Division of labor among gonadotropes

The Musashi RNA binding proteins direct the translational activation of key pituitary mRNAs

The pituitary functions as a master endocrine gland that secretes hormones critical for regulation of a wide variety of physiological processes including reproduction, growth, metabolism and stress responses. The distinct hormone-producing cell lineages within the pituitary display remarkable levels of cell plasticity that allow remodeling of the relative proportions of each hormone-producing cell population to meet organismal demands. The molecular mechanisms governing pituitary cell plasticity have not been fully elucidated. Our recent studies have implicated a role for the Musashi family of sequence-specific mRNA binding proteins in the control of pituitary hormone production, pituitary responses to hypothalamic stimulation and modulation of pituitary transcription factor expression in response to leptin signaling. To date, these actions of Musashi in the pituitary appear to be mediated through translational repression of the target mRNAs. Here, we report Musashi1 directs the translational activation, rather than repression, of the Prop1, Gata2 and Nr5a1 mRNAs which encode key pituitary lineage specification factors. We observe that Musashi1 further directs the translational activation of the mRNA encoding the glycolipid Neuronatin (Nnat) as determined both in mRNA reporter assays as well as in vivo. Our findings suggest a complex bifunctional role for Musashi1 in the control of pituitary cell function.

Musashi Exerts Control of Gonadotrope Target mRNA Translation During the Mouse Estrous Cycle

The anterior pituitary controls key biological processes, including growth, metabolism, reproduction, and stress responses through distinct cell types that each secrete specific hormones. The anterior pituitary cells show a remarkable level of cell type plasticity that mediates the shifts in hormone-producing cell populations that are required to meet organismal needs. The molecular mechanisms underlying pituitary cell plasticity are not well understood. Recent work has implicated the pituitary stem cell populations and specifically, the mRNA binding proteins of the Musashi family in control of pituitary cell type identity. In this study we have identified the target mRNAs that mediate Musashi function in the adult mouse pituitary and demonstrate the requirement for Musashi function in vivo. Using Musashi RNA immunoprecipitation, we identify a cohort of 1184 mRNAs that show specific Musashi binding. Identified Musashi targets include the Gnrhr mRNA, which encodes the gonadotropin-releasing hormone receptor (GnRHR), and the Fshb mRNA, encoding follicle-stimulating hormone (FSH). Reporter assays reveal that Musashi functions to exert repression of translation of the Fshb mRNA, in addition to the previously observed repression of the Gnrhr mRNA. Importantly, mice engineered to lack Musashi in gonadotropes demonstrate a failure to repress translation of the endogenous Gnrhr and Fshb mRNAs during the estrous cycle and display a significant heterogeneity in litter sizes. The range of identified target mRNAs suggests that, in addition to these key gonadotrope proteins, Musashi may exert broad regulatory control over the pituitary proteome in a cell type-specific manner.

Plasticity in medaka gonadotropes via cell proliferation and phenotypic conversion

Follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) produced by the gonadotropes play a major role in control of reproduction. Contrary to mammals and birds, Lh and Fsh are mostly produced by two separate cell types in teleost. Here, we investigated gonadotrope plasticity, using transgenic lines of medaka (Oryzias latipes) where DsRed2 and hrGfpII are under the control of the fshb and lhb promotors respectively. We found that Fsh cells appear in the pituitary at 8 dpf, while Lh cells were previously shown to appear at 14 dpf. Similar to Lh cells, Fsh cells show hyperplasia from juvenile to adult stages. Hyperplasia is stimulated by estradiol. Both Fsh and Lh cells show hypertrophy during puberty with similar morphology. They also share similar behavior, using their cellular extensions to make networks. We observed bi-hormonal gonadotropes in juveniles and adults but not in larvae where only mono-hormonal cells are observed, suggesting the existence of phenotypic conversion between Fsh and Lh in later stages. This is demonstrated in cell culture, where some Fsh cells start to produce Lhβ, a phenomenon enhanced by gonadotropin-releasing hormone (Gnrh) stimulation. We have previously shown that medaka Fsh cells lack Gnrh receptors, but here we show that with time in culture, some Fsh cells start responding to Gnrh, while fshb mRNA levels are significantly reduced, both suggestive of phenotypic change. All together, these results reveal high plasticity of gonadotropes due to both estradiol-sensitive proliferation and Gnrh promoted phenotypic conversion, and moreover, show that gonadotropes lose part of their identity when kept in cell culture.

