Differential actions of corticosterone on luteinizing hormone and follicle-stimulating hormone biosynthesis and release in cultured rat anterior pituitary cells: interactions with estradiol

Raging elephants: effects of human disturbance on physiological stress and reproductive potential in wild Asian elephants

Human disturbance has become a widespread threat to wildlife viability. The Asian elephant (Elephas maximus), an endangered and disturbance-prone species, is under severe threat from habitat loss and fragmentation, human-elephant conflict and poaching. Establishing connections between human disturbance, stress responses and reproduction is crucial for assessing the long-term survivability of a species and will provide critical information for conservation management. The current study investigated the effects of human disturbance on population-level stress responses and stress-related effects on reproductive potential of wild Asian elephants in Xishuangbanna Dai Autonomous Prefecture, China. We used a radioimmunoassay to measure the concentration of fecal cortisol and estradiol in 257 samples collected from five local populations at 15 sites over 4 years. Human disturbance in Xishuangbanna was quantified based on the Ecological-Niche Factor Analysis model. We found that fecal cortisol concentrations were strongly positively correlated with the degree of human disturbance and increased markedly with the expansion of tea plantations. Percentage of non-stressed individuals in a population was higher depending on the extend of undisturbed area in their home ranges. Fecal estradiol concentrations decreased significantly with increasing stress levels. Our results suggest that human disturbance poses environmental challenges to wild Asian elephant populations, and chronic exposure to human disturbance could lead to population decline. The study demonstrates the efficacy of non-invasive endocrine monitoring for further informing management decisions and developing conservation strategies.

Hormonal Regulation of Glucocorticoid Inactivation and Reactivation in αT3-1 and LβT2 Gonadotroph Cells

The regulation of reproductive function by glucocorticoids occurs at all levels of the hypothalamo-pituitary-gonadal axis. Within the pituitary, glucocorticoids have been shown to directly alter gene expression in gonadotrophs, indicating that these cell types are sensitive to regulation by the glucocorticoid receptor. Whilst the major glucocorticoid metabolising enzymes, 11β-hydroxysteroid dehydrogenase (11βHSD; HSD11B1 and HSD11B2), have been described in human pituitary adenomas, the activity of these enzymes within different pituitary cell types has not been reported. Radiometric conversion assays were performed in αT3-1, LβT2 (gonadotrophs), AtT-20 (corticotrophs) and GH3 (somatolactotrophs) anterior pituitary cell lines, using tritiated cortisol, corticosterone, cortisone or 11-dehydrocorticosterone as substrates. The net oxidation of cortisol/corticosterone and net reduction of cortisone/11-dehydrocorticosterone were significantly higher in the two gonadotroph cells lines compared with the AtT-20 and GH3 cells after 4 h. Whilst these enzyme activities remained the same in αT3-1 and LβT2 cells over a 24 h period, there was a significant increase in glucocorticoid metabolism in both AtT-20 and GH3 cells over this same period, suggesting cell-type specific activity of the 11βHSD enzyme(s). Stimulation of both gonadotroph cell lines with either 100 nM GnRH or PACAP (known physiological regulators of gonadotrophs) resulted in significantly increased 11β-dehydrogenase (11βDH) and 11-ketosteroid reductase (11KSR) activities, over both 4 and 24 h. These data reveal that gonadotroph 11βHSD enzyme activity can act to regulate local glucocorticoid availability to mediate the influence of the HPA axis on gonadotroph function.

Reproductive axis function and gonadotropin microheterogeneity in a male rat model of diet-induced obesity

Obesity causes complex metabolic and endocrine changes that may lead to adverse outcomes, including hypogonadism. We herein studied the reproductive axis function in male rats under a high-fat diet and analyzed the impact of changes in glycosylation of pituitary LH on the bioactivity of this gonadotropin. Rats were fed with a diet enriched in saturated fat (20% of total calories) and euthanized on days 90 or 180 of diet. Long-term (180 days), high-fat feeding rats exhibited a metabolic profile compatible with insulin resistance and metabolic syndrome; they concomitantly showed decreased intrapituitary and serum LH concentrations, low serum testosterone levels, and elevated serum 17beta-estradiol concentrations. A fall in biological to immunological ratio of intrapituitary LH was detected in 180 days control diet-treated rats but not in high-fat-fed animals, as assessed by a homologous in vitro bioassay. Chromatofocusing of pituitary extracts yielded multiple LH charge isoforms; a trend towards decreased abundance of more basic isoforms (pH 9.99-9.0) was apparent in rats fed with the control diet for 180 days but not in those that were fed the diet enriched in saturated fat. It is concluded that long-term high-fat feeding alters the function of the pituitary-testicular axis, resulting in hypogonadotropic hypogonadism. The alterations in LH function found in these animals might be subserved by changes in hypothalamic GnRH output and/or sustained gonadotrope exposure to an altered sex steroid hormone milieu, representing a distinctly different regulatory mechanism whereby the pituitary attempts to counterbalance the effects of long-term obesity on reproductive function.

