Luteinizing hormone-releasing hormone (LH-RH) antagonist Cetrorelix down-regulates the mRNA expression of pituitary receptors for LH-RH by counteracting the stimulatory effect of endogenous LH-RH

Structural Determinants of the Stereoinverting Activity of Pseudomonas stutzeri d-Phenylglycine Aminotransferase

Aromatic d-amino acids are key precursors for the production of many small molecule therapeutics. Therefore, the development of biocatalytic methods for their synthesis is of great interest. An enzyme that has great potential as a biocatalyst for the synthesis of d-amino acids is the stereoinverting d-phenylglycine aminotransferase (DPAT) from Pseudomonas stutzeri ST-201. This enzyme catalyzes a unique l to d transamination reaction that produces d-phenylglycine and α-ketoglutarate from benzoylformate and l-glutamate, via a mechanism that is poorly understood. Here, we present the crystal structure of DPAT, which shows that the enzyme folds into a two-domain structure representative of class III aminotransferases. Guided by the crystal structure, we performed saturation mutagenesis to probe the substrate binding pockets of the enzyme. These experiments helped us identify two arginine residues (R34 and R407), one in each binding pocket, that are essential to catalysis. Together with kinetic analyses using a library of amino acid substrates, our mutagenesis and structural studies allow us to propose a binding model that explains the dual l/d specificity of DPAT. Our kinetic analyses also demonstrate that DPAT can catalyze the transamination of β- and γ-amino acids, reclassifying this enzyme as an ω-aminotransferase. Collectively, our studies highlight that the DPAT active site is amenable to protein engineering for expansion of its substrate scope, which offers the opportunity to generate new biocatalysts for the synthesis of a variety of valuable optically pure d-amino acids from inexpensive and abundant l-amino acids.

Concurrence of chromosome 3 and 4 aberrations in human uveal melanoma

Uveal melanoma (UM) is the most common primary intraocular malignancy with a very poor prognosis. The most frequent chromosome aberration in UM is the monosomy of chromosome 3. Previously, we demonstrated that ~50% of UMs express type-I receptor for luteinizing hormone‑releasing hormone (LH-RH-R). The gene encoding LH-RH-R is located in chromosome 4 (location: 4q21.2); however, the occurrence of numerical aberrations of chromosome 4 have never been studied in UM. In the present study, we investigated the abnormalities of chromosome 3 and 4, and the possible correlation between them, as well as with LH-RH-R expression. Forty-six specimens of UM were obtained after enucleation. Numerical aberrations of chromosome 3 and 4 were studied by fluorescence in situ hybridization (FISH). Chromosome 4 was detected in normal biparental disomy only in 14 (30%) samples; however, 32 cases (70%) showed more than 2 signals/nucleus. Monosomy of chromosome 3 could be found in 16 (35%) samples. In 6 specimens (13%), more than 2 copies of chromosome 3 were found, while normal biparental disomy was detected in 24 (52%) samples. Statistical analysis indicated a statistically significant (p<0.05) correlation between the copy number of chromosome 3 and 4. Moreover, moderate difference was revealed in the survival rate of the UM patients with various pathological profiles. No correlation was found between chromosome aberrations and LH-RH-R expression. Our results clearly demonstrate abnormalities in chromosome 3 and 4 and the incidence of the monosomy of chromosome 3 in human UM. In summary, our results provide new incite concerning the genetic background of this tumor. Our findings could contribute to a more precise determination of the prognosis of human UM and to the development of new therapeutic approaches to this malignancy.

Luteinizing hormone: Evidence for direct action in the CNS

This article is part of a Special Issue "SBN 2014". Hormonal dysfunction due to aging, especially during menopause, plays a substantial role in cognitive decline as well as the progression and development of neurodegenerative diseases. The hypothalamic-pituitary-gonadal (HPG) axis has long been implicated in changes in behavior and neuronal morphology. Most notably, estrogens have proven beneficial in the healthy brain through a host of different mechanisms. Recently, luteinizing hormone (LH) has emerged as a candidate for further investigation for its role in the CNS. The basis of this is that both LH and the LH receptor are expressed in the brain, and serum levels of LH correlate with cognitive deficits and Alzheimer's disease (AD) incidence. The study of LH in cognition and AD primarily focuses on evaluating the effects of downregulation of this peptide. This literature has shown that decreasing peripheral LH, through a variety of pharmacological interventions, reduces cognitive deficits in ovariectomy and AD models. However, few studies have researched the direct actions of LH on neurons and glial cells. Here we summarize the role of luteinizing hormone in modulating cognition, and we propose a mechanism that underlies a role for brain LH in this process.

