Negative Feedback Effect Of Estrogen Research Articles

Endocrine regulation of puberty in cows and ewes

Sexual maturation in cows and ewes is modulated through changes in hypothalamic inhibition. This inhibition results in little or no stimulation of the release of gonadotrophins from the anterior pituitary. The ovary has a primary role in inhibiting gonadotrophin secretion during the prepubertal period and the responsiveness to the negative feedback effects of oestrogen decreases during the peripubertal period. There is also an increased secretion of ovarian progesterone during the peripubertal period but its role in the process of sexual maturation is not clear. Photoperiodic cues and dietary intake act upon the hypothalamus to modulate gonadotrophin secretion during sexual maturation and, in turn, influence the time when puberty occurs.

The etiology of menopause: not just ovarian dysfunction but also a role for the central nervous system

Background: The hormonal changes which occur leading up to menopause have been described in multiple cross-sectional and longitudinal studies. Results from these studies document the occurrence of marked alterations in pituitary-hypothalamic function, along with the changes in the ovary, which play a role in the etiology of menopause. However, the role of pituitary-hypothalamic function in the menopausal transition (MT) has been generally overlooked. Methods: Literature searches were performed using PubMed, Scopus, and Medline, for articles with content related to menopause and the MT. The searches were restricted to English language observational studies and reviews of human studies and clinical trials. Results: During the MT, in addition to a reduction in the number of ovarian follicles, there is a marked reduction in the sensitivity of the central nervous system to both the positive and negative feedback effects of estrogen. These changes in sensitivity explain menstrual irregularities and systemic systems which are present in the time before the final menstrual period. Although the timing of menopause is difficult to predict, broad patterns in hormonal changes are seen, including a slow drop in inhibin B levels, followed by a monotropic rise in follicle-stimulating hormone during the late reproductive period. The MT is marked by irregular cycles with unpredictable luteinizing hormone levels and unpredictable ovulation. The levels of estrogen remain stable, or are even elevated through most of the MT, and levels of estrogen do not begin to decrease until late in the MT. The Stages of Reproductive Aging Workshop guidelines have improved the homogeneity of classifications in studies. Conclusions: On the basis of longitudinal hormone studies, it is clear that desensitization of the hypothalamic-pituitary-ovarian feedback loop plays an important role, in addition to a decrease in follicle number and insensitivity of remaining follicles to the positive feedback of estrogen, to trigger ovulation. Further studies in humans are needed to elucidate the mechanisms responsible for these perimenopausal changes if proper therapeutic modalities for the symptoms associated with menopause can be developed.

Effect of clomiphene citrate and human chorionic gonadotropin (hCG) on ovulation induction in prepubertal Sahiwal heifers

Abstract Objective To evaluate the ovulation inducing effect of clomiphene citrate along with human chorionic gonadotropin (hCG) in prepubertal Sahiwal heifers. Clomiphene citrate (an anti-estrogen) causes gonadotropin secretion from pituitary gland by removing the negative feedback effect of estrogen. This causes more FSH secretion which ultimately stimulates follicular growth and upon hCG administration (luteinizing hormone activity) ovulation occurs. Methods Twelve prepubertal Sahiwal heifers were divided into experimental and control groups, with six animals in each group. The grouping of animals was made on the basis of plane ovary. The status of ovarian puberty was checked through rectal palpation and history of previous estrus signs. Experimental heifers were given clomiphene citrate at 300 mg/heifer for 9 d orally, while heifers in control group were given placebo tablets. The dose of clomiphene citrate was selected on the basis of previous literature. On day 10, hCG (IVF-C 5000 iu (hCG) LG Life Sciences, Korea) was given to all heifers of both groups at 2 500 IU/heifer through intravenous route. Two base line blood samples were collected before start of the treatment at each rectal palpation examination for selection of animals. Subsequently, during and after treatment three blood samples per week (on alternate days) for 4 weeks were collected for measuring the progesterone (P4) in serum. The concentration of progesterone was measured though enzyme linked immunosorbent assay. For the comparison of variables, student t -test was applied. Results The P4 concentrations in five heifers (83.3%) in the experimental group with (P4 > 1 ng/mL) and one heifer (16.7%) in the control group along with corpus luteum on ovary showed ovulation. One heifer in control group ovulated that might be due to hCG. Mean progesterone concentration was significantly higher ( P Conclusion Administration of clomiphene citrate and hCG can induce ovulation induction in prepubertal Sahiwal heifers.

Ovulation induction in pre-pubertal Sahiwal-Friesian cross bred heifers by the use of clomiphene citrate and hCG

In this study, an attempt was made to induce ovulation in crossbred pre-pubertal heifers by the use of clomiphene citrate (an anti-estrogen) and hCG. The hypothesis was that the clomiphene citrate, being anti-estrogen, would remove negative feedback effect of estrogen on the release of pituitary gonadotropins. Elevated gonadotropins would support ovarian follicular growth to a point where hCG would induce ovulation. Six pre-pubertal heifers were given clomiphene citrate orally at 300mg/heifer for 9 days. At the same time, six control heifers were given placebo tablets. On following day, hCG was given to all heifers at a dose of 2500IU. Blood was collected before, during and after treatment and plasma was assayed for progesterone (P4) and estradiol (E2) concentrations. Five heifers (83.3%) in the experimental group had E2 concentrations above 10pg/ml compared to one heifer (16.7%) in control group. Due to a wide variation, mean E2 concentration did not differ significantly between experimental and control heifers. The P4 assays showed that four heifers (66.7%) in the experimental group had ovulated (P4>1ng/ml plasma) compared to one heifer (16.7%) in control group, and mean P4 concentration differed (P<0.05) when concentrations were greatest in the clomiphene citrate-treated heifers. In conclusion, clomiphene citrate along with hCG can be used to induce ovulation in pre-pubertal heifers.

Carcinogenicity and hormone studies with the tissue‐selective estrogen receptor modulator bazadoxifene

Bazedoxifene Acetate (BZA) is a selective estrogen receptor modulator (SERM) that is approved for the prevention and/or treatment of osteoporosis in postmenopausal women. To assess for carcinogenic potential, BZA was administered ad libitum in the diet to rats for 2 years. BZA caused an increase in benign ovarian tumors in female rats and decreased incidences of mammary tumors (females) and pituitary tumors (males and females). In addition, BZA provided a significant survival benefit at all dosages