Gonadotropin secretory abnormalities

Normal physiology of puberty and normal GnRH, LH, FSH, and hCG secretion have been reviewed. Systemic disorders can affect the neuroendocrine axis and cause varying degrees of hypogonadism by acting at different levels in the axis. As both hypothalamic abnormalities and intrinsic pituitary abnormalities can cause an abnormal FSH/LH response to GnRH, this test does not distinguish hypothalamic from pituitary mechanisms of hypogonadism. Therefore, only in disorders that have been demonstrated to have a structural pituitary abnormality (e.g., iron or granulomatous infiltration of the gonadotrophs) can we be certain that the disorder has its effect at the level of the pituitary. Abnormalities leading to hypersecretion (both ectopic and eutopic) of gonadotropins have also been described. To date, ectopic production of FSH and LH has not been unequivocally demonstrated. Systemic disorders cause mainly hypogonadism, many of the symptoms of which are reversible with control or cure of the disease. The effect of hypersecretion of gonadotropins on the reproductive system depends on the age at which the tumor (ectopic/eutopic) occurs.     

Simultaneous induction of neuropeptide Y and gonadotropin-releasing hormone release in the rabbit hypothalamus

Neuropeptide Y (NPY) can induce the release of endogenous mediobasal hypothalamic gonadotropin-releasing hormone (MBH-GnRH) and pituitary gonadotropins, especially LH. In these studies, we monitored changes in endogenous NPY concentrations at 20-min intervals for 6-8 h during push-pull perfusion (PPP) in both the mediobasal hypothalamus (MBH) and the third cerebroventricle (3VT) of ovarian intact, conscious rabbits. Because previous studies had shown that copper ion can induce hypothalamic GnRH release, cupric acetate (CuAc) was administered either intravenously or intraventricularly during the PPP to manipulate changes in NPY concentrations. Our results show that NPY concentrations in both MBH and 3VT PPP samples were detectable by radioimmunoassay. Administration of CuAc sharply increased hypothalamic NPY release within the same time interval as that for induction of hypothalamic GnRH release. The results are consistent with the hypotheses that NPY may act as a neuromodulator for hypothalamic GnRH secretion, or that common mechanisms drive secretion of these two neuropeptides.     

Gonadotropin-releasing hormone and its receptor in normal and malignant cells

Gonadotropin-releasing hormone (GnRH) is the hypothalamic factor that mediates reproductive competence. Intermittent GnRH secretion from the hypothalamus acts upon its receptor in the anterior pituitary to regulate the production and release of the gonadotropins, LH and FSH. LH and FSH then stimulate sex steroid hormone synthesis and gametogenesis in the gonads to ensure reproductive competence. The pituitary requires pulsatile stimulation by GnRH to synthesize and release the gonadotropins LH and FSH. Clinically, native GnRH is used in a pump delivery system to create an episodic delivery pattern to restore hormonal defects in patients with hypogonadotropic hypogonadism. Agonists of GnRH are delivered in a continuous mode to turn off reproductive function by inhibiting gonadotropin production, thus lowering sex steroid production, resulting in medical castration. They have been used in endocrine disorders such as precocious puberty, endometriosis and leiomyomata, but are also studied extensively in hormone-dependent malignancies. The detection of GnRH and its receptor in other tissues, including the breast, ovary, endometrium, placenta and prostate suggested that GnRH agonists and antagonists may also have direct actions at peripheral targets. This paper reviews the current data concerning differential control of GnRH and GnRH receptor expression and signaling in the hypothalamic-pituitary axis and extrapituitary tissues. Using these data as a backdrop, we then review the literature about the action of GnRH in cancer cells, the utility of GnRH analogs in various malignancies and then update the research in novel therapies targeted to the GnRH receptor in cancer cells to promote anti-proliferative effects and control of tumor burden.     

