The central effect of beta-endorphin and naloxone on the expression of GnRH Gene and GnRH receptor (GnRH-R) gene in the hypothalamus, and on GnRH-R gene in the anterior pituitary gland in follicular phase ewes.

The effect of prolonged intermittent infusion of beta-endorphin or naloxone into the third cerebral ventricle in ewes during the follicular phase of the estrous cycle on the expression of GnRH gene and GnRH-R gene in the hypothalamus and GnRH-R gene in the anterior pituitary gland was examined by Real time-PCR. Activation of micro opioid receptors decreased GnRH mRNA levels in the hypothalamus and led to complex changes in GnRH-R mRNA: an increase of GnRH-R mRNA in the preoptic area, no change in the anterior hypothalamus and decrease in the ventromedial hypothalamus and stalk/median eminence. In beta-endorphin treated ewes the levels of GnRH-R mRNA in the anterior pituitary gland also decreased significantly. These complex changes in the levels of GnRH mRNA and GnRH-R mRNA were reflected in the decrease of LH secretion. Blockade of micro opioid receptors affected neither GnRH mRNA and GnRH-R mRNA nor LH levels secretion. These results indicate that beta-endorphin displays a suppressive effect on the expression of the GnRH gene in the hypothalamus and GnRH-R gene in the anterior pituitary gland, but affects GnRH-R gene expression in a specific manner in the various parts of hypothalamus; altogether these events lead to the decrease in GnRH/LH secretion.     

Proenkephalin and opioid mu-receptor mRNA expression in ovine hypothalamus across the estrous cycle

The neural mechanism underlying the preovulatory surge of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) is thought to include reduced opioid inhibition of GnRH secretion (disinhibition). Possible mechanisms for disinhibition include reduced endogenous opioid peptide or receptor mRNA expression. Proenkephalin and opioid mu-receptor mRNA expression were measured by in situ hybridization using 35S-labeled cRNA probes and computer-assisted grain counting in hypothalamic nuclei of ovary-intact ewes (n = 4) killed on day 10 of the luteal phase or 24 or 48 h into the follicular phase. In a second experiment, proenkephalin and mu-receptor mRNA expression were compared in ewes killed on day 10 of the luteal phase or during the preovulatory LH surge. Cells expressing proenkephalin mRNA were more widely distributed in ovine hypothalamus than previously described. In the periventricular nucleus, there was a significant reduction in the grain count per cell and the number of labeled cells during the follicular phase and during the LH surge, as compared to the luteal phase. In the ventromedial hypothalamus, there was a significant reduction in the grain count per cell during the follicular phase and LH surge as compared to the luteal phase, but no change in the number of labeled cells. No differences in proenkephalin mRNA expression were detected in the medial septum, diagonal band of Broca, preoptic area, anterior hypothalamic area or paraventricular nucleus across the estrous cycle. Cells expressing opioid mu-receptor mRNA were also widely distributed. No difference in mu-receptor mRNA expression was detected in the medial septum, diagonal band, medial preoptic area, anterior hypothalamus or bed nucleus of the stria terminalis across the cycle. We conclude that in sheep, proenkephalin gene expression in the periventricular nucleus and ventromedial hypothalamus is reduced during the follicular phase and at the time of the LH surge. This may be part of the neural mechanism underlying the GnRH/LH surge in this species.     

GnRH antagonist-induced down-regulation of the mRNA expression of pituitary receptors: comparisons with GnRH agonist effects

