Pituitary GnRH Receptor Research Articles

Nucleotide sequence analyses predict that human pituitary and human placental gonadotropin-releasing hormone receptors have identical primary structures.

Gonadotropin-releasing hormone (GnRH) interacts with a putative receptor in human placenta to cause the dose-dependent release of human chorionic gonadotropin (hCG) in a manner analogous to hypothalamic GnRH stimulation of luteinizing hormone (LH), and follicle-stimulating hormone (FSH) by the pituitary gland. However, GnRH agonists bind a placental binding site at a lower affinity than they bind the pituitary GnRH receptor, suggesting the two sites differ. To address this issue, human placental GnRH receptor mRNA from an 8-wk placental sample was amplified using primers based on the sequence of the human pituitary receptor, cloned, and sequenced. The sequence of the placental transcript was found to be identical to its pituitary counterpart. Thus, as with the pituitary receptor, this placental receptor transcript appears to encode a single polypeptide chain with seven intramembranous domains and two glycosylation sites. However, it is possible these receptors differ in their posttranslational processing or there is more than one placental GnRH receptor. In addition, subsequent gene amplification studies, modified to increase the sensitivity of the method, revealed a band of the predicted size (approximately 1051 bp) in a 6-wk and two 8-wk placental samples, but not in 5-wk and 7-wk samples. The failure to amplify GnRH receptor mRNA at these gestational ages did not appear to be an issue of cell health or sample degradation, since GnRH, alpha hCG, and beta hCG mRNA were amplified from each of these tissues. Instead these data suggest GnRH receptor mRNA may have a short half-life or the mRNA may be translated as rapidly as it is transcribed, with the result that it has not accumulated in the cell.

Differential regulation of gonadotropin-releasing hormone (GnRH) receptor expression in the posterior mediobasal hypothalamus by steroid hormones: implication of GnRH neuronal activity

The present study is designed to evaluate the relationship between gonadotropin-releasing hormone (GnRH) and GnRH receptor ( GnRHR) gene expression during the steroid-induced LH surge. One week after ovariectomy (OVX), a capsule containing 17β-estradiol (E) or vehicle (V) was implanted into OVX rats, and 2 days later a single injection of progesterone (P) or V was administered s.c. at 10:00 h. Poly(A)-rich RNA samples were isolated from the micropunches of the preoptic area (POA) and the posterior mediobasal hypothalamus (pMBH) from both sides of individual brain slices. Using competitive reverse transcription–polymerase chain reaction (RT-PCR) procedures, three parameters (POA GnRH, pMBH GnRHR and pituitary GnRHR mRNA levels) were simultaneously determined in each individual animal. POA GnRH mRNA and pituitary GnRHR mRNA levels were decreased by treatment with E, but increased by a combination of E and P. In contrast, pMBH GnRHR mRNA levels were clearly augmented by treatment with E, and decreased by the combination of E and P. Temporal changes in such parameters were determined in OVX+E+V- and OVX+E+P-treated rats. P augmented POA GnRH mRNA levels at the time of the LH surge (17:00 h) and the increased GnRH mRNA levels were remained until 22:00 h, while E alone failed to alter POA GnRH mRNA levels. In the pMBH micropunch samples, P substantially decreased E-induced increase in GnRHR mRNA levels at 17:00 h and further lowered those until 22:00 h. Antisense oligonucleotides of GnRHR mRNA administered into the lateral ventricle of OVX+E-treated rats blocked the E-induced increase in pMBH GnRHR mRNA levels. The antisense oligonucleotides also prevented the LH surge as well as the increase in pituitary GnRHR mRNA levels in the OVX+E+P-treated group. However, administration of this antisense oligonucleotides failed to alter POA GnRH mRNA levels. In conclusion, the present study demonstrated that there is an inverse relationship between POA GnRH mRNA levels and pMBH GnRHR mRNA levels in response to E and/or P, and that the blockade of the E-induced increase in pMBH GnRHR mRNA levels effectively nullified the P-induced LH surge. These results indicate that pMBH GnRHR gene expression is involved in synchronizing the GnRH neuronal activity, which is crucial for the generation of the LH surge.