Cellular mechanisms and integrative timing of neuroendocrine control of GnRH secretion by kisspeptin

The hypothalamus integrates endogenous and exogenous inputs to control the pituitary–gonadal axis. The ultimate hypothalamic influence on reproductive activity is mediated through timely secretion of GnRH in the portal blood, which modulates the release of gonadotropins from the pituitary. In this context neurons expressing the RF-amide neuropeptide kisspeptin present required features to fulfill the role of the long sought-after hypothalamic integrative centre governing the stimulation of GnRH neurons. Here we focus on the intracellular signaling pathways triggered by kisspeptin through its cognate receptor KISS1R and on the potential role of proteins interacting with this receptor. We then review evidence implicating both kisspeptin and RFRP3 – another RF-amide neuropeptide – in the temporal orchestration of both the pre-ovulatory LH surge in female rodents and the organization of seasonal breeding in photoperiodic species.     

Orphanin FQ: evidence for a role in the control of the reproductive neuroendocrine system

Orphanin FQ (OFQ), also known as nociceptin, is a member of the endogenous opioid peptide family that has been functionally implicated in the control of pain, anxiety, circadian rhythms, and neuroendocrine function. In the reproductive system, endogenous opioid peptides are involved in the steroid feedback control of GnRH pulses and the induction of the GnRH surge. The distribution of OFQ in the preoptic area and hypothalamus overlaps with GnRH, and in vitro evidence suggests that OFQ can inhibit GnRH secretion from hypothalamic fragments. Using the sheep as a model, we examined the potential anatomical colocalization between OFQ and GnRH using dual-label immunocytochemistry. Confocal microscopy revealed that approximately 93% of GnRH neurons, evenly distributed across brain regions, were also immunoreactive for OFQ. In addition, almost all GnRH fibers and terminals in the external zone of the median eminence, the site of neurosecretory release of GnRH, also colocalized OFQ. This high degree of colocalization suggested that OFQ might be functionally important in controlling reproductive endocrine events. We tested this possibility by examining the effects of intracerebroventricular administration of [Arg(14), Lys(15)] OFQ, an agonist to the OFQ receptor, on pulsatile LH secretion. The agonist inhibited LH pulse frequency in both luteal phase and ovariectomized ewes and suppressed pulse amplitude in the latter. The results provide in vivo evidence supporting a role for OFQ in the control of GnRH secretion and raise the possibility that it acts as part of an ultrashort, autocrine feedback loop controlling GnRH pulses.     

Does cortisol inhibit pulsatile luteinizing hormone secretion at the hypothalamic or pituitary level?

Elevations in glucocorticoids suppress pulsatile LH secretion in sheep, but the neuroendocrine sites and mechanisms of this disruption remain unclear. Here, we conducted two experiments in ovariectomized ewes to determine whether an acute increase in plasma cortisol inhibits pulsatile LH secretion by suppressing GnRH release into pituitary portal blood or by inhibiting pituitary responsiveness to GnRH. First, we sampled pituitary portal and peripheral blood after administration of cortisol to mimic the elevation stimulated by an immune/inflammatory stress. Within 1 h, cortisol inhibited LH pulse amplitude. LH pulse frequency, however, was unaffected. In contrast, cortisol did not suppress either parameter of GnRH secretion. Next, we assessed the effect of cortisol on pituitary responsiveness to exogenous GnRH pulses of fixed amplitude, duration, and frequency. Hourly pulses of GnRH were delivered to ewes in which endogenous GnRH secretion was blocked by estradiol. Cortisol, again, rapidly and robustly suppressed the amplitude of GnRH-induced LH pulses. We conclude that, in the ovariectomized ewe, cortisol suppresses pulsatile LH secretion by inhibiting pituitary responsiveness to GnRH rather than by suppressing hypothalamic GnRH release.     

Metabolic interfaces between growth and reproduction. I. Nutritional modulation of gonadotropin, prolactin, and growth hormone secretion in the growth-limited female lamb