Plasticity of Anterior Pituitary Gonadotrope Cells Facilitates the Pre-Ovulatory LH Surge

Gonadotropes cells located in the anterior pituitary gland are critical for reproductive fitness. A rapid surge in the serum concentration of luteinizing hormone (LH) secreted by anterior pituitary gonadotropes is essential for stimulating ovulation and is thus required for a successful pregnancy. To meet the requirements to mount the LH surge, gonadotrope cells display plasticity at the cellular, molecular and morphological level. First, gonadotrope cells heighten their sensitivity to an increasing frequency of hypothalamic GnRH pulses by dynamically elevating the expression of the GnRH receptor (GnRHR). Following ligand binding, GnRH initiates highly organized intracellular signaling cascades that ultimately promote the synthesis of LH and the trafficking of LH vesicles to the cell periphery. Lastly, gonadotrope cells display morphological plasticity, where there is directed mobilization of cytoskeletal processes towards vascular elements to facilitate rapid LH secretion into peripheral circulation. This mini review discusses the functional and organizational plasticity in gonadotrope cells including changes in sensitivity to GnRH, composition of the GnRHR signaling platform within the plasma membrane, and changes in cellular morphology. Ultimately, multimodal plasticity changes elicited by gonadotropes are critical for the generation of the LH surge, which is required for ovulation.

Leptin Regulation of Gonadotrope Gonadotropin-Releasing Hormone Receptors As a Metabolic Checkpoint and Gateway to Reproductive Competence

The adipokine leptin signals the body's nutritional status to the brain, and particularly, the hypothalamus. However, leptin receptors (LEPRs) can be found all throughout the body and brain, including the pituitary. It is known that leptin is permissive for reproduction, and mice that cannot produce leptin (Lep/Lep) are infertile. Many studies have pinpointed leptin's regulation of reproduction to the hypothalamus. However, LEPRs exist at all levels of the hypothalamic-pituitary-gonadal axis. We have previously shown that deleting the signaling portion of the LEPR specifically in gonadotropes impairs fertility in female mice. Our recent studies have targeted this regulation to the control of gonadotropin releasing hormone receptor (GnRHR) expression. The hypotheses presented here are twofold: (1) cyclic regulation of pituitary GnRHR levels sets up a target metabolic checkpoint for control of the reproductive axis and (2) multiple checkpoints are required for the metabolic signaling that regulates the reproductive axis. Here, we emphasize and explore the relationship between the hypothalamus and the pituitary with regard to the regulation of GnRHR. The original data we present strengthen these hypotheses and build on our previous studies. We show that we can cause infertility in 70% of female mice by deleting all isoforms of LEPR specifically in gonadotropes. Our findings implicate activin subunit (InhBa) mRNA as a potential leptin target in gonadotropes. We further show gonadotrope-specific upregulation of GnRHR protein (but not mRNA levels) following leptin stimulation. In order to try and understand this post-transcriptional regulation, we tested candidate miRNAs (identified with in silico analysis) that may be binding the Gnrhr mRNA. We show significant upregulation of one of these miRNAs in our gonadotrope-Lepr-null females. The evidence provided here, combined with our previous work, lay the foundation for metabolically regulated post-transcriptional control of the gonadotrope. We discuss possible mechanisms, including miRNA regulation and the involvement of the RNA binding protein, Musashi. We also demonstrate how this regulation may be vital for the dynamic remodeling of gonadotropes in the cycling female. Finally, we propose that the leptin receptivity of both the hypothalamus and the pituitary are vital for the body's ability to delay or slow reproduction during periods of low nutrition.

Selective deletion of leptin receptors in gonadotropes reveals activin and GnRH-binding sites as leptin targets in support of fertility