Preserved inhibitory effect of recurrent hypoglycaemia on the male gonadotrophic axis

OBJECTIVE

Hypoglycaemia-induced decreases in male LH and testosterone concentrations are possibly mediated by activation of the hypothalamus-pituitary-adrenal (HPA) axis or by an increase in PRL. As counterregulatory stress hormone release is attenuated during recurrent hypoglycaemia, we questioned whether the gonadotrophic axis and PRL adapt similarly.

DESIGN

We performed two consecutive hypoglycaemic clamps on day 1 and one clamp on the following day in 15 healthy men. Blood concentrations of gonadotrophins, PRL, testosterone, ACTH and cortisol were measured during the first and the third clamp, taking place at the same time of day.

RESULTS

During hypoglycaemia, serum concentrations of LH and testosterone decreased (P < 0.003 for both), PRL, ACTH and cortisol increased (P < 0.001), and FSH remained unchanged (P = 0.90). The hypoglycaemia-induced decreases in LH and testosterone concentrations were similar during the first and the last clamp (P > 0.28 for both) whereas the increase in PRL, ACTH and cortisol was markedly attenuated during the third clamp (P < 0.001).

CONCLUSIONS

LH and testosterone responses do not adapt to recurrent hypoglycaemia, whereas the increase in PRL is attenuated, indicating adaptation. Considering the marked decrease in the responses of PRL and the HPA axis after antecedent hypoglycaemia, the data suggest that the hypoglycaemia-induced decreases in LH and testosterone concentrations, not adapting to recurrent hypoglycaemia, are mediated independently, probably by blood glucose itself.

Transcriptional regulation of the GnRH receptor gene by glucocorticoids

Expression of the gonadotropin-releasing hormone (GnRH) receptor gene is stimulated by dexamethasone in GnRH-deficient rodents. In this study we identify a 1226 bp sequence at the 5'-flanking region of the mouse GnRH-R gene that confers dexamethasone responsiveness when expressed in host cells. Further, a glucocorticoid antagonist blocks transcriptional activity of the mGnRHR promoter. Progressive 5'-deletion of the mGnRHR promoter localized the response sequence between the -331/-255 region. Analysis of this region revealed binding sites for the AP-1 transcription factor. Mutation in AP-1 modified the functional activity of the mGnRHR promoter following GnRH agonist or dexamethasone-stimulation. Using an electrophoretic mobility shift assay, a protein complex is shown to bind to the AP-1 site. These results suggest that AP-1 proteins and glucocorticoid receptor regulate transcription of the GnRH-R promoter in a heterologous system.

Effects of perinatal maternal food restriction on pituitary-gonadal axis and plasma leptin level in rat pup at birth and weaning and on timing of puberty

The effects of maternal 50% food restriction (FR) during the last week of gestation and/or lactation on pituitary-gonadal axis (at birth and weaning), on circulating levels of leptin (at weaning), and on the onset of puberty have been determined in rats at birth and at weaning. Maternal FR during pregnancy has no effect at term on the litter size, on the basal level of testosterone in male pups, and on the drastic surge of circulating testosterone that occurs 2 h after birth. At weaning, similar retardation of body growth is observed in male and female pups from mothers exposed to FR. This undernutrition induces the most drastic effects when it is performed during both gestation and lactation or during lactation alone. Drastic retardation of testicle growth with reduction of cross-sectional area and intratubular lumen of the seminiferous tubules is observed in male pups from mothers exposed to undernutrition during both gestation and lactation or during lactation alone. Maternal FR during the perinatal period reduces circulating levels of FSH in male pups without affecting LH and testosterone concentrations. Maternal FR does not affect circulating levels of LH, estradiol, and progesterone in female pups. Female pups from mothers exposed to FR during both gestation and lactation show a significant increase of plasma FSH as well as a drastic retardation of ovarian growth. The follicular population was also altered. The number of antral follicles of small size (vesicular follicles) was increased, although the number of antral follicles of large size (graafian follicles) was reduced. Maternal FR occurring during both late gestation and lactation (male and female pups), during lactation alone (male and female pups), or during late gestation (female pups) induces a drastic reduction of plasma leptin and fat mass in pups at weaning. The onset of puberty is delayed in pups of both sexes from mothers exposed to FR during lactation and during both gestation and lactation. In conclusion, these data demonstrate that a perinatal growth retardation induced by maternal FR has long-term consequences on both size and histology of the genitals, on plasma gonadotropins and leptin levels, on fat stores at weaning, and on the onset of puberty.

