Regulation of pituitary gonadotropin-releasing hormone receptors by pulsatile gonadotropin-releasing hormone injections in male rats. Modulation by testosterone

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.

GnRH-Induced Ca(2+) Signaling Patterns and Gonadotropin Secretion in Pituitary Gonadotrophs. Functional Adaptations to Both Ordinary and Extraordinary Physiological Demands

PITUITARY GONADOTROPHS ARE A SMALL FRACTION OF THE ANTERIOR PITUITARY POPULATION, YET THEY SYNTHESIZE GONADOTROPINS: luteinizing (LH) and follicle-stimulating (FSH), essential for gametogenesis and steroidogenesis. LH is secreted via a regulated pathway while FSH release is mostly constitutive and controlled by synthesis. Although gonadotrophs fire action potentials spontaneously, the intracellular Ca(2+) rises produced do not influence secretion, which is mainly driven by Gonadotropin-Releasing Hormone (GnRH), a decapeptide synthesized in the hypothalamus and released in a pulsatile manner into the hypophyseal portal circulation. GnRH binding to G-protein-coupled receptors triggers Ca(2+) mobilization from InsP3-sensitive intracellular pools, generating the global Ca(2+) elevations necessary for secretion. Ca(2+) signaling responses to increasing (GnRH) vary in stereotyped fashion from subthreshold to baseline spiking (oscillatory), to biphasic (spike-oscillatory or spike-plateau). This progression varies somewhat in gonadotrophs from different species and biological preparations. Both baseline spiking and biphasic GnRH-induced Ca(2+) signals control LH/FSH synthesis and exocytosis. Estradiol and testosterone regulate gonadotropin secretion through feedback mechanisms, while FSH synthesis and release are influenced by activin, inhibin, and follistatin. Adaptation to physiological events like the estrous cycle, involves changes in GnRH sensitivity and LH/FSH synthesis: in proestrus, estradiol feedback regulation abruptly changes from negative to positive, causing the pre-ovulatory LH surge. Similarly, when testosterone levels drop after orquiectomy the lack of negative feedback on pituitary and hypothalamus boosts both GnRH and LH secretion, gonadotrophs GnRH sensitivity increases, and Ca(2+) signaling patterns change. In addition, gonadotrophs proliferate and grow. These plastic changes denote a more vigorous functional adaptation in response to an extraordinary functional demand.

Mechanism delineating differential effect of an antiestrogen, tamoxifen, on the serum LH and FSH in adult male rats

Tamoxifen, a synthetic non-steroidal antiestrogen with residual estrogenic activity, administered to adult male rats reduces their fertility. A decrease in the circulating LH and testosterone levels with a transient rise or no change in circulating FSH levels was observed. The present study was carried out to delineate the mechanism causing the differential effect of tamoxifen on circulating gonadotropins by correlating it to changes in the hypothalamic LHRH, pituitary gonadotropins and testicular inhibin/activin. Hypothalamus, pituitary-hypothalamus complex (PHC) and intact pituitary (PI) from control and tamoxifen-treated male rats were superfused in vitro, and pulsatile release of LHRH by hypothalamus and that of LH and FSH by the PHC and PI were studied. Concomitantly, testicular immunoexpression of alpha and betaB subunits of inhibin/activin were studied by immunohistochemistry and enzyme-linked immunosorbent assays (ELISA). At 0.4 mg/kg/day dose of tamoxifen a decrease in mean hypothalamic LHRH and LH pulse frequency from PHC construct was observed. FSH pulse frequency was not affected under the same experimental conditions. At the same dose of tamoxifen, testicular expression of both alpha and betaB subunits of inhibin/activin was upregulated. The study demonstrated that reduced circulating LH levels were due to a decrease in hypothalamic LHRH concentration and in LH pulsatility following tamoxifen treatment. The lack of effect on circulating FSH under the same experimental conditions was likely due to its modulation by inhibin and activin.

