosterone, as well as the nor-aromatizable androgen dihydrotestosterone, exhibited enhanced LH but not FSH responses to intermittent delivery of GnRH boluses under conditions in which endogenous LH (GnRH) pulses were suppressed. Luteinizing hormone hypersecretion was more evident in adults than in prepubertal females, and it was associated with development of acyclicity. Measurement of pituitary mRNA concentrations revealed that prenatal testosterone excess induced developmental changes in gene expression of pituitary GnRH and E2 receptors and paracrine modulators of LH and FSH synthesis in a manner consistent with subsequent amplification of LH release. Together, this series of studies suggests that prenatal testosterone excess, by its androgenic action, amplifies GnRH-induced LH response, leading to LH hypersecretion and acyclicity in adulthood, and that this programming involves developmental changes in expression of pituitary genes involved in LH and FSH release.     

Regulation of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) gene expression by gonadotropin-releasing hormone (GnRH) and sexual steroids in the Mediterranean Sea bass

The secretion of gonadotropins, the key reproductive hormones in vertebrates, is controlled from the brain by the gonadotropin-releasing hormone (GnRH), but also by complex steroid feedback mechanisms. In this study, after the recent cloning of the three gonadotropin subunits of sea bass (Dicentrarchus labrax), we aimed at investigating the effects of GnRH and sexual steroids on pituitary gonadotropin mRNA levels, in this valuable aquaculture fish species. Implantation of sea bass, in the period of sexual resting, for 12 days with estradiol (E2), testosterone (T) or the non-aromatizable androgen dihydrotestosterone (DHT), almost suppressed basal expression of FSHbeta (four to 15-fold inhibition from control levels), while slightly increasing that of alpha (1.5-fold) and LHbeta (approx. twofold) subunits. Further injection with a GnRH analogue (15 microg/kg BW; [D-Ala6, Pro9-Net]-mGnRH), had no effect on FSHbeta mRNA levels, but stimulated (twofold) pituitary alpha and LHbeta mRNA levels in sham- and T-implanted fish, and slightly in E2- and DHT-implanted fish (approx. 1.5-fold). The GnRHa injection, as expected, elevated plasma LH levels with a parallel decrease on LH pituitary content, with no differences between implanted fish. In conclusion, high circulating steroid levels seems to exert different action on gonadotropin secretion, inhibiting FSH while stimulating LH synthesis. In these experimental conditions, the GnRHa stimulate LH synthesis and release, but have no effect on FSH synthesis.     

Studies of the effectiveness of gonadotrophin-releasing hormone, steroids and follicular fluid in modulating ovine gonadotrophin output in vivo and in vitro

Gonadotrophin-releasing hormone (GnRH) readily stimulated LH output by sheep pituitary cells in vitro, and raised plasma LH concentrations in vivo in sheep, in a dose-dependent fashion. However, increases in FSH levels were only marginal by comparison. Dose-dependent decreases in sheep pituitary cell FSH output and in plasma FSH concentrations were caused by sheep follicular fluid and oestradiol-17 beta in vitro, and by bovine follicular fluid and oestradiol benzoate in vivo. In contrast, LH concentrations were only reduced slightly at the higher doses of these reagents. Cumulative suppressive effects of follicular fluid and oestradiol-17 beta (oestradiol benzoate) on FSH levels were observed both in vitro and in vivo. The transient positive feedback effect of oestradiol benzoate on FSH output negated the suppressive effect of bovine follicular fluid on plasma FSH concentrations. Progestagens, androgens and catechol oestrogens also suppressed mean FSH output in vitro, though not as effectively as oestradiol-17 beta. While only 1-5 pg/ml of oestradiol-17 beta was needed to suppress significantly mean FSH output in vitro, greater than 500 pg/ml of the other steroids was required. Seminal plasma inhibin-like peptide failed to suppress mean FSH output by cultured sheep pituitary cells at doses from 1 pg/ml to 500 ng/ml. At higher doses, both FSH and LH output was suppressed and this was accompanied by morphological deterioration of the cells. It is suggested that, to raise plasma FSH concentrations with a view to increasing ovulation rates in sheep, the development of means to reduce the negative feedback effects of steroids, notably oestradiol-17 beta, and inhibin on FSH secretion may be a more appropriate pharmacological strategy than increasing pituitary exposure to GnRH.     