In order to compare the mechanism for the down regulation of the mRNA expression of pituitary receptors induced by GnRH antagonist (GnRHant) to that by GnRH agonist (GnRHa), we examined the effects of GnRHant (Cetrorelix, 333 mug/kg/day), GnRHa (leuprolide depot, 333 microg/kg), and GnRHant combined with GnRHa on LH response to exogenous GnRH, pituitary LH content, LH beta subunit mRNA, and GnRH receptor (GnRH-R) mRNA levels at 2, 5, 24, 72 hours, and 7 days after the treatment in ovariectomized rats. GnRHant significantly decreased serum LH, the LH response of the pituitary to exogenous GnRH, and the pituitary LH content compared to the control treatment, though GnRHa significantly increased serum LH. GnRHant with GnRHa significantly diminished the GnRHa-induced flare-up phenomenon. GnRHant significantly decreased LH beta mRNA and GnRH-R mRNA levels, but the magnitude of the decrease in these mRNA levels by GnRHant was significantly less than those by GnRHa until 72 hours following treatment. Prolonged treatment of GnRHant caused a marked inhibition of LH beta mRNA and GnRH-R mRNA expression, similar to that caused by GnRHa. Combination treatment with GnRHa and GnRHant was demonstrated to decrease LH beta mRNA and GnRH-R mRNA levels as much as GnRHa alone and GnRHant alone over 7 days of the treatment. The present study showed differences between GnRHant and GnRHa treatment in the reduction of GnRH-R mRNA levels up to 72 hours after the treatment, and indicated that the suppression of GnRH-R mRNA by GnRHant was the maximal by GnRHa 7 days after the treatment because more profound suppression was not observed upon additional treatment with GnRHa. The findings in the present study support the hypothesis that the mechanism by which GnRHant leads to down-regulation of the mRNA expression of pituitary receptors is similar to that of GnRHa.     

Differential gonadotropin-releasing hormone (GnRH) and GnRH receptor messenger ribonucleic acid expression patterns in different tissues of the female rat across the estrous cycle

GnRH, the main regulator of reproduction, is produced in a variety of tissues outside of the hypothalamus, its main site of synthesis and release. We aimed to determine whether GnRH produced in the female rat pituitary and ovaries is involved in the processes leading to ovulation. We studied the expression patterns of GnRH and GnRH receptor (GnRH-R) in the same animals throughout the estrous cycle using real-time PCR. Hypothalamic levels of GnRH mRNA were highest at 1700 h on proestrus, preceding the preovulatory LH surge. No significant changes in the level of hypothalamic GnRH-R mRNA were detected, although fluctuations during the day of proestrus are evident. High pituitary GnRH mRNA was detected during the day of estrus, in the morning of diestrus 1, and at noon on proestrus. Pituitary GnRH-R displayed a similar pattern of expression, except on estrus, when its mRNA levels declined. Ovarian GnRH mRNA levels increased in the morning of diestrus 1 and early afternoon of proestrus. Here, too, GnRH-R displayed a somewhat similar pattern of expression to that of its ligand. To the best of our knowledge, this is the first demonstration of a GnRH expression pattern in the pituitary and ovary of any species. The different timings of the GnRH peaks in the three tissues imply differential tissue-specific regulation. We believe that the GnRH produced in the anterior pituitary and ovary could play a physiological role in the preparation of these organs for the midcycle gonadotropin surge and ovulation, respectively, possibly via local GnRH-gonadotropin axes.     

Daily GnRH and GnRH-receptor mRNA expression in the ovariectomized and intact rat

We recently described patterns of GnRH and GnRH receptor (GnRH-R) expression in the hypothalamus, pituitary and ovary throughout the rat estrus cycle. Here, we wished to distinguish between regulatory effects of ovarian factors and underlying circadian rhythmicity. We quantified GnRH and GnRH-R mRNA in the pituitary and hypothalamus of long-term ovariectomized (OVX) rats, at different times of day, using real-time PCR. Furthermore, we expanded our previous study of hypothalamic and pituitary GnRH and GnRH-R expression in intact rats by including more time points throughout the estrus cycle. We found different daily patterns of GnRH and GnRH-R expression in intact versus OVX rats, in both tissues. In the hypothalamus of OVX rats, GnRH mRNA peaked at 12, 16 and 20 h, whereas in the hypothalamus of intact rats we observed somewhat higher GnRH mRNA concentrations at 19 h on every day of the estrus cycle except proestrus, when the peak occurred at 17 h. In this tissue, GnRH-R fluctuated less significantly and peaked at 16 h in OVX rats. During the estrus cycle, we observed higher levels in the afternoon of each day except on estrus. In OVX rats, pituitary GnRH mRNA rose sharply at 9 h, with low levels thereafter. In these animals, pituitary GnRH-R also peaked at 9h followed by a second rise at 22 h. In intact rats pituitary GnRH was high at noon of diestrus-II and on estrus, whereas GnRH-R mRNA was highest in the evening of diestrus-II. This is the first demonstration of daily GnRH and GnRH-R mRNA expression patterns in castrated animals. The observed daily fluctuations hint at underlying tissue-specific circadian rhythms. Ovarian factors probably modulate these rhythms, yielding the observed estrus cycle patterns.     

