Desensitization of cultured pituitary cells to gonadotropin-releasing hormone: evidence for a post-receptor mechanism

Dynamic and Sex-Specific Changes in Gonadotropin-Releasing Hormone Neuron Activity and Excitability in a Mouse Model of Temporal Lobe Epilepsy

Reproductive endocrine disorders are prominent comorbidities of temporal lobe epilepsy (TLE) in both men and women. The neural mechanisms underlying these comorbidities remain unclear, but hypothalamic gonadotropin-releasing hormone (GnRH) neurons may be involved. Here, we report the first direct demonstrations of aberrant GnRH neuron function in an animal model of epilepsy. Recordings of GnRH neuron firing and excitability were made in acute mouse brain slices prepared two months after intrahippocampal injection of kainate (KA) or control saline, a well-established TLE model in which most females develop comorbid estrous cycle disruption. GnRH neurons from control females showed elevated firing and excitability on estrus compared with diestrus. By contrast, cells from KA-injected females that developed prolonged, disrupted estrous cycles (KA-long) showed the reverse pattern. Firing rates of cells from KA-injected females that maintained regular cycles (KA-regular) were not different from controls on diestrus, but were reduced on estrus. In KA-injected males, only GnRH neurons in the medial septum displayed elevated firing. In contrast to the diestrus versus estrus and sex-specific changes in firing, GnRH neuron intrinsic excitability was elevated in all KA-injected groups, indicating a role for afferent synaptic and neuromodulatory inputs in shaping overall changes in firing activity. Furthermore, KA-injected females showed cycle-stage-specific changes in circulating sex steroids on diestrus and estrus that also differed between KA-long and KA-regular groups. Together, these findings reveal that the effects of epilepsy on the neural control of reproduction are dynamic across the estrous cycle, distinct in association with comorbid estrous cycle disruption severity, and sex-specific.

Evaluation of the Biological Properties and the Enzymatic Stability of Glycosylated Luteinizing Hormone-Releasing Hormone Analogs

The enzymatic stability, antitumor activity, and gonadotropin stimulatory effects of glycosylated luteinizing hormone-releasing hormone (LHRH) analogs were investigated in this study. Conjugation of carbohydrate units, including lactose (Lac), glucose (GS), and galactose (Gal) to LHRH peptide protected the peptide from proteolytic degradation and increased the peptides' half-lives in human plasma, rat kidney membrane enzymes, and liver homogenate markedly. Among all seven modified analogs, compound 1 (Lac-[Q(1)][w(6)]LHRH) and compound 6 (GS(4)-[w(6)]LHRH) were stable in human plasma during 4 h of experiment. The half-lives of compounds 1 and 6 improved significantly in kidney membrane enzymes (from 3 min for LHRH to 68 and 103 min, respectively). The major cleavage sites for most of the glycosylated compounds were found to be at Trp(3)-Ser(4) and Ser(4)-Tyr(5) in compounds 1-5. Compound 6 was hydrolyzed at Ser(4)-Tyr(5) and the sugar conjugation site. The antiproliferative activity of the glycopeptides was evaluated on LHRH receptor-positive prostate cancer cells. The glycosylated LHRH derivatives had a significant growth inhibitory effect on the LNCaP cells after a 48-h treatment. It was demonstrated that compound 1 significantly increased the release of luteinizing hormone (LH) at 5 and 10 nM concentrations and compound 5 (GS-[Q(1)]LHRH) stimulated the release of follicle-stimulating hormone (FSH) at 5 nM concentration in dispersed rat pituitary cells (p < 0.05). In our studies, compound 1-bearing lactose and D-Trp was the most stable and active and is a promising candidate for future preclinical investigations in terms of in vitro biological activity and metabolic stability.

Predominant suppression of follicle-stimulating hormone β-immunoreactivity after long-term treatment of intact and castrate adult male rats with the gonadotrophin-releasing hormone agonist deslorelin

Gonadotrophin-releasing hormone (GnRH) agonists are used to treat gonadal steroid-dependent disorders in humans and to contracept animals. These agonists are considered to work by desensitising gonadotrophs to GnRH, thereby suppressing follicle-stimulating hormone (FSH) and luteinising hormone (LH) secretion. It is not known whether changes occur in the cellular composition of the pituitary gland after chronic GnRH agonist exposure. Adult male Sprague-Dawley rats were treated with a sham, deslorelin, or deslorelin plus testosterone implant for 41.0 ± 0.6 days. In a second experiment, rats were castrated and treated with deslorelin and/or testosterone. Pituitary sections were labelled immunocytochemically for FSHβ and LHβ, or gonadotrophin α subunit (αGSU). Deslorelin suppressed testis weight by two-thirds and reduced plasma FSH and LH in intact rats. Deslorelin decreased the percentage of gonadotrophs, although the effect was specific to the FSHβ-immunoreactive (-ir) cells. Testosterone did not reverse the deslorelin-induced reduction in the overall gonadotroph population. However, in the presence of testosterone, the proportion of gonadotrophs that was FSHβ-ir increased in the remaining gonadotrophs. There was no effect of treatment on the total LHβ-ir cell population, although the loss of FSHβ in bi-hormonal cells increased the proportion of mono-hormonal LHβ-ir gonadotrophs. The castration-induced plasma LH and FSH increases were suppressed by deslorelin, testosterone or both. Castration increased both LH-ir and FSH-ir without increasing the overall gonadotroph population, thus increasing the proportion of bi-hormonal cells. Deslorelin suppressed these increases. Testosterone increased FSH-ir in deslorelin-treated castrate rats. Deslorelin did not affect αGSU immunoreactivity, suggesting that the gonadotroph population per se is not eliminated by deslorelin, although the ability of gonadotrophs to synthesise FSHβ is compromised. We hypothesise that the FSH dominant suppression may be central to the long-term contraceptive efficacy of deslorelin in the male.

Pituitary responses of seasonally anoestrous ewes to long-term continuous infusion of low doses of GnRH

The response to long-term continuous infusion of gonadotrophin releasing hormone(GnRH)was monitored in progesterone-treated seasonally anoestrous ewes. Using osmotic pumps, groups of five ewes eachreceived 0(controls),125, 250, 500 or 1000 ng GnRH per h subcutaneously for a period of 21 days. Bloodsamples were collecte dat 30-minintervals from 6 h before until 24 h after the start of treatment and then for 8-h periods on days 2, 8, 15 and 21. After 21 days of treatment all the ewes were slaughtered to determine pituitary GnRH receptor numbers. Continuous infusion of GnRH resulted in a short-lived (2day)increase in plasma LH and oestradiol concentrations after which they were not different from the pre-treatment values. Over the later period of treatment when the pituitary gland was not responding to the exogenous GnRH(days 8, 15 and 21), LH episodes(presumably due to endogenous GnRH secretion fromthe hypothalamus) were observed. Continuous infusion of GnRH was also associated with a suppression in plasma FSH concentrations, the duration of which was dose-dependent. Only at the highest GnRH dose level(1000 ng/h)was there a significant reductionin pituitary GnRH receptor content.