The acute and long term effects of dietary restrictions on gonadotropin secretion were studied in ovariectomized female lambs. Nutritionally growth-restricted lambs which were chronically maintained at a body weight comparable to that at weaning (approximately 20 kg) became hypogonadotropic, exhibiting a low frequency of episodic LH discharges. Repeated administration of physiological doses of GnRH to these females at hourly intervals produced corresponding LH pulses, leading to the hypothesis that the dietary-induced hypogonadotropism arises from a deficiency in endogenous GnRH release, rather than an inability of the pituitary gland to secrete gonadotropins in response to hypothalamic stimulation. In such growth-restricted females receiving a single meal daily, initiation of ad libitum feeding led to a spontaneous LH pulse within 1 h. After 14 days of increased food intake, hourly LH pulses were evident; a marked reduction in LH pulse frequency was associated with the return to limited nutrition. No effects on pulse amplitude were evident. Changes in circulating FSH followed a pattern similar to that for LH, namely an increase in concentration with improved nutrition and a decrease with reduced nutrition. The rate of response of FSH secretion to these alterations in nutrition was slower than that for LH. PRL levels were not altered by changes in nutrition, and a clear annual rhythm of secretion was observed. GH concentrations changed inversely with the level of nutrition; high secretion was associated with periods of restricted feeding, and low secretion with increased nutrition. These findings indicate that dietary restriction in the developing female lamb depresses gonadotropin secretion without reducing other anterior pituitary gland secretions, such as PRL and GH. That these changes occur in the absence of the ovaries implies that metabolic and growth-related modulation of neuroendocrine function can occur independently of changes in sensitivity to the feedback actions of ovarian steroids and polypeptides.     

Neuroendocrine abnormalities in hypothalamic amenorrhea: spectrum, stability, and response to neurotransmitter modulation.

To characterize the neuroendocrine patterns of abnormal GnRH secretion in hypothalamic amenorrhea (HA), 49 women with primary and secondary HA underwent frequent sampling of LH in a total of 72 baseline studies over 12-24 h. A subset of women participated in more than one study to address 1) the variability of LH pulse patterns over time; and 2) the impact of modulating opioid, dopaminergic, and adrenergic tone on LH secretory patterns. The frequency and amplitude of LH secretion was compared with that seen in the early follicular phase (EFP) of normally cycling women. The spectrum of abnormalities of LH pulses was 8% apulsatile, 27% low frequency/low amplitude, 8% low amplitude/normal frequency, 43% low frequency/normal amplitude, 14% normal frequency/normal amplitude. Of patients studied overnight, 45% demonstrated a pubertal pattern of augmented LH secretion during sleep. Of patients studied repeatedly, 75% demonstrated at least 2 different patterns of LH secretion, and 33% reverted at least once to a normal pattern of secretion. An increase in LH pulse frequency was seen in 12 of 15 subjects in response to naloxone (opioid receptor antagonist). Clonidine (alpha-2 adrenergic agonist) was associated with a decrease in mean LH in 3 of 3 subjects. An increase in LH pulse frequency was seen in 4 of 8 subjects in response to metoclopramide (dopamine receptor antagonist), but the response was not statistically significant. Baseline abnormalities in LH secretion did not appear to influence response to neurotransmitter modulation. Conclusions: 1) HA represents a spectrum of disordered GnRH secretion that can vary over time; 2) LH pulse patterns at baseline do not appear to influence the ability to respond to neurotransmitter modulation; 3) Opioid and adrenergic tone appear to influence the hypothalamic GnRH pulse generator in some individuals with HA.     

Pulsatile GnRH secretion: Roles of G protein-coupled receptors,second messengers and ion channels

The pulsatile secretion of GnRH from normal and immortalized hypothalamic GnRH neurons is highly calcium-dependent and is stimulated by cAMP. It is also influenced by agonist activation of the endogenous GnRH receptor (GnRH-R), which couples to multiple G proteins. This autocrine mechanism could serve as a timer to determine the frequency of pulsatile GnRH release by regulating Ca²⁺- and cAMP-dependent signaling and GnRH neuronal firing. The firing of individual and/or bursts of action potentials (APs) in spontaneously active GnRH neurons is followed by afterhyperpolarization (AHP) that lasts from several milliseconds to several seconds. GnRH-induced activation of GnRH neurons causes a significant increase in medium AHP that is partially sensitive to apamin. GnRH-induced modulation of Ca²⁺ influx and the consequent changes in AHP current suggest that the GnRH receptors expressed in hypothalamic GnRH neurons are important modulators of their neuronal excitability. The coexistence of multiple regulatory mechanisms could provide a high degree of redundancy in the maintenance of this crucial component of the reproductive process. It is also conceivable that this multifactorial system could reflect the gradation from simple to more complex neuroendocrine control systems for regulating hypothalamo-pituitary function and gonadal activity during the evolution of the GnRH pulse generator.     