LH down-regulates gonadotropin-releasing hormone (GnRH) receptor, but not GnRH, mRNA levels in the rat testis.

The demonstration of an inhibitory effect of gonadotropin-releasing hormone (GnRH) agonists upon steroidogenesis in hypophysectomized rats and the presence of mRNA coding for GnRH and GnRH receptors (GnRH-R) in rat gonads suggests that GnRH can act locally in the gonads. To assess this hypothesis, we investigated the effects of GnRH analogs, gonadotropins and testosterone on the levels of both GnRH and GnRH-R mRNA in the rat testis. Using dot blot hybridization, we measured the mRNA levels 2 to 120 h after the administration of the GnRH agonist, triptorelin. We observed an acute reduction of both GnRH and GnRH-R mRNAs 24 h after the injection (about 38% of control). However, the kinetics for testis GnRH-R mRNA were different from those previously found for pituitary GnRH-R mRNA under the same conditions. Initially, the concentrations of serum LH and FSH peaked, then declined, probably due to the desensitization of the gonadotrope cells. In contrast, the GnRH antagonist, antarelix, after 8 h induced a 2.5-fold increase in GnRH-R mRNA, but not in GnRH mRNA, while gonadotropins levels were reduced. Human recombinant FSH had no significant effect on either GnRH or GnRH-R mRNA levels. Inversely, GnRH-R mRNA levels markedly decreased by 21% of that of control 24 h after hCG injection. Finally, 24 h after testosterone injection, a significant increase in GnRH-R mRNA levels (2.3 fold vs control) was found, but a reduction in the concentration of serum LH, probably by negative feedback on the pituitary, was observed. In contrast, GnRH mRNA levels were not significantly altered following testosterone treatment. Since LH receptors, GnRH-R and testosterone synthesis are colocalized in Leydig cells, our data suggest that LH could inhibit the GnRH-R gene expression or decrease the GnRH-R mRNA stability in the testis. However, this does not exclude the possibility that GnRH analogs could also affect the GnRH-R mRNA levels via direct binding to testicular GnRH-R. In contrast, the regulation of GnRH mRNA levels appeared to be independent of gonadotropins. Taken together, our results suggest a regulation of GnRH and GnRH-R mRNA specific for the testis.     

Pituitary content of gonadotropins and receptors for gonadotropin-releasing hormone (GnRH) and hypothalamic content of GnRH during the periovulatory period of the ewe