Maternal pituitary gonadotroph function in relation to GnRH receptor and LH mRNA content during pregnancy in ewes

To investigate the mechanism controlling the fall in maternal pituitary responsiveness to GnRH, LH synthesis and pituitary GnRH receptor content during pregnancy, maternal pituitaries were collected from sheep on days 35, 45, 60, 90, 110, 125 and 135 of pregnancy. Circulating steroids and gonadotrophins were determined in blood samples collected from these ewes immediately before death. Pituitary blocks from each ewe were perifused with either medium alone (control) or medium supplemented with oestradiol, oestradiol plus progesterone or oestradiol plus RU486, for 150 min before administration of two 15 s GnRH pulses 90 min apart. The amounts of mRNA encoding LH beta and GnRH receptor were determined in pituitary tissue fragments snap-frozen in liquid N2 at the time of collection from the ewes. While basal LH secretion fell during pregnancy, pituitary responsiveness to GnRH remained high (up to seven times basal LH concentrations). After day 90, the first GnRH pulse elicited LH peaks equivalent to the LH peaks produced by the second GnRH pulse. Therefore, GnRH self-priming was not evident possibly because the pituitaries were constantly primed by increased concentrations of maternal oestradiol. Around day 90, circulating concentrations of progesterone rose from 7.8 +/- 1.5 to 12.2 +/- 3.8 ng ml-1. Up to day 60, oestradiol in the perifusion buffer had stimulatory effects on LH secretion although this was reduced by RU486. By day 125, the content of mRNA encoding LH beta had declined during pregnancy to 7% of the content on day 35, although the content of mRNA encoding GnRH receptor remained unchanged. From these data, there appears to be a transitional period at around day 90 of gestation when pituitary sensitivity to steroids in vitro is lost together with detectable GnRH self-priming. In conclusion, the marked decline in pituitary amounts of mRNA encoding LH beta, but not in GnRH responsiveness or expression of GnRH receptor, after day 45 of pregnancy suggests that the principal effect of pregnancy on gonadotroph function is mediated via a mechanism other than reduced pituitary amounts of GnRH receptors. Two possible mechanisms are (1) a reduction in GnRH output leading to lowered LH synthesis, or (2) the presence of an inhibitory factor with a short half-life in the maternal circulation.

-Difluoromethylornithine Modifies Gonadotropin-Releasing Hormone Release and Follicle-Stimulating Hormone Secretion in the Immature Female Rat

Polyamines play an essential role in tissue growth and differentiation, in body weight increment, in brain organization, and in the molecular mechanisms of hormonal action, intracellular signaling, and cell-to-cell communication. In a previous study, inhibition of their synthesis by alpha-difluoromethylornithine (DFMO), a specific and irreversible inhibitor of ornithine decarboxylase, during development in female rats, was followed by prolonged high follicle-stimulating hormone (FSH) serum level and a delayed puberty onset. Those changes were relatively independent of body mass and did not impair posterior fertility. The present work studies the mechanisms and site of action of polyamine participation in FSH secretion during development. DFMO was injected in female rats between Days 1 and 9 on alternate days. At 10 days of age, hypothalami from control and DFMO rats were perifused in vitro, and basal and potassium-induced gonadotropin-releasing hormone (GnRH) release were measured. The response to membrane depolarization was altered in DFMO hypothalami. Increased GnRH release in response to a low K+ concentration was evidenced. Adenohypophyses of the same treated prepubertal rats were perifused in vitro and the response to GnRH pulses was checked. In DFMO-treated rats, higher FSH release was observed, with no changes in LH or PRL secretion. Finally, pituitary GnRH receptor number in adenohypophyseal membranes from treated and control groups was quantified. A significant reduction in specific binding was evident in hypophyses from DFMO-treated rats when compared with binding in the control group. In summary, DFMO treatment in a critical developmental period in the female rat impacts the immature GnRH neuronal network and immature gonadotropes. A delay in maturation is evidenced by a higher sensitivity to secretagogs in both pituitary glands and hypothalamic explants. These events could explain the prolonged high FSH serum levels and delayed puberty onset seen in this experimental model.

Brain sexual differentiation and gonadotropins secretion in the rat.