New developments in the use of peptide gonadotropin-releasing hormone antagonists versus agonists

Gonadotropin-releasing hormone (GnRH) stimulates the pituitary secretion of both luteinising hormone (LH) and follicle-stimulating hormone (FSH), and thus controls the hormonal and reproductive functions of the gonads. The blockade of the effects of GnRH may be sought for a variety of reasons; for example, to control premature LH surges and to reduce the cancellation rate with the aim of improving the pregnancy rate per treatment cycle or in the treatment of sex hormone-dependent disorders. Selective blockade of LH/FSH secretion and subsequent chemical castration have previously been achieved by desensitising the pituitary to continuously administered GnRH or by giving long-acting GnRH agonists. GnRH analogues are indicated for clinical situations in which the suppression of endogenous gonadotropins (precocious puberty, contraception and controlled ovarian hyperstimulation) or sexual steroids (endometriosis, prostate hyperplasia, cancer and uterine fibroids) is desired. The immediate suppression of the pituitary that is achieved by GnRH antagonists without an initial stimulatory effect is the main advantage of these compounds over the agonists. GnRH antagonists have been developed for clinical use with acceptable pharmacokinetic, safety and commercial profiles. In assisted reproduction, these compounds seem to be as effective as established therapy, but with shorter treatment times, less use of gonadotropic hormones, improved patient acceptance, and fewer follicles and oocytes. All of the current indications for GnRH agonist desensitisation may prove to be indications for a GnRH antagonist, including endometriosis, leiomyoma and breast cancer in women, benign prostatic hypertrophy and prostatic carcinoma in men, and central precocious puberty in children. However, the best clinical evidence has been in assisted reproduction and prostate cancer.

GnRH-antagonists in reproductive medicine

Suppression of sex steroid production based on desensitisation and down-regulation of pituitary gonadotropin-releasing hormone (GnRH)-receptors by agonistic GnRH-analogues resulting in the blockage of gonadotropin release from the anterior pituitary gland is a well-established approach in a variety of clinical conditions. Antagonistic analogues of GnRH exert their effect by competing with endogenous GnRH for pituitary binding sites. Because of the lack of any intrinsic activity of these compounds, the characteristic initial 'flare-up' effect of GnRH-agonist administration is absent. A more rapid suppression of gonadotropin release from the pituitary gland can be achieved, enabling shorter treatment regimes in ovarian hyperstimulation for assisted reproduction. As yet, GnRH-antagonists have attained market approval only for the indication of premature luteinizing hormone (LH) surge prevention in controlled ovarian hyperstimulation and palliative treatment of advanced prostatic cancer. However, GnRH-antagonists may be useful in a variety of other malignant and non-malignant indications where rapid sex steroid suppression is desired, such as uterine leiomyomas, endometriosis, gynaecological cancers or benign prostatic hyperplasia. In the context of infertility treatment, available data on the application of GnRH-antagonists in the treatment of endometriosis and uterine leiomyomas are reviewed.

Gonadotropin-Releasing Hormone Antagonists for Assisted Reproductive Techniques

Gonadotropin-releasing hormone (GnRH) antagonists have been tested extensively in ovarian stimulation protocols for assisted reproductive techniques (ART). GnRH antagonists immediately and rapidly inhibit gonadotropin release by the anterior pituitary gland by competitive blockage of the GnRH receptor, preventing and interrupting luteinising hormone surges in controlled ovarian hyperstimulation for infertility treatment. A review of the available literature on GnRH antagonists for ART is presented, focusing on the pharmacological and clinical properties of the two compounds available on the market, cetrorelix and ganirelix. Both cetrorelix and ganirelix are well tolerated and effective drugs for controlled ovarian hyperstimulation and are of comparable value for infertility treatment. Cetrorelix is available as a 0.25mg preparation for daily injections and as a 3mg intermediate depot preparation. Ganirelix is available as a 0.25mg preparation for daily injections.Currently, two treatment protocols are used in clinical practice: the GnRH antagonist multiple-dose protocol and the GnRH antagonist single-dose protocol. Both protocols are effective and well tolerated. Cetrorelix and ganirelix have not yet been directly compared in a clinical trial; nor have the single-dose and the multiple-dose approaches been compared in a randomised, controlled trial. Data to compare these compounds in clinical terms can be extrapolated only from results of phase II dose-finding studies and phase III studies comparing GnRH agonist cycles with GnRH antagonists in single- and multiple-dose protocols. Therefore, all conclusions on clinical differences between cetrorelix and ganirelix should remain tentative, as they are based on a limited amount of available data.Randomised, controlled trials comparing cetrorelix and ganirelix are warranted to further evaluate benefits and drawbacks of individual GnRH antagonists. Furthermore, more data are needed to determine the efficacy and safety of cetrorelix and ganirelix in established treatment protocols in patients other than those included in clinical trials investigating new drugs, such as "poor responders", patients with polycystic ovaries, patients with a history of allergy or overweight patients.

Gonadotropin-releasing hormone-desensitized LbetaT2 gonadotrope cells are refractory to acute protein kinase C, cyclic AMP, and calcium-dependent signaling

Sustained exposure of gonadotropes to GnRH causes a pronounced desensitization of gonadotropin release, but the mechanisms involved are poorly understood. It is known that desensitization is associated with decreased GnRH receptor and Gq/11 levels in alphaT3-1 cells, but it is not known whether downstream signaling is impaired. We have shown previously that chronic stimulation of signaling via expression of an active form of Galphaq causes GnRH resistance in LbetaT2 cells. In this study we investigated whether chronic GnRH treatment could down-regulate protein kinase C (PKC), cAMP, or Ca2+-dependent signaling in LbetaT2 cells. We found that chronic GnRH treatment desensitizes cells to acute GnRH stimulation not only by reducing GnRH receptor and Gq/11 expression but also by down-regulating PKC, cAMP, and calcium-dependent signaling. Desensitization was observed for activation of ERK and p38 MAPK and induction of c-fos and LHbeta protein expression. Activation of individual signaling pathways was able to partially mimic the desensitizing effect of GnRH on ERK, p38 MAPK, c-fos, and LHbeta but not on Gq/11. Chronic stimulation with phorbol esters reduced GnRH receptor expression to the same extent as chronic GnRH. Sustained GnRH also desensitized PKC signaling by down-regulating the delta, epsilon, and theta isoforms of PKC. We further show that chronic GnRH treatment causes heterologous desensitization of other Gq-coupled receptors.

Luteal phase support

Assisted reproductive techniques have become a routine treatment for infertility. The extended use of gonadotrophin-releasing hormone analogues in assisted reproductive techniques has made luteal phase support mandatory, as it has been clearly demonstrated that they alter luteal LH pulsatility. For luteal support, HCG administration, though effective, has a high risk of ovarian hyperstimulation syndrome. Progesterone continues to be the gold standard for supplementation. Vaginal progesterone represents a highly effective alternative to painful intramuscular injections. The vaginal route is mainly characterized by direct delivery of the progesterone to the endometrium, thus producing high levels at the target tissue and a very low incidence of side effects.

Stimulation of combinatorial expression of prolactin and glycoprotein hormone alpha-subunit genes by gonadotropin-releasing hormone and estradiol-17beta in single rat pituitary cells during aggregate cell culture

Previously we showed the existence of rat and mouse anterior pituitary cells coexpressing mRNA from two or more hormone genes in which production and/or storage of the corresponding hormones were not detectable. To substantiate a putative function for these cells, we investigated whether these phenotypes were retained during long-term reaggregate cell culture and whether protagonist regulatory factors could expand cell populations expressing particular hormone mRNA combinations. After 4-wk culture and treatments, aggregates were trypsinized and single cells collected by means of a fluo-rescence-activated cell sorter. Hormone mRNAs were detected by single-cell RT-PCR. Combinatorial hormone mRNA expression was retained in culture. Both estradiol (E2) and GnRH (1 nM) markedly augmented the proportion of cells expressing prolactin (PRL) mRNA together with other hormone mRNAs and cells expressing glycoprotein subunit (GSU)-alpha mRNA together with other hormone mRNAs. GnRH strongly increased the proportion of cells containing alphaGSU mRNA alone, but E2 did not. GnRH and (E2) affected the expansion of a population (approximately 20% of all cells) coexpressing PRL and alphaGSU mRNA without betaGSUs. Immunostaining of stored hormone on tissue sections revealed colocalization of PRL and alphaGSU in the E2- but not in the GnRH-treated cells. The present findings suggest that cells coexpressing different pituitary hormone mRNAs form a distinct population that survives without extrapituitary factors. Their occurrence can be markedly modified by regulatory factors. Certain hormone regimens favor unique coexpressions distinctly at mRNA and protein level. These peculiar characteristics support the notion that combinatorial expression of hormone genes in the pituitary serves a biological role.