Excessive testosterone treatment and castration induce reactive astrocytes and fos immunoreactivity in suprachiasmatic nucleus of mice

The suprachiasmatic nucleus (SCN) has long been recognized as the central mammalian circadian pacemaker that controls behavioral and physiological processes. The role of the SCN in circadian rhythms has been the subject of a wide range of physiological and behavioral studies, although the influence of homeostasis rhythms (such as fluctuating hormone levels) on the SCN of the hypothalamus is not entirely clear. The present study was undertaken to examine the morphological interactions between astroglial and neuronal elements in the SCN of mice after either a short-term excessive testosterone treatment (ETT) or castration, using glial fibrillary acidic protein (GFAP), and immediate early gene c-fos as well as calbindin-D28k (CB) immunohistochemistry. Both ETT and castration resulted in a significant increase in the accumulation of reactive astrocytes and Fos-imunoreactivity (IR), especially in the dorsomedial (DM) sub-region of the SCN. However, CB-IR neurons in the examined brain regions showed little change. These findings indicate that the DM sub-region of the SCN may be a possible center of hormonal regulation via a hypothalamic neuroendocrine circuit, and that a non-photic stimuli mechanism might play a role in circadian rhythm regulation.

Fetal alcohol exposure and effects of LHRH and PMA on LH beta-mRNA expression in the female rat

Fetal alcohol exposure (FAE) is associated with a variety of physiological and behavioral dysfunctions. Effects of FAE on reproduction have been described that include delayed puberty, altered gonadotrophin secretion and steroidogenesis, and altered sexual behavior. Earlier work suggested that pituitary function was compromised in adult fetal alcohol-exposed female rats. This study examined the effects of LHRH and PMA in vitro on LH beta-mRNA expression in pituitary fragments from FAE animals; a separate experiment examined the effects of estradiol-17 beta on LH beta-mRNA under similar conditions. The results indicate that the pituitary glands of FAE females have a reduced ability to respond to these three stimuli. The reason for this reduced responsiveness to LHRH, PMA, and estradiol-17 beta is not clear, but the alterations suggest that LH synthesis is impeded following fetal ethanol exposure.

Gonadotropin-releasing hormone pulses: regulators of gonadotropin synthesis and ovulatory cycles

The data reviewed present evidence that the pattern of GnRH secretion is an important factor in the regulation of gonadotropin subunit gene expression, gonadotropin synthesis, and secretion. The information on regulation of mRNA expression by GnRH pulses should be considered with some caution, as the experiments were performed in male rats and may not accurately reflect events in female primates or humans. However, an overall pattern emerges which suggests that common factors may be involved in all mammalian species. If current evidence is correct, and only a single gonadotropin-releasing hormone exists, then mechanisms to differentially regulate the three gonadotropin genes may involve changes in GnRH secretion. Alterations in GnRH pulse frequency and amplitude are recognized by the pituitary gonadotrope cell and could be the mechanism used to effect differential expression of the gonadotropin subunit genes. Differential regulation of subunit gene expression would be expected to be critically important in the establishment of pubertal maturation, and subsequently in the maintenance of ovulatory cycles in women. Our hypotheses, proposing a major role of pulsatile GnRH secretion in the regulation of human reproduction, are summarized in schematic form in Fig. 14 for men and Fig. 15 for women. In utero and during the first few months of life, GnRH is secreted at a relatively fast frequency (approximately 1 pulse/hour). During the first year, GnRH secretion is inhibited and both the amplitude and apparent frequency of pulsatile release is markedly reduced. The mechanisms involved in inhibiting GnRH release remain unclear in humans. Similarly, the mechanisms involved in the disinhibition of GnRH secretion, which first occurs during sleep at the initiation of puberty, are unclear, but in humans do not appear to involve opiates. In males, the increased frequency and amplitude of GnRH secretion favor LH synthesis and release, which in turn stimulates testosterone secretion (Fig. 14). Testosterone acts at the hypothalamus, perhaps through opioid mechanisms, to inhibit GnRH pulse frequency and to maintain a regular pattern of pulses occurring approximately every 90-110 min in adult males. In females, the mechanisms involving alterations in the patterns of GnRH secretion to regulate reproduction appear more complex. This may reflect the need to differentially synthesize and secrete FSH and LH at different times during reproductive cycles to allow orderly follicular maturation and ovulation. As shown in Fig. 15, we hypothesize that the events during the first decade of life and through the initiation of nocturnal GnRH secretion at puberty are similar in both sexes.(ABSTRACT TRUNCATED AT 400 WORDS)

Temperature and protein kinase C modulation of gonadotropin-releasing hormone receptors in pituitary cells from intact or castrated male rats