1. The present work deals with sexual differences in gonadotropin regulation in the rat and the role of sexual organization of the hypothalamus in determining such differences. 2. Sex differences between male and female rats, with regard to their control of gonadotropin secretion, go beyond whether or not gonadotropins are released cyclically. Rats show additional sex differences (a) in the response of gonadotropins to removal and imposition of negative feedback signals and (b) in the ontogeny of gonadotropin regulation from birth to puberty. 3. There is a sensitive developmental period during which sexual differentiation of neural substrates proceeds irreversibly under the influence of gonadal hormones. In the rat this period starts a few days before birth and ends approximately 10 days after birth. Female rats treated during this sensitive period with androgens or estrogens will permanently lose the capacity to release GnRH in response to estrogenic stimulation. 4. Nevertheless although sexual differentiation is dramatically affected by events during the neonatal period, recent data question the "critical" nature of this period, as it has been shown that testosterone can still act on neural substrates well beyond (15 to 30 days of age) the neonatal period to defeminize and masculinize endocrine and behavioral functions. 5. Furthermore, the capacity for the normal display of female sexual behavior and for the cyclic release of gonadotropins is not, as has been assumed, inherent to central nervous tissue but depends on active hormonal estrogenic induction during a sensitive period of development. 6. Besides, during differentiation of male sexual brain function estrogens may be supportive, rather than directive, to the primary action of androgens. 7. Serotonergic, noradrenergic, and opioid systems participate in the sexual dimorphism in gonadotropin control in adult rats. 8. The sex difference in the postcastration LH rise is dependent on the early sexual organization of the hypothalamus, even though in adulthood it can also be influenced by a variety of factors such as the stage of the estrous cycle, age of the animal, estradiol pretreatment, and history of release from feedback inhibition. 9. The characteristic pattern of gonadotropin secretion in the female infantile rat, which is sexually differentiated, can be related to an increase in hypophyseal receptors coupled to an increase in the intracellular calcium response to GnRH. Such events depend on the sexual organization of the hypothalamus. In males the greater sensitivity to GnRH at 30 days is reflected in an increase in pituitary GnRH receptors but not in an increase in the magnitude of Ca2+ mobilization induced by GnRH, therefore it is probable that in this situation alternative second messengers may modulate high sensitivity. Neonatal androgenization of the hypothalamus may decrease the hypophyseal response to GnRH by an alteration in receptor concentration and signal transduction during the infantile period. 10. Finally, serotonergic, dopaminergic, opioid, and noradrenergic regulation of GnRH varies with increasing age, and the sexual organization of the hypothalamus by testosterone or estrogens is a determinant in such regulation.

Primary Cultures of Dispersed Pituitary Cells from Estradiol-Pretreated Female Silver Eels (Anguilla anguillaL.): Immunocytochemical Characterization of Gonadotropic Cells and Stimulation of Gonadotropin Release

To analyze the multihormonal control mechanisms of GTH secretion in the eel, primary culture of pituitary cells from control or estradiol-treated female silver eels, a treatment known to stimulatein vivoGTH synthesis, was developed. Dispersed eel pituitary cells obtained by enzymatic (trypsin/DNAse) and mechanical dispersion were cultured in Earles M199, at 18°. Immunoreactive GTH (ir-GTH) cells were characterized by the immunoperoxidase method, using antiserum to carp GTH beta subunit. Ir-GTH cells from control silver eels were small and represented 14% of the dispersed pituitary cells. In contrast, ir-GTH cells from estradiol-treated eels were larger (cell area ×2.5) and represented a higher proportion (21%) of the pituitary cells. Intracellular and medium contents of GTH were measured by radioimmunoassay for the GTH beta subunit.In vivoestradiol-treatment increased more than 100-fold the GTH content of cell cultures. GTH release, studied over 1 to 4 hr, was undetectable in cultures from normal eels. In contrast, GTH release was low (less than 2% of cell content) but measurable in cultures from estradiol-treated eels. Subsequent experiments examined effects of various secretagogues on GTH release from primary cultures of pituitary cells from estradiol-pretreated eels. GTH release was significantly increased (×1.5 to ×3 basal release) by 10−6MGnRH-A as well as by both native GnRHs in the eel (mammalian GnRH, mGnRH, and chicken GnRH-II, cGnRH-II), at the same concentration. Lower GnRH concentrations had no significant effect, indicating a low sensitivity of gonadotrophs to GnRH, likely to be related to their immature state at the silver stage. The similar efficacy of mGnRH and cGnRH-II suggested that the pituitary GnRH receptor had a low specificity toward various molecular forms, in the eel as in the other nonmammalian species. The protein kinase C (PKC) activator (phorbol ester: PMA) also stimulated GTH secretion, with a maximal effect at 10−8M,indicating that the PKC pathway was functional. In contrast, a depolarizing agent (50 mMKCl) had no significant effect on GTH release, suggesting lack of a functional voltage-sensitive calcium channel (VSCC) secretory pathway. Perifusion experiments on whole pituitary confirmed the lack of effect of KCl on gonadotrophs from E2-pretreated eels and indicated that an additionalin vivotreatment with GnRH agonist and dopamine antagonist could induce the differentiation of a functional VSCC pathway. These characteristics of the transduction mechanisms may be related to the immature state of the eel gonadotrophs at the silver stage.