Effect of the concurrent LHRH antagonist administration with a LHRH superagonist in rats

PURPOSE

The purpose of this study was to investigate the effect of a novel LHRH antagonist, Orntide acetate, on the initial testosterone elevation in rats during treatment with a LHRH superagonist, Leuprolide acetate.

METHODS

Thirteen groups of a rat animal model were administered either liquid Orntide or Orntide PLGA microspheres before or simultaneously with Leuprolide injections. Serum levels of testosterone were monitored during the time course of the study using a radioimmunoassay method.

RESULTS

Administration of a single daily dose of liquid Orntide resulted in testosterone suppression within 6 h to levels below 0.5 ng/ml (castration level). However, combined administration of liquid Orntide and liquid Leuprolide did not have a significant effect on the initial testosterone elevation in studied rats. Similarly, there was no effect when liquid Orntide was co-administered with Leuprolide microspheres. Administration of Orntide microspheres 48 h before Leuprolide microspheres suppressed testosterone levels below the castration level within 24 h, however, did not prevent a rise in testosterone serum concentration upon administration of Leuprolide microspheres. Also, a second testosterone peak was observed between days 3 and 15 in the animals which were simultaneously treated with Orntide microspheres and Leuprolide microspheres.

CONCLUSIONS

Orntide acetate was found to be an effective LHRH antagonist with a rapid onset of pharmacological action and a short biological half-life. Administration of a single dose of liquid Orntide or Orntide microspheres, resulted in rapid testosterone suppression without an initial elevation, as seen with LHRH superagonists. However, combined administration of Orntide and Leuprolide did not have an effect on the initial testosterone elevation in rats.

Modulation of gonadotropin levels by peptides acting at the anterior pituitary gland

There are several lines of evidence that point to peptides participating in the regulation of LH and/or FSH levels by action at the pituitary. This evidence includes altered secretion of gonadotropins from the anterior pituitary cells or tissue in vitro when exposed to the peptide. Additionally, modification of GnRH-stimulated LH/FSH secretion has been observed. Furthermore, there is potential for a separately modulated interaction with the primed response. Another potential of action is by interaction among non-GnRH peptides on gonadotropin-regulating processes, although there are no good data available on this aspect. Other observations, consistent with a pituitary role for the peptides in modulation of LH, include detection of the peptides in portal blood, detection of high-affinity receptors or receptor mRNA in the pituitary, and detection of intrapituitary peptide or peptide mRNA in the pituitary. The modulation by steroids of both concentrations and type of activities provides a further level of physiological refinement. There is, however, some confusion regarding the involvement of these peptides in gonadotropin control. The reasons can be seen by considering aspects of investigations. There are experimental variations such as 1) species studied, e.g., NPY has been reported to have an effect on LH secretion from rat cells (168) but not on sheep anterior pituitary tissue (64), and substance P inhibits GnRH-stimulated release from rat cells (182) but potentiates the response in prepubertal porcine cells (92); 2) the steroidal conditions under which the study is performed, e.g., NPY has opposite effects in certain endocrine environments, augmenting GnRH-stimulated LH release in proestrus-like conditions (168), and inhibiting in metestrus-like environment (66); 3) the type of cell preparation, e.g., responsiveness to substance P might depend on whether cells in overnight culture were in separated or clustered state (91); 4) the time course considered, e.g., oxytocin that might induce marked LH release from pituitary cells after a longer length of incubation than GnRH requires (68); 5) length of exposure to peptide, e.g., endothelin that augmented or inhibited GnRH-stimulated LH release (50); 6) In addition, it is possible that the traditional endpoint selected in such studies, namely, observation of gonadotropin secretion, is not necessarily the most important for these peptides (56, 81, 117). Unfortunately, at this stage a definitive answer to the question "What do the peptides actually do?" cannot be provided and we remain tantalized by the glimpses of potential roles. Perhaps in a few years an updated review will be able to include a more complete answer. It is necessary for the full understanding of LH control that not only the properties of the peptides in isolation be characterized but also their interactions.

Effects of lead exposure on GnRH and LH secretion in male rats: response to castration and alpha-methyl-p-tyrosine (AMPT) challenge

Animal and clinical studies suggest that lead exposure disrupts the hypothalamic-pituitary axis. To define more precisely the toxic action of lead on the hypothalamic-pituitary unit, a series of in vivo and in vitro experiments were performed. The first experiment was designed to determine whether lead exposure exerts an inhibitory effect on GnRH secretion as reflected by an enhanced inhibition of luteinizing hormone (LH) secretion in response to the tyrosine hydroxylase inhibitor methyl-p-tyrosine (AMPT). In the control animals, the AMPT dose had no significant effect on LH secretion, whereas LH fell significantly in the lead-treated animals. In experiments designed to evaluate the effects of lead exposure on the pattern of pulsatile release of gonadotropins castrated control and lead-dosed animals were cannulated, and serial blood sampling was performed. Baseline LH and follicle-stimulating hormone values were not statistically different between the control and lead-treated group. There were no significant differences noted in pulsatile patterns when the data were analyzed as groups. Pituitary cells harvested from lead-treated animals released significantly more LH that did the control animals. These data are consistent with the hypothesis that the signals between the hypothalamus and pituitary gland are disrupted by lead exposure in the intact animal. However, the lead-exposed castrated rat's hypothalamic-pituitary unit is able to adapt to the toxic effects of lead.

Effect of age of donor on the responsiveness of dispersed and cultured chicken anterior pituitary cells to GnRH-I

1. The aim of this study was to devise a method to prepare and culture anterior pituitary cells from juvenile and adult chickens in order to investigate mechanisms controlling gonadotrophin-releasing hormone-I (GnRH-I)-induced luteinising hormone (LH) release in vitro. 2. The optimum culture medium for maintaining gonadotroph responsiveness to GnRH-I was bicarbonate-buffered and phenol red-free Medium 199 supplemented with 10% foetal calf serum. 3. Cultured pituitary cells from juvenile chickens were more responsive to GnRH-I than cells from adult cockerels, while no LH was released in response to GnRH-I from pituitary cells from laying hens. 4. Cultured pituitary cells from adult chickens of both sexes released LH in response to 12-O-tetradecanoyl-13-phorbol acetate (TPA), an activator of an enzyme involved in intracellular signalling, protein kinase C. 5. It is concluded that freshly-dispersed and cultured gonadotrophs from adult chickens do not regain their responsiveness to GnRH-I as well as freshly-dispersed and cultured gonadotrophs from juvenile chickens. It appears that the stimulus-secretion coupling pathway between the GnRH-receptor and the activation of protein kinase C in gonadotrophs from adult chickens is more easily disrupted by dispersion and culture than in gonadotrophs from juvenile chickens.