Gonadal regulation of hypothalamic gonadotropin-releasing hormone release in primates

This review has examined, in primates, the action of sex steroids on the neural timing mechanism that governs the intermittent release of GnRH by the hypothalamus, the so-called GnRH pulse generator. Determinants of hypothalamic GnRH pulse generator frequency have generally been examined indirectly by monitoring moment to moment fluctuations in circulating LH concentrations. Studies using this approach have led to the hypothesis that negative feedback control of LH secretion by the testes is mediated by the action of T, or of one of its metabolites, to retard the frequency of the GnRH pulse generator. P also appears capable of decelerating GnRH pulse frequency during the luteal phase of the menstrual cycle, but the physiological significance of this phenomenon remains to be clarified. To date, a cognate action of E2 on the GnRH pulse generator has not been described. Because of limitations in contemporary technology, the factors underlying amplitude modulation of the GnRH pulse generator are less well understood. In addition to the ability of certain sex steroids to decelerate the frequency of pulsatile GnRH discharge, E2 and P appear capable of facilitating the secretion of this hypothalamic releasing factor. However, the increase in GnRH pulse frequency and/or GnRH pulse amplitude that must underlie these stimulatory actions remains to be fully defined. An inhibitory action of high levels of circulating cortisol on the hypothalamic GnRH pulse generator has also been noted.     

A potential apulsatile mode of GnRH release in the male rhesus monkey (Macaca mulatta).

The hypothalamic component of the reproductive axis in vertebrates is comprised of a pulse generator that stimulates the release of GnRH. Several lines of evidence are in agreement that the activity of this pulse generator is intermittent and results in the pulsatile pattern of GnRH and LH release. During a recent investigation of the re-initiation of LH secretion in the agonadal, prepubertal male monkey, we observed a daytime profile of LH secretion, which suggests an apulsatile mode of GnRH release. The first purpose of this study was to describe this observation of apulsatile LH release during the peripubertal transition. Furthermore, we have explored the dependence of this form of LH secretion on GnRH release. Five male rhesus monkeys (Macaca mulatta) were castrated prepubertally and were treated with an intermittent infusion of GnRH to prematurely sensitize the juvenile pituitary to endogenous GnRH release. Alternate daytime (1100-1800 h) and nighttime (1900-0200 h) assessments of LH release were performed at 10-day intervals throughout the peripubertal transition with samples taken every 12 min. In a second experiment, four agonadal males which demonstrated an apulsatile profile of LH release were maintained on an infusion of physiological saline and were treated with the GnRH antagonist Nal-Glu (i.m., 500 microgram/kg). Circulating levels of LH were determined 22 h after antagonist treatment. In peripubertal animals, circulating levels of LH were similar between morning and evening assessments. However, pulse frequency was significantly lower during the daytime. GnRH antagonist reduced LH levels by 72% and a similar reduction in response to an exogenous GnRH test stimulus occurred. These findings suggest an apulsatile mode of GnRH release.     

Pulsatile gonadotropin secretion in women with hypothalamic amenorrhea: evidence that reduced frequency of gonadotropin-releasing hormone secretion is the mechanism of persistent anovulation