Dexamethasone or triamcinolone increases follicular development in immature female rats

We have previously reported that dexamethasone increased follicle-stimulating hormone (FSH) secretion via suppression of inhibin in immature female rats. In the present study, we investigated the effects of dexamethasone or triamcinolone on follicular development and ovarian functions (estradiol and inhibin secretion) in equine chorionic gonadotropin (eCG)-primed immature female rats. Dexamethasone significantly increased the number of ovulated oocytes in immature female rats treated with 5 i.u. eCG. Serum concentration of FSH in eCG-treated female rats was increased by administration of dexamethasone or triamcinolone, showing the peak value at 9 h after its administration, although the levels of inhibin markedly decreased at that time. Serum concentrations of inhibin and estradiol in eCG-treated female rats increased at 24 h after administration of dexamethasone or triamcinolone. These results demonstrate that dexamethasone or triamcinolone increases FSH secretion, and the excess amount of FSH strongly stimulates follicular development cooperating with exogenous eCG.

How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions

The secretion of glucocorticoids (GCs) is a classic endocrine response to stress. Despite that, it remains controversial as to what purpose GCs serve at such times. One view, stretching back to the time of Hans Selye, posits that GCs help mediate the ongoing or pending stress response, either via basal levels of GCs permitting other facets of the stress response to emerge efficaciously, and/or by stress levels of GCs actively stimulating the stress response. In contrast, a revisionist viewpoint posits that GCs suppress the stress response, preventing it from being pathologically overactivated. In this review, we consider recent findings regarding GC action and, based on them, generate criteria for determining whether a particular GC action permits, stimulates, or suppresses an ongoing stress-response or, as an additional category, is preparative for a subsequent stressor. We apply these GC actions to the realms of cardiovascular function, fluid volume and hemorrhage, immunity and inflammation, metabolism, neurobiology, and reproductive physiology. We find that GC actions fall into markedly different categories, depending on the physiological endpoint in question, with evidence for mediating effects in some cases, and suppressive or preparative in others. We then attempt to assimilate these heterogeneous GC actions into a physiological whole.

Dexamethasone increases follicle-stimulating hormone secretion via suppression of inhibin in rats

In the present study, the effects of dexamethasone on follicle-stimulating hormone (FSH) secretion in immature female rats were investigated. Dexamethasone increased the selective secretion of FSH and decreased plasma concentrations of inhibin in immature female rats. The effects of dexamethasone on FSH secretion were not confirmed in rats treated with ovariectomy or immunoneutralization against inhibin. In addition to the direct effect of dexamethasone on FSH synthesis in gonadotrophs, the present study has clearly demonstrated that the increased level of FSH in dexamethasone-treated rats is mediated by suppression of ovarian function, especially by the inhibition of inhibin secretion.

Glucocorticoid receptor immunoreactivity in neurons and pituitary cells implicated in reproductive functions in rainbow trout: a double immunohistochemical study

In order to identify the nature of the glucocorticoid receptor (GR)-expressing neurons and pituitary cells that potentially mediate the negative effects of stress on reproductive performance, double immunohistochemical stainings were performed in the brain and pituitary of the rainbow trout (Oncorhynchus mykiss). To avoid possible cross-reactions during the double staining studies, combinations of primary antibodies raised in different species were used, and we report here the generation of an antibody raised in guinea pig against the rainbow trout glucocorticoid receptor (rtGR). The results obtained in vitellogenic females showed that GnRH-positive neurons in the caudal telencephalon/anterior preoptic region consistently exhibited rtGR immunoreactivity. Similarly, in the anterior ventral preoptic region, a group of tyrosine hydroxylase-positive neurons, known for inhibiting gonadotropin (GTH)-2 secretion during vitellogenesis, was consistently shown to strongly express GR. Finally, we show that a large majority of the GTH-1 (FSH-like) and GTH-2 (LH-like) cells of the pituitary exhibit rtGR immunoreactivity. These results indicate that cortisol may affect the neuroendocrine control of the reproductive process of the rainbow trout at multiple sites.