Molecular characterisation of the gonadotropin-releasing hormone receptor gene

The gonadotropin-releasing hormone (GnRH) receptor is a plasma membrane associated receptor and a member of the GTP-binding protein coupled receptor family. The interaction of GnRH and its receptor is a critical event in the endocrine control of reproduction. We have recently isolated the gene encoding the human pituitary GnRH receptor. The genomic clones obtained encompassed the full-length coding region and substantial portions of the 5′ and 3′ non-coding regions. Analysis of the clones revealed that the gene contains three exons and two introns which are distributed over 18.9 kb. Five putative promoters and transcription initiation sites have been identified in the 5′ end of the gene. Several consensus cis-acting regulatory sequences, such as sites for PEA-3, AP-1 and Pit-1, were identified within the 5′ flanking region. At the 3′ end of the gene, five classical polyadenylation signals were found scattered over a region of 800 bp. The mouse, sheep and rat GnRH receptor genes have also been characterised partially; these genes show similar structure and identical exon-intron splicing sites to the human GnRH receptor gene. However, the transcription start sites of the mouse and rat GnRH receptor genes are much closer to the translation start site when compared with the human GnRH receptor gene and multiple TATA boxes were not found in the promoter region of the mouse and rat GnRH receptor genes. On the other hand, initial characterisation of the ovine GnRH receptor gene indicates that, like the human gene, it contains multiple transcription initiation sites and far more 5′ untranslated region than the murine gene. Characterisation of the GnRH receptor gene provides the basis for further study of the complex mechanisms involved in the transcriptional regulation of GnRH receptor gene expression.

Dynamic Changes in Gonadotropin Releasing Hormone Receptor mRNA Content in the Mediobasal Hypothalamus during the Rat Estrous Cycle

The purpose of the present study was to determine if GnRH receptor mRNA levels in the rat brain undergo changes during the estrous cycle. We focused on the arcuate and ventromedial nuclei of the hypothalamus and on the hippocampus which are sites in the rat central nervous system that have been shown to contain measurable amounts of GnRH receptor mRNA. Groups of regularly cycling female rats were decapitated at 08.00 and 17.00 h of each day of the estrous cycle, trunk blood was collected for radioimmunoassay analysis of circulating LH levels, and the brains were processed for 'in situ' hybridization. A cDNA probe encoding the rat pituitary GnRH receptor was transcribed 'in vitro' in the presence of (33)P-alpha UTP and used under saturating conditions to label GnRH receptor mRNA. The results show that in the arcuate and ventromedial nuclei GnRH receptor mRNA levels are relatively high during diestrus 1, they decline slightly during diestrus 2 before they rise to the highest levels at 08.00 h of proestrus. By 17.00 h of proestrus, GnRH receptor mRNA levels had declined to the lowest levels of the estrous cycle where they remain through the morning of estrus. The GnRH receptor mRNA levels rise again sharply during the afternoon of estrus. The changes in the hippocampus follow a similar pattern in that a decline in GnRH receptor mRNA levels to its lowest levels occurs between 08.00 and 17.00 h of proestrus. However, the changes in the hippocampus did not reach statistical significance. It is concluded that GnRH receptor mRNA levels in the arcuate and ventromedial nuclei are upregulated in the morning of proestrus, probably by rising estradiol levels, in preparation for the GnRH-LH preovulatory surge while this effect of estradiol is not apparent in the hippocampus.

Estradiol up-regulation of pituitary progesterone binding is required for progesterone inhibition of luteinizing hormone release

This study examines the regulation of progesterone receptor (PR) in the inhibition of pituitary luteinizing hormone (LH) secretion. Ovariectomized ewes underwent hypothalamic-pituitary disconnection, were pulsed with gonadotropin-releasing hormone (GnRH) and received 1 of 4 treatments: estradiol alone (E), estradiol priming before progesterone (E+P), E removed and replaced with P (E-P), or no steroids (C). P treatment for 24 h, with E or following E-priming, reduced LH pulse amplitude by 55% (p<0.05). E alone did not affect LH release. E increased pituitary cytosolic P binding capacity fourfold over controls (p<0.01) and P further increased binding to eight times controls (p<0.01). Pituitary PR mRNA increased to 149 and 171% of C in E and E+P groups, respectively (p<0.05), but E removal resulted in PR mRNA levels not different from controls. Pituitary receptors for GnRH were tripled by E alone compared to C (p<0.01), whereas P alone or with E had no effect. These data suggest an E-induced, direct pituitary inhibition of LH secretion by P and that this effect of P is associated with E-enhanced binding of P in the pituitary. Additionally, the direct pituitary effects of P on LH secretion cannot be accounted for by influences on GnRH receptor numbers.

Measurement of luteal and placental gonadotrophin-releasing hormone (GnRH) binding sites: role of inactivation of GnRH tracer.

The ability of radiolabelled gonadotrophin-releasing hormone (GnRH) and GnRH analogues to bind to homogenates and membranes of rat, rabbit and sheep pituitary and to luteal homogenates and membranes from a number of species was measured. In addition, inactivation of tracer during binding incubation was estimated by measurement of the ability of the unbound tracer fraction to bind to fresh GnRH-receptor or anti-GnRH antibody. Following incubation of rat, sheep and rabbit pituitary gland with a radiolabelled GnRH agonist tracer, appreciable amounts of specific binding of GnRH agonist to fresh rat pituitary or human placental GnRH receptors could still be demonstrated. In contrast, no specific binding of [125I]-labelled GnRH analogues were measured following incubation of the tracer with homogenates or membranes of bovine, sheep and porcine luteal tissue, nor with rabbit and rat placental homogenates. However, during incubation of GnRH tracers with these tissues, almost complete inactivation of GnRH tracers occurred. There was an inverse relationship between binding and inactivation of [125I]-labelled GnRH for a number of human corpora lutea. We conclude that degradation of GnRH tracers may prevent the measurement of specific GnRH binding sites in some tissues.