Effects of administration of oxytocin in association with gonadotropin-releasing hormone on luteinizing hormone levels in rats in vivo

Gonadotropin-releasing hormone (GnRH) and oxytocin both stimulate the secretion of luteinizing hormone (LH), although with different characteristics. Therefore, interaction between oxytocin and GnRH in the control of LH may be postulated. We developed models for investigating whether oxytocin can modulate GnRH action on LH in vivo. Pentobarbitone is known to pharmacologically isolate the pituitary from hypothalamic GnRH. We found that after pentobarbitone anesthesia of female rats at proestrus, oxytocin caused a synergistically enhanced LH response to administered GnRH (p < 0.04). In a second series of experiments, female proestrous rats were anesthetized with althesin. This anesthetic allows transport of endogenous GnRH from the hypothalamus to the pituitary. In control animals, which received no exogenous hormone, there was a surge in the mean LH concentration on the evening of proestrus, indicating the presence of endogenous GnRH activity. Thus, the novel model enables detection of interactions of administered hormones with endogenous GnRH. Administration of GnRH plus oxytocin in the afternoon of proestrus caused a reduction (p < 0.01) in the mean level of LH observed in the evening. The reduction was larger than if GnRH alone was administered. Following althesin anesthesia, rats sometimes had low LH levels on the afternoon of proestrus. There was a statistically significant difference between the number of rats that received oxytocin plus GnRH and had low LH levels and the number with low LH levels in the control group (p < 0.02). Neither of the hormones administered alone had a significant effect. Thus, it appears that oxytocin accentuated the effect of GnRH in reducing LH concentrations in althesin-anesthetized rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Desensitization of pituitary cells by gonadotrophin-releasing hormone or its analogues in the superfusion system: different pattern for males and females

The effect of a 6-h infusion of gonadotrophin-releasing hormone (GnRH) or its analogues on dispersed anterior pituitary cells from male or female rats was investigated. The cells were stimulated with 3-min pulses of K(+) and GnRH. Thereafter GnRH (1 nM) or GnRH analogues ([D-Trp(6) ]GnRH-ethylamide ([D-Trp(6) ]GnRH, 50 pM), [D-Phe(6) , Gln(8) ]GnRH-ethylamide (Folligen, 100 pM) and [Asu(6) ]GnRH-ethylamide ([Asu(6) ]GnRH, 33 pM)) were applied for 6 h. In cells from female rats this treatment resulted in a 20-fold increase in luteinizing hormone (LH) secretion during the first 90-min period of the 6-h incubation. Following this a gradual decrease in LH release occurred, and during the fourth 90-min period the amount of LH secreted was only one-third or less of the initial value. The pituitary cells of male rats responded to the same treatment with only a 7-fold rise of LH secretion during the first period. In the second 90-min of the 6-h incubation a 20% to 30% increase was observed. Even in the fourth 90-min period the amount of LH secreted was two-thirds or more greater than that of the first 90-min period. When using 10-fold greater concentrations of the same peptides in males, the increase in hormone secretion in the second 90-min was not seen and the hormone release decreased to around 50%. We found definite differences in the responses of male and female rat pituitary cells to the 6-h infusion of GnRH or its analogues: the initial amplitude of the response in females was higher but desensitization was stronger. In males, the initial response was weaker; however, even using doses one magnitude greater, the level of desensitization did not reach the values obtained in females. The results were similar both with GnRH and the analogues. The responses to 3-min K(+) and GnRH stimuli given after the 6-h incubation were strongly reduced in cells from female rats compared to the initial responses; however, in cells from male rats the reaction was higher or unchanged. The ratio of LH released by the final K(+) stimulus relative to the actual LH content of the cells decreased in females but increased in males. Our data show that the differences between the pattern of desensitization in cells from male and female rats may be caused by the differences in the amount and ratio of immediately releasable hormone and the hormone replenishment into these pools.

Desensitization to native molecular forms of gonadotropin-releasing hormone in the goldfish pituitary: dependence on pulse frequency and concentration

Homologous desensitization of gonadotropin-releasing hormone (GnRH) was investigated using goldfish pituitary fragments in vitro. The two native GnRH peptides, sGnRH [( Trp7, Leu8]-GnRH) and cGnRH-II [( His5, Trp7, Tyr8]-GnRH) were administered either continuously or in pulsatile fashion at different frequencies and concentrations. Continuous treatment (60 min) with either sGnRH or cGnRH-II at 10(-7), 10(-8), and 10(-9) M resulted in desensitization of goldfish pituitary in a biphasic fashion, characterized by an initial rapid peak of GTH release (phase 1), followed by a lower sustained release of GTH remaining at a stable concentration above the basal level (phase 2). Pititary fragments were then washed for 60 min and further treated continuously (60 min) with the same concentrations of sGnRH or cGnRH-II (second treatment). Total sGnRH- or cGnRH-II-induced GTH release during the second treatment period was significantly lower than that observed during the initial treatment period, depending upon the concentration of the peptides. The second phase of GTH release was more pronounced at lower concentrations compared to that observed following 10(-7) M treatment, especially for sGnRH. Pulsatile treatment with either sGnRH or cGnRH-II (2-min pulses of 10(-7), 10(-8), and 10(-9) M given every 20 min) resulted in significant desensitization of the pituitary GTH release. Reduction of pulse frequency to 2 min treatment every 60 min resulted in a lower degree of desensitization; little or no desensitization was observed following treatment with 10(-8) and 10(-9) M cGnRH-II or 10(-9) M sGnRH. A further reduction in frequency to 2-min pulses of sGnRH or cGnRH-II (10(-7) or 10(-8) M) given every 90 min did not result in desensitization of the pituitary GTH release. In summary, the present study demonstrates that GnRH-induced desensitization is dependent on both pulse frequency and concentration in the goldfish pituitary. These findings support the hypothesis that pulsatile secretion of the native GnRH peptides may be essential for maintenance of normal pituitary GTH release in goldfish.

Gonadotrophin-releasing hormone: physiological significance and relevance to cancer

Gonadotrophin releasing hormone (GnRH) is a decapeptide released by the hypothalamus. The binding of the peptide to pituitary receptors leads to the activation of second messenger systems. The physiological outcome of the exposure of pituitary cells to GnRH is the release of luteinising hormone (LH) and follicle-stimulating hormone (FSH). Continued exposure of these receptors to high concentrations of the peptide desensitises the receptor, thus inhibiting the release of gonadotrophins. This paradoxical effect has proved to be beneficial in the clinic where long-acting and enzyme-resistant analogues are used to inhibit the pituitary-gonadal axis, for example in the treatment of advanced prostatic cancer. In addition GnRH-analogues may affect tumour cells directly as observed in vitro. These direct effects have been described as inhibitory but recent data suggests that low concentrations of GnRH-analogues may stimulate short term growth of prostatic cancer cells in vitro. GnRH shares many other common characteristics with peptide growth factors, including common second messenger systems and receptor desensitisation on prolonged exposure to the ligand. It is possible that the direct inhibitory effects of GnRH-analogues are mediated through the desensitisation of tumour GnRH receptors, as suggested by recent observations. The nature and mechanism of the direct anti-tumour effect is important to understand and to promote the therapeutic efficacy of GnRH-analogues in the clinic.

Frequency encoding of pulsatile signals of cAMP based on receptor desensitization in Dictyostelium cells

Dictyostelium discoideum amoebae represent a prototype for the study of periodic signaling in intercellular communication. These cells synthesize cAMP in response to cAMP pulses. Cell responsiveness in Dictyostelium can be characterized by the capability to generate a large number of significant responses to cAMP signals in a given amount of time. The existence of a frequency of pulsatile cAMP signals yielding maximum responsiveness is demonstrated by analysis of a realistic model for cAMP synthesis, based on receptor desensitization. The optimal frequency of stimulation closely depends on the kinetics of receptor desensitization and resensitization in target cells. Synthesis of cAMP is determined both in conditions where cells are not excitable and in conditions where they relay suprathreshold pulses of cAMP. Moreover, the effect of the stimulus waveform is investigated, and several measures of cell responsiveness are compared. The results provide an explanation for the effectiveness of cAMP pulses delivered at 5 min intervals, and for the failure of pulses delivered at 2 min intervals, in inducing slime mold development. Besides applying to intercellular communication in Dictyostelium, the present analysis bears on patterns of pulsatile signaling observed for hormones and growth factors. In all these cases, it appears that pulsatile signals can be encoded in terms of their frequency on the basis of desensitization in target cells.

