Induction of ovulation with chronic intermittent (pulsatile) administration of Gn-RH in women with hypothalamic amenorrhoea

The physiological and pathophysiological basis of hypothalamic amenorrhoea are reviewed as well as the clinical results of chronic intermittent (pulsatile) administration of Gn-RH in the treatment of infertility. Hypothalamic amenorrhoea is considered to be the result of a deficient hypothalamic secretion of Gn-RH. By pulsatile administration of Gn-RH, which is a pre-requisite of normal pituitary gonadotrophic function, deficient endogenous Gn-RH is replaced. If an adequate dose of Gn-RH is provided, which takes into account the degree of impairment of hypothalamic function in the individual case, follicular maturation, ovulation and corpus luteum formation are achieved in nearly every treatment cycle. Although dependent also on factors other than the treated dysfunction, a high conception rate is achieved.     

Kisspeptin Signalling in the Hypothalamic Arcuate Nucleus Regulates GnRH Pulse Generator Frequency in the Rat

Background

Kisspeptin and its G protein-coupled receptor (GPR) 54 are essential for activation of the hypothalamo-pituitary-gonadal axis. In the rat, the kisspeptin neurons critical for gonadotropin secretion are located in the hypothalamic arcuate (ARC) and anteroventral periventricular (AVPV) nuclei. As the ARC is known to be the site of the gonadotropin-releasing hormone (GnRH) pulse generator we explored whether kisspeptin-GPR54 signalling in the ARC regulates GnRH pulses.

Methodology/Principal Findings

We examined the effects of kisspeptin-10 or a selective kisspeptin antagonist administration intra-ARC or intra-medial preoptic area (mPOA), (which includes the AVPV), on pulsatile luteinizing hormone (LH) secretion in the rat. Ovariectomized rats with subcutaneous 17β-estradiol capsules were chronically implanted with bilateral intra-ARC or intra-mPOA cannulae, or intra-cerebroventricular (icv) cannulae and intravenous catheters. Blood samples were collected every 5 min for 5–8 h for LH measurement. After 2 h of control blood sampling, kisspeptin-10 or kisspeptin antagonist was administered via pre-implanted cannulae. Intranuclear administration of kisspeptin-10 resulted in a dose-dependent increase in circulating levels of LH lasting approximately 1 h, before recovering to a normal pulsatile pattern of circulating LH. Both icv and intra-ARC administration of kisspeptin antagonist suppressed LH pulse frequency profoundly. However, intra-mPOA administration of kisspeptin antagonist did not affect pulsatile LH secretion.

Conclusions/Significance

These data are the first to identify the arcuate nucleus as a key site for kisspeptin modulation of LH pulse frequency, supporting the notion that kisspeptin-GPR54 signalling in this region of the mediobasal hypothalamus is a critical neural component of the hypothalamic GnRH pulse generator.     

Localization of hormone secreting pathways in the brain by immunohistochemistry and light microscopy: a review.

Immunocytochemical techniques are now being used to localize hypothalamic neurosecretory hormones and related peptides in the mammalian brain. The data are probably incomplete, due primarily to false negative results. A number of previous assumptions concerning these pathways have been confirmed while other unexpected results were obtained. As expected, vasopressin and oxytocin and their associated proteins, neurophysins, were found in the magnocellular cell bodies of the hypothalamus and in their axonal projections to the neural lobe of the pituitary. Gonadotropin-releasing hormone (Gn-RH), somatostatin, and thyrotropin-releasing hormone (TRH) were located in what appears to be parvicellular nerve terminals on portal capillaries. Gn-RH has been found in perikarya in the arcuate nucleus, which is considered a source of fibers to the portal capillary bed. An extensive network of cell bodies and fibers in the preoptic area was also found to contain Gn-RH, and others in the periventricular nucleus in the anterior hypothalamus reacted with antiserum to somatostatin. Unexpected was considerable evidence that vasopressin is secreted directly into hypophyseal portal blood. This hormone and its neurophysin were also found in parvicellular neurons in the suprachiasmatic nucleus of rodents. All the hormones were found in fibers in the organum vasculosum of the lamina terminalis and in the posterior pituitary gland.     