Regulation of testicular function in the stallion: an intricate network of endocrine, paracrine and autocrine systems

It is well established in many mammalian species, including the horse that normal testicular function is dependent upon a functional hypothalamic-pituitary-testicular (HPT) axis, which involves classic feedback mechanisms. The major HPT hormones involved in the stallion are gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), follicle stimulating hormone (FSH), testosterone (T), estrogens (Es) and inhibin (INH). Although prolactin (PRL) fluctuates with season in the stallion and both PRL and thyroid hormone (TH) affect reproduction in other male species, their effects on stallion reproduction have not been elucidated. Growth hormone (GH) in the stallion may be involved in sperm motility, production and secretion of insulin-like growth factor-1 (IGF-1) and LH-induced testosterone release. The action of these hormones and the products involved for normal spermatogenesis require cell to cell communication within the testis. The somatic cell types, Leydig, Sertoli and peritubular myoid cells, all support germ cell development, maturation and release into the seminiferous tubule lumen. The cell to cell crosstalk involves an intricate network of paracrine-autocrine systems that support the endocrine input to modulate cell function. In other male species, researchers have demonstrated the reproductive effects of such paracrine-autocrine factors as IGF-1, transferrin, androgens, estrogens, inhibin, insulin like peptide 3 (INSL3), beta-endorphin and oxytocin. The specific nature and relative contribution of these various factors on testicular function in fertile and subfertile stallions are under investigation. This review summarizes current information regarding the nature of the multiple endocrine-paracrine-autocrine systems that may be necessary for normal testicular function in the stallion.     

Negative feedback regulation of the secretion and actions of gonadotropin-releasing hormone in males

This minireview considers the state of knowledge regarding the interactions of testicular hormones to regulate the secretion and actions of GnRH in males, with special focus on research conducted in rams and male rhesus monkeys. In these two species, LH secretion is under the negative feedback regulation of testicular steroids that act predominantly within the central nervous system to suppress GnRH secretion. The extent to which these actions of testicular steroids result from the direct actions of testosterone or its primary metabolites, estradiol or dihydrotestosterone, is unclear. Because GnRH neurons do not contain steroid receptors, the testicular steroids must influence GnRH neurons via afferent neurons, which are largely undefined. The feedback regulation of FSH is controlled by inhibin acting directly at the pituitary gland. In male rhesus monkeys, the feedback regulation of FSH secretion is accounted for totally by the physiologically relevant form of inhibin, which appears to be inhibin B. In rams, the feedback regulation of FSH secretion involves the actions of inhibin and testosterone and interactions between these hormones, but the physiologically relevant form of inhibin has not been determined. The mechanisms of action for inhibin are not known.     

Neonatal androgenization of hypogonadal (hpg) male mice does not abolish estradiol-induced FSH production and spermatogenesis

Background  

Testicular development is arrested in the hypogonadal (hpg) mouse due to a congenital deficiency in hypothalamic gonadotropin-releasing hormone (GnRH) synthesis. Chronic treatment of male hpg mice with estradiol induces FSH synthesis and secretion, and causes testicular maturation and qualitatively normal spermatogenesis. As estradiol negative feedback normally inhibits FSH production in the male, this study tested whether this paradoxical response to estradiol in the male hpg mouse might be due to inadequate masculinisation or incomplete defeminization in the neonatal period. Previous studies have demonstrated that treatment of hpg mice with testosterone propionate in the immediate neonatal period is necessary to allow full reproductive behaviors to be expressed following suitable endocrine stimulation at adult ages.     