GnRH receptors in rat brain, pituitary and testis; modulation following surgical and gonadotropin-releasing hormone agonist-induced castration

The regulation of rat gonadotropin-releasing hormone (GnRH) receptors in male rat pituitary, hippocampus and testis was studied, in vivo, under steady-state conditions during treatment with D-Trp6 GnRH (triptorelin, slow-release form, at 300 micrograms/kg/month). GnRH receptors were characterized on tissue sections by quantitative autoradiography using 125I-GnRHa as a tracer. Castrating doses of triptorelin strongly down-regulated pituitary GnRH receptors (50% of reduction after 8 h, 80% on days 1-30); in contrast, only a transient decrease (20% at 8 h) was observed in the hippocampus with a rapid return to control levels. Triptorelin induced a marked (2-fold) increase in GnRH receptors in testicular interstitial tissue during 5 days with a return to control value by day 20. Administration of a GnRH antagonist (BIM 21009, 1 mg/kg/24 h) induced a rapid reduction of pituitary and testicular receptors to undetectable levels at 24 h, while hippocampal receptors were strongly reduced only. This indicates that GnRH receptors with similar pharmacology are differently controlled in various tissues and that brain receptors are likely to be also regulated by GnRH agonists and antagonists.

Frequency specificity in intercellular communication. Influence of patterns of periodic signaling on target cell responsiveness

Cells often communicate by means of periodic signals, as exemplified by a large number of hormones and by the aggregation of Dictyostelium discoideum amebas in response to periodic pulses of cyclic AMP. Periodic signaling allows bypassing the phenomenon of desensitization brought about by constant stimuli. To gain further insight into the efficiency of pulsatile signaling, we analyze the effect of periodic stimulation on the dynamic behavior of a receptor system capable of desensitization toward its ligand. We first show that the receptor system adapts to square-wave stimuli, i.e., the response eventually reaches a steady, periodic pattern after a transient phase. By analyzing the dependence of the response on the characteristics of the square-wave stimulation, we show that there exist a waveform and a period of that signal that result in maximum responsiveness of the target system. Similar results are obtained when the signal takes the more realistic form of a periodically repeated stimulation followed by exponential decay of the ligand. The results are discussed with respect to the role of pulsatile secretion of gonadotropin-releasing hormone (GnRH) by the hypothalamus and of periodic signaling by cyclic AMP pulses in Dictyostelium. The analysis accounts for the existence, in both cases, of an optimal frequency and waveform of the periodic stimulus that correspond to maximum target cell responsiveness.

Restoration of the LH secretory response in desensitized gonadotropes

The time-dependent recovery of gonadotropin-releasing hormone (GnRH) responsiveness in desensitized gonadotropes was examined under conditions of altered membrane fluidity and GnRH exposure. Cultured pituitary cells were treated for 3 h with GnRH (10(-9) M; to provoke homologous desensitization) or vehicle alone (controls). When cells were washed and immediately rechallenged for 3 h with GnRH, gonadotrope responsiveness (assessed by luteinizing hormone (LH) release) was significantly lower in GnRH-pretreated cells than controls. If gonadotropes were allowed to recover in medium alone, membrane fluidity agents 2-(2-methoxyethoxy)-ethyl-8-(cis-2-n-octylcyclopropyl)-octanoate (A2C; 10(-4) M) or cis-vaccenic acid (CVA; 0.5 mM) or a low dose of GnRH (10(-10) M) for up to 48 h prior to rechallenging with GnRH, responsiveness in all cases was significantly lower in GnRH-pretreated cells than controls. However, if cells were treated with either A2C or CVA in the presence of GnRH (10(-10) M) during the recovery period, gonadotrope responsiveness to a subsequent challenge with GnRH was partially restored by 24 h; by 48 h no differences in the LH secretory response to GnRH was detected between GnRH-pretreated cells and controls. The possibility that restoration of the GnRH receptor-linked Ca2+ channel is associated with recovery of the desensitized gonadotrope was also examined. Identical protocols to those described above were used except that the functional integrity of the Ca2+ channel was assessed by measuring LH release in response to increasing doses of maitotoxin (MTX; a specific Ca2+ channel activator). Again, GnRH-pretreated cells were significantly less responsive to MTX than controls when allowed to recover for 48 h in medium alone, A2C (10(-4) M) or GnRH (10(-10) M). However, allowing cells to recover for 48 h under a condition of increased membrane fluidity and basal GnRH levels completely restored the MTX-stimulated LH secretory response in GnRH-pretreated gonadotropes. Taken together, these studies suggest that the physical state of the gonadotrope plasma membrane together with the appropriate hormonal milieu provide an important environment for the gonadotrope to recover from desensitization. Additionally, our results suggest that functional recovery of the GnRH-linked Ca2+ channel may play a requisite role in restoring GnRH responsiveness to the desensitized gonadotrope.

Desensitization of pituitary gonadotropes by mediators of LH release

Desensitization of pituitary gonadotropes by exposure to 10 nM gonadotropin-releasing hormone (GnRH) for 6 h severely impaired the luteinizing hormone (LH) response to a second 3-h treatment with GnRH, and reduced the secretory responses to 50 microM arachidonic acid (AA), 100 nM tetradecanoyl phorbol-13-acetate (TPA), and AA + TPA. Pretreatment with AA blocked subsequent responses to AA but not to other secretagogues. Pretreatment with TPA attenuated the LH response to TPA, but not to GnRH, AA, and AA + TPA. After exposure to AA + TPA, all subsequent responses were abolished. Each of the secretagogues reduced GnRH receptor binding, but only GnRH-induced receptor loss and desensitization were reversed by simultaneous incubation with a GnRH antagonist. Similar results were obtained when 16-h pretreatment periods were used, or when the data were normalized for the concomitant reduction of cellular LH content. These findings indicate that GnRH-receptor loss and depletion of LH content are not the sole causes of GnRH-induced desensitization. Receptor uncoupling and impairment of AA- and protein kinase C-dependent pathways may also be involved in this process.

Endocrine evaluation in the assessment of male reproductive hazards

Male reproductive function requires the integrated functioning of the hypothalamus, pituitary, and testis. The disturbance of endocrine function at any of these levels may result in hypogonadism and infertility. The clinical and laboratory evaluation of these disorders is reviewed here.

Membrane fluidity regulates development of gonadotrope desensitization to GnRH

Development of GnRH-mediated gonadotrope desensitization was examined under conditions in which membrane fluidity was altered by temperature and/or chemical means. Cultured pituitary cells were preincubated at temperatures ranging from 4 degrees C to 37 degrees C for 3 h with a desensitizing concentration of GnRH (10(-9) M) or with vehicle alone. Cells were then rinsed and responsiveness assessed by a second 3 h incubation with GnRH at 37 degrees C. As preincubation temperatures decreased from 37 degrees C to 23 degrees C, development of desensitization in gonadotropes was progressively reduced. At 23 degrees C and below, gonadotropes failed to become desensitized to GnRH. Decreases in membrane fluidity occurred over the same temperature range as measured directly by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene incorporated into plasma membrane. When membrane fluidity was increased by incubating cells with the membrane mobilizing agent 2-(2-methoxy-ethoxy)-ethyl-8-(cis-2-n-octylcyclopropyl)-octanoate (A2C), low temperature blockade of GnRH-mediated gonadotrope desensitization was reversed. A2C had no measurable effects on either GnRH receptor binding or number and caused no cytotoxic effects. These studies suggest that development of gonadotropine desensitization to GnRH can be regulated by the state of membrane fluidity.