Hypothalamic-pituitary-gonadal axis involvement in learning and memory and Alzheimer's disease: more than "just" estrogen

Accumulating studies affirm the effects of age-related endocrine dysfunction on cognitive decline and increasing risk of neurodegenerative diseases. It is well known that estrogens can be protective for cognitive function, and more recently androgens and luteinizing hormone have also been shown to modulate learning and memory. Understanding the mechanisms underlying hypothalamic-pituitary-gonadal axis-associated cognitive dysfunction is crucial for therapeutic advancement. Here, we emphasize that reproductive hormones are influential in maintaining neuronal health and enhancing signaling cascades that lead to cognitive impairment. We summarize and critically evaluate age-related changes in the endocrine system, their implications in the development of Alzheimer's disease, and the therapeutic potential of endocrine modulation in the prevention of age-related cognitive decline.

The Impact of Pituitary Blockage with GnRH Antagonist and Gonadotrophin Stimulation Length on The Outcome of ICSI Cycles in Women Older than 36 Years

BACKGROUND

The objective of this retrospective cohort study was to evaluate whether the length of pituitary blockage with gonadotrophin-releasing hormone (GnRH) antagonists or the stimulation period influence intracytoplasmic sperm injection (ICSI) outcomes in patients older than 36 years of age.

MATERIALS AND METHODS

In this retrospective study, a total of 138 couples with maternal age >36 years undergoing ICSI with an antagonist protocol were included. The influences of stimulation and suppression length on the response to ovarian stimulation and ICSI outcomes were investigated. Receiver operating characteristic curve (ROC) analysis was performed to assess the predictive value of the stimulation period for achievement of implantation and pregnancy.

RESULTS

THE GONADOTROPHIN STIMULATION LENGTH NEGATIVELY INFLUENCED THE IMPLANTATION RATE (RC: -4.200; p=0.023). The area under ROC curve (AUC) could distinguish between women with positive and negative implantation (AUC: 0.611; CI: 0.546-0.673) and pregnancy (AUC: 0.593; CI: 0.528-0.656). The threshold value demonstrated a high negative predictive value on likelihood of implantation (p=0.0032, 90% sensitivity) and pregnancy (p=0.0147, 87.1% sensitivity) when patients underwent more than 10 days of stimulation.

CONCLUSION

The stimulation period negatively influences the implantation rate in women older than 36 years. A stimulation interval greater than 10 days is associated with a negative predictive value for the chance of implantation and pregnancy.

Strategies for Pituitary Down-regulation to Optimize IVF/ICSI Outcome in Poor Ovarian Responders

The ovarian stimulation of poor responders still remains a challenging task for clinicians. There are numerous strategies that have been suggested to improve the outcome in poor responders but there is still no one pituitary down-regulation protocol that best suits all women with such condition. Traditional GnRH agonist flare and long luteal phase protocols do not appear to be advantageous. Reduction of GnRH agonist doses, "stop" protocols, and microdose GnRH agonist flare regimes all appear to improve outcomes, although the proportional benefit of one approach over another has not been convincingly established. GnRH antagonists improve outcomes in this patient population, although, in general, pregnancy rates appear to be lower in comparison to microdose GnRH agonist flare regimes.

Will GnRH antagonists improve prostate cancer treatment?

Androgen ablation forms a basis for treating prostate cancer and is achieved either by surgical castration, or pharmacologically using oestrogens, anti-androgens and/or gonadotropin-releasing hormone (GnRH) analogues. GnRH antagonists (or blockers) offer a new means of treatment by directly blocking GnRH receptors. Advantages of GnRH antagonists include lack of the initial stimulation of gonadotropin and testosterone production, lack of gonadotropin microsurges and sustained follicle-stimulating hormone suppression; disadvantages include increased histamine release. This review discusses advantages and disadvantages of the GnRH antagonists currently in development, in light of receptor physiology and pre-clinical and clinical data. Comparative clinical trials will ultimately establish their efficacy in comparison to other pharmacotherapies. Therefore, continuing development and refinement is needed to improve prostate cancer treatment.