Studies were undertaken to determine if the number of hypophyseal receptors for GnRH changes at the time of the preovulatory surge of LH in ewes. Concentrations of LH, FSH, progesterone, and estradiol in serum and concentrations of LH and FSH in pituitary were measured. The content of GnRH in the hypothalamus was also determined. Estrus was synchronized in 35 cross-bred ewes by injecting prostaglandin F2 alpha (PGF2 alpha) at 0 and 4 h (7.5 mg each, im) on day 14 of a naturally occurring estrous cycle, followed 30 h later by the injection of estradiol (25 micrograms in safflower oil, im). Five ewes were killed at each of the following times relative to the first injection of PGF2 alpha: 0, 24, 32, 44, 50, 56 and 96 h. Blood samples were collected throughout the course of the experiment. Concentrations of progesterone in serum decreased markedly by 8 h after PGF2 alpha and were uniformly undetectable (less than 300 pg/ml) by 34 h. Concentrations of estradiol in serum increased after the injection of estradiol and returned to basal values 10 h later. Surges of LH, which were usually coincident with surges of FSH, occurred between 43 and 53 h. Concentrations of both LH and FSH in the pituitary declined after the LH surge. There were no significant changes in the amount of GnRH contained in the preoptic area, the median eminence, or the hypothalamus. The number of receptors for GnRH increased at 24 and 32 h compared to the 0 h value and remained elevated at 44 and 50 h. After the LH surge (56 h), the number of GnRH receptors declined and at 96 h was not different from the number measured at 0 h. Since an increase in the number of receptors will result in the formation of more receptor-hormone complex and may lead to an augmented response, these data suggest that an increase in the number of hypophyseal receptors for GnRH may contribute to the preovulatory LH surge in ewes.     

Immunocytochemical localization of beta endorphin and gonadal steroid regulation of proopiomelanocortin messenger ribonucleic acid in the ewe.

In the ewe, estradiol and progesterone inhibit luteinizing hormone (LH) secretion during the breeding season. Endogenous opioid peptides (EOP) are also inhibitory to LH secretion, and both estrogen and progesterone have been reported to enhance EOP inhibition of LH release. Which EOP are involved in this inhibition is unclear. In this study, we concentrated on beta-endorphin because evidence for its ability to inhibit LH secretion exists in ewes. We first studied the distribution of beta-endorphin-immunoreactive neurons in 4 cycling ewes using immunocytochemistry. Cell bodies were found only within the medial basal hypothalamus (MBH) and were concentrated in arcuate nucleus and mammillary recess of the third ventricle, with a few in the median eminence. Extensive fiber tracts were seen in preoptic area (POA) and median eminence. We next tested the hypothesis that gonadal steroids increase the synthesis of EOP by measuring levels of mRNA for proopiomelanocortin (POMC), the precursor to beta-endorphin. Ovariectomized ewes were treated with no steroids (n = 7) or given subcutaneous Silastic implants containing either estradiol (n = 6) or progesterone (n = 6). After 4 days of treatment, EOP inhibition of LH secretion was measured by determining the LH response to WIN 44,441-3 (WIN), an EOP antagonist. LH pulse frequency and pulse amplitude were determined in blood samples collected at 12-min intervals for 3 h before and after intravenous administration of 12.5 mg WIN. WIN injection increased (p < 0.01) the LH pulse-frequency only in progesterone-treated and pulse amplitude only in estradiol-treated ewes. After blood sampling, the ewes were killed, and POA, MBH, and pituitary gland were removed. Total RNA was extracted from these tissues and dot blotted onto nitrocellulose membranes for hybridization with a DNA probe complementary to the POMC mRNA. The resulting autoradiographs were quantified densitometrically. Levels of POMC mRNA in the MBH were increased (p < 0.01) by both estradiol and progesterone as compared with the no steroid group. There was no detectable POMC mRNA in the POA. These results suggest that estrogen and progesterone enhance EOP inhibition of LH secretion by increasing POMC mRNA levels and thus synthesis of beta-endorphin.     

Dopamine infusion into the third ventricle increases gene expression of hypothalamic vasoactive intestinal peptide and pituitary prolactin and luteinizing hormone beta subunit in the turkey