Wheat germ agglutinin behaves as a GnRH antagonist but induces gonadotrope desensitization

Preincubation of cultured rat pituitary cells with 10 micrograms/ml of either wheat germ agglutinin (WGA) or concanavalin A inhibited LH release stimulated with GnRH (0.5 nM) by 55% and 40%, respectively. WGA-inhibition of LH release stimulated by GnRH was dose-dependent, reaching a plateau of 75% inhibition at 50 micrograms/ml. Concomitantly, WGA induced a dose-dependent inhibition of 125I-Buserelin specific binding to pituitary cells, with a maximal inhibition of 45%. The inhibition of 125I-Buserelin binding by WGA is due to GnRH receptor internalization and not to persistent occupancy of the receptors. In addition to the effect of WGA on receptor internalization, WGA also induced partial desensitization of pituitary cells but was ineffective in modulating GnRH-induced desensitization. These findings indicate that WGA has all the characteristics of a GnRH antagonist, nevertheless, it does induce desensitization of cultured rat pituitary cells to further stimulation with GnRH.

Effects of temperature and mode of delivery on responses to gonadotropin-releasing hormone by superfused frog pituitaries

An in vitro superfusion system was used to examine the effects of temperature on the responsiveness of frog, Rana pipiens, hemipituitaries to chronic superfusion with gonadotropin-releasing hormone (GnRH). The effects of pulsatile as opposed to continuous delivery of GnRH on luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion were also studied at 30 degrees. Secretion of both gonadotropins (FSH and LH) increased significantly in a dose-dependent manner in response to a brief exposure to 0.1-1000 ng/ml (nM) GnRH at both 18 and 32 degrees, but the magnitude of response at each dose was temperature dependent. Moreover, the temporal pattern of the chronic response to a standard dose of GnRH (100 ng/ml) also varied markedly between 10 and 32 degrees: Hemipituitaries superfused at 10 or 18 degrees showed greatly attenuated responsiveness to GnRH after the first hour of GnRH treatment, whereas responsiveness was maintained or even continued to increase for many hours at higher temperatures. These temperature effects may be related to seasonal changes in reproductive activity. Pulsatile administration of GnRH was more effective in inducing elevated gonadotropin (GtH) secretion during the first few hours of superfusion, but continuous superfusion with GnRH was more effective at maintaining elevated GtH levels over long periods. The results support and extend previous studies by indicating that, unlike the situation in many mammals and birds, the frog pituitary is highly resistant to desensitization by even high doses of GnRH, and in fact, self-priming and maintained responsiveness to GnRH is better supported by chronic treatment than by pulsatile delivery of GnRH.

Interpretation of dose-response curves for luteinizing hormone release by gonadotropin-releasing hormone, related peptides, and leukotriene C4 according to a hormone/receptor/effector model

The dose-response curves for pituitary luteinizing hormone (LH) release in response to gonadotropin-releasing hormone and its agonists are unusually broad. It appears, however, that these ligands bind to a single class of receptors. It is shown that these dose-response data can be explained by either of two models in which ligand-receptor complexes stimulate LH secretion by interacting with either of two different effector systems or by interacting with a single effector system but forming monomeric and dimeric active effector complexes. A combination of these two basic models can account for the very broad, biphasic dose-response curve reported for LH release in response to leukotriene C4.

Copper induces luteinizing hormone release and desensitization of pituitary gonadotropes

Copper stimulated LH release from cultured rat pituitary cells in a dose-and time-dependent manner. After 4 h of incubation with 10 mu M Cu2+, LH release was stimulated by 3-fold. The release of LH stimulated by Cu2+ was Ca2+ dependent, thus excluding the possibility that the releasing activity of this divalent cation was due to a toxic effect on pituitary cells. The stimulatory action of Cu2+ is substantially mediated via the GnRH-receptors since Cu2+ inhibited 125I-Buserelin binding and since GnRH-antagonist blocked most of the Cu2+-stimulated LH release (80%). Both GnRH (1 microM) and Cu2+ (10 microM) induced desensitization of pituitary cells to a subsequent stimulation of either GnRH (0.5 nM) or Cu2+ (10 microM). However, in contrast to GnRH, Cu2+ did not induce down regulation of GnRH receptors. These findings suggest that the Cu2+ effects are mainly mediated through the GnRH receptors.

GnRH-stimulated LH release from rat anterior pituitary cells in culture: refractoriness and recovery

Previous reports have demonstrated that long-term continuous administration of gonadotropin-releasing hormone (GnRH) to pituitary cells results in a decreased level of gonadotropin secretion. The present report demonstrates that cultured rat anterior pituitary cells preincubated for 6 h in 10(-9) or 10(-7) M GnRH became refractory to further stimulation by the releasing hormone. Cells required 2-4 days to recover from the refractory condition. Cells also became refractory to GnRH when luteinizing hormone (LH) release was blocked by Ca2+ chelation. Drugs such as veratridine, ionophore A23187, or high K+, which stimulated LH release without GnRH receptor occupancy, were also capable of causing refractoriness to GnRH in long-term exposure. These data suggest that the refractory state observed after stimulation with GnRH is a result of the combined effects of a Ca2+-independent receptor-mediated mechanism for desensitization and some other postreceptor mechanism. Tunicamycin interfered with recovery, whereas cycloheximide did not. This evidence presents a potential role for protein glycosylation in the restoration of responsiveness. Phorbol myristate acetate did not cause subsequent refractoriness to GnRH, and dibutyryl adenosine 3',5'-cyclic monophosphate had no measurable effect on the rate of recovery.

Gonadotrophin releasing hormone receptor regulation in relationship to gonadotrophin secretion

The relationship between pituitary GnRH receptors (GnRH-R) and LH responsiveness to GnRH stimulation is not straightforward. In some circumstances, e.g. post-gonadectomy of rats, in lactating rats, during the rat, hamster and monkey oestrous cycles there appears to be a good positive correlation between GnRH-R, basal serum LH values and LH responses to exogenous GnRH. However, in mice following gonadectomy GnRH-R fall by 50% while serum LH levels rise by 10-fold, and in cultured pituitary cells, GnRH exposure increases GnRH-R yet desensitizes cellular responsiveness to subsequent GnRH stimulation. Thus, our original hypothesis that GnRH-R regulation was closely coupled to gonadotroph secretory function does not always hold. Further, we and others, using the rat as an experimental model, hypothesised that the pituitary GnRH receptor content reflected the level of previous pituitary exposure to endogenous GnRH. This view is supported with studies in the GnRH deficient hypogonadotrophic hypogonadal (hpg) mouse in which exogenous GnRH rapidly normalises GnRH-R from very low levels, and is accompanied by rapid activation of pituitary FSH synthesis. However, the post-castration fall in GnRH-R in mice, which is opposite to that in rats, does not appear to be so closely related to endogenous GnRH secretion and cannot be reversed by exogenous GnRH. Using the ovariectomised mouse as an experimental model, evidence has been obtained that estradiol, in addition to GnRH, is essential for maintenance of pituitary GnRH-R in this species. Exogenous estradiol stimulates GnRH-R in OVX mice while it reduces the high values in OVX rats. In female mice estradiol and GnRH have additive stimulatory effects on GnRH-R. Thus, there is species variability in the predominant hormonal regulation of GnRH receptors. In rat pituitary cells in vitro up-regulation of GnRH-R can be effected by several agents which stimulate LH release (GnRH, KCl, DbCAMP) as well as some which do not (Ca ionophore at low concentrations). Receptor up-regulation requires Ca2+ mobilisation and protein synthesis. The data obtained from several in vivo and in vitro model systems supports the conclusion that GnRH receptor changes represent another, medium-term, consequence of GnRH action on the gonadotroph and are not always a locus for the modulation of gonadotrophin secretion and synthesis.