Abstract Binding constants of [(125) I]Des-Gly(10)-(D-Ala(6))-gonadotropin-releasing hormone-ethylamide (GnRHa) to dispersed pituitary cells were evaluated in a 4-day culture. Cells were sampled either from intact or from castrated male rats and binding was measured at various temperatures before or after treatment with phorbol-12-myristate-13-acetate (PMA) or 1-5-(isoquinolinyl-sulfoxyl) 2-methylpiperazine (H7), which activate or inhibit, respectively, protein kinase C (PKC). In cells from intact rats incubated with increasing concentrations of ligand at 21 degrees C for 25 min, the Scatchard plot was not linear and calculation of the Hill coefficient (N(H)) was indicative of positive cooperativity (N(H)= 1.26 +/- 0.02). Such non-linearity was not observed when cells were incubated at 0.5 degrees C for 3 h. In that condition the maximal number of binding sites measured at equilibrium (B(max)) increased (15.1 +/- 0.05 versus 9.3 +/- 0.5 fmoles x mg(-1) proteins at 21 degrees C). Two control experiments permitted us to rule out the possibility that lower B(max) at 21 degrees C might reflect internalization: 1) Cells were first incubated with the ligand at 21 degrees C for 25 min and subsequently for 3 additional hours at 0.5 degrees C. Preincubation did not affect the B(max) obtained at 0.5 degrees C; 2) when the radioligand bound to the cell surface was washed out with an acidic buffer, only 13% of the specific radioactivity was retained irrespective of the ligand concentration applied, a much lower value than the 40% binding difference observed between 0.5 degrees C and 21 degrees C. When the cells were incubated with PMA, the Scatchard plot was linearized and the B(max) recorded at 21 degrees C increased by 50% over control cells (13 +/- 0.7 fmoles x mg(-1) proteins). Conversely, inhibition of PKC by H7, a preferential PKC inhibitor, was ineffective. In contrast, cells sampled from castrates exhibited linear and comparable Scatchard plots at either 0.5 degrees or 21 degrees C, with B(max) values of 14.4 +/- 0.3 and 15 +/- 0.34 fmoles x mg(-1) proteins, respectively. PKC activation did not affect binding in that model, but H7 decreased the number of sites (B(max)= 10.7 +/- 0.9) and induced appearance of positive cooperativity (N(H)= 1.36 +/- 0.07). Taken together, these experiments reveal a pool of GnRH receptors in the pituitary of intact rats that recognizes the ligand in a phosphorylation-dependent manner. These binding sites also became evident when biological properties of the membrane were modified by temperature or cell homogenization. After castration, PKC activation was no longer a prerequisite for recruitment of the total population of receptors whereas protein kinase inhibition resulted in a reduction of maximal binding. Finally, our observations demonstrate that GnRH binding can exhibit positive cooperativity, either on normal cells or on cells from castrates after protein kinase inhibition, suggesting that such a cooperativity is not related to a GnRH-dependent phosphorylation.

Aging and the hypothalamo-pituitary-testicular axis in the rat

Several experiments have been performed in order to clarify the mechanisms through which aging in male rats brings about profound modifications of the neuroendocrine system (reduced pulsatile secretion of LH and FSH, decreased serum levels of gonadotropins and testosterone, etc.). (1) It has been found that the number of mu opioid receptors decreases significantly in the hypothalami of old male rats; the substitution therapy with testosterone is ineffective in increasing the number of mu opioid receptors. These data suggest that the decrease of hypothalamic mu opioid receptors is not due to a decline of serum testosterone levels, but appears to be an independent phenomenon. (2) K opioid receptors increase significantly in the amygdala and in the thalamus of old male rats. These results show that aging, in addition to mu receptors, affects also the number of K receptors in selected areas of the brain. The increase of the number of K receptors in the amygdala might have some bearing on the decrease of serum gonadotropins observed in aged rats, since the amygdala is involved in the nervous circuitry influencing the hypothalamo-pituitary-gonadal axis. (3) The study of the release of LHRH from the hypothalamus of old male rats with an in vitro perfusion system shows that the release of the hormone is comparable in young and old animals, both in basal and in K+ stimulated conditions. These results indicate that the hypothalamus of old male rats retains the capacity of releasing LHRH both in basal and in stimulated conditions. (4) It has been observed that the number of LHRH receptors at the level of the anterior pituitary is significantly reduced in old male rats. This finding might explain the low serum levels of gonadotropins and testosterone in aged rats, due to a lack of an adequate response of the pituitary to hypothalamic LHRH.

