Gonadotropin gene expression and secretion in gonadotropin-releasing hormone antagonist-treated male rats: effect of sex steroid replacement

Reactive oxygen species and male reproductive hormones

Reports of the increasing incidence of male infertility paired with decreasing semen quality have triggered studies on the effects of lifestyle and environmental factors on the male reproductive potential. There are numerous exogenous and endogenous factors that are able to induce excessive production of reactive oxygen species (ROS) beyond that of cellular antioxidant capacity, thus causing oxidative stress. In turn, oxidative stress negatively affects male reproductive functions and may induce infertility either directly or indirectly by affecting the hypothalamus-pituitary-gonadal (HPG) axis and/or disrupting its crosstalk with other hormonal axes. This review discusses the important exogenous and endogenous factors leading to the generation of ROS in different parts of the male reproductive tract. It also highlights the negative impact of oxidative stress on the regulation and cross-talk between the reproductive hormones. It further describes the mechanism of ROS-induced derangement of male reproductive hormonal profiles that could ultimately lead to male infertility. An understanding of the disruptive effects of ROS on male reproductive hormones would encourage further investigations directed towards the prevention of ROS-mediated hormonal imbalances, which in turn could help in the management of male infertility.

Down-regulation of pituitary receptors for luteinizing hormone-releasing hormone (LH-RH) in rats by LH-RH antagonist Cetrorelix

Antagonists of luteinizing hormone-releasing hormone (LH-RH), unlike the LH-RH agonists, suppress gonadotropins and sex steroid secretion immediately after administration, without initial stimulatory effects. [Ac-D-Nal(2)1,D-Ph(4Cl)2,D-Pal(3)3,D-Cit6,D-Ala10]LH-R H (SB-75; Cetrorelix) is a modern, potent antagonistic analog of LH-RH. In this study, the binding characteristics of receptors for LH-RH in membrane fractions from rat anterior pituitaries were investigated after a single injection of Cetrorelix at a dose of 100 microg per rat. To determine whether the treatment with Cetrorelix can affect the concentration of measurable LH-RH binding sites, we applied an in vitro method to desaturate LH-RH receptors by chaotropic agents such as manganous chloride (MnCl2) and ammonium thiocyanate (NH4SCN). Our results show that the percentages of occupied LH-RH receptors at 1, 3, and 6 h after administration of Cetrorelix were approximately 28%, 14%, and 10%, respectively, of total receptors. At later time intervals, we could not detect occupied LH-RH binding sites. Ligand competition assays, following in vitro desaturation, demonstrated that rat pituitary LH-RH receptors were significantly (P < 0.01) down-regulated for at least 72 h after administration of Cetrorelix. The lowest receptor concentration was found 3-6 h after Cetrorelix treatment and a recovery in receptor number began within approximately 24 h. The down-regulation of LH-RH binding sites induced by Cetrorelix was accompanied by serum LH and testosterone suppression. Higher LH-RH receptor concentrations coincided with elevated serum hormone levels at later time intervals. Our results indicate that administration of LH-RH antagonist Cetrorelix produces a marked down-regulation of pituitary receptors for LH-RH and not merely an occupancy of binding sites.

Testosterone inhibits the basal and gonadotropin-releasing hormone-stimulated synthesis and release of newly synthesized alpha- and lutropin (LH) beta-subunit but not release of stored LH in cultured rat pituitary cells

To further explore the mechanism of steroid feedback in male, the effects of testosterone (T) and gonadotropin-releasing hormone (GnRH) on the rates of alpha- and lutropin (LH)beta-chain synthesis, neosynthesized subunits and radioimmunoassayable LH release into the medium were studied in the cultures of anterior pituitary cells from orchiectomized and intact rats. Polypeptides were [35S]methionine-labeled, immunoprecipitated separately in the medium and cells, then after SDS-PAGE precisely quantified. The total (medium + cells) radioactivity incorporated in the absence of GnRH into alpha- and LH beta-subunit was increased in orchiectomized rat cells vs. intact rat cells. GnRH stimulated the synthesis of both subunits, whether cells were from normal or castrated rat. T suppressed basal and GnRH-enhanced synthesis of both subunits in castrated rat cells. The values became closed to those observed in the normal rat cells. Also release of neosynthesized subunits from castrated rat cells into the culture medium was inhibited by T. In contrast, T did not change the basal and GnRH-induced radioimmunoassayed LH release. These results show that T can inhibit directly, at the pituitary level, alpha- and LH beta-subunit synthesis and neosynthesized but not stored LH release. They could explain, at least in part, no correlation between modifications of GnRH and LH secretion observed in vivo in response to T replacement.