Turkey prolactin (PRL) secretion is controlled by vasoactive intestinal peptide (VIP) neurons residing in the infundibular nuclear complex (INF) of the hypothalamus. The VIPergic activity is modulated by dopamine (DA) via stimulatory D(1) DA receptors. DA (10 nmol/min for 40 min) was infused into the third ventricle of laying turkey hens to study its effect on circulating PRL, hypothalamic VIP and pituitary PRL and LHbeta subunit mRNA levels. Plasma PRL was significantly elevated after 20 min of DA infusion and remained elevated 30 min after cessation of infusion. Hypothalamic VIP mRNA content was significantly greater in the INF of DA-infused birds than it was in the INF of vehicle-infused control birds. No increase in VIP mRNA due to DA infusion was noted in the preoptic area. Pituitary PRL and LHbeta subunit mRNAs were increased in DA-infused hens as compared to vehicle-infused controls but the rate of increase was more in PRL than LHbeta subunit. This study demonstrates that exogenous DA activates hypothalamic VIP gene expression and this increased expression is limited exclusively to the avian INF. The increased VIP mRNA in the INF is correlated with increased levels of circulating PRL and PRL and LHbeta mRNAs in the anterior pituitary.     

Differential effects of hypothalamic IGF-I on gonadotropin releasing hormone neuronal activation during steroid-induced LH surges in young and middle-aged female rats

Interactions between brain IGF-I receptors and estrogen receptors regulate female reproductive physiology and behavior. The present study investigated potential mechanisms by which IGF-I receptors in the neuroendocrine hypothalamus regulate GnRH neuronal activation and LH release in young and middle-aged female rats under estradiol (E2) positive feedback conditions. We infused vehicle, IGF-I, or JB-1, a selective antagonist of IGF-I receptors, into the third ventricle of ovariectomized female rats primed with E2 and progesterone or vehicle. In young females, blockade of IGF-I receptors attenuated the steroid hormone-induced LH surge, reduced the percent of GnRH neurons expressing c-fos on the day of the LH surge, and decreased the total number of neurons expressing c-fos in the preoptic area. Middle-aged females had fewer GnRH neurons expressing c-fos during the LH surge than young females, and the LH surge amplitude was attenuated. Infusion of an IGF-I dose previously shown to increase LH surge amplitude did not increase the percent of GnRH neurons expressing c-fos in middle-aged females. Brain IGF-I receptor blockade did not modify E2 induction of progestin receptor-immunoreactive neurons in the preoptic area, arcuate, or ventromedial hypothalamus of young rats. These findings indicate that brain IGF-I receptors are required for E2 activation of GnRH neurons in young rats and for robust GnRH release from axon terminals in middle-aged females. IGF-I likely exerts its effects by actions on E2-sensitive neurons that are upstream of GnRH neurons and terminals.     

Colocalization of kisspeptin and gonadotropin-releasing hormone in the ovine brain

Kisspeptin is a peptide that has been implicated in the regulation of GnRH cells in the brain. Immunohistochemical studies were undertaken to examine the distribution of kisspeptin-immunoreactive (IR) cells in the ovine diencephalon and determine the effect of ovariectomy in the ewe. We report that kisspeptin colocalizes to a high proportion of GnRH-IR cells in the preoptic area, which is a novel finding. A high level of colocalization of kisspeptin and GnRH was also seen in varicose neuronal fibers within the external, neurosecretory zone of the median eminence. Apart from the kisspeptin/GnRH cells, a population of single-labeling kisspeptin-IR cells was also observed in the preoptic area. Within the hypothalamus, kisspeptin-IR cells were found predominantly in the arcuate nucleus, and there was an increase in the number of immunohistochemically identified cell within this nucleus after ovariectomy. Kisspeptin-IR cells were also found in the periventricular nucleus of the hypothalamus, but the number observed was similar in gonad-intact and ovariectomized ewes. The colocalization of GnRH and kisspeptin within cells of the preoptic area and GnRH neurosecretory terminals of the median eminence suggests that the two peptides might be cosecreted into the hypophyseal portal blood to act on the pituitary gland. Effects of ovariectomy on the non-GnRH, Kisspeptin-IR cells of the hypothalamus suggest that kisspeptin production is negatively regulated by ovarian steroids.     