Continuous stimulation of gonadotropin-releasing hormone (GnRH) receptors by GnRH agonist decreases pituitary GnRH receptor messenger ribonucleic acid concentration in immature female rats.

Although it is well recognized that continuous administration of gonadotropin-releasing hormone agonist (GnRHa) induces pituitary desensitization, the precise molecular mechanism of this phenomenon is still unclear. To test the hypothesis that pituitary gonadotroph desensitization is mediated by a change in GnRH receptor (GnRH-R) gene expression, the GnRH-R mRNA concentration was analyzed in immature female rats during GnRHa treatment. Northern blot hybridization was used to determine the GnRH-R mRNA concentration several times after an injection of TAP-144-SR, a slow releasing GnRHa. The GnRH-R mRNA readings were 92.7 +/- 9.5%, 49.9 +/- 5.0%, 35.7 +/- 2.3% and 73.8 +/- 5.7% (Mean +/- SD) compared to each control value at 1, 2, 4 and 8 weeks, respectively, after a single injection of 0.94 mg TAP-144 SR. These changes in GnRH-R mRNA coincided with the changes in gonadotropin secretion and LH-beta mRNA in response to GnRH in the results of our previous report. The present results indicate that the reduction of the number of pituitary GnRH-R sites induced by continuous stimulation with GnRHa is regulated at a transcriptional level.

Immunolocalization of a gonadotropin-releasing hormone receptor site in murine endometrium that mediates apoptosis.

Circumstantial evidence from a previous study indicated that antibodies generated against a synthetic N-terminal extracellular domain mouse pituitary gonadotropin-releasing hormone (GnRH) receptor peptide acted directly on the murine uterus affecting endometrial regression. Affinity-purified polyclonal sheep antibodies were used to assess tissue-specificity of antibody reactions in diestrous mice. Antibody binding was localized by immunofluorescence staining to anterior pituitary gland and endometrium. Ovary, brain, liver, kidneys, heart, lungs, spleen, gastrointestinal tract, adrenal glands, thymus, thyroid gland, muscle, and adipose were unreactive. Fragmented deoxyribonucleic acid, a marker of programmed cell death/apoptosis, was detected by digoxigenin labeling-immunoperoxidase in endometrial (but not pituitary) glands of animals injected with antipeptide antibodies or native ligand. It appears that luteal phase endometrium of mice expresses a GnRH receptor moiety that is coupled to a cell death (endonuclease) transduction pathway.

Expression of gonadotropin-releasing hormone and gonadotropin-releasing hormone receptor mRNAs in various non-reproductive human tissues

Recently, cloning of the gonadotropin-releasing hormone (GnRH) receptor from the human breast tumor cell line (MCF-7) and from an ovarian tumor, and its expression in various other human tumors, tumor cell lines and reproductive organs have been reported (Kakar et al., Mol. Cell. Endocrinol., 106 (1994) 145–149). In the present studies, we investigated the expression of GnRH and GnRH receptor mRNAs in normal human non-reproductive tissues. Using reverse transcriptase-polymerase chain reaction (RT-PCR) techniques and specific oligonucleotide primers derived from the placental GnRH cDNA sequence, PCR products of the expected size were obtained from human liver, heart, sekeletal muscle, kidney, placenta, and pituitary. The authenticity of the PCR products was confirmed by Southern blot analysis with an internal oligonucleotide primer as probe. Similarly, using specific oligonucleotide primers for the GnRH receptor selected from the human pituitary GnRH receptor cDNA sequence, PCR products of the expected size were amplified from human liver, heart, skeletal muscle, kidney, placenta, and pituitary, and these strongly hybridized with the human GnRH receptor cDNA on Southern blot. Cloning and nucleotide sequencing of the PCR products for the GnRH and GnRH receptor from heart revealed identical sequences when compared to the human placental GnRH and pituitary GnRH receptor cDNAs, respectively. These data demonstrate for the first time the existence of GnRH and GnRH receptor mRNAs in normal human non-reproductive tissues and suggest that GnRH and its receptor may play an important role in the regulation of cellular functions in an autocrine or paracrine manner, in addition to regulating the secretion of gonadotropins from the anterior pituitary.