The adipokine, leptin (LEP), is a hormonal gateway, signaling energy stores to appetite-regulatory neurons, permitting reproduction when stores are sufficient. Dual-labeling for LEP receptors (LEPRs) and gonadotropins or GH revealed a 2-fold increase in LEPR during proestrus, some of which was seen in LH gonadotropes. We therefore investigated LEPR functions in gonadotropes with Cre-LoxP technology, deleting the signaling domain of the LEPR (Lepr-exon 17) with Cre-recombinase driven by the rat LH-β promoter (Lhβ-cre). Selectivity of the deletion was validated by organ genotyping and lack of LEPR and responses to LEP by mutant gonadotropes. The mutation had no impact on growth, body weight, the timing of puberty, or pregnancy. Mutant females took 36% longer to produce their first litter and had 50% fewer pups/litter. When the broad impact of the loss of gonadotrope LEPR on all pituitary hormones was studied, mutant diestrous females had reduced serum levels of LH (40%), FSH (70%), and GH (54%) and mRNA levels of Fshβ (59%) and inhibin/activin β A and β B (25%). Mutant males had reduced serum levels of GH (74%), TSH (31%), and prolactin (69%) and mRNA levels of Gh (31%), Ghrhr (30%), Fshβ (22%), and glycoprotein α-subunit (Cga) (22%). Serum levels of LEP and ACTH and mRNA levels of Gnrhr were unchanged. However, binding to GnRH receptors was reduced in LEPR-null LH or FSH gonadotropes by 82% or 89%, respectively, in females (P < .0001) and 27% or 53%, respectively, in males (P < .03). This correlated with reductions in GnRH receptor protein immunolabeling, suggesting that LEP's actions may be posttranscriptional. Collectively, these studies highlight the importance of LEP to gonadotropes with GnRH-binding sites and activin as potential targets. LEP may modulate population growth, adjusting the number of offspring to the availability of food supplies.

Social dominance in tilapia is associated with gonadotroph hyperplasia

Tilapias are emerging as one of the most important fish in worldwide aquaculture and are also widely used as model fish in the study of reproduction and behavior. During the reproductive season, male tilapia are highly territorial and form spawning pits in which the dominant males court and spawn with available females. Non-territorial males stand a much lower chance of reproducing. Using transgenic tilapia in which follicle stimulating hormone (FSH) gonadotrophs were fluorescently labeled with enhanced green fluorescent protein (EGFP), we studied the effect of social dominance on the hormonal profile and pituitary cell populations in dominant and non-dominant males. Immunofluorescence studies showed that FSH-EGFP-transgenic fish reliably express EGFP in FSH-secreting cells. EGFP expression pattern differed from that of luteinizing hormone. Dominant males had larger gonads as well as higher levels of androgens and gonadotropins in the plasma. Pituitaries of dominant males exhibited higher gonadotropin content and gene expression. Flow cytometry revealed pituitary hyperplasia as well as FSH cell hyperplasia and increased granulation. Taken together, these findings suggest that gonadotroph hyperplasia as well as increased production by individual cells underlie the increased reproductive activity of dominant tilapia males.

Normal testicular function and spermatogenesis

The testis performs two basic functions, sperm production and testosterone secretion. Formation of the testis is genetically controlled; expression of the SRY gene directs the embryonic gonads into the pathway leading to the development of testes. By the fourth week of gestation in humans, the primordial germ cells derived from pluripotent cells of the embryonic epiblast proliferate and migrate from the endoderm of the yolk sac into the undifferentiated gonad, which becomes morphologically distinct during the seventh week of gestation in humans. Histological development of the testis is largely completed by the end of the third month of gestation.

Regional differences in the pituitary distribution of luteinizing hormone in the gonadectomized and proestrous female rat

Previous data have shown that regional differences in the presence of anterior pituitary luteinizing hormone (LH) generally correlate with the comparable disparities in distribution of gonadotropes throughout the gland. In female rats, the differences are apparent over the estrous cycle, but are more prominent during the hours preceding the proestrus surge of LH. The current experiments examined (1) if such regional disparities are present throughout the surge window, (2) if differences are mirrored by release of LH in vitro and (3) if the appearance of regional differences is altered in ovariectomized females. Results showed that a comparative elevation in the rostral portion of the pituitary during the pre-surge period diminishes and finally disappears concurrent with the rise in circulating LH. This increase in rostral LH concentrations is reflected in this region by a comparable effect in vitro on stimulated LH secretion from pituitary fragments, although the effect is somewhat diminished by referencing release against tissue concentrations of LH present in a contralateral rostral fragment. Ovariectomies conducted at 1500h on proestrus, at a time when a significant regional difference has faded, resulted in a prompt increase in LH across all areas of the pituitary, and the emergence of a marked augmentation in rostral concentrations over the ensuing 72h. The effect was not seen when ovariectomies were performed on estrus. These data show that, while a regional disparity in anterior pituitary LH is present as circulating concentrations of estradiol rise prior to the LH surge, the removal of this steroid feedback at a time when LH synthesis is normally amplified accentuates the difference between the rostral region and other areas of the pituitary.