Molecular mechanism of gonadotropin releasing hormone action

This work summarizes some of our studies related to the mechanism of action of gonadotropin releasing hormone including those leading to identification of the three steps of the gonadotropin releasing process: receptor binding, mobilization of extracellular calcium, and granule exocytosis. Evidence is also presented to suggest how these steps are integrated one to another and how they are integrated to other actions of the releasing hormone such as regulation of target cell sensitivity, and receptor regulation.

Increased gonadotrophin releasing hormone receptors on pituitary gonadotrophs: effect on subsequent LH secretion

GnRH, high potassium concentrations, and cAMP derivatives have been previously shown to increase GnRH receptor levels (GnRH-R) in cultured rat pituitary cells. However, the effect of these changes in receptor number on subsequent stimulated LH release has not been investigated. In this study pretreatment of pituitary cells with either 1 nM GnRH, 58 mM KCl, or 1 mM dibutyryl cAMP (dbcAMP) resulted in a 70-100% increase in GnRH-R 7-10 h later. Subsequent LH responses to GnRH in those cells pretreated with GnRH and KCl were markedly reduced and the dose-response characteristics altered such that the curves were non-sigmoidal. When corrected for depletion of cellular LH during the pretreatment period these GnRH response curves were similar to control, implying that hormone depletion was the explanation for apparent desensitisation. By contrast, dbcAMP and low-dose calcium ionophore (0.1 microM A23187) pretreatment, which did not deplete cellular LH, neither enhanced nor decreased subsequent sensitivity to GnRH. Thus, 4 agents which all, under these conditions, increased GnRH receptors did not sensitise gonadotrophs to GnRH. By contrast, pretreatment with 10(-9) and 10(-8) M GnRH for either 12 or 16 h rendered cells completely or partially refractory to further GnRH stimulation, despite an increase in GnRH receptors. This desensitisation could not be explained by cellular LH depletion, and was specific to the homologous ligand since dose-responses to the Ca2+ ionophore A23187 and KCl were normal when corrected for LH depletion. Non-receptor-mediated depletion of cellular LH during A23187 pretreatment (10 microM for 10 h) did not alter subsequent GnRH dose-responses, after correction for LH content. These data indicate that, under these in vitro conditions, the increased GnRH receptors are not functionally linked to the secretory apparatus of the gonadotroph. Furthermore, homologous ligand-induced desensitisation is both time- and concentration-dependent and is mediated largely by post-receptor cellular events independent of cellular LH content. Therefore, post-receptor cellular processes may be more important than changes in GnRH receptors in regulating gonadotrophin secretion. It is suggested that an increase in GnRH receptors may represent a cellular response to generalised gonadotroph activation by a variety of agents, and does not necessarily signify enhanced responsiveness to GnRH.

Sexual maturation of the hypothalamus: pathophysiological aspects and clinical implications

Sexual maturation in humans begins early in fetal life and culminates in adulthood when the gonads have acquired a full capacity for reproduction. It is remarkable that during this long process, the pituitary gonadal function, hence its hypothalamic control presents an alternative of activation and inhibition periods, during which the interrelations of the 3 components of the hypothalamic-pituitary-gonadal axis change gradually and inversely. The ontogeny of the hypothalamic-pituitary system, the varying activity of the reproductive endocrine system throughout sexual maturation and the developmental changes in the interrelations of the hypothalamic-pituitary-gonadal axis are reviewed: the most striking feature of human sexual development is the long inhibition of hypothalamo-pituitary function during childhood. Much indirect evidence points to the determining role of the CNS in the maturation of hypothalamic function: the occurrence of rhythms of secretion, the amplitude of secretions and peripubertal specific sleep-related nycthemeral rhythm of secretion at the onset of puberty. Despite the reality of a negative feedback control, these changes do occur independently of gonadal secretions since they are observed (qualitatively if not strictly quantitatively) in agonadal children. It is likely that neurotransmitters (dopamine, serotonine) and opiates have an inhibitory effect on Gn-RH release. But we still don't know their evolution during sexual maturation. It does not appear that melatonine plays any determinant role in the onset of human puberty. The clinical implications of our present understanding of the physiological events occurring during sexual maturation are several. Considering the major problems related to abnormal sexual maturation we will discuss successively: (1) diagnosis of hypogonadotrophic hypogonadism in early infancy; (2) differential diagnosis between premature thelarche and true sexual precocity; (3) the usefulness of endocrine investigations in the evaluation of hypothalamic-pituitary function; and (4) the new developments in the treatment of precocious puberty, delayed puberty or hypogonadism.

Photoaffinity labeling of gonadotropin releasing hormone receptors in control and desensitized pituitary cells

Gonadotropin releasing hormone (GnRH), preincubated with cultured rat pituitary cells, induced down regulation of GnRH receptors in a time- and dose-dependent manner. The specific binding was inhibited by 50% after 30 min and maximal inhibition (70%) was obtained after 75 min preincubation with 1 microM GnRH. Preincubation of the cells for 2 h with 10 nM GnRH inhibited the specific binding by 20%, reaching a plateau of 70% inhibition with 0.1 microM GnRH. Concomitantly, exposure of the cells to GnRH caused a time- and dose-dependent desensitization of LH release. The responsiveness of the desensitized cells was not parallel to the binding capacity and was inhibited to a greater extent (93%). Photoactivation of GnRH receptors with iodinated [azidobenzoyl-D-Lys6]GnRH in control and desensitized cells resulted in the identification of a single specific band with the same apparent molecular weight of 60K daltons. These results indicate that structural alterations of GnRH receptors are not associated with GnRH-induced desensitization. Therefore, desensitization may involve conformational changes in the receptor or more likely a post-receptor mechanism.

Pituitary LH responsiveness to GnRH in vitro as related to GnRH receptor number

The objective of this in vitro study was to determine whether the increase in the augmented phase of the biphasic luteinizing hormone (LH) response to gonadotrophin-releasing hormone (GnRH) and its enhancement by estradiol (E2) were associated with GnRH-stimulated increases in pituitary GnRH receptor concentration. Pituitary glands from 72 h ovariectomized (OVX), OVX + E2, or proestrous rats were perifused continuously with GnRH (12 ng/h). LH release was measured at 10-min intervals, and pituitary GnRH-binding capacity (GnRH-BC) was assessed at 0, 40, 80, 120, and 240 min after addition of GnRH. All treatment groups exhibited a biphasic pattern of LH release; initial (20-70 min) and augmented (120-240 min) mean rates of LH secretion (micrograms/h) were 1.78 and 3.92 (OVX), 6.40 and 16.67 (OVX + E2), and 2.79 and 18.64 (proestrus), respectively. Total LH release was significantly greater in the OVX + E2 and proestrous groups (44.0 and 45.8 micrograms) vs. the OVX group (12.4 micrograms). Throughout the GnRH infusion period, GnRH-BC did not change significantly in any of the treatment groups with the exception of the OVX group in which there was a transient small decrease at 80 min post-GnRH infusion. There were no significant differences between treatment groups in GnRH-BC at any time after infusion of GnRH. These results demonstrate that the acute and augmented phases of GnRH-stimulated LH release and the enhancement of this biphasic response by E2 occurs independent of any increase in GnRH-BC and suggest that these events are mediated by postreceptor mechanisms.