Hypothalamic amenorrhea (HA) is a clinical disorder of unknown etiology. The diagnosis is made by exclusion of known abnormalities of pituitary and ovarian function. To determine if abnormalities of GnRH secretion could account for the anovulation and amenorrhea, we measured plasma gonadotropins every 20 min for 10- to 24-h periods in 19 women with HA. Ovarian steroids and gonadotropin responses to an iv bolus dose of GnRH (25 ng/kg) were also measured. The results were compared to those obtained during the early follicular (EF) and late luteal (LL) phases of ovulatory cycles in normal women. Plasma estradiol was lower (mean +/- SE, 52 +/- 5 pg/ml) than either cycle stage in normal women. Mean plasma LH was lower than EF values and FSH was higher than LL values. The amplitude of LH pulses in HA was similar to that in normal women. LH pulse frequency was the same as that present during the LL, but lower than that during the EF (HA, 4.7 pulses/12 h; EF, 7.7 pulses/12 h; P less than 0.05). In addition to the similar frequency, the patterns of LH secretion in HA resembled that of LL in that the amplitude of LH pulses was highly variable and pulses occurred at irregular intervals. Consistent changes in diurnal gonadotropin secretion were not found, and LH secretion was greater at night in 9 studies and during the day in 5 studies. Repeat studies in three patients (5-13 months later) revealed that LH pulse frequency was variable, being unchanged in 1, increased in 1, and decreased in the third patient. Thus, LH pulse frequency and, by inference, GnRH pulse frequency are similar in HA to those in the normal luteal phase despite a different steroid milieu. GnRH pulse frequency increases from the luteal to the follicular phases of normal cycles and may be important in the initiation of ovarian follicular maturation. These data suggest that the absence of cyclical gonadotropin secretion and anovulation in HA result from a decreased frequency and irregular amplitude of GnRH secretion and consequent absence of ovarian follicular maturation.     

Evidence from the rhesus monkey (Macaca mulatta) for the view that negative feedback control of luteinizing hormone secretion by the testis is mediated by a deceleration of hypothalamic gonadotropin-releasing hormone pulse frequency

The site and mode of the feedback actions of testicular hormones on gonadotropin secretion in the adult rhesus monkey were investigated using the arcuate-lesioned preparation previously employed by others to study cognate problems in the female. The negative feedback loop that governs LH and FSH release in the male monkey was opened without changing either the frequency or amplitude of intermittent GnRH stimulation of the pituitary gonadotrophs, which was clamped by exogenous GnRH replacement at a level that approximated the intact or closed loop hypophysiotropic signal. In this manner, the relative importance of adenohypophysial vs. hypothalamic sites of feedback action of testicular hormones on LH and FSH secretion was assessed. To accomplish the foregoing, radiofrequency lesions were placed in the region of the arcuate nucleus to abolish endogenous hypothalamic GnRH secretion. Patterns of temporally coupled episodes of pituitary LH and testicular testosterone discharge that in nonlesioned animals characteristically occur, on the average, once every 3 h throughout the 24-h light-dark cycle were restored in lesioned animals by an intermittent iv infusion of GnRH (0.1 micrograms/min for 3 min every 3 h). Bilateral orchidectomy in this experimental paradigm elicited only small increments in LH pulse amplitude and mean plasma LH concentration, a response in striking contrast to the dramatic postcastration LH hypersecretion observed in animals with intact hypothalami that respond to the opening of the negative feedback loop with an apparent acceleration in the endogenous frequency of intermittent GnRH secretion. A marked rise in mean plasma LH concentration in arcuate-lesioned males, however, was forth-coming when the frequency of intermittent exogenous GnRH stimulation was increased 2-3 weeks after castration from one pulse every 3 h (intact frequency) to one pulse per h (castrate frequency). These findings fail to provide evidence for a major inhibitory feedback action of the testes on LH secretion at the level of the adenohypophysis. They are entirely consistent, however, with the hypothesis that the negative feedback control of LH release by the male gonad is mediated, principally, via the central nervous system by an action of testicular hormone, most probably testosterone, to retard the frequency of the neural timing mechanism that governs the intermittent pattern of GnRH release by the hypothalamus.(ABSTRACT TRUNCATED AT 400 WORDS)     

Abrogation by a potent gonadotropin-releasing hormone antagonist of the estrogen/progesterone-stimulated surge-like release of luteinizing hormone and follicle-stimulating hormone in postmenopausal women.

In both the rodent and primate, administration of progesterone elicits an acute surge-like release of LH in the setting of prior estrogen treatment. Whether these facilitative effects of estrogen and progesterone on gonadotropin secretion reside at pituitary or hypothalamic loci is not known. To further investigate the mechanisms by which estrogen combined with progesterone amplifies gonadotropin secretion, we studied the responses of seven estrogen-primed postmenopausal women to progesterone administration with or without cotreatment with a potent GnRH antagonist, [Ac-D2Nal1,D4ClPhe2,D3Pal3,Arg5,DGlu6(AA), DAla10]GnRH. Repetitive blood sampling for the later measurement of serum concentrations of LH, FSH, and PRL was begun 4 h before the administration of progesterone and continued for 36 h. We observed that progesterone administration after 72 h of priming with ethinyl estradiol resulted in a surge-like release of LH and FSH in all subjects. Concomitant administration of the GnRH antagonist abolished the surge-like release of both gonadotropins in all subjects. In contrast, administration of the antagonist had no effect on PRL release. These results indicate that endogenous GnRH action is an obligatory component of the progesterone-induced surge-like release of both gonadotropic hormones in the estrogen-primed human.     