Anterior pituitary leptin expression changes in different reproductive states: in vitro stimulation by gonadotropin-releasing hormone

This study was designed to learn more about the changes in expression of rat anterior pituitary (AP) leptin during the estrous cycle. QRT-PCR assays of cycling rat AP leptin mRNA showed 2-fold increases from metestrus to diestrus followed by an 86% decrease on the morning of proestrus. Percentages of leptin cells increased in proestrus and pregnancy to 55-60% of AP cells. Dual labeling for leptin proteins and growth hormone (GH) or gonadotropins showed that the rise in leptin protein-bearing cells from diestrus to proestrus was mainly in GH cells. Only 10-20% of leptin cells in male or cycling female rats coexpress gonadotropins. In contrast, 50-73% of leptin cells from pregnant or lactating females coexpress gonadotropins and only 19% coexpress GH, indicating plasticity in the distribution of leptin. Leptin cells expressed GnRH receptors, and estrogen and GnRH together increased the coexpression of leptin mRNA and gonadotropins. GnRH increased cellular leptin proteins three to four times and mRNA 9.8 times in proestrous rats and stimulated leptin secretion in cultures from diestrous, proestrous, and pregnant rats. These regulatory influences, and the high expression of AP leptin during proestrus and pregnancy, suggest a supportive role for leptin during key events involved with reproduction.

Androgen receptors in gonadotrophs in pituitary cultures from adult male monkeys and rats

There is substantial evidence demonstrating that the principal feedback action of androgens to decrease LH secretion in male primates, including man, is to slow the GnRH pulse generator, whereas in male rats androgens not only decrease GnRH but also suppress LH synthesis and secretion through a direct pituitary effect. Previous experiments in our laboratory revealed that testosterone (T) suppresses LH secretion and decreases alpha-subunit mRNA levels in male rat pituitary cell cultures perifused with pulses of GnRH but not in pituitary cells from adult male monkeys. In the present study, we sought to determine whether the lack of responsiveness of gonadotrophs to androgens in the primate is androgen receptor (AR) related. Primary cultures were prepared from the anterior pituitary glands of adult male monkeys and rats. Cells were identified as gonadotrophs if they were immunoreactive for LH-beta or FSH-beta. Of these cells in the monkey, 80% contained both gonadotropins, 17% contained only LH-beta, and 3% contained only FSH-beta. AR immunoreactivity (IR) was nuclear in 22% and 15%, respectively, of monkey and rat FSH-beta-positive cells in the absence of T. Following T treatment, nuclear AR IR was identified in 79% of monkey and 81% of rat gonadotrophs. T treatment similarly intensified AR IR in mouse gonadotroph alphaT3-1 and LbetaT2 cells and in monkey and rat fibroblasts. Single-cell RT-PCR confirmed coexpression of LH-beta and AR mRNA as well as LH-beta and GH mRNA in monkey gonadotrophs. Our data reveal that most monkey, as well as rat, gonadotrophs are AR-positive with nuclear localization in the presence of T. GH expression is not required for AR expression in gonadotrophs. We conclude that the failure of T to inhibit LH secretion and decrease alpha-subunit mRNA expression in the male primate is not due a disturbance in AR nuclear shuttling.

Molecular, structural, and cellular biology of follitropin and follitropin receptor

Follitropin and the follitropin receptor are essential for normal gamete development in males and females. This review discusses the molecular genetics and structural and cellular biology of the follitropin/follitropin receptor system. Emphasis is placed on the human molecules when possible. The structure and regulation of the genes for the follitropin beta subunit and the follitropin receptor is discussed. Control of systemic and cellular protein levels is explained. The structural biology of each protein is described, including protein structure, motifs, and activity relationships. Finally, the follitropin/follitropin receptor signal transduction system is discussed.