Relationship between pituitary responsiveness to Gn-RH and number of Gn-RH-binding sites in pituitary glands of beef cows

Changes in the ability of Gn-RH to induce gonadotrophin release with time after synchronization of oestrus was determined in 4 groups of 6 cows each. Cows were given Gn-RH at 40-min intervals for 6 h beginning at -24, 0, 18 or 36 h (time 0 = removal of progestagen implant). Changes in concentration (ng/ml) of serum LH after Gn-RH averaged 2.9, 6.2, 6.4 and 33.4, whereas serum FSH averaged 25.7, 35.8, 35.8 and 97.3. Thus the responsiveness of the pituitary to Gn-RH had increased by 36 h after implant removal. Other groups of cows subjected to the same synchronization scheme were slaughtered at 0 h, 24 h or at various times after onset of oestrous behaviour. Gn-RH binding to crude pituitary membrane preparations was assessed. There was no apparent change in the affinity constant of Gn-RH-binding sites with time after synchronization. The number of Gn-RH-binding sites remained unchanged until the period of oestrus when a significant decline with time was detected. We conclude that the increase in pituitary responsiveness to Gn-RH that occurs before the preovulatory gonadotrophin surge was not directly associated with changes in number or affinity of pituitary Gn-RH-binding sites in crude pituitary membrane preparations.     

Studies on factors affecting release of gonadotrophins during the superfusion of isolated rat pituitaries.

Isolated pituitary glands from adult male rats were maintained in a continuous flow system. Gn-RH (1000 pmol/ml) caused a characteristic release of cyclic AMP, LH, and FSH. Cyclic AMP (1000 nmol/ml) liberated a similar amount of both gonadotrophins. Theophylline (1 mmol/ml) enhanced the effect of cyclic AMP by 21% for LH and 41% for FSH. The infusion of oestradiol (184 pmol/ml) alone or before Gn-RH infusion did not produce a significant effect on the secretion of either gonadotrophin or cyclic AMP. In contrast, there was a significant reduction in the amount of LH (P less than 0-025) and FSH (P less than 0-05) released by pituitaries infused with oestradiol and cyclic AMP.     

Pulsatile administration of gonadotropin releasing hormone (Gn-RH) to lactating sows using a portable automatic pump (“Zyklomat”)

The object of this investigation was to study the feasibility of using a “Zyklomat roller peristaltic pump” for pulsatile administration of gonadotropin releasing hormone (Gn-RH) in primiparous lactating sows. Four primiparous sows were used. The pump catheter was inserted into a jugular vein on day 8 or day 22 of lactation. The pump delivered a 1 min Gn-RH pulse of 10 μg every 89 min for 7 days. The pump worked without any complication or detrimental effect on the sows throughout the experimental period. Two sows showed standing oestrus during lactation and one of them ovulated. It can be concluded that the “Zyklomat” pump can be used for pulsatile infusion of Gn-RH in lactating sows.     

Oestrogen and progesterone interactions in the control of gonadotrophin and prolactin secretion

Oestrogen and progesterone have marked effects on the secretion of the gonadotrophins and prolactin. During most of the oestrous or menstrual cycle the secretion of gonadotrophin is maintained at a relatively low level by the negative feedback of oestrogen and progesterone on the hypothalamic-pituitary system. The spontaneous ovulatory surge of gonadotrophin is produced by a positive feedback cascade. The cascade is initiated by an increase in the plasma concentration of oestradiol-17 beta which triggers a surge of luteinizing hormone releasing hormone (LHRH) and an increase in pituitary responsiveness to LHRH. The facilitatory action of oestrogen on pituitary responsiveness is reinforced by progesterone and the priming effect of LHRH. How oestrogen and progesterone exert their effects is not clear but the facilitatory effects of oestrogen take about 24 h, and the stimulation of LHRH release is produced by an indirect effect of oestradiol on neurons which are possibly opioid, dopaminergic or noradrenergic and which modulate the activity of LHRH neurons. In the rat, a spontaneous prolactin surge occurs at the same time as the spontaneous ovulatory gonadotrophin surge. The prolactin surge also appears to involve a positive feedback between the brain-pituitary system and the ovary. However, the mechanism of the prolactin surge is poorly understood mainly because the neural control of prolactin release appears to be mediated by prolactin inhibiting as well as releasing factors, and the precise role of these factors has not been established. The control of prolactin release is further complicated by the fact that oestradiol stimulates prolactin synthesis and release by a direct action on the prolactotrophes. Prolactin and gonadotrophin surges also occur simultaneously in several experimental steroid models. A theoretical model is proposed which could explain how oestrogen and progesterone trigger the simultaneous surge of LH and prolactin.     