Effect of acupuncture on pubertal development of rats and rabbits at different developmental stages

Physiological and endocrine studies on sexual development in animals and effects of acupuncture on sexual development are limited. Therefore, we investigated the effect of electro-acupuncture (EA) on the arcuate nucleus (Arc) and release of gonadotropin-releasing hormone (GnRH) in animals at different developmental stages. In Experiment 1, EA stimulation (30 Hz) was performed for 30 min per day in EA group of rabbits for 48 days, while the control group (mature rabbits) was not given EA. Arc discharges in those two groups were measured after the 48-day treatment. Arc discharge was also measured in the pre-pubertal group (as control) without EA treatment. Then, all three groups were treated with transient EA for 30 min and Arc discharges were determined again. In Experiment 2, EA (3 Hz) at the same acupoints or non-acupoints as that in the rabbits was performed for 20 min per day in different developmental group of Sprague-Dawley rats for 10 days. GnRH mRNA expression in the hypothalamus of rats was determined using RT-PCR and real-time PCR. The serum sexual hormone, sperm count, and body weight was measured. The results showed that the Arc discharge (P<0.01), testosterone (T) (P<0.01) and sperm count (P<0.01) in male rabbits were reduced by repeated EA. However, the body weight of rabbits was not changed after EA compared to the control in Experiment 1. In Experiment 2, GnRH mRNA expression in rats of the early pubertal group (EPG) and adult group (AG) were significantly depressed after repeated EA at acupoints (P<0.01). The sexual hormones were negatively influenced by repeated EA during puberty. Sperm count was reduced significantly after repeated EA at time of puberty (P<0.01). Repeated EA did not influence body weight of rats (P>0.01) and structures of the gonadial tissues during development. The results suggested that repeated EA is a good option that can be considered for regulating the function of the hypothalamus-pituitary-gonad (HPG) axis during puberty.

Role of gonadotropin-releasing hormone antagonists in poor responders

OBJECTIVE

To evaluate the role of GnRH antagonists in poor-responder protocols.

DESIGN

Literature review.

CONCLUSION(S)

The optimum stimulation protocol for poor responders is unknown. Although many IVF programs currently use GnRH antagonists for poor responders, there have been only four prospective, randomized trials comparing GnRH antagonists to alternate protocols. None of these studies had sufficient power to evaluate a difference in pregnancy rates (PRs), and in all four cases, IVF outcomes were comparable. Nevertheless, interest in the use of GnRH antagonists in poor responders has continued. GnRH antagonists may be associated with simpler stimulation protocols, lower gonadotropin requirements, reduced patient costs, and shorter downtimes between consecutive cycles. However, the greatest advantage of GnRH antagonists may lie in the ability to assess ovarian reserves immediately prior to deciding whether or not to initiate gonadotropin stimulation. The ability to respond to cycle-to-cycle variation in antral follicle counts may allow the optimization of oocyte yield and reduce cycle cancellation rates. It remains to be seen if this approach (initiating gonadotropins only in cycles where an adequate antral follicle count is present) also translates into higher clinical PRs for poor responders.

Actions of gonadotropin-releasing hormone analogues in pituitary gonadotrophs and their modulation by ovarian steroids