Effect of stress-like concentrations of cortisol on gonadotroph function in orchidectomized sheep

The effect of stress-like concentrations of cortisol (C) on the feedback potency of estradiol (E2) was assessed using 32 orchidectomized sheep (wethers) assigned at random to 1 of 4 treatment groups in a 2 x 2 factorial design. Wethers received C (3. 6 mg/50 kg per hour; groups 2 and 4) or a comparable volume of C delivery vehicle (groups 1 and 3) as a continuous infusion for 7 days. During the final 48 h of infusion, wethers received E2 (0.3 microg/50 kg/h; groups 3 and 4) or E2 delivery vehicle (groups 1 and 2). The pattern of LH secretion was assessed during a 4-h period of intensive blood collection beginning 44 h after initiation of E2 infusion. Gonadotroph responsiveness (LH secretion induced by GnRH challenge [500 ng, i.v.]) was determined 48 h after E2 delivery was begun. Although the frequency of secretory episodes of LH was not affected (p > 0.05) by infusion of C or E2 alone, LH pulse frequency was significantly decreased in wethers receiving C and E2 in combination. In contrast, neither the magnitude of basal gonadotroph responsiveness nor the extent of E2-dependent augmentation of responsiveness was significantly affected by stress-like concentrations of C. In a second experiment, the effect of C on the magnitude of E2-induced increase in pituitary concentration of GnRH receptor and GnRH receptor mRNA was assessed using 32 additional wethers. Continuous infusion of E2 for 48 h increased (p < 0.05) tissue concentrations of GnRH receptor and GnRH receptor mRNA. Concurrent delivery of C did not affect (p > 0.05) E2-induced increase in GnRH receptor mRNA but significantly reduced the magnitude of the E2-dependent increase in pituitary concentration of GnRH receptor. Collectively, these data indicate that stress-like concentrations of C enhance the negative feedback potency of E2 and reduce estrogen-dependent augmentation of the concentration of GnRH receptor in pituitary tissue.

Effect of stress-like concentrations of cortisol on estradiol-dependent expression of gonadotropin-releasing hormone receptor in orchidectomized sheep

The effect of stress-like concentrations of cortisol (C) on estrogen-dependent expression of GnRH receptor was evaluated using orchidectomized sheep (wethers; n = 6 animals per group). C (5.0 mg/50 kg per hour; groups 1-4) or a comparable volume of vehicle (groups 5-8) was delivered by continuous infusion for 48 h. During the final 24 h of infusion, animals received concurrent infusion of estradiol (E2) at rates of 0 (groups 1 and 5), 0.5 (groups 2 and 6), 2.0 (groups 3 and 7), or 8.0 (groups 4 and 8) microg/50 kg per hour. Pituitary tissue was collected at the end of infusion. Although C did not affect (p > 0.05) the basal concentration of GnRH receptor or GnRH receptor mRNA, it reduced (p < 0.05) the increase in receptor and receptor mRNA induced by concurrent administration of 0. 5 microg E2/50 kg per hour. In contrast, the increase in GnRH receptor expression induced by higher levels of estrogen stimulation was not affected (p > 0.05) by concurrent administration of C. The effect of C on the temporal pattern of E2-dependent increase in GnRH receptor expression was assessed using wethers receiving E2 (0.5 microg/50 kg per hour) by continuous infusion for 0 (groups 1 and 5), 24 (groups 2 and 6), 48 (groups 3 and 7), or 72 h (groups 4 and 8). Animals received C (5.0 mg/50 kg per hour; groups 1-4) or vehicle (groups 5-8) beginning 24 h before, and continuing throughout, the E2 delivery period. Stress-like concentrations of C reduced (p < 0. 05) the increase in GnRH receptor and receptor mRNA induced after 24 h of E2 stimulation. However, the suppressive effect of C was transient, and tissue levels of GnRH receptor and receptor mRNA were comparable after 72 h of E2 infusion in animals receiving C or vehicle alone. Collectively these observations demonstrate that C suppresses estrogen-dependent increase in tissue concentrations of GnRH receptor and receptor mRNA. However, this effect of C is transient and not evident in animals receiving moderate to high levels of estrogen stimulation. This transient suppression of GnRH receptor expression may account, at least in part, for the anti-gonadal effect of glucocorticoids.