Follicular development and a single ovulation induced with pulsatile administration of Gn-RH in anovulatory women: studies of hormonal analysis and follicular sonometry

The method of pulsatile administration of gonadotropin-releasing hormone (Gn-RH) has been proven as a useful means for induction of ovulation in anovulatory women. In our series of clinical trials, 23 out of 29 anovulatory patients ovulated with pulsatile administration of Gn-RH. Seven patients who ovulated volunteered for the present study with daily hormonal analysis and follicular sonometory . Two patients have oligomenorrhea, 3 patients secondary amenorrhea-1st grade (the sole administration of gestagen required for withdrawal bleeding) and the remaining 2 patients secondary amenorrhea-2nd grade (the combined administration of estrogen and gestagen required for withdrawal bleeding). A diagnosis of hyperprolactinemia was made for one patient with secondary amenorrhea-1st grade. Pulsatile administration of Gn-RH was performed by the use of a self-administered infuser . The infuser was connected to an i.v. indwelling catheter via a specially designed blood backflow eliminater . Five micrograms or less of Gn-RH was given every 2 hr from 07:00 to 23:00 hr daily. Five patients received HCG during the preovulatory period. In one patient, a short term treatment of HMG was added to Gn-RH treatment. Follicular sonometry revealed the development of a single dominant follicle which reached between 20 and 28 mm (23.7 +/- 0.12 mm, mean +/- S.E.) in diameter at the preovulatory period. Disappearance of a dominant follicle was recognized in the early luteal phase. Characteristic increases in estradiol were recognized concomitantly with the development of a dominant follicle. Progesterone levels after ovulation were within the limits of its normal "luteal phase" rise. The present data suggest that pulsatile administration of low dose Gn-RH with nocturnal interruption of treatment is effective for normal progress of follicular development in various types of anovulatory patients, culminating in single ovulation. This paper includes the discussion on our method which may be responsible for a high success rate of ovulation induction.     

Changes in plasma inhibin levels following pulsatile gonadotrophin-releasing hormone therapy in a man with idiopathic hypogonadotrophic hypogonadism

Changes in circulating inhibin levels were related to changes in testosterone (T) and the gonadotrophins luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in a hypogonadotrophic hypogonadal man before and during pulsatile gonadotrophin-releasing hormone therapy which resulted in normal spermatogenesis. Before treatment, the plasma inhibin levels in the patient (210 +/- 50 U/l; mean +/- SD of four samples) were lower than in normal controls (552 +/- 150 U/l; p less than 0.01), as were T (1.1 nmol/l) and gonadotrophin (less than 1.0 IU/l) levels. Within 1 week of gonadotrophin-releasing hormone treatment, plasma LH (14.1 +/- 0.7 IU/l) and FSH (14.4 +/- 0.6 IU/l) reached supraphysiological levels. In response, T and inhibin concentrations increased progressively to reach high normal levels (27.7 +/- 1.6 nmol/l and 609 +/- 140 U/l) at 4 weeks, by which time the gonadotrophin levels stared to decline and gradually returned to the normal range between 12 and 24 weeks of treatment. There was a concomitant decrease in T and inhibin levels which remained within the normal range. The decline in the FSH level following the rise in testicular hormones was earlier and steeper than that of LH (37.5% decrease at 4 weeks vs. 30.4% at 12 weeks), suggesting that T and inhibin may act together to inhibit pituitary FSH secretion as opposed to LH secretion which is primarily controlled by T. It is concluded that, in man, during maturation of the pituitary-testicular axis, changes in circulating inhibin parallel those of T, and quantitatively normal inhibin secretion is dependent on gonadotrophin stimulation. FSH secretion may be regulated through negative feedback control, by both T and inhibin.     

Clinical uses of gonadotropin-releasing hormone analogues.