Gonadotropin-releasing hormone increases the amount of messenger ribonucleic acid for gonadotropins in ovariectomized ewes after hypothalamic-pituitary disconnection

To investigate the role of GnRH in regulating the synthesis and secretion of gonadotropins, GnRH (250 ng/6 min every other hour for 7 days) or saline was administered to ovariectomized (OVX) ewes after hypothalamic-pituitary disconnection (HPD). Blood samples were collected from all HPD ewes on the day before and the day after HPD and on days 1 and 7 of GnRH or saline. At the end of day 7, anterior pituitary glands were removed for analysis of hormone, receptor, and mRNA content. The amount of mRNA for gonadotropins was lower (P less than 0.05) in saline-treated HPD ewes than in GnRH-treated HPD or OVX ewes. Administration of GnRH restored the amount of mRNA for FSH beta and alpha-subunits to levels similar (P greater than 0.05) to those measured in OVX ewes. The amount of mRNA for LH beta was higher (P less than 0.05) in GnRH-treated HPD ewes than in saline-treated HPD ewes, but lower (P less than 0.05) than that in OVX ewes. The pituitary content of LH and FSH was lower (P less than 0.05) in saline-treated HPD ewes than in OVX ewes. Administration of GnRH to HPD ewes maintained the ewes. Administration of GnRH to HPD ewes maintained the pituitary content of LH, but not FSH, compared to the pituitary gonadotropin content in OVX ewes. There were no differences (P greater than 0.05) in the amount of mRNA for GH or PRL or the pituitary content of these hormones among treatments. The number of hypophyseal receptors for GnRH was reduced in saline-treated HPD ewes (P less than 0.05) compared to that in OVX ewes and GnRH-treated HPD ewes. The number of hypophyseal receptors for 17 beta-estradiol was lower (P less than 0.05) in GnRH- and saline-treated HPD ewes than in OVX ewes. Serum LH concentrations were lower (P less than 0.05) after HPD than before HPD, but were restored to normal (P greater than 0.05) by GnRH replacement. Serum concentrations of FSH were lower (P less than 0.05) after HPD and were not affected by GnRH replacement. Serum PRL concentrations in all ewes were higher (P less than 0.05) after HPD than before HPD. Serum GH concentrations in all ewes were similar (P greater than 0.05) before and after HPD. Since synthesis and secretion of GH and PRL were not diminished after HPD, it was considered that the pituitary gland remained viable and functioned independently of hypothalamic input in OVX ewes after HPD.(ABSTRACT TRUNCATED AT 400 WORDS)     

Expression, structure, function, and evolution of gonadotropin-releasing hormone (GnRH) receptors GnRH-R1SHS and GnRH-R2PEY in the teleost, Astatotilapia burtoni

Multiple GnRH receptors are known to exist in nonmammalian species, but it is uncertain which receptor type regulates reproduction via the hypothalamic-pituitary-gonadal axis. The teleost fish, Astatotilapia burtoni, is useful for identifying the GnRH receptor responsible for reproduction, because only territorial males reproduce. We have cloned a second GnRH receptor in A. burtoni, GnRH-R1(SHS) (SHS is a peptide motif in extracellular loop 3), which is up-regulated in pituitaries of territorial males. We have shown that GnRH-R1(SHS) is expressed in many tissues and specifically colocalizes with LH in the pituitary. In A. burtoni brain, mRNA levels of both GnRH-R1(SHS) and a previously identified receptor, GnRH-R2(PEY), are highly correlated with mRNA levels of all three GnRH ligands. Despite its likely role in reproduction, we found that GnRH-R1(SHS) has the highest affinity for GnRH2 in vitro and low responsivity to GnRH1. Our phylogenetic analysis shows that GnRH-R1(SHS) is less closely related to mammalian reproductive GnRH receptors than GnRH-R2(PEY). We correlated vertebrate GnRH receptor amino acid sequences with receptor function and tissue distribution in many species and found that GnRH receptor sequences predict ligand responsiveness but not colocalization with pituitary gonadotropes. Based on sequence analysis, tissue localization, and physiological response we propose that the GnRH-R1(SHS) receptor controls reproduction in teleosts, including A. burtoni. We propose a GnRH receptor classification based on gene sequence that correlates with ligand selectivity but not with reproductive control. Our results suggest that different duplicated GnRH receptor genes have been selected to regulate reproduction in different vertebrate lineages.     