A study on co-localization of FSH and its receptor in rat hippocampus

It has been known that GnRH, LH and their receptors exist in hippocampal neurons. However, whether FSH and its receptor also exist in hippocampal neurons remained unknown yet. In situ hybridization, double-labeled immunofluorescence stain and double-labeled immunohistochemistry stain in adjacent sections were used in our research to study the distribution, co-localization of FSH and its receptor and co-localization of FSH and GnRH receptor in rat hippocampus. The result found that pyramidal neurons from CA1 to CA4 region and granule neurons in dentate gyrus could express FSH and its receptor, majority of hippocampal neurons co-expressed FSH and its receptor, FSH and GnRH receptor. These suggested that hippocampal neurons not only express FSH but also act as FSH target cells. FSH may regulate the function of hippocampal neurons by ways of paracrine or autocrine. At the same time, GnRH may regulate the function of FSH neuron in hippocampus through GnRH receptor.     

Physiological role of putative testicular gonadotrophin releasing hormone (GnRH)

Evidence suggests that exogenous GnRH and agonist analogues have short-term stimulatory effects on rat Leydig cell function - when administered intratesticularly. Since rat Leydig cells possess GnRH receptors and their endogenous ligand has not yet been identified the physiological importance of the observations for testis function is unknown. To address this issue we have determined the consequences of blockade of testis GnRH receptors on Leydig cell function under both normogonadotrophic and hypogonadotrophic stimulation of the testis in vivo. A GnRH antagonist (ANT) was used to achieve receptor blockade but during continuous systemic infusion ANT occupied pituitary GnRH receptors and markedly reduced serum LH, FSH, testosterone, and intratesticular testosterone in adult and 30 d old immature male rats. These results were similar to those obtained by administration of a GnRH antiserum which did not bind to testis GnRH receptors. Thus, blockade of testis GnRH receptors during hypogonadotrophism did not produce additional inhibition of steroidogenesis by Leydig cells. However, direct continuous infusion of ANT into one testis produced greater than 90% occupancy of GnRH receptors while reducing GnRH receptors by only 50% in the contralateral testis. Unilateral intratesticular infusion did not reduce serum LH, FSH, Prolactin or testosterone levels despite 75% occupancy of pituitary GnRH receptors. Thus, both ANT infused and saline infused testes were exposed to the same gonadotrophic stimulants but in the former GnRH-R were essentially non-existent. Compared to the control testis, the ANT infused testis showed a 20-30% reduction in LH, FSH, lactogen receptors and 30-40% fall in testosterone content. Identical results were obtained in adult and 30 d-old male rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Progesterone does not affect the amount of mRNA for gonadotropins in the anterior pituitary gland of ovariectomized ewes

To evaluate the effect of progesterone on the synthesis and secretion of gonadotropins, ovariectomized ewes either were treated with progesterone (n = 5) for 3 wk or served as controls (n = 5) during the anestrous season. After treatment for 3 wk, blood samples were collected from progesterone-treated and ovariectomized ewes. After collection of blood samples, hypothalamic and hypophyseal tissues were collected from all ewes. Half of each pituitary was used to determine the content of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), and the number of receptors for gonadotropin-releasing hormone (GnRH). The amounts of mRNA for LH beta subunit, FSH beta subunit, alpha subunit, growth hormone, and prolactin were measured in the other half of each pituitary. Treatment with progesterone reduced mean serum concentrations of LH (p less than 0.001) but ot FSH (p greater than 0.05). Further, progesterone decreased (p less than 0.05) the total number of pulses of LH. We were unable to detect pulsatile release of FSH. Hypothalamic content of GnRH, number of receptors for GnRH, pituitary content of gonadotropins and mRNA for LH beta subunit, FSH beta subunit, alpha subunit, growth hormone, and prolactin were not affected (p greater than 0.05) by treatment with progesterone. Thus, after treatment with progesterone, serum concentrations of LH (but not FSH) are decreased. This effect, however, is not due to a decrease in the steady-state amount of mRNA for LH beta or alpha subunits.     