Changes in rat pituitary gonadotropes after immunization with peptides derived from the receptor for gonadotropon-releasing hormone

Gonadotropin-releasing hormone (GnRH) is a neuropeptide which regulates hormone release from pituitary gonadotropes. The recently cloned pituitary GnRH receptor consists of a linear chain of amino acids in which seven transmembrane segments separate 4 extracellular and 3 intracellular (cytoplasmic) sequences. The rat receptor is approximately 90% homologous with that of the human.Observations made in several autoimmune diseases and experimental models suggest that anti-receptor antibodies can interfere with receptor function. The present studies were designed to test this hypothesis in female rats (n=9×3) after immunization with synthetic peptides whose overlapping sequences replicated all putative extracellular domains of the cloned human GnRH receptor (Table 1). The peptides (A-H) were conjugated to keyhole limpet hemocyanin and the conjugates were injected i.m. at monthly intervals for four months. A control group (K) received adjuvant only. Anti-peptide antibodies in the sera of immunized rats were detected by ELISA. Recognition of the “native” GnRH receptor was tested in immunocytochemistry on normal pituitary and brain tissue.

Expression of gonadotropin-releasing hormone (GnRH) receptor gene is altered by GnRH agonist desensitization in a manner similar to that of gonadotropin beta-subunit genes in normal and castrated rat pituitary.

It was previously established that the administration of a potent GnRH agonist such as triptorelin (D-Trp6-GnRH) induced desensitization of pituitary gonadotropic cells, resulting in decreased expression of gonadotropin beta-subunit genes and the suppression of LH and FSH synthesis and release. Binding of GnRH to the pituitary is also affected by agonist treatment. To examine the desensitizing effects of GnRH agonist on the expression of the pituitary GnRH receptor (GnRH-R) gene, male rats were given triptorelin (long-acting formulation, 300 micrograms/kg), and levels of GnRH-R messenger RNA (mRNA) were determined by Northern and dot blot hybridization to a 32P-labeled rat complementary DNA probe. Abundances of gonadotropin alpha-subunit, LH beta, and FSH beta mRNAs were examined in parallel, using appropriate probes. A rapid time-dependent decrease in the level of GnRH-R mRNA was observed in rats after triptorelin administration. A minimum residual level of mRNA, in the range of 20-25% of the initial value, was attained as early as 5 h after treatment. Levels further stabilized to 25-30% after a small transient increase to 45% on day 5. A single injection was effective for at least 30 days, after which GnRH-R mRNA levels slowly returned to normal, suggesting a progressive abolition of agonist effects. A concomitant acute depletion of mRNA levels was observed for LH beta and FSH beta (50% decrease in about 48 and 3 h, respectively), whereas the alpha-subunit message increased (rapidly reaching a level 1.8-fold that in control rats after 1-2 days). Castration induced a 3.8-fold elevation in the amounts of GnRH-R mRNA after 3 weeks, whereas alpha, LH beta, and FSH beta mRNAs increased by 6.2-, 7.9-, and 4.2-fold, respectively, compared to corresponding values in intact animals. Administration of the GnRH agonist readily prevented, for as long as 3 weeks, the stimulatory effects of castration on the GnRH-R mRNA and mRNAs for the beta-subunit of gonadotropins, but not for the alpha mRNA, which remained at a high level. When triptorelin was administered 3 weeks postoperatively, the castration-induced increase in LH beta and FSH beta was totally abolished, and no significant effect was noted on alpha-subunit mRNA. In conclusion, these data demonstrate that expression of the GnRH-R gene is subject to regulation and depends on GnRH stimulation, in a manner that indicates susceptibility to desensitizing action by the long-acting GnRH analog, triptorelin.(ABSTRACT TRUNCATED AT 400 WORDS)

Roles of estrogen, progesterone, and gonadotropin-releasing hormone (GnRH) in the control of pituitary GnRH receptor gene expression at the time of the preovulatory gonadotropin surges