Neuroendocrine vs. paracrine control of follicle-stimulating hormone

In spite of the pivotal role FSH plays in the regulation of gametogenesis, we are far from understanding the regulatory mechanisms involved in the control of its synthesis, secretion, and functions. Part of the problem relates to its molecular heterogeneity and the unavailability of assay methods capable of distinguishing the various isoforms of FSH. Recent work has confirmed the existence of two modes of FSH secretion, the basal and the episodic modes. The major portion of FSH secretion appears to be in the basal mode. The episodic mode appears to consist of both GnRH-associated and non-GnRH-associated pulses of FSH. The intracellular mechanisms by which differential release of LH and FSH are facilitated by GnRH are just beginning to be unraveled and may involve different second-messenger systems. Local pituitary regulators such as activins, inhibins, and follistatins are receiving considerable attention in recent years as a means by which differential release of LH and FSH can be facilitated by GnRH and other neuroendocrine factors. In parallel, the search for a selective FSH-releasing factor (FSH-RF) continues. Identification of variant forms of GnRH in recent years has opened up the possibility that one GnRH variant may be the long-sought-after FSH-RF. From a functional aspect, an understanding of how FSH heterogeneity is regulated is also important, as the different mixes of FSH isoforms have the ability to fine-tune the follicular recruitment and selection process. This review focuses on the recent advances made in the neuroendocrine and paracrine regulation of FSH synthesis/secretion/heterogeneity and pinpoints areas of gaps in our knowledge.

Expression of proadrenomedullin derived peptides in the mammalian pituitary: co-localization of follicle stimulating hormone and proadrenomedullin N-20 terminal peptide-like peptide in the same secretory granules of the gonadotropes

Expression of proadrenomedullin-derived peptides in the rat, cow and human pituitary was studied by a variety of techniques. Immunocytochemical detection showed a widespread expression of adrenomedullin peptide in the adenohypophysis and the neural lobe, with low expression in the intermediate pituitary. Proadrenomedullin N-20 terminal peptide (PAMP)-immunoreactivity was also present in the anterior pituitary but showed a more marked heterogeneous distribution, with cells going from very strong to negative immunostaining. Lower levels of PAMP were found in the neural lobe. Interestingly, the distribution of adrenomedullin and PAMP immunoreactivity in the anterior pituitary did not completely overlap. In the present study, we concentrated our efforts to determine which cell type of the adenohypophysis expresses PAMP. Paraffin and semithin serial sections immunostained for PAMP and the classical pituitary hormones revealed that a subpopulation of the gonadotropes expresses high levels of PAMP-immunoreactive material. Ultrastructural analysis clearly showed PAMP-immunoreactivity in the follicle stimulating hormone (FSH)-containing large secretory granules of the gonadotropes, suggesting simultaneous secretion of PAMP and FSH by this cell type. Three mouse adenohypophysis-derived cell lines (AtT20, GH3, and alphaT3-1 derived from corticotropes, lacto/somatotropes and gonadotropes, respectively) were also analysed and showed expression of both proadrenomedullin-derived peptides and their mRNA. Functional studies in these three cell lines showed that neither adrenomedullin nor PAMP was able to stimulate cAMP production in our experimental conditions. Taken together, our results support that proadrenomedullin derived peptides are expressed in the pituitary in cell-specific and not overlapping patterns, that could be explained by differences in postranslational processing. Our data showing costorage of PAMP and FSH in the same secretory granules open a way by which PAMP could be involved in the control of reproductive physiology in a coordinated manner with FSH.

Gonadotropin-releasing hormone receptor initiates multiple signaling pathways by exclusively coupling to G(q/11) proteins

The agonist-bound gonadotropin-releasing hormone (GnRH) receptor engages several distinct signaling cascades, and it has recently been proposed that coupling of a single type of receptor to multiple G proteins (G(q), G(s), and G(i)) is responsible for this behavior. GnRH-dependent signaling was studied in gonadotropic alphaT3-1 cells endogenously expressing the murine receptor and in CHO-K1 (CHO#3) and COS-7 cells transfected with the human GnRH receptor cDNA. In all cell systems studied, GnRH-induced phospholipase C activation and Ca(2+) mobilization was pertussis toxin-insensitive, as was GnRH-mediated extracellular signal-regulated kinase activation. Whereas the G(i)-coupled m2 muscarinic receptor interacted with a chimeric G(s) protein (G(s)i5) containing the C-terminal five amino acids of Galpha(i2), the human GnRH receptor was unable to activate the G protein chimera. GnRH challenge of alphaT3-1, CHO#3 and of GnRH receptor-expressing COS-7 cells did not result in agonist-dependent cAMP formation. GnRH challenge of CHO#3 cells expressing a cAMP-responsive element-driven firefly luciferase did not result in increased reporter gene expression. However, coexpression of the human GnRH receptor and adenylyl cyclase I in COS-7 cells led to clearly discernible GnRH-dependent cAMP formation subsequent to GnRH-elicited rises in [Ca(2+)](i). In alphaT3-1 and CHO#3 cell membranes, addition of [alpha-(32)P]GTP azidoanilide resulted in GnRH receptor-dependent labeling of Galpha(q/11) but not of Galpha(i), Galpha(s) or Galpha(12/13) proteins. Thus, the murine and human GnRH receptors exclusively couple to G proteins of the G(q/11) family. Multiple GnRH-dependent signaling pathways are therefore initiated downstream of the receptor/G protein interface and are not indicative of a multiple G protein coupling potential of the GnRH receptor.