Gonadotropin-releasing hormone (Gn-RH) analogues are synthetic derivatives of the native hypothalamic peptide with alterations in their chemical structure that result in changes in biologic activity. Several Gn-RH agonists are available for clinical use, and all act through the same mechanism: first to stimulate and then to inhibit gonadotropin and gonadal steroid secretion by downregulating the pituitary Gn-RN receptors. This review should provide clinicians with a working knowledge of the physiologic and pharmacokinetic features of Gn-RH agonists. Although over 2000 articles concerning Gn-RH analogues have been published I chose to review only those that were the first to report a novel clinical application. Gn-RH agonists have proved to be extremely efficacious in treating gonadal steroid-dependent problems such as endometriosis, uterine leiomyoma, precocious puberty and prostate and breast cancers, and they have resulted in very few side effects. Long-term use may, however, lead to skeletal calcium loss in women as a consequence of hypoestrogenism. Further research is needed to prevent this and maintain clinical efficacy.     

Pulsatile infusion of gonadotrophin releasing hormone (GnRH): investigative and therapeutic applications

Normal gonadotrophin secretion, and therefore normal ovarian function, depend on delivery to the pituitary of the hypothalamic neuropeptide gonadotrophin releasing hormone (GnRH) in a pulsatile pattern. In the mid-follicular phase of the menstrual cycle, for example, discrete pulses of luteinizing hormone (LH) can be observed at approximately 90 min intervals. Many disorders of ovulation are caused by abnormalities of this natural pulsed signal. We have developed and used a small portable infusion pump to deliver GnRH to women with hypothalamic amenorrhoea; our studies, and those of other groups, have shown that successful ovulation and pregnancy result from such treatment. The results of treatment at St Mary's Hospital show that 16 women with hypogonadotrophic amenorrhoea received a total of 31 cycles of treatment with pulsatile GnRH; 25 (81%) of these cycles were ovulatory and 11 of the 14 women who were trying to conceive became pregnant. There was only one multiple pregnancy (twins).     

Control of ovulation in farm animals

Examination of hormonal changes occurring in farm species at the onset of puberty, during the follicular phase of the oestrous cycle, and at those times when ovarian activity is re-established after periods of seasonal or lactational anoestrus, provides circumstantial evidence that the final phases of follicular development are dependent on a pattern of tonic (episodic) LH secretion. A suppression of episodic LH secretion is associated with periods of anovulation. Stimulation of tonic LH secretion by repeated injections of small doses of synthetic Gn-RH or purified LH restores normal reproductive function in all but deeply anoestrous animals. Continuous infusion of Gn-RH is as effective as repeated injections. It is suggested that an additional inadequacy, possibly endocrine, contributes to the anovulatory state in deep anoestrus.     

The role of gonadotropin releasing hormone (Gn-RH) in the regulation of gonadal functions of birds. Review article

Gonadotrop hormone secretion is regulated by the central nervous system through the hypothalamus. This neuro-hormonal regulation was first verified in birds by Follett /21/ who was able to increase the LH secretion of hypophysis in vitro by crude extract of quail hypothalamus. His results supported the indirect statements of earlier neuroendocrine studies and emphasized the importance of bird hypothalamus in the regulation of gonadal function /1, 62/. A neurohormone fundamental in the central regulation of gonadic function, luteinizing hormone releasing hormone (abbreviated earlier as LH-RH, but recently, and, thus, hereinafter as Gn-RH) has first been isolated from porcine hypothalamus in Schally's Laboratory /41/, and, following the determination of its amino-acid sequence, it has been synthesized in the same year /42/. It has been stated that this peptide, consisting of 10 amino acids (p Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2), increases the LH and FSH secretion of the hypophysis both in vitro and in vivo. One year later, a decapeptide, similar in its structure to porcine Gn-RH was produced from sheep hypothalamus. Investigations of the two teams suggested that decapeptide containing arginine on place 8 was the physiological Gn-RH of mammals.     

Anorexia nervosa: hypogonadotrophic hypogonadism and bone mineral density

Anorexia nervosa is a chronic illness that involves a reduction in caloric intake, loss of weight and amenorrhoea, either primary or secondary. In addition to prolonged amenorrhoea, osteopenia and osteoporosis are the most frequent complications. Patients exhibit an alteration in the hypothalamic-pituitary-gonadal axis, which is responsible for the menstrual disorders. The increase in gonadotrophin secretion that can be observed after ponderal recuperation suggests that malnutrition could be the most important mechanism involved in the decrease in gonadotrophin secretion. The loss of fat tissue, as a consequence of the restriction of nutrients, has been associated with hypoleptinaemia, abnormal secretion of peptides implicated in food control (neuropeptide Y, melanocortins and corticotrophin-releasing hormone, among others) and diminution of the amount of total body fat. Despite oestrogen therapy, the severe loss of bone mass may progress. Other factors such as weight loss, duration of amenorrhoea and low insulin-like growth factor-I (IGF-I) levels could contribute to the loss of bone mass in women with anorexia nervosa. The recuperation of weight and, in particular, the amount of total body fat could lead to the spontaneous recuperation of menstruation.     