Recently, GnRH antagonists (GnRHant) like cetrorelix and ganirelix have been introduced in protocols of controlled ovarian hyperstimulation for assisted reproductive techniques to prevent premature luteinizing hormone (LH) surges. Here we tested, whether the actions of cetrorelix and the GnRH agonist (GnRHag) triptorelin in gonadotrophs are dependent on the steroid milieu. Furthermore, we characterized the actions of cetrorelix and triptorelin on LH secretion and the total LH pool. Female rat pituitary cells were treated either with 0.1 nM triptorelin for 1, 2, 4 and 6 days or for 1, 3, 5 and 6 h or with 1, 10 or 100 nM cetrorelix for 1, 2, 3 and 5 h or for 10 min. Cells were stimulated for 3h with different concentrations of GnRH (10 pM-1 microM). For analysis of the total LH pool, which is composed of stored and released LH, cells were lysed with 0.1% Triton X-100 at -80 degrees C overnight. To test, whether the steroid milieu affects the actions of cetrorelix and triptorelin, cells were incubated for 52 h with 1 nM estradiol (E) alone or with combinations of 100 nM progesterone (P) for 4 or 52 h, respectively. Cells were then treated with 0.1 nM triptorelin for 9 h or 1 nM cetrorelix for 3 h and stimulated for 3 h with different concentrations of GnRH (10 pM-1 microM). The suppressive effect of triptorelin on LH secretion was fully accomplished after 3 h of treatment, for cetrorelix only 10 min were sufficient. The concentration of cetrorelix must be at least equimolar to GnRH to block LH secretion. Cetrorelix shifted the EC50s of the GnRH dose-response curve to the right. Triptorelin suppressed total LH significantly (from 137 to 36 ng/ml) after 1 h in a time-dependent manner. In contrast, only high concentrations of cetrorelix increased total LH. In steroid treated cells the suppressive effects of triptorelin were more distinct. One nanomolar cetrorelix suppressed GnRH-stimulated LH secretion of cells not treated with steroids from 10.1 to 3.5 ng/ml. In cells, additionally treated with estradiol alone or estradiol and short-term progesterone, LH levels were higher (from 3.5 to 5.4 or 4.5 ng/ml, respectively). In cells co-treated with estradiol and progesterone for 52 h LH secretion was only suppressed from 10.1 to 9.5 ng/ml. Steroid treatments diminished the suppressive effect of cetrorelix on LH secretion. In conclusion, the depletion of the total LH pool contributes to the desensitizing effect of triptorelin. The actions of cetrorelix and triptorelin are dependent on the steroid milieu.

Comparison of embryo implantation in Wistar rats that underwent ovarian stimulation using exogenous gonadotropins associated with cetrorelix acetate or leuprolide acetate

OBJECTIVE

To compare embryo implantation in Wistar rats submitted to ovarian stimulation using recombinant FSH (rFSH) with cetrorelix acetate or leuprolide acetate.

DESIGN

Experimental study.

SETTING

Faculty of medicine animal facility.

PATIENT(S)

Fifty-six female Wistar rats with normal estrus cycles and 30 male.

INTERVENTION(S)

Ovarian stimulation and laparotomy (by the day 13 of gestation).

MAIN OUTCOME MEASURE(S)

Embryo implantation.

RESULT(S)

The female rats were subdivided into four groups: group 1, medicated with rFSH, hCG, and cetrorelix acetate; group 2, medicated with rFSH, hCG, and leuprolide acetate; group 3, medicated with rFSH and hCG; and group 4, in which only saline solution was administered. The female rats were mated with fertile male rats on the day of hCG administration with copulation confirmed through cytologic vaginal analysis. The females were killed on the 13th day of gestation. After laparotomy, comparison and identification was done regarding the numbers of corpora lutea and embryo implantations and gestation rates. Group 1 presented lower numbers of corpora lutea and embryo implantations in comparison to the other groups (P<.05). A difference was not found in the gestation rates between the groups.

CONCLUSION(S)

The number of embryo implantations in Wistar rats medicated with rFSH and cetrorelix acetate is lower than that of rats medicated with rFSH and leuprolide acetate.

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.

Endocrine treatment of prostate cancer

Although androgen deprivation as a treatment for patients with prostate cancer was described more than 60 years ago its optimal use remains controversial. The widespread use of prostate-specific (PSA) assay has lead to earlier diagnosis and earlier detection of recurrent disease. This means that the systemic side effects of androgen deprivation and quality of life have become more important. Debates continue regarding the proper use and timing of endocrine therapy with orchiectomy, oestrogen agonists, gonadotropin hormone-releasing hormone (GnRH) agonists, GnRH antagonists, and androgen antagonists. A critical review of the literature was performed. Data support that androgen deprivation is an effective treatment for patients with advanced prostate cancer. However, although it improves survival, it is not curative, and creates a spectrum of unwanted effects that influence quality of life. Castration remains the frontline treatment for metastatic prostate cancer, where orchiectomy, oestrogen agonists and GnRH agonists produce equivalent clinical responses. Maximum androgen blockade (MAB) is not significantly more effective than single agent GnRH agonist or orchiectomy. Nonsteroidal antiandrogen monotherapy is as effective as castration in treatment of locally advanced prostate cancer offering quality of life benefits. Adjuvant endocrine treatment is able to delay disease progression at any stage. There is, however, controversy of the possible survival benefit of such treatment, including patients having PSA relapse after definitive local treatment for prostate cancer. Neoadjuvant endocrine treatment has its place mainly in the external beam radiotherapy setting. Intermittent androgen blockade is still considered experimental. The decision regarding the type of androgen deprivation should be made individually after informing the patient of all available treatment options, including watchful waiting, and on the basis of potential benefits and adverse effects. Several large studies are under way to investigate the role of adjuvant endocrine treatment in the field of early prostate cancer, intermittent androgen deprivation and endocrine therapy alone compared with endocrine therapy with radiotherapy. The real challenge, however, is to develop better means to avert hormone-refractory prostate cancer and better treatments for patients with hormone-refractory disease when it occurs.