Decreased hypothalamic gonadotropin-releasing hormone secretion in male marathon runners

Hypogonadotropic hypogonadism due to a deficiency in hypothalamic gonadotropin-releasing hormone is common in female athletes ("hypothalamic amenorrhea"). It is not known, however, whether a similar phenomenon occurs in male athletes. We investigated the integrity of the hypothalamic-pituitary-gonadal axis in six highly trained male marathon runners (who were running 125 to 200 km per week). The mean (+/- SEM) frequency of spontaneous luteinizing hormone pulses was diminished in the runners, as compared with healthy controls (2.2 +/- 0.48 vs. 3.6 +/- 0.24 pulses per eight hours, P less than 0.05). The amplitude of the pulses was also low in the runners (0.9 +/- 0.24 vs. 1.6 +/- 0.15 mlU per milliliter; P less than 0.05), and the responses of luteinizing hormone to gradually increasing doses of exogenous gonadotropin-releasing hormone were decreased. Plasma testosterone levels were similar in the two groups and increased equally in response to an intramuscular injection of 2000 units of human chorionic gonadotropin. During short-term intense physical exercise (a treadmill run at 72 percent of maximal oxygen consumption for two hours), the plasma gonadotropin levels in the athletes remained stable, but significant elevations in plasma levels of cortisol, prolactin, and testosterone occurred. We conclude that highly trained male athletes, like their female counterparts, may have a deficiency of hypothalamic gonadotropin-releasing hormone. This condition may be caused by the prolonged, repetitive elevations of gonadal steroids and other hormones known to suppress gonadotropin-releasing hormone secretion that are elicited by their daily exercise.

Gonadotropin-releasing hormone differentially regulates expression of the genes for luteinizing hormone alpha and beta subunits in male rats

Gonadotropin-releasing hormone (GnRH) and gonadal steroids regulate synthesis and release of luteinizing hormone (LH). GnRH is secreted intermittently by the hypothalamus, producing pulsatile LH release, and a pulsatile GnRH stimulus is required to maintain LH secretion. We report the regulatory effects of GnRH pulse injections on pituitary concentrations of LH alpha and beta subunit mRNAs in a castrated/testosterone-replaced male rat model. Replacement with physiologic amounts of testosterone decreased concentrations of both LH subunit mRNAs. GnRH pulse injections (10-250 ng per pulse given every 30 min for 48 hr) increased both mRNA concentrations, but the dose response patterns were markedly different. alpha subunit mRNA was increased by all GnRH doses but not the levels seen after castration alone. In contrast, LH beta subunit mRNA concentrations showed a marked dependence on GnRH dose. Maximal responses, to values similar to those in castrates, occurred after 25-ng GnRH pulses, and larger doses produced a smaller increase in LH beta subunit mRNA. Both the acute LH secretory response to GnRH and the number of GnRH receptors followed a pattern similar to the LH beta subunit mRNA concentration and were maximal after the 25-ng GnRH dose. These results show that GnRH can differentially regulate LH subunit mRNAs and suggest that concentrations of LH beta subunit mRNA may be a limiting factor in GnRH-stimulated LH release.

Reduction of pituitary GnRH receptors in immature rats treated with monosodium glutamate

The number of pituitary gonadotropin-releasing hormone (GnRH) receptors increases during sexual maturation in the rat and probably reflects changes in hypothalamic GnRH secretion. As GnRH is synthesized in various hypothalamic nuclei, including the arcuate nucleas (ARC), we investigated the effects of monosodium glutamate (MSG)-induced lesions of the ARC in the rat. In males and females treated with MSG during the first 10 days of life, GnRH receptor content (GnRH-RC) was unchanged from controls at 10 days but was decreased at 20 and 30 days of age (P less than 0.01). Serum concentrations of luteinizing hormone (LH) were similar in MSG-treated and control males but were significantly lower in 10-day-old females (P less than 0.01). Injections of GnRH (3 micrograms every 8 h on days 18 and 19) restored GnRH-RC to control values in MSG-treated rats. Both MSG and untreated control rats showed similar LH responses to acute injections of GnRH, but responses were attenuated (P less than 0.05) after 2 days pretreatment with GnRH in rats that had received MSG. Ovarian GnRH-RC was similar in both MSG-treated and untreated controls. These data indicate that MSG-induced lesions of the ARC reduce pituitary GnRH-RC in immature rats, and the more marked effects in females suggest a more significant role in the ARC in the control of GnRH secretion during maturation in females. The lack of MSG-induced changes in ovarian GnRH-RC indicates that GnRH from the arcuate nucleus is not responsible for the increase in ovarian GnRH receptors seen during sexual maturation.