Influence of different neural systems on the secretion of sex hormones in potassium deficient mice

The influence of different neural systems that modulate GnRH secretion by hypothalamic neurons was investigated in mice exposed to hypokalemic conditions, in which the pulsatile release of GnRH has been shown to be altered and associated with a significant decrease of plasma sex steroids. Our results demonstrate that the potentiation of the inhibitory pathways mediated by opiates and GABA may be implicated in the decrease of sex hormones secretion produced by hypokalemia since treatment with higher doses of naloxone or flumazenil are required to restore progesterone or testosterone levels in potassium deficient mice. The combination treatment of prazoxin and naloxone suggests that the inhibitory action of opiates take place through its action on noradrenergic neurons. It is also possible that the inhibition of GnRH release could be due to a decrease in the tonic stimulatory action of noradrenergic pathway implicated in the control of GnRH release. Our results also reveal that it is unlikely that the glutamatergic system may play any relevant direct role in the decrease of sex steroid secretion observed in potassium deficient mice. Finally, these results together with the normal pattern of estradiol levels found along the estrus cycle in potassium deficient mice indicate that factors different from estradiol and acting on neural systems implicated in the regulation of GnRH-secreting neurons participate in the generation of the preovulatory surge of GnRH.     

Comparison of insulin and glucagon pulsatile secretion between the rat and dog pancreas in-vitro

Sustained pulses of insulin and glucagon were obtained from the isolated perfused in vitro rat pancreas. The respective periodicity of hormone release (peak to peak interval) was calculated by the Pulsar computer algorithm as insulin 5.8 +/- 0.3 min and glucagon 6.5 +/- 0.25 min. Because pulsatile insulin secretion is absent in type II diabetics, pulsatile islet hormone secretion could theoretically be regulated directly by intra-islet hormone interactions or indirectly by hormone sensitive nerve feedback, possibly from a venous hormone sensitive receptor system within the pancreas. To test the possible contributions of these systems in pulse regulation, the direction of perfusion was reversed in both rat and dog pancreata to prevent hormone contact with putative venous hormone receptors. The periodicity of hormone secretion was unchanged by reversed perfusion in both species. As vascular perfusion of islet cells is normally B to A to D, these results suggest that neither intra-islet hormone interactions nor intra-pancreatic insulin or glucagon sensitive nerve feedback systems are responsible, on an acute basis, for the regulation of pulsatile insular secretion from the normal pancreas. Insulin regulates net glucagon secretion but does not acutely influence glucagon pulses. The presence of pulses during retrograde perfusion may be the result of the entrainment of the pacemaker-islet system. These observations are consistent with the presence of an independent pacemaker and neural coordinating system within the dog and rat pancreas which may influence both the A- and B-cell.     

Deterministic construct of amplifying actions of ghrelin on pulsatile growth hormone secretion

Ghrelin is a native ligand for the growth hormone secretagogue (GHS) receptor that stimulates pulsatile GH secretion markedly. At present, no formal construct exists to unify ensemble effects of ghrelin, GH-releasing hormone (GHRH), somatostatin (SRIF), and GH feedback. To model such interactions, we have assumed that ghrelin can stimulate pituitary GH secretion directly, antagonize inhibition of pituitary GH release by SRIF, oppose suppression of GHRH neurons in the arcuate nucleus (ArC) by SRIF, and induce GHRH secretion from ArC. The dynamics of such connectivity yield self-renewable GH pulse patterns mirroring those in the adult male and female rat and explicate the following key experimental observations. 1) Constant GHS infusion stimulates pulsatile GH secretion. 2) GHS and GHRH display synergy in vivo. 3) A systemic pulse of GHS stimulates GH secretion in the female rat at any time and in the male more during a spontaneous peak than during a trough. 4) Transgenetic silencing of the neuronal GHS receptor blunts GH pulses in the female. 5) Intracerebroventricular administration of GHS induces GH secretion. The minimal construct of GHS-GHRH-SRIF-GH interactions should aid in integrating physiological data, testing regulatory hypotheses, and forecasting innovative experiments.     