Effects of corticosterone and testosterone on pituitary gonadotropin content, secretion, bioactivity and messenger RNA levels in the presence or absence of GnRH in male rats

The effects of corticosterone (B) and testosterone (T) on pituitary and serum bioactive and immunoreactive gonadotropins and on gonadotropin hormone subunit messenger RNA levels were compared in the absence of GnRH. Male rats were implanted with pellets of either cholesterol, B or T. At implantation, 2 and 4 days later half of each group received GnRH antagonist and animals were killed 5 days after implantation. As expected, GnRH antagonist lowered bioactive and immunoreactive serum FSH and LH, pituitary FSH, LHβ and FSHβ mRNA. B treatment alone lowered bioactive and immunoreactive serum FSH and immunoreactive serum LH. B reversed the antagonist effect on bioactive and immunoreactive pituitary FSH and FSHβ mRNA. T alone lowered bioactive and immunoreactive serum FSH and LH levels. T reversed the antagonist effect on bioactive and immunoreactive pituitary FSH. T lowered bioactive and immunoreactive pituitary LH and LHβ mRNA and partially reversed the antagonist effect on FSHβ mRNA. The data suggest that either B or T enhance FSH synthesis by acting directly at the gonadotrope, but that B does not affect LH variables to the same extent as T. The results suggest that in stressed animals, when T levels are reduced, B can substitute for T in sustaining FSH synthesis.

Regulation of anterior pituitary gonadotropin subunit mRNA levels during the preovulatory gonadotropin surge: a physiological role of progesterone in regulating LH-beta and FSH-beta mRNA levels

In a previous study we demonstrated that in the ovariectomized estrogen-primed immature rat, progesterone induced a gonadotropin surge while the gonadotropin mRNA subunit levels were either suppressed or unaltered. This observation has now been confirmed using more frequent time points. Progesterone administered at 0900 h was found to suppress LH-beta mRNA levels at 1300, 1400, and 0800 h the next day, with no subsequent effects at 1000, 1200 or 1600 h. FSH-beta mRNA levels were unaffected by progesterone except for a slight elevation at 1400 h and a suppression at 0800 h. Progesterone was either suppressive or had no effect on alpha mRNA levels. Since elevations in LH-beta and FSH-beta mRNA levels were observed in the cycling rat, the observed differences in the ovariectomized estrogen-primed rat could be due to a higher basal synthesis occurring due to ovariectomy. This was indeed the case because LH-beta and FSH-beta mRNA levels were 3.7- and 42.7-fold higher in such animals as compared to intact estrogen-primed rats. In contrast to the ovariectomized estrogen-primed rats, in intact estrogen-primed rats LH-beta mRNA levels were increased at 1000 h and FSH-beta mRNA levels were increased at 1000, 1200 and 1300 h after the administration of progesterone. In pregnant mare's serum gonadotropin-primed immature rats, LH-beta, FSH-beta and alpha-subunit mRNA levels were significantly elevated at 1800 and 2000 h, paralleling the serum LH and FSH surge. The progesterone antagonist RU486 (0.2 and 1.0 mg) significantly reduced serum LH and FSH levels at 2000 h. The lower dose reduced LH-beta and alpha-subunit mRNA levels at 2000 h and FSH-beta mRNA levels at 1800 h. The higher dose caused an increase in LH-beta mRNA levels at 1200 and 1800 h and a decrease in FSH-beta mRNA levels at 1800 and 2000 h. In conclusion, the present study provides evidence that preovulatory progesterone plays an important role in the increase in FSH-beta mRNA levels as well as the release of LH and FSH during the normal preovulatory gonadotropin surge. This relationship appears to be dependent on the ongoing rate of synthesis because this does not occur in the ovariectomized estrogen-primed rat in which synthesis is at a high basal level. Furthermore, the correlation with FSH appears to be tighter as compared to LH.

Glucocorticoid receptor colocalization with pituitary hormones in the rat pituitary gland

The presence of glucocorticoid receptor (GR) in the anterior lobe of the pituitary gland has previously been demonstrated, but the exact cell types expressing GR have not yet been characterized. In this study, we demonstrate the colocalization of GR and pituitary hormones in the rat pituitary gland by using an immunocytochemical double-labelling method. The majority of anterior lobe corticotropin-immunoreactive and growth hormone-immunoreactive cells contained GR-like immunoreactivity. Cells of the intermediate lobe showed intensive ACTH-like immunoreactivity but did not express GR. The glycoprotein hormones thyroid-stimulating hormone, follicle-stimulating hormone and luteinizing hormone were colocalized with GR to a lesser degree; approximately one-half of the cells exhibited immunoreactivity to these hormones contained GR. By contrast, only a minority of the prolactin-immunoreactive cells expressed GR. Our results suggest that glucocorticoids may differentially regulate the secretion and/or synthesis of these pituitary hormones by directly affecting the hormone-producing cells of the anterior pituitary.