Homologous upregulation of GnRH receptor mRNA by continuous GnRH in cultured rat pituitary cells

Phenotypic differences based on lateralization of intrahippocampal kainic acid injection in female mice

Clinical evidence indicates that patients with temporal lobe epilepsy (TLE) often show differential outcomes of comorbid conditions in relation to the lateralization of the seizure focus. A particularly strong relationship exists between the side of seizure focus and the propensity for distinct reproductive endocrine comorbidities in women with TLE. Therefore, here we evaluated whether targeting of left or right dorsal hippocampus for intrahippocampal kainic acid (IHKA) injection, a model of TLE, produces different outcomes in hippocampal granule cell dispersion, body weight gain, and multiple measures of reproductive endocrine dysfunction in female mice. One, two, and four months after IHKA or saline injection, in vivo measurements of estrous cycles and weight were followed by ex vivo examination of hippocampal dentate granule cell dispersion, circulating ovarian hormone and corticosterone levels, ovarian morphology, and pituitary gene expression. IHKA mice with right-targeted injection (IHKA-R) showed greater granule cell dispersion and pituitary Fshb expression compared to mice with left-targeted injection (IHKA-L). By contrast, pituitary expression of Lhb and Gnrhr were higher in IHKA-L mice compared to IHKA-R, but these values were not different from respective saline-injected controls. IHKA-L mice also showed an increased rate of weight gain compared to IHKA-R mice. Increases in estrous cycle length, however, were similar in both IHKA-L and IHKA-R mice. These findings indicate that although major reproductive endocrine dysfunction phenotypes present similarly after targeting left or right dorsal hippocampus for IHKA injection, distinct underlying mechanisms based on lateralization of epileptogenic insult may contribute to produce similar emergent reproductive endocrine outcomes.

Gonadotrope plasticity at cellular, population and structural levels: A comparison between fishes and mammals

Often referred to as "the master gland", the pituitary is a key organ controlling growth, maturation, and homeostasis in vertebrates. The anterior pituitary, which contains several hormone-producing cell types, is highly plastic and thereby able to adjust the production of the hormones governing these key physiological processes according to the changing needs over the life of the animal. Hypothalamic neuroendocrine control and feedback from peripheral tissues modulate pituitary cell activity, adjusting levels of hormone production and release according to different functional or environmental requirements. However, in some physiological processes (e.g. growth, puberty, or metamorphosis), changes in cell activity may be not sufficient to meet the needs and a general reorganization of cell composition and pituitary structure may occur. Focusing on gonadotropes, this review examines plasticity at the cellular level, which allows precise and rapid control of hormone production and secretion, as well as plasticity at the population and structural levels, which allows more substantial changes in hormone production. Further, we compare current knowledge of the anterior pituitary plasticity in fishes and mammals in order to assess what has been conserved or not throughout evolution, and highlight important remaining questions.

Divergent expression patterns of pituitary gonadotropin subunit and GnRH receptor genes to continuous GnRH in vitro and in vivo

Continuous, as opposed to pulsatile, delivery of hypothalamic gonadotropin-releasing hormone (GnRH) leads to a marked decrease in secretion of pituitary gonadotropins LH and FSH and impairment of reproductive function. Here we studied the expression profile of gonadotropin subunit and GnRH receptor genes in rat pituitary in vitro and in vivo to clarify their expression profiles in the absence and continuous presence of GnRH. Culturing of pituitary cells in GnRH-free conditions downregulated Fshb, Cga, and Gnrhr expression, whereas continuous treatment with GnRH agonists upregulated Cga expression progressively and Gnrhr and Fshb expression transiently, accompanied by a prolonged blockade of Fshb but not Gnrhr expression. In contrast, Lhb expression was relatively insensitive to loss of endogenous GnRH and continuous treatment with GnRH, probably reflecting the status of Egr1 and Nr5a1 expression. Similar patterns of responses were observed in vivo after administration of a GnRH agonist. However, continuous treatment with GnRH stimulated LH secretion in vitro and in vivo, leading to decrease in LH cell content despite high basal Lhb expression. These data suggest that blockade of Fshb expression and depletion of the LH secretory pool are two major factors accounting for weakening of the gonadotroph secretory function during continuous GnRH treatment.