Pituitary GnRH receptor (GnRH-R) messenger RNA (mRNA) levels are regulated dynamically during the rat estrous cycle. GnRH-R mRNA levels increase 3-fold on the morning of proestrus and remain elevated throughout the gonadotropin surges, after which they decline rapidly. Because the day of proestrus is characterized by complex changes in the steroidal milieu and increased release of hypothalamic peptides such as GnRH, a series of in vivo steroid replacement and in vitro perifusion studies was used to assess the relative contributions of estrogen (E), progesterone (P), and GnRH to the induction and decline of GnRH-R gene expression during the gonadotropin surges. Steroid replacement studies in ovariectomized (OVX) E-primed rats demonstrated that GnRH-R mRNA levels were elevated before and during the E-induced LH surge (1000-1800 h). Receptor mRNA levels declined after the peak of the LH surge and were significantly lower by 2000 h. Pentobarbital treatment, which inhibits hypothalamic input and the LH surge, prevented the gonadotropin surge-associated increase in GnRH-R mRNA levels in E-primed OVX rats. Although GnRH-R mRNA levels did not change throughout the day of experiments in OVX unprimed rats, treatment with pentobarbital significantly reduced GnRH-R mRNA expression in these animals. P treatment of E-primed OVX rats had no significant effect on GnRH-R mRNA expression during the LH surge, but significantly reduced mRNA levels immediately after the LH surge (2000 h). Data from in vitro perifusion experiments using either metestrous or proestrous pituitary glands demonstrated that pulsatile GnRH up-regulates the expression of its own receptor mRNA at both estrous cycle stages. Based on these results, we conclude that enhanced GnRH-R mRNA expression observed on the day of proestrus is largely due to the actions of E, exerted indirectly via hypothalamic routes (presumably through enhanced GnRH secretion). Furthermore, preovulatory P secretion may account for the rapid decline in GnRH-R mRNA levels observed on the evening of proestrus.

Roles of estrogen, progesterone, and gonadotropin-releasing hormone (GnRH) in the control of pituitary GnRH receptor gene expression at the time of the preovulatory gonadotropin surges.

Pituitary GnRH receptor (GnRH-R) messenger RNA (mRNA) levels are regulated dynamically during the rat estrous cycle. GnRH-R mRNA levels increase 3-fold on the morning of proestrus and remain elevated throughout the gonadotropin surges, after which they decline rapidly. Because the day of proestrus is characterized by complex changes in the steroidal milieu and increased release of hypothalamic peptides such as GnRH, a series of in vivo steroid replacement and in vitro perifusion studies was used to assess the relative contributions of estrogen (E), progesterone (P), and GnRH to the induction and decline of GnRH-R gene expression during the gonadotropin surges. Steroid replacement studies in ovariectomized (OVX) E-primed rats demonstrated that GnRH-R mRNA levels were elevated before and during the E-induced LH surge (1000-1800 h). Receptor mRNA levels declined after the peak of the LH surge and were significantly lower by 2000 h. Pentobarbital treatment, which inhibits hypothalamic input and the LH surge, prevented the gonadotropin surge-associated increase in GnRH-R mRNA levels in E-primed OVX rats. Although GnRH-R mRNA levels did not change throughout the day of experiments in OVX unprimed rats, treatment with pentobarbital significantly reduced GnRH-R mRNA expression in these animals. P treatment of E-primed OVX rats had no significant effect on GnRH-R mRNA expression during the LH surge, but significantly reduced mRNA levels immediately after the LH surge (2000 h). Data from in vitro perifusion experiments using either metestrous or proestrous pituitary glands demonstrated that pulsatile GnRH up-regulates the expression of its own receptor mRNA at both estrous cycle stages. Based on these results, we conclude that enhanced GnRH-R mRNA expression observed on the day of proestrus is largely due to the actions of E, exerted indirectly via hypothalamic routes (presumably through enhanced GnRH secretion). Furthermore, preovulatory P secretion may account for the rapid decline in GnRH-R mRNA levels observed on the evening of proestrus.

Gonadotropin-Releasing Activities of the Three Native Forms of Gonadotropin-Releasing Hormone Present in the Brain of Gilthead Seabream, Sparus aurata

Three forms of gonadotropin-releasing hormone (GnRH) have been recently identified in the brain of gilthead seabream (Sparus aurata): salmon GnRH (sGnRH), chicken GnRH-II (cGnRH-II), and a novel form, Ser8-mammalian GnRH, named seabream GnRH (sbGnRH). shGnRH is the most abundant form in the pituitaries of sexually mature seabream during the spawning season. The present study investigated the gonadotropin-releasing activities of the three native forms of GnRH found in seabream brains, as well as of two structural analogs of sbGnRH. All native forms of GnRH stimulated gonadotropin-II (GtH-II) secretion in preovulatory female seabream. cGnRH-II was found to be 7 to 8 times more potent than sbGnRH and 2 times more potent than sGnRH in inducing GtH-II release. sGnRH was found to he 3.5 to 5 times more potent than sbGnRH in inducing GtH-II secretion. These data demonstrate that cGnRH-II, which is not present in pituitaries of sexually mature seabream, is the most potent GtH-II releaser, whereas sbGnRH, 500 times more abundant than sGnRH in the pituitary of maturing fish, is the least potent. The lower potency of sbGnRH may suggest faster enzymatic breakdown, more rapid clearance from the circulation, or a lower binding affinity to the pituitary GnRH receptor. The lower bioactivity of sbGnRH may be compensated for by its high levels in the pituitary. The two analogs of sbGnRH, [d-Nal(2)6,Pro9-NEt]-sbGnRH and [d-Arg6,Pro9-NEt]-sbGnRH, were equipotent to each other and 5 times more potent than sbGnRH in inducing GtH-II release in preovulatory seabream. However, they were 5 to 6 times less active than the analog of mammalian GnRH, [d-Ala6,Pro9-NEt]-mGnRH. Strategies for designing superactive analogs of sbGnRH are discussed.