High efficiency method for gene transfer in normal pituitary gonadotropes: adenoviral-mediated expression of G protein-coupled receptor kinase 2 suppresses luteinizing hormone secretion

The level of LH secretion is determined by both alterations in gonadotrope responsiveness and alterations in GnRH secretion. The molecular mechanisms underlying gonadotrope responsiveness are unknown, but may include G protein-coupled receptor kinases (GRKs). Typically, GRKs phosphorylate the intracellular regions of seven-transmembrane receptors permitting beta-arrestin to bind, which prevents receptor activation of its G protein. Previously, we reported that heterologous expression of GRK2, -3, and -6 in GnRH receptor-expressing COS cells by complementary DNA transfection suppressed GnRH-stimulated inositol trisphosphate production, and that coexpression of GRK2 and beta-arrestin-2 was more inhibitory than either expressed alone. Here, we have investigated the effect of GRK2 on GnRH-stimulated LH secretion using adenovirus-mediated gene transfer in normal pituitary gonadotropes. Pituitary cells were infected with adeno-GRK2 or adeno-beta-galactosidase constructs at a multiplicity of infection of 60 (number of viral particles per cell). Seventy-two hours later, GRK2 expression was measured by enzyme-linked immunosorbent assay, and GnRH-stimulated LH secretion (10(-7) M GnRH-A for 90 min) was assayed by RIA. Adeno-beta-galactosidase infected 96-99% of the cells based on X-Gal staining. Uninfected and adeno-beta-galactosidase-infected cells exhibited endogenous GRK immunoreactivity of about 0.5 (OD405), and LH secretion of 14.8-17.7 ng/ml. Adeno-GRK2-infected cells showed a GRK2 immunoreactivity of about 2.5 (OD405) and LH secretion of 2.5 ng/ml. Therefore, adeno-GRK2 infection resulted in a 5-fold increase in the GRK2 OD405 value, which was accompanied by an 80-85% decrease in GnRH-stimulated LH secretion. GnRH-stimulated inositol trisphosphate production by gonadotropes also was inhibited, suggesting a site of action for GRK2 at phospholipase Cbeta or earlier in the signal transduction pathway. The significance of these findings is 2-fold: 1) adenoviral-mediated gene transfer permits investigation of the regulatory role of gene products in the cell of interest, the gonadotrope, rather than in heterologous cell systems; and 2) additional, stronger evidence is provided that supports a role for GRKs in setting the responsiveness of GnRH receptor signaling.

A mathematical model of luteinizing hormone release from ovine pituitary cells in perifusion

We model the effect of gonadotropin-releasing hormone (GnRH) on the production of luteinizing hormone (LH) by the ovine pituitary. GnRH, released by the hypothalamus, stimulates the secretion of LH from the pituitary. If stimulus pulses are regular, LH response will follow a similar pattern. However, during application of GnRH at high frequencies or concentrations or with continuous application, the pituitary delivers a decreased release of LH (termed desensitization). The proposed mathematical model consists of a system of nonlinear differential equations and incorporates two possible mechanisms to account for this observed behavior: desensitized receptor and limited, available LH. Desensitization was provoked experimentally in vitro by using ovine pituitary cells in a perifusion system. The model was fit to resulting experimental data by using maximum-likelihood estimation. Consideration of smaller models revealed that the desensitized receptor is significant. Limited, available LH was significant in three of four chambers. Throughout, the proposed model was in excellent agreement with experimental data.