Somatostatin desensitization in rat anterior pituitary cells

The possibility that desensitization to the inhibitory effects of somatostatin (SS) might develop following chronic exposure to this tetradecapeptide was examined in cultured rat anterior pituitary cells. Pretreatment with 1 microM SS for 48 h caused a shift in the IC50 of SS to inhibit 3-isobutyl-1-methylxanthine (IBMX) or growth hormone-releasing factor (GRF)-induced growth hormone (GH) and TRH + IBMX-induced thyroid-stimulating hormone (TSH) release by more than 2 orders of magnitude. Refractoriness developed after 12 h of exposure to doses of SS of 10 nM or more and became maximal at 48 h. Restoration of SS responsiveness followed a similar time-course upon removal of the peptide. In superfused cells, 10 nM SS lowered GH secretion rates to less than 5 ng/min within 2 h, but GH release began to rise after 16 h despite the continued presence of SS. However, when somatostatin was delivered in pulses, it remained fully effective for more than 36 h. Somatostatin-28 was also capable of inducing refractoriness in cultured pituitary cells. However, cells made refractory to either SS-14 or SS-28 were not made refractory to the same extent to the other form of somatostatin. These results indicate that the pituitary can become desensitized to the inhibitory actions of somatostatin just as it does to the stimulatory actions of the other hypothalamic releasing hormones.

Homologous ligand induction of pituitary gonadotrophin-releasing hormone receptors in vivo is protein synthesis dependent

This study demonstrates that a single subcutaneous injection of gonadotrophin-releasing hormone (GnRH) (60 ng) to GnRH-deficient (hpg) male mice causes a doubling of pituitary GnRH receptors (GnRH-R). No change in GnRH-R occurs during the time of LH release (15-60 min) or up until 4 h post-GnRH. Between 4 and 12 h there is a progressive increase in GnRH-R, which is still apparent 24 h later. No induction of GnRH-R occurs after the same treatment of intact adult normal mice. The same degree of GnRH-R induction occurs 12 h after a single GnRH injection (60 ng) to orchidectomized hpg male mice, indicating that this effect is mediated by a direct action of GnRH on the pituitary gonadotroph, rather than being secondary to stimulation of some gonadal product. Homologous ligand GnRH-R induction in hpg mouse pituitaries in vivo is prevented by prior treatment with cycloheximide, a non-specific protein synthesis inhibitor. Cycloheximide alone had no effect on GnRH-R in normal male mice but when combined with GnRH caused a 40% depletion of receptors, implying ligand-induced receptor loss without subsequent replenishment. The similarity between the extent, time-course, and dependence on protein synthesis of GnRH induction of its own receptors in vivo and in cultured pituitary cells in vitro indicates that the hpg mouse pituitary behaves like an in vivo pituitary cell culture system in this respect. Similarity of data derived from this in vivo model provides direct support for the view that in vitro studies on the cellular mechanism of GnRH action can be physiologically relevant to the intact animal.

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

The pattern of the gonadotropin-releasing hormone (GnRH) stimulus is critically important in the regulation of pituitary gonadotropin secretion and continuous infusions down-regulate secretion while intermittent pulses maintain luteinizing hormone (LH) and follicle-stimulating hormone (FSH) responsiveness. We examined the effects of pulsatile GnRH administration on pituitary GnRH receptors (GnRH-R) and gonadotropin secretion in the presence of physiological concentrations of testosterone (T) to elucidate the mechanisms and sites of action of GnRH and T on the pituitary gonadotroph. Castrate male rats received one, two, or four testosterone (T) implants (serum T concentrations of 1.1, 2.4, and 5.2 ng/ml, respectively) to suppress endogenous GnRH secretion. Subsequently, intracarotid pulse injections of GnRH (5-250 ng/pulse) or saline in controls were given every 30 min for 48 h, after which gonadotropin responses and pituitary GnRH-R were measured. In control rats, the T implants prevented the rise in GnRH-R that was seen in castrates (empty implant--600 fmol/mg protein) and maintained receptors at the level that was present in intact animals (300 fmol/mg). Pulsatile GnRH administration increased GnRH-R in castrate T-implanted rats, but the response was dependent on the serum T concentration. With one T implant, increasing GnRH doses per pulse stimulated GnRH-R in a linear manner and the maximum receptor concentration (703 +/- 99 fmol/mg) was seen after the 250 ng GnRH dose. In the presence of two T implants, GnRH-R was maximal (705 +/- 45 fmol/mg) after the 25-ng dose and higher doses did not increase receptors above control values. With four T implants, GnRH doses of 5 ng induced a maximum response, 17-50 ng/pulse did not increase GnRH-R, but receptors were again increased by the 250-ng dose (633 +/- 86 fmol/mg). After 48 h of pulsatile GnRH administration there was no correlation between the number of GnRH-R and LH responses to GnRH. In rats with one or two T implants, LH responses were absent after all but the 250-ng doses. In contrast, LH responsiveness was not impaired in the presence of four implants. Thus, low dose GnRH pulses down-regulate LH secretion by an action at a post GnRH-R site, and this effect is regulated by testosterone. The results show that GnRH, given in a pulsatile manner, regulates its own receptor, and physiological increases in serum T produce a 50-fold increase in the sensitivity of GnRH-R stimulation by GnRH.

The effects of long term growth hormone releasing factor (GRF 1-40) administration on growth hormone secretion and synthesis in vitro

The ability of human pancreatic GH releasing factor 1-40 (hpGRF 1-40) to release GH has been studied in rat anterior pituitary cells in primary culture. Over 24 hours hpGRF (1-40) increased total (cell content and secretion) production 2-fold with an ED50 of 20 pM. Subsequent hpGRF (1-40) stimulation of GH release was not affected by pretreatment when the fall in stored GH was taken into account. In contrast LH responses to gonadotrophin releasing hormone (GnRH) were markedly desensitized after 24 hours GnRH pretreatment in the same experimental system and using the same analysis. hpGRF (1-40) responses were not desensitized when pretreatment was for 3, 12 or 24 hours. The data show that hpGRF (1-40) responses do not desensitize in our experimental conditions under which GnRH responses show marked desensitization.

Induction of ovulation with pulsatile luteinising hormone releasing hormone

Ovulation was successfully induced with luteinising hormone releasing hormone in 28 women with hypothalamic amenorrhoea who had failed to respond to treatment with clomiphene. Luteinising hormone releasing hormone was administered in a pulsatile manner with miniaturised automatic infusion systems. The rate of ovarian follicular maturation, as monitored by serial pelvic ultrasonography, was similar to that observed in spontaneous cycles. Endocrine assessment by serial measurement of gonadotrophin, oestradiol, and progesterone concentrations showed hormone concentrations to be within the normal range. Intravenous treatment was required in only two patients, the remainder responding satisfactorily to subcutaneous infusion. All patients conceived within six cycles of treatment, and only one multiple pregnancy occurred.

Receptor-mediated binding and uptake of GnRH agonist and antagonist by pituitary cells

The intracellular pathway of an enzyme resistant GnRH agonist (D-Lys6-GnRH) conjugated to ferritin or to colloidal gold was followed in cultured pituitary cells. After an initial uniform distribution over the cell surface of gonadotropes, the electrondense marker was internalized, either individually or in small groups. Some, but not all marker was associated with invaginated membrane specializations showing a proteineous coat at their cytoplasmic site. After longer incubation times, the marker appeared in the lysosomal compartment and the Golgi apparatus, where it could be found in the vesicular as well as cisternal portion. In addition, the receptor-mediated endocytosis of the GnRH antagonist D-p-Glu1-D-Phe2-D-Trp3-D-Lys6-GnRH was studied by light and electron microscopic autoradiography after 30 and 60 min of incubation to ensure uptake. At both time points, in in vitro as well as in vivo studies, silver grains were localized over cytoplasmic organelles of castration cells, including dilated endoplasmic reticulum, lysosomes, and clear vesicles. No consistent association with cell nuclei, mitochondria, or secretory vesicles could be observed. The results suggest that both agonist and antagonist are binding selectively to the plasma membrane of gonadotropes and subsequently are taken up via receptor-mediated endocytosis for degradation or possible action on synthetic processes.