Electrophysiological studies on the hypothalamic GnRH pulse generator]

Gonadal function in mammals depends on gonadotropins secreted from the pituitary gland in a pulsatile manner. This pulsatility is governed by the periodic activation of the hypothalamic GnRH pulse generator. By means of multiple unit activity (MUA) recording techniques, characteristics increases in the neuronal activity, each of which is associated with the initiation of pulsatile LH secretion, have been recorded in the medial basal hypothalamus of the monkey, rat and goat. An unambiguous unitary relationship between the increased electrical activity (volley) and the LH pulse under a variety of physiological and experimental conditions indicates that the MUA volleys represent the electrical activity of the GnRH pulse generator. Hypothalamic MUA recordings provide direct access to the central component of the neuroendocrine control system which governs reproductive function.     

Inherited disorders of the gonadotropin hormones.

Pulsatile GnRH acts at the GnRH receptor on gonadotropes to stimulate gonadotropin gene expression, hormone synthesis and secretion. The pituitary gonadotropins, LH and FSH, stimulate steroid production and gametogenesis in males and in females. Gonadotropin production thus requires the normal development and function of hypothalamic GnRH-producing neurons and pituitary gonadotrope cells. Genes involved in gonadotrope development and/or gene expression include SF1, DAX1, KAL, GNRHR, PC1, HESX1, LHX3, PROP1, LH beta, and FSH beta. Given the complex control of gonadotropin biosynthesis and secretion, it is not surprising that genetic abnormalities have been identified at several of these steps. Some of the mutations that will be reviewed include: (1) SF1 and DAX1-orphan nuclear receptors that are expressed at multiple levels throughout the reproductive axis; (2) KAL-X-linked Kallmann syndrome, where there is abnormal development of hypothalamic GnRH-producing neurons; (3) PC1-causing abnormal processing of GnRH and GNRHR mutations that impair action at the GnRH receptor; (4) HESX1, LHX3, PROP1-abnormal development/function of the gonadotrope cell lineage; (5) LH beta and FSH beta-mutations in the gonadotropin genes that cause structural abnormalities in the hormones. Although all of these gene defects lead to gonadotropin deficiency, each disorder is associated with unique phenotypic or hormonal features. Characterization of the molecular basis of gonadotropin deficiency is useful for directing therapy and for genetic counseling. Identification of these mutations also provides insight into the pathways that govern reproduction.     

Genetics of hypogonadotropic hypogonadism

Determining the physiologic influences that modulate GnRH secretion, the prime initiator of reproductive function in the human, is fundamental not only to our understanding of the rare condition of congenital idiopathic hypogonadotropic hypogonadism (IHH), but also common disorders such as constitutional delay of puberty and hypothalamic amenorrhea. IHH is characterized by low levels of sex steroids and gonadotropins, normal findings on radiographic imaging of the hypothalamic-pituitary regions, and normal baseline and reserve testing of the remainder of the hypothalamic-pituitary axes. Failure of the normal pattern of episodic GnRH secretion results in delay of puberty and infertility. IHH is characterized by rich clinical and genetic heterogeneity, variable modes of inheritance, and association with other anomalies. To date, 4 genes have been identified as causes of IHH in the human; KAL [the gene for X-linked Kallmann syndrome (IHH and anosmia)], DAX1 [the gene for X-linked adrenal hypoplasia congenita (IHH and adrenal insufficiency)], GNRHR (the GnRH receptor), and PC1 (the gene for prohormone convertase 1, causing a syndrome of IHH and defects in prohormone processing). As these mutations account for less than 20% of all IHH cases, discovery of additional gene mutations will continue to advance our understanding of this intriguing syndrome.