Transcriptional and translational regulation of LH, prolactin and their testicular receptors by hCG and bromocriptine treatments in adult and neonatal rats

Effects of altered gonadotropin and prolactin (PRL) secretion on luteinizing hormone (LH), PRL and their testicular receptors (R) were studied in neonatal and adult rats. Changes in gene expression were monitored by measurements of steady-state mRNA levels. Five-day and 90-day-old male rats received a single s.c. injection of hCG (600 IU/kg), 1 mg/kg bromocriptine (BR) twice daily, or their combination. After 2 or 8 days, the responses of LH, PRL, their testicular R, and testosterone (T) were assessed, including measurements of the appropriate mRNA levels. Vehicle-treated age-matched animals served as controls. hCG suppressed serum LH in 2 days in adult rats from 0.85 +/- 0.16 to 0.04 +/- 0.01 microg/l, and in neonates from 0.59 +/- 0.29 to levels below 0.01 microg/l (p < 0.01 for both). This was accompanied at both ages by a 60% decrease in pituitary content of the LH beta-subunit mRNA (p < 0.01), but a decrease in the alpha-chain (40%, p < 0.05) occurred only in neonates. hCG increased serum PRL in adult rats in 8 days over 2-fold (p < 0.01); this did not occur in neonates. In neonates, BR increased the LH subunit mRNAs 2-fold in 8 days (p < 0.01) without a concomitant effect on serum LH; no BR effects on the LH parameters were seen in adult animals. BR decreased pituitary PRL protein and mRNA levels at both ages (p < 0.01-0.05), but serum PRL decreased only in the adults. The homologous down-regulation of testicular LHR (near 100%) was accompanied in adults by a 30% decrease in LHR mRNA (p < 0.05). Also BR at this age decreased LHR binding (75% in 8 days, p < 0.01), but in this case no change occurred in the cognate mRNA. hCG and BR slightly up-regulated in adults PRLR binding, but only the 2-day effect of BR was accompanied by a 60% increase in PRLR mRNA (p < 0.05). In neonates, both hCG and BR increased testicular LHR and PRLR mRNA levels (p < 0.01-0.05). In adult animals, both hCG and BR suppressed testicular and serum T levels after 8 days (40-70%, p < 0.01-0.05); only BR was inhibitory to T by 8 days in the neonates (p < 0.05). In conclusion, the homologous and heterologous regulatory effects of hCG and BR on LH, PRL and their testicular R levels were only partly explained by changes in steady-state levels of the respective mRNAs. In general, the autoregulatory effects on LHR and PRLR appeared to affect steady-state levels of cognate mRNAs, whereas heteroregulation predominately involved changes at the protein level. The responses of the neonatal pituitary-gonadal axis to hCG and/or BR differed greatly from those observed in the adult, indicating that the mechanisms involved in these regulatory events in adult animals are a result of gradual postnatal development.

Biphasic effect of exogenous testosterone on follicle-stimulating hormone gene expression and synthesis in the male rat