Effect of long-term treatment with low doses of the LHRH antagonist Cetrorelix on pituitary receptors for LHRH and gonadal axis in male and female rats

Our previous studies showed that treatment of female rats with large doses of Cetrorelix, an antagonist of luteinizing hormone-releasing hormone (LHRH), reduces levels of serum LH, estradiol, progesterone, and the concentration of pituitary LHRH receptors (LHRH-Rs) and their mRNA expression. Serum LH and testosterone levels and pituitary LHRH-R in male rats are also decreased by high doses of Cetrorelix. This approach can be used for therapy of sex hormone-dependent cancers. However, in conditions where an incomplete hormone deprivation is indicated, lower doses of Cetrorelix may suffice. Thus, we investigated the effect of a 30-day treatment with a low-dose depot formulation of Cetrorelix (20-24 microg per kg per day) on the pituitary-gonadal axis of male and female rats. In both sexes, lower serum LH levels were observed on day 4 after administration. In males, LH returned to control levels by day 10, whereas in females, a rebound LH elevation occurred. Testosterone levels in male rats were decreased up to day 20, but on day 30, the values were similar to controls. In females, serum estradiol was reduced on day 4; however, by day 10 it returned to normal. Progesterone levels were diminished through the entire period. Female rats showed diestrous smears during the first week of treatment and prolonged estrous periods thereafter. The weights of testes and ovaries were significantly lower, but not the weights of prostate, seminal vesicles, and uterus. Pituitary LHRH-R mRNA and LHRH-R protein levels were not significantly different from the controls. Thus, the treatment with low doses of Cetrorelix did not seriously impair gonadal functions. The results suggest that Cetrorelix in low doses induces only a partial pituitary-gonadal inhibition and might be indicated for treatment of endometriosis, leiomyomas, and benign prostatic hyperplasia.

Effects of long-term treatment with the luteinizing hormone-releasing hormone (LHRH) agonist Decapeptyl and the LHRH antagonist Cetrorelix on the levels of pituitary LHRH receptors and their mRNA expression in rats

The effects of depot formulations of the luteinizing hormone-releasing hormone (LHRH) agonist Decapeptyl (25 microg/day) for 30 days or LHRH antagonist Cetrorelix pamoate (100 microg/day) for 30 days and daily injections of 100 microg of Decapeptyl for 10 days on the expression of mRNA for pituitary LHRH receptor (LHRH-R) and the levels of LHRH-R protein were evaluated in rats. Serum sex steroid concentrations and the weights of the reproductive organs were greatly reduced in all groups treated with analogs, demonstrating an efficient blockade of the pituitary-gonadal axis. Decapeptyl microcapsules elevated serum LH in female rats, but decreased it in male rats. LHRH-R mRNA expression in female pituitaries was reduced to 41% and 56-65% on days 10 and 30, respectively, whereas LHRH-R protein was 64% of control on day 10 and returned to pretreatment levels on day 30. Decapeptyl microcapsules reduced LHRH-R mRNA expression in male pituitaries to 58% on day 30 but not LHRH-R protein. Daily injections of Decapeptyl caused a desensitization of LH responses in female rats, while raising LHRH-R mRNA expression in female rats by 23% and LHRH-R protein levels by 119%. Cetrorelix pamoate reduced serum LH in female rats and diminished LHRH-R mRNA to 30% and 26% and LHRH-R protein to 57% and 48% on days 10 and 30, respectively. Elevated LHRH-R protein levels of ovariectomized rats were reduced after 10-day treatment with Cetrorelix or 100 microg/day Decapeptyl. Thus, changes in the mRNA expression after treatment with Cetrorelix, but not always Decapeptyl, paralleled those of LHRH-R protein. The inhibitory effect of Cetrorelix on serum LH, pituitary LHRH-R mRNA, and LHRH-R protein was greater than that of Decapeptyl.