Evidence for a pituitary site of gonadal steroid stimulation of GnRH receptors in female mice

Treatment of GnRH-deficient (hpg) female mice with oestradiol-17 beta (E2) for 7 days increased GnRH receptors from 4.1 +/- 0.4 fmol/pituitary (control) to 7.2 +/- 0.7 fmol/pituitary (GnRH-treated), and consistently increased pituitary FSH content. Treatment of hpg female mice with E2 plus progesterone (P) for 14 days stimulated GnRH receptors more than did E2 alone, although values still remained lower than those of normal intact female mice. In contrast, GnRH treatment of intact hpg female mice alone, or combined with E2 + P, increased GnRH receptors to values similar to those of intact normal female mice. In contrast, the receptor rise after GnRH treatment alone of ovariectomized hpg mice was significantly less than in intact hpg mice similarly treated. However, the combination of GnRH + E2 + P treatment of ovariectomized hpg mice increased GnRH receptors to normal intact female values, indicating the synergistic actions of these hormones on GnRH receptor up-regulation at the pituitary. Oestradiol treatment of ovariectomized normal female mice prevented the receptor fall after ovariectomy, and when combined with exogenous GnRH further increased receptors to values identical to those of intact female mice receiving GnRH alone. Ovariectomy of hpg mice had no effect on GnRH receptor, serum or pituitary LH and FSH values. There was no change in serum LH concentration after GnRH treatment of hpg female mice, but serum FSH increased and this was accentuated by ovariectomy, indicating that in intact mice an ovarian factor(s) normally inhibits GnRH-stimulated FSH release. This factor did not appear to be an ovarian steroid since serum FSH was not suppressed in intact or ovariectomized GnRH-treated hpg mice concurrently receiving E2 + P treatment. These results suggest that: (1) gonadal steroids alone have a major direct stimulatory action on the pituitary to increase GnRH receptors; (2) the oestrogen-induced increase in GnRH receptors is enhanced in the presence of GnRH; (3) steroids exert inhibitory feedback on gonadotrophin secretion that is mediated at some cellular regulatory locus other than the GnRH-receptor complex.     

Reciprocal cross talk between gonadotropin-releasing hormone (GnRH) and prostaglandin receptors regulates GnRH receptor expression and differential gonadotropin secretion

The asynchronous secretion of gonadotrope LH and FSH under the control of GnRH is crucial for ovarian cyclicity but the underlying mechanism is not fully resolved. Because prostaglandins (PG) are autocrine regulators in many tissues, we determined whether they have this role in gonadotropes. We first demonstrated that GnRH stimulates PG synthesis by induction of cyclooxygenase-2, via the protein kinase C/c-Src/phosphatidylinositol 3'-kinase/MAPK pathway in the LbetaT2 gonadotrope cell line. We then demonstrated that PGF(2alpha) and PGI2, but not PGE2 inhibited GnRH receptor expression by inhibition of phosphoinositide turnover. PGF(2alpha), but not PGI2 or PGE2, reduced GnRH-induction of LHbeta gene expression, but not the alpha-gonadotropin subunit or the FSHbeta subunit genes. The prostanoid receptors EP1, EP2, FP, and IP were expressed in rat gonadotropes. Incubations of rat pituitaries with PGF(2alpha), but not PGI2 or PGE2, inhibited GnRH-induced LH secretion, whereas the cyclooxygenase inhibitor, indomethacin, stimulated GnRH-induced LH secretion. None of these treatments had any effect on GnRH-induced FSH secretion. The findings have thus elaborated a novel GnRH signaling pathway mediated by PGF(2alpha)-FP and PGI2-IP, which acts through an autocrine/paracrine modality to limit autoregulation of the GnRH receptor and differentially inhibit LH and FSH release. These findings provide a mechanism for asynchronous LH and FSH secretions and suggest the use of combination therapies of GnRH and prostanoid analogs to treat infertility, diseases with unbalanced LH and FSH secretion and in hormone-dependent diseases such as prostatic cancer.