Time- and dose-related effects of a gonadotropin-releasing hormone agonist and dopamine antagonist on reproduction in the Northern leopard frog (Lithobates pipiens)

Gonadotropin-releasing hormone (GnRH) stimulates luteinizing hormone release to control ovulation and spermiation in vertebrates. Dopamine (DA) has a clear inhibitory role in the control of reproduction in numerous teleosts, and emerging evidence suggests that similar mechanisms may exist in amphibians. The interactions between GnRH and DA on spawning success and pituitary gene expression in the Northern leopard frog (Lithobates pipiens) were therefore investigated. Frogs were injected during the natural breeding season with a GnRH agonist [GnRH-A; (Des-Gly¹⁰, D-Ala⁶, Pro-NHEt⁹)-LHRH; 0.1μg/g and 0.4μg/g] alone and in combination with the dopamine receptor D2 antagonist metoclopramide (MET; 5μg/g and 10μg/g). Injected animals were allowed to breed in outdoor mesocosms. Time to amplexus and oviposition were assessed, and egg mass release, incidences of amplexus, egg mass weight, total egg numbers and fertilization rates were measured. To examine gene expression, female pituitaries were sampled at 12, 24 and 36h following injection of GnRH-A (0.4μg/g) alone and in combination with MET (10μg/g). The mRNA levels of the genes lhb, fshb, gpha, drd2 and gnrhr1 were measured using quantitative real-time PCR. Data were analyzed by a two-way ANOVA. Both GnRH-A doses increased amplexus, oviposition and fertilization alone. Co-injection of MET with GnRH-A did not further enhance spawning success. Injection of GnRH-A alone time-dependently increased expression of lhb, fshb, gpha and gnrhr1. The major effect of MET alone was to decrease expression of drd2. Importantly, the stimulatory effects of GnRH-A on lhb, gpha and gnrhr1 were potentiated by the co-injection of MET at 36h. At this time, expression of fshb was increased only in animals injected with both GnRH-A and MET. Spawning success was primarily driven by the actions of GnRH-A. The hypothesized inhibitory action of DA was supported by pituitary gene expression analysis. The results from this study provide a fundamental framework for future time- and dose-response investigations to improve current spawning methods in amphibians.

Intrinsic and Regulated Gonadotropin-Releasing Hormone Receptor Gene Transcription in Mammalian Pituitary Gonadotrophs

The hypothalamic decapeptide gonadotropin-releasing hormone (GnRH), acting via its receptors (GnRHRs) expressed in pituitary gonadotrophs, represents a critical molecule in control of reproductive functions in all vertebrate species. GnRH-activated receptors regulate synthesis of gonadotropins in a frequency-dependent manner. The number of GnRHRs on the plasma membrane determines the responsiveness of gonadotrophs to GnRH and varies in relation to age, sex, and physiological status. This is achieved by a complex control that operates at transcriptional, translational, and posttranslational levels. This review aims to overview the mechanisms of GnRHR gene (Gnrhr) transcription in mammalian gonadotrophs. In general, Gnrhr exhibits basal and regulated transcription activities. Basal Gnrhr transcription appears to be an intrinsic property of native and immortalized gonadotrophs that secures the presence of a sufficient number GnRHRs to preserve their functionality independently of the status of regulated transcription. On the other hand, regulated transcription modulates GnRHR expression during development, reproductive cycle, and aging. GnRH is crucial for regulated Gnrhr transcription in native gonadotrophs but is ineffective in immortalized gonadotrophs. In rat and mouse, both basal and GnRH-induced Gnrhr transcription rely primarily on the protein kinase C signaling pathway, with subsequent activation of mitogen-activated protein kinases. Continuous GnRH application, after a transient stimulation, shuts off regulated but not basal transcription, suggesting that different branches of this signaling pathway control transcription. Pituitary adenylate cyclase-activating polypeptide, but not activins, contributes to the regulated transcription utilizing the protein kinase A signaling pathway, whereas a mechanisms by which steroid hormones modulate Gnrhr transcription has not been well characterized.