Agonist activity of mammalian gonadotropin-releasing antagonists in chicken gonadotropes reflects marked differences in vertebrate gonadotropin-releasing receptors

The pharmacology of mammalian and avian gonadotropin-releasing (GnRH) receptors differs for agonist analogues. We have therefore compared the activities of mammalian-based GnRH antagonists in sheep and chicken gonadotropes to further elucidate the different structural requirements of the receptors. The antagonist activities of ten GnRH analogues were compared in cultured sheep and chicken pituitary cells by determining the dose required to cause a 50% inhibition of luteinizing hormone secretion (IC 50) induced by GnRH at its half-maximal concentration (EC 50). Nine analogues showed high antagonist activity in the sheep bioassay. Analogue IC 50s varied between half and twice ((1.22 – 6.06) × 10 −10 M) the GnRH EC 50 (3 × 10 −10 M). One of these peptides exhibited partial agonist activity. In contrast, eight of the analogues showed low antagonist activity in chicken pituitary cells, with IC 50s varying from 46 to 1490 times ((1.4–44.7) × 10 −7 M) the GnRH EC 50 (3 × 10 −9 M) and had a different order of potencies compared with that in the sheep. Furthermore, two analogues did not display antagonist activity at all in the chicken bioassay, but acted as pure agonists, stimulating LH secretion. These findings demonstrate marked differences in pharmacology between the avian and mammalian pituitary GnRH receptors and emphasize that GnRH antagonists, selected for their efficacy in mammals, cannot necessarily be used for physiological studies in non-mammalian vertebrates. The distinctly different pharmacology of the receptors and structural requirements of analogues for agonist/antagonist activity establish a basis for identifying receptor features involved in ligand-induced signal propagation using chimaeras of cloned sheep and chicken receptors.

A transient effect of estrogen on calcium currents and electrophysiological responses to gonadotropin-releasing hormone in ovine gonadotropes.

Episodic release of luteinizing hormone (LH) by the pituitary gland is controlled by hypothalamic gonadotropin-releasing hormone (GnRH). In the period leading up to the preovulatory surge of LH, estrogen increases the number of pituitary receptors for GnRH and sensitises the gonadotropes to GnRH. The postreceptor events that are responsible for the increase in responsiveness to GnRH are not clearly delineated, but LH release is known to be Ca2+ dependent. The present study addressed the question as to whether or not estrogen may act to modify voltage-dependent Ca2+ entry in normal gonadotropes. Primary cultures enriched in gonadotropes or somatotropes were produced from anestrous female sheep. Conventional whole-cell patch-clamp recording was used to measure inward membrane current in the absence of GnRH treatment, with and without 10 nM estradiol-17beta (E2) treatment for 0 to 36 h. Nystatin-perforated whole-cell patch-clamp recording was used to record membrane voltage responses to GnRH. Ca2+ current density (ICa, pA/pF) began to increase after 2 h exposure to E2, and reached peak values of about 200% of control by 16-20 h (p < 0.005), then declined. If E2 was withdrawn at 24 h, ICa returned towards control values by 36 h. If E2 treatment was continued beyond 24 h, however, ICa fell to about 75% of control by 36 h (p < 0.005). Actinomycin D prevented the enhancement of ICa. E2 was without effect on Na+ current density in gonadotropes, or on ICa in somatotropes. The proportion of ICa carried by L-type and N-type channels in gondadotropes was not changed by E2. Ovine gonadotropes respond to GnRH with membrane potential fluctuations driven by periodic activation of Ca2+-dependent K+ channels, and synchronised action potential generation. This response was found to be sensitive to E2. Responses were categorised according to the pattern of activity evoked by 10 nM GnRH. Without E2 treatment, 11/14 cells responded with oscillations and 3/14 cells responded with spiking (hyperpolarizations following single action potentials). After 20 h 10 nM E2, just 1/14 cells responded with hyperpolarizing oscillations while 13/14 cells showed spiking activity. The predominance of the spiking pattern in E2-treated cells is consistent with the increased Ca2+ flux, and with enhanced LH release. We conclude that E2 has a transient effect on gonadotropes to enhance voltage-gated Ca2+ channel function. The time-course and biphasic nature of the influence of E2 on ICa may be physiologically appropriate to the preovulatory LH surge. Enhanced Ca2+ influx may participate in increased Ca2+-dependent hormone release, while the delayed inhibitory action of E2 on ICa may serve to limit the duration of the surge.