Effects of 2-week treatments with increasing doses of testosterone (T) on gonadotropin gene expression and secretion were studied in intact and acutely castrated male rats. T was administered in silastic capsules with lengths of 2, 4, 8 or 16 cm, and control animals received empty capsules (eight per treatment). The treatments increased serum T up to 3-fold of control levels. In intact animals, the 2-8 cm capsules suppressed pituitary follicle-stimulating hormone-beta (FSH beta) mRNA contents by 40-50% (p < 0.01), but 16 cm of T returned the levels back to control range. Castration alone increased the FSH beta mRNA level 2.3-fold (p < 0.01) and, after T treatment, the FSH beta message returned to control levels indistinguishable from intact controls but higher than in intact animals receiving the same T dose. Pituitary luteinizing hormone-beta (LH beta) mRNA displayed a dose-dependent suppression in response to T, to 32-35% of controls (p < 0.01) with the 8 and 16 cm capsules. Castration increased this message 10-fold, and additional T treatment suppressed the levels to the range of T-treated intact animals. Pituitary common-alpha mRNA decreased to 30-31% of controls by 2, 4 and 8 cm of T (p < 0.01), but the highest dose of T increased the common-alpha contents, in comparison to the other doses, to 54% of controls (p < 0.01). Castration alone increased the common-alpha contents 4.4-fold, and there was a dose-dependent suppression of this parameter by T down to the range of T-treated intact rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of antiandrogens and ethane dimethane sulphonate (EDS) on gene expression, free subunits, bioactivity and secretion of pituitary gonadotrophins in male rats

In the male rat, testosterone has been shown to regulate gonadotrophin synthesis and secretion under experimental conditions such as castration or gonadotrophin-releasing hormone (GnRH) antagonist with or without testosterone. The present study aims at clarifying the effects of non-steroidal antiandrogens, Casodex and flutamide, and ethane dimethane sulphonate (EDS) on the regulation of gonadotropin synthesis and secretion. To enable a direct comparison within this study to expected effects of testosterone, a GnRH antagonist-treated group and a castrated group were included. The gene expression of the subunits was correlated with changes in the pituitary and plasma content of immunoreactive luteinizing hormone (LH) and follicle-stimulating hormone (FSH), free subunits and pituitary content of in vitro bioactive LH and FSH. Groups of ten male rats each received the following treatments for 7 days: (1) vehicle; (2) castration; (3) EDS (75 mg/kg); (4) GnRH antagonist (Cetrorelix 250 micrograms/kg/day), (5) Casodex (20 mg/kg/day) or (6) flutamide (20 mg/kg/day). The effectiveness of testosterone deprivation was demonstrated by the reduction of weight in androgen-dependent organs such as epididymides and seminal vesicles in the treated groups. Treatment with flutamide, EDS or castration significantly increased (p < 0.05) serum levels of LH, FSH and alpha-subunit, whereas serum gonadotrophin levels were decreased in the GnRH antagonist-treated group. alpha-Subunit mRNA levels were elevated in the castrated, EDS and flutamide group and LH-beta mRNA levels were increased in the castrated and EDS group. FSH-beta mRNA levels were increased in the castrated group and decreased in the GnRH antagonist group, but remained unchanged in the flutamide and EDS group.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential regulation of anterior pituitary prodynorphin and gonadotropin-subunit gene expression by steroid hormones

Prodynorphin is expressed by neurons of the hypothalamus and gonadotrophs of the anterior pituitary gland (AP) and plays a role in the negative feedback regulation of the reproductive neuroendocrine axis. The present study examined whether gonadal steroid hormones are capable of modulating pituitary prodynorphin expression in immature, female rats. Steroids were administered via subcutaneous Silastic implants and rats were killed at 29 days of age. Northern blot analysis was used to measure AP prodynorphin, luteinizing hormone-beta (LH beta), follicle-stimulating hormone-beta (FSH beta), and common alpha-subunit mRNA levels (normalized to 18S ribosomal RNA). Treatment groups (n = 5-6) consisted of control (CNT; empty implants), estradiol (E2; 4 days), E2 + progesterone (E2 + P4; 8 days and 4 days, respectively), and dihydrotestosterone (DHT; 4 days). Pituitary prodynorphin mRNA was significantly suppressed in only the DHT-treated animals (26 +/- 10% of CNT, p < 0.01). LH beta mRNA was suppressed by all steroid treatments (p < 0.01), FSH beta was lower in only the E2 group, and alpha-subunit was reduced in both the E2 + P4 and DHT groups (p < 0.01). Serum LH was suppressed by all steroid treatments but FSH was reduced in only the E2 and E2 + P4 groups (p < 0.01). Treatment of prepubescent rats with continuous high levels of gonadal steroids is known to severely reduce endogenous hypothalamic gonadotropin releasing hormone (GnRH) release and this is supported by our observation of reduced gonadotropin-subunit gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)

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)