Gonadotropin-releasing hormone gene expression during the rat estrous cycle: effects of pentobarbital and ovarian steroids.

Although hypothalamic GnRH release is known to be modulated by neural and hormonal factors, the relationship between altered GnRH secretion and GnRH synthesis remains unclear. In an attempt to address this question, we examined GnRH gene expression in the rat hypothalamus using in situ hybridization histochemistry. An 25S-labeled antisense RNA probe was used to identify neurons expressing GnRH mRNA in an area that included the diagonal band of Broca, the organum vasculosum of the lamina terminalis, and the preoptic area. The number of GnRH mRNA-expressing cells was determined at various times during the rat estrous cycle. During proestrus, the number of GnRH mRNA-expressing cells decreased somewhat at 1400-1600 h, increased significantly at 1800 h (the time of the LH surge), then gradually returned to basal levels at 2200 h. Expression did not change substantially at other times during the estrous cycle. To understand this close temporal relationship between the LH surge and increased GnRH mRNA levels, we examined GnRH gene expression in proestrous animals in which the LH surge was blocked with pentobarbital. Pentobarbital treatment blocked the increase in the number of GnRH mRNA-expressing cells normally observed at 1800 h in saline-treated controls, suggesting that the increase in GnRH gene expression is closely coupled to secretion of GnRH from the hypothalamus. Finally, we addressed the question of whether ovarian steroids have direct effects on GnRH gene expression by examining GnRH mRNA levels in ovariectomized steroid-treated rats at a time before (1100 h) and a time after (1800 h) hypothalamic GnRH hypersecretion. At 1100 h, no significant changes were observed, but at 1800 h, estrogen-treated rats showed a significant increase in both the number of GnRH mRNA-expressing cells and serum LH levels. This suggests that estrogen influences GnRH gene expression indirectly, perhaps by altering hypothalamic GnRH release. Our results in each of these models suggest that GnRH mRNA levels increase in response to GnRH hypersecretion at the time of the LH surge.     

Dexamethasone alters responses of pituitary gonadotropin-releasing hormone (GnRH) receptors, gonadotropin subunit messenger ribonucleic acids, and gonadotropins to pulsatile GnRH in male rats.

Dexamethasone (Dex), when administered in high doses, has been shown to suppress spontaneous and GnRH-induced gonadotropin secretion, but the level and the mechanism(s) of this effect are unknown. We administered Dex to castrate testosterone-replaced male rats to determine if gonadotropin gene expression is affected and whether Dex differentially influences GnRH-modulated parameters of gonadotrope function: induction of GnRH receptors (GnRH-R) and gonadotropin synthesis and secretion. GnRH was given iv at 25 ng/pulse at 8, 30, and 120 min intervals for 48 h. Rapid GnRH injection frequency preferentially increased alpha and LH-beta messenger RNA (mRNA) responses to GnRH as well as LH secretion. Slower GnRH injection frequencies were required to increase levels of GnRH-R, FSH-beta mRNA, and FSH secretion. Dex selectively inhibited the serum LH, alpha, and LH-beta mRNA responses to GnRH, but not the serum FSH or FSH-beta mRNA responses. Additionally, it augmented the GnRH-induced increase in GnRH-R. We conclude: 1) induction of GnRH-R, gonadotropin synthesis, and secretion require different modes of GnRH stimulation; 2) Dex acts directly on the gonadotrope to differentially modulate GnRH-induced increases in GnRH-R levels, gonadotropin gene expression, and gonadotropin secretion; and 3) GnRH effects upon induction of GnRH-R, LH, and FSH synthesis and secretion are likely to be mediated via different cellular pathways.