Effects of sex steroids on secretory granule formation in gonadotropes of castrated male rats with respect to granin expression

Pituitary gonadotropes show sex-related differences in their ultrastructure. Typical gonadotropes of male rats exhibit both large granules, which contain chromogranin A (CgA), and small granules, which contain secretogranin II (SgII). In contrast, typical female rat gonadotropes show only a very few large granules among the numerous small granules. To clarify the nature of the biogenesis of these secretory granules and the effects of sex steroids, the ultrastructural and immunocytochemical changes in gonadotropes were examined in castrated male rats supplied with a testosterone or estradiol implant. In castrated rats, pituitary expression and plasma levels of LH increased drastically, but the pituitary content of CgA decreased. The majority of gonadotropes then showed features of "castration cells" containing many small secretory granules. A testosterone implant to castrated rats remarkably suppressed the expression and circulating levels of LH and increased the CgA content in the pituitary to near-normal levels. In this situation, immunocytochemical studies demonstrated that gonadotropes again exhibited large and small secretory granules with the respective localization of CgA and SgII. On the contrary, in castrated rats supplied with an estradiol implant, the expression and content of CgA in the pituitary were remarkably suppressed, and large secretory granules disappeared from gonadotropes. These results suggest that the expression of CgA in gonadotropes is regulated differently by male and female sex steroids. These different effects of androgen and estrogen on the expression level of CgA are closely associated with the sex-related differences in the ultrastructure of secretory granules within gonadotropes.

A temporally intermediate mode of gonadotropin releasing hormone-induced desensitization of luteinizing hormone secretion

The classical mode of luteinizing hormone (LH) secretory desensitization in the rat appears after 3-6 h of continuous in vitro administration of gonadotropin (GnRH). A second mode has been reported to occur very rapidly (< 2 min) after the onset of GnRH administration, and to reverse within 3 min after its withdrawal. Here, the existence of a third mode of desensitization is reported. occurring at 40-50 min after initiation of continuous GnRH administration. Rat pituitary cells were perifused with 10(-8) M GnRH for 6 h: 10 min samples were collected for LH measurements by radioimmunoassay. As expected, the pattern of LH release was biphasic: LH levels peaked in the first phase at 30 min, decreased at 40-50 min, increased in the second phase to maximal levels at 90-110 min, and then decreased in the classical desensitization mode to near-baseline values by 300-360 min. Static incubations of pituitary cells in Petri dishes in the presence of high (10(-8) M) or submaximal (10(-9) M) GnRH concentrations confirmed the decrease in LH secretion at 40-50 min. Measurement of LH by reverse hemolytic plaque assay (RHPA) confirmed the existence of this new mode of desensitization; since 93% of all gonadotropes had become secretory at 40-50 min, the possibility of two subpopulations of gonadotropes accounting for the two phases of LH secretion appears to be ruled-out. GnRH receptor binding studies demonstrated a approximately 50% decrease in cell-surface binding in association with the desensitization at 40-50 min. These studies suggest the existence of a third mode of GnRH-induced LH secretory desensitization that is not due to gonadotrope subpopulations but may be causally associated with decreased GnRH receptor binding.

Estrogen regulation of gonadotropin-releasing hormone receptor messenger RNA in female rat pituitary tissue

Gonadotropin releasing hormone (GnRH) is crucial in regulating the reproductive system of female vertebrates. In the present study we have analyzed the estrogen regulation of the GnRH receptor mRNA at the cellular level in Sprague-Dawley female rats. Northern blot analysis detected 3 species (5.0, 4.5 and 1.4 kb) of GnRH receptor mRNA in pituitary tissues. The GnRH receptor mRNA levels of these 3 species were increased by estrogen. By in situ hybridization we observed a 3.5-fold increase in GnRH receptor mRNA levels after 48 h of estrogen treatment when compared to ovariectomized (OVX) rats, 12 h of estrogen treatment did not change the GnRH mRNA levels. Similar increases in GnRH receptor mRNA levels by estrogen were also found in Wistar-Imamichi female rat pituitary tissue. In situ hybridization analysis identified clusters of anterior pituitary cells that expressed the GnRH receptor mRNA. The estradiol effect depends on increased mRNA levels in these clusters. Moreover, a significant increase in the number of pituitary cells that expressed GnRH receptor was observed after 48 h of estrogen treatment. These findings suggest that the mechanisms for estrogen regulation of GnRH receptor include changing levels of GnRH receptor mRNA in the rat pituitary.