Non-dopaminergic neurons expressing dopamine synthesis enzymes: differentiation and functional importance

The study has evaluated in vivo, ex vivo and in vitro ontogenesis and functional significance of the arcuate nucleus neurons expressing either individual enzymes of dopamine synthesis, tyrosine hydroxylase or aromatic L-amino acid decarboxylase as well as both of them (dopaminergic neurons) in rats from the 17th embryonic day to adulthood. Monoenzymatic tyrosine hydroxylase-containing neurons were initially observed on the 18th embryonic day. On the 20-21 day, the monoenzymatic tyrosine hydroxylase- or aromatic L-amino acid decarboxylase-expressing neurons comprised more than 99% of the whole neuron population expressing the dopamine-synthesizing enzymes. The dopamine production in the fetus arcuate nucleus was sufficient to provide an inhibitory control of prolactin secretion like in adults. The data suggest a possibility of the dopamine synthesis in the fetus arcuate nucleus by the monoenzymatic neurons containing either tyrosine hydroxylase or aromatic L-amino acid decarboxylase-expressing neurons in co-operation.     

Reproductive axis response to repeated lipopolysaccharide administration in peripubertal female rats

Immune system disorders are often accompanied by alterations in the reproductive axis. Several reports have shown that administration of bacterial lipopolysaccharide (LPS) has central inflammatory effects and activates cytokine release in the hypothalamus where the luteinizing hormone releasing hormone (Gn-RH) neurons are located. The present study was designed to investigate the effect of repeated LPS administration on the neuroendocrine mechanisms of control of the reproductive axis in peripubertal female rats (30-day-old rats). With this aim, LPS (50 μg/kg weight) was administered to the animals during 25, 27 and 29 days of age and sacrificed on 30 day of life. Gn-RH, γ?amino butyric acid (GABA) and glutamic acid (GLU), two amino acids involved in the regulation of Gn-RH secretion, hypothalamic content were measured. LH and estradiol serum levels were also determined and the day of vaginal opening examined. The results showed a significant increase in Gn-RH and GLU content (p?<?0.0001), shared by a reduction of GABA one (p?<?0.0001). LH and estradiol serum levels were decreased (p?<?0.01, p?<?0.001) and delay in the day of vaginal opening was also observed in treated animals. Present results show that repeated LPS administration impaired reproductive function, modifying the neuroendocrine mechanisms of control of the axis in peripubertal female rats.     

Passive immunization with anti-oestradiol antibodies during the luteal phase of the menstrual cycle potentiates the perimenstrual rise in serum gonadotrophin concentrations and stimulates follicular growth in the cynomolgus monkey (Macaca fascicularis)

Three unilaterally ovariectomized cynomolgus monkeys, in which menstrual cycles were driven by pulsatile infusion of synthetic GnRH at a fixed frequency of 1 pulse/h, were provided with a continuous infusion of ovine anti-oestradiol gamma-globulin beginning 13 days after ovulation and continuing for 7 days thereafter. Plasma concentrations of both FSH and LH rose at the start of the antibody infusion and remained elevated throughout the 7-day treatment regimen when compared with control (non-immune gamma-globulin-treated or untreated) animals. Morphometric examination of ovaries at the end of the experimental and control infusions revealed a significant difference (P less than 0.05) in the average size of the largest non-atretic antral follicle in each of the experimental animals when compared with that of the control animals (2.45 +/- 0.23 vs 1.30 +/- 0.53 mm). Collectively, the 3 control animals possessed 9 non-atretic antral follicles greater than 1.0 mm diameter, none of which exceeded a diameter of 2.0 mm. In contrast, the experimental animals had 28 non-atretic follicles of greater than 1.0 mm diameter, 8 of which exceeded 2.0 mm. These observations are consistent with the hypothesis that oestrogen and progesterone are the primary agents responsible for the restraint of gonadotrophin secretion and preovulatory follicular growth during the luteal phase of the primate menstrual cycle.