Changes in subcellular distribution of pituitary receptors for luteinizing hormone-releasing hormone (LH-RH) after treatment with the LH-RH antagonist cetrorelix

Treatment with antagonists of luteinizing hormone-releasing hormone (LH-RH) leads to down-regulation of pituitary LH-RH receptors. Thus, the effect of LH-RH antagonists is similar to that of the LH-RH agonists, but the mode of action of antagonists is not completely understood. The aim of this study was to investigate the effects of LH-RH antagonist cetrorelix on the binding characteristics and subcellular localization of receptors for LH-RH in rat pituitaries. Radioligand binding studies, performed after in vitro desaturation, revealed that a single s.c. injection of cetrorelix at a dose of 100 microg per rat significantly decreased the number of pituitary membrane receptors for LH-RH in a time-dependent manner with the nadir occurring at 6 h. In contrast, 2-6 h after cetrorelix treatment, the concentration of binding sites for LH-RH in the nuclei of rat pituitaries was significantly higher (P < 0.01) than in controls. Chronic administration of cetrorelix also decreased the level of membrane receptors for LH-RH by 83% (P < 0.01) after 7 days, and 86% (P < 0.01) after 14 days. The number of LH-RH binding sites in the nuclear pellet was increased 3-fold (P < 0.01) by days 7 and 14 after the initiation of treatment with cetrorelix. A single injection or prolonged treatment with LH-RH antagonist also decreased the mRNA expression of pituitary receptors for LH-RH. Our results demonstrate that the down-regulation of LH-RH receptors on the cell membranes of rat pituitaries after therapy with antagonist cetrorelix is associated with an increase in receptor concentration in the nuclei. These phenomena could be related to the internalization and subcellular translocation of LH-RH receptors.

Comparison of mechanisms of action of luteinizing hormone-releasing hormone (LHRH) antagonist cetrorelix and LHRH agonist triptorelin on the gene expression of pituitary LHRH receptors in rats

The mechanisms through which luteinizing hormone (LH)-releasing hormone (LHRH) antagonists suppress pituitary gonadotroph functions and LHRH-receptor (LHRH-R) expression are incompletely understood. Consequently, we investigated the direct effect of LHRH antagonist cetrorelix in vitro on the expression of the pituitary LHRH-R gene and its ability to counteract the exogenous LHRH and the agonist triptorelin in the regulation of this gene. We also compared the effects of chronic administration of cetrorelix and triptorelin on the LHRH-R mRNA level and gonadotropin secretion in ovariectomized (OVX) and normal female rats. The exposure of pituitary cells in vitro to 3-min pulses of 1 nM LHRH or 0.1 nM triptorelin for 5 h increased the LHRH-R mRNA level by 77-88%. Continuous perfusion of the cells with 50 nM cetrorelix did not cause any significant changes, but prevented the stimulatory effect of LHRH pulses on the receptor mRNA expression. In OVX rats, 10 days after administration of a depot formulation of cetrorelix, releasing 100 microg of peptide daily, the elevated LHRH-R mRNA level was decreased by 73%, whereas daily injection of 100 microg of triptorelin caused a 41% suppression. In normal female rats, cetrorelix treatment suppressed the LHRH-R mRNA level by 33%, but triptorelin increased it by 150%. The highly elevated serum LH levels in OVX rats and the normal LH concentration of cycling rats were rapidly and completely suppressed by cetrorelix. Triptorelin decreased the serum LH in OVX rats to the precastration level, but had no effect on basal LH in normal rats. Our results confirm that LHRH antagonists, such as cetrorelix, inhibit the gene expression of pituitary LHRH-R indirectly, by counteracting the stimulatory effect of LHRH. A rapid suppression of serum LH by LHRH antagonists would be advantageous in the treatment of sex hormone-dependent tumors and other conditions.