The relationship between basal and regulated Gnrhr expression in rodent pituitary gonadotrophs

Hypothalamic GnRH together with gonadal steroids and activins/inhibin regulate its receptor gene (Gnrhr) expression in vivo, which leads to crucial changes in GnRHR numbers on the plasma membrane. This is accompanied by alterations in the gonadotroph sensitivity and responsiveness during physiologically relevant situations. Here we investigated basal and GnRH-regulated Gnrhr expression in rodent pituitary gonadotrophs in vitro. In pituitary cells from adult animals cultured in the absence of GnRH and steroid hormones, the Gnrhr expression was progressively reduced but not completely abolished. The basal Gnrhr expression was also operative in LβT2 immortalized gonadotrophs never exposed to GnRH. In both cell types, basal transcription was sufficient for the expression of functional GnRHRs. Continuous application of GnRH transiently elevated the Gnrhr expression in cultured pituitary cells followed by a sustained fall without affecting basal transcription. Both basal and regulated Gnrhr transcriptions were dependent on the protein kinase C signaling pathway. The GnRH-regulated Gnrhr expression was not operative in embryonal pituitary and LβT2 cells and was established neonatally, the sex-specific response patterns were formed at the juvenile-peripubertal stage and there was a strong correlation between basal and regulated gene expression during development. Thus, the age-dependent basal and regulated Gnrhr transcription could account for the initial blockade and subsequent activation of the reproductive system during development.

Mice harboring Gnrhr E90K, a mutation that causes protein misfolding and hypogonadotropic hypogonadism in humans, exhibit testis size reduction and ovulation failure

GnRH, produced in the hypothalamus, acts on pituitary gonadotropes to stimulate release of the gonadotropins LH and FSH. Reduced responsiveness of gonadotropes to GnRH is a primary cause of hypogonadotropic hypogonadism (HH), a disease characterized by gonadal dysfunction and low blood levels of gonadotropins. Loss-of-function mutations in the gene encoding the receptor for GnRH (GNRHR) are a common cause of HH. Sequencing of the GNRHR gene in patients with HH revealed mainly point mutations producing single amino acid substitutions that cause misfolding and misrouting of this G protein-coupled receptor. To generate a mouse model that mimics the human disease, we introduced a single amino acid substitution (E90K) into the mouse Gnrhr gene, which is identical to a known human recessive mutation. In humans, E90K causes severe HH by preventing formation of the E90-K121 salt bridge, which is essential for correct folding. In cell cultures, E90K causes misfolding that leads to almost complete retention by the protein quality control system and subsequent degradation. Here we report that the primary phenotype of mice homozygous for E90K is female infertility due to ovulation failure. Mutant males are fertile despite reduced gonadotropin levels and smaller testes. These results suggest decreased GnRH receptor signaling in the mutant animal, compared with wild type. Our findings suggest that a threshold level of GnRH receptor activity is required for ovulation.

Expression and localization of gonadotropin-releasing hormone receptor in the rat oviduct during pregnancy

A recent study from our laboratory has shown cellular and ultrastructural distribution of the gonadotropin-releasing hormone (GnRH) and the relative expression of its mRNA in the rat oviduct during the postimplantation period of pregnancy (days 7, 9, 16, and 20). To determine the possible autocrine/paracrine involvement of the oviductal GnRH during pregnancy in rats, the present investigation aims at the study of the relative expression of GnRH receptor (GnRHR) mRNA by real-time PCR followed by immunolocalization of the peptide in the oviduct during pregnancy. Semiquantitative analysis of the oviductal GnRHR expression by Western blot was done by densitometry of the signal intensity. Our results indicate the expression of GnRHR mRNA in the rat oviduct throughout the postimplantation period of pregnancy with no significant difference in expression between the selected time points. Immunoreactive GnRHR peptide was localized predominantly in the cytoplasm of the luminal epithelial cells, with less expression in the cytoplasm of the stromal cells and the smooth muscles throughout the oviduct. Signal intensity of GnRHR was significantly lower during day 16 when compared to days 7 and 20. These results, for the first time, support the transcription of GnRHR mRNA and its translation to protein in the rat oviduct throughout the postimplantation period of pregnancy. The lower protein content of GnRHR by day 16 may be indicative of ligand-induced downregulation of the receptor expression. The present investigation thus strengthens our previously postulated hypothesis regarding the receptor-mediated autocrine/paracrine role of oviductal GnRH during pregnancy in rats.

Regulation of GnRH I receptor gene expression by the GnRH agonist triptorelin, estradiol, and progesterone in the gonadotroph-derived cell line alphaT3-1

The secretion of luteinizing hormone (LH) and the GnRH receptor (GnRH-R) concentration are modulated by ovarian steroids and GnRH. To elucidate whether this regulation is due to alterations at the transcriptional level, we examined the GnRH I-R mRNA expression in the gonadotroph-derived cell line alphaT3-1 treated with different estradiol and progesterone paradigms and the GnRH I agonist triptorelin. alphaT3-1 cells were treated with different steroid paradigms: 1 nM estradiol or 100 nM progesterone for 48 h alone or in combination. Cells were exposed to 10 nM or 100 pM triptorelin for 30 min, 3 h, 9 h, or, in pulsatile way, with a 5-min pulse per hour. The GnRH I-R mRNA was determined by Northern blot analysis. GnRH I-R mRNA from cells treated with continuous triptorelin decreased in a time- and concentration-dependent manner. Pulsatile triptorelin increased GnRH I-R gene expression. Progesterone alone further enhanced this effect, whereas estradiol and its combination with progesterone diminished it. Continuous combined treatment with estradiol and progesterone lead to a significant decrease of GnRH I-R mRNA by 30% and by 35% for estradiol alone. The addition of 10 nM triptorelin for 30 min or 3 h could not influence that steroid effect. In conclusion, estradiol and progesterone exclusively decreased GnRH I-R mRNA in alphaT3-1 cells no matter whether they are treated additionally with the GnRH I agonist triptorelin. The enhanced sensitivity of gonadotrophs and GnRH I-R upregulation by estradiol is not due to increased GnRH I gene expression because GnRH I-R mRNA is downregulated by estradiol and progesterone. Other pathways of the GnRH I-R signal transduction might be involved.

Regulation of gonadotropin-releasing hormone (GnRH)-receptor gene expression in tilapia: effect of GnRH and dopamine

The present work was designed to study certain aspects of the endocrine regulation of gonadotropin-releasing hormone receptor (GnRH-R) in the pituitary of the teleost fish tilapia. A GnRH-R was cloned from the pituitary of hybrid tilapia (taGnRH-R) and was identified as a typical seven-transmembrane receptor. Northern blot analysis revealed a single GnRH-R transcript in the pituitary of approximately 2.3 kilobases. The taGnRH-R mRNA levels were significantly higher in females than in males. Injection of the salmon GnRH analog (sGnRHa; 5-50 microg/kg) increased the steady-state levels of taGnRH-R mRNA, with the highest response recorded at 25 microg/kg and at 36 h. At the higher dose of sGnRHa (50 microg/kg), taGnRH-R transcript appeared to be down-regulated. Exposure of tilapia pituitary cells in culture to graded doses (0.1-100 nM) of seabream (sbGnRH = GnRH I), chicken II (cGnRH II), or salmon GnRH (sGnRH = GnRH III) resulted in a significant increase in taGnRH-R mRNA levels. The highest levels of both LH release and taGnRH-R mRNA levels were recorded after exposure to cGnRH II and the lowest after exposure to sbGnRH. The dopamine-agonist quinpirole suppressed LH release and mRNA levels of taGnRH-R, indicating an inhibitory effect on GnRH-R synthesis. Collectively, these data provide evidence that GnRH in tilapia can up- regulate, whereas dopamine down-regulates, taGnRH-R mRNA levels.

Progesterone together with estrogen attenuates homologous upregulation of gonadotropin-releasing hormone receptor mRNA in primary cultured rat pituitary cells

In a previous study, we clearly demonstrated that an application of gonadotropin-releasing hormone (GnRH) to cultured rat pituitary cells increased the expression of GnRH receptor (GnRH-R) mRNA through transcriptional activation of GnRH-R gene rather than suppression of the turnover rate of GnRH-R mRNA. Along with GnRH, gonadal steroids seem to be an important regulator for GnRH-R expression in the pituitary gland. Recent in vivo studies reported that an application of gonadal steroids to gonadectomized animals modulated GnRH-R mRNA expression in the pituitary gland. However, it has not been clearly understood whether steroids may act directly at the pituitary or indirectly via modulation of hypothalamic GnRH release. Therefore, we assessed the effects of estrogen and progesterone on GnRH-R mRNA expression in primary cultured female rat pituitary cells. Neither estradiol nor progesterone modulates the basal expression of GnRH-R mRNA in primary cultured pituitary cells. When cultured pituitary cells were exposed to different doses of estradiol in combination with GnRH (0.2 nM), the GnRH-stimulated increment of GnRH-R mRNA expression was not significantly changed by estradiol at any given doses. However, when different doses of progesterone were added to primary cultured pituitary cells in combination with GnRH (0.2 nM), GnRH-induced increases in GnRH-R mRNA levels were reduced in a dose-related manner, showing a significant reduction at 100 nM progesterone. Furthermore, the addition of estradiol reinforced the suppressive effect of progesterone on the homologous upregulation of GnRH-R mRNA expression. Collectively, our results clearly demonstrated that progesterone directly attenuates the homologous upregulation of GnRH-R mRNA expression at the pituitary level, and that estradiol potentiates the effect of progesterone.

Homologous upregulation of gonadotropin-releasing hormone receptor mRNA occurs through transcriptional activation rather than modulation of mRNA stability

In a previous study, we showed that even continuous application of gonadotropin-releasing hormone (GnRH) could increase the steady-state levels of GnRH receptor (GnRH-R) mRNA if treated for a relatively short period (6 h). Therefore, in the present study we examined whether GnRH-induced increment of GnRH-R mRNA is owing to stabilization of the preexisting GnRH-R mRNA or new synthesis of GnRH-R mRNA or both. Initially, to examine the effect on new RNA synthesis, the transcription inhibitor, actinomycin D (2 microM), was added to primary cultured rat anterior pituitary cells. In the presence of transcription inhibitor, GnRH-induced augmentation of GnRH-R mRNA levels was completely abolished. This result indicates that homologous upregulation of GnRH-R mRNA expression occurs at least through new RNA synthesis of GnRH-R gene. We further assessed the effects of GnRH on the turnover rate of GnRH-R mRNA using actinomycin D (2 microM). The basal half-life of GnRH-R mRNA was estimated to be approx 21 h. The application of GnRH tended to slightly suppress the basal turnover rate of GnRH; however, there was no statistically significant difference, compared with the group treated with actinomycin D alone. Collectively, our results suggest that the homologous upregulation of GnRH-R mRNA may occur through transcriptional activation of GnRH-R gene rather than enhancement of GnRH-R mRNA stability, although we did not examine the transcription rate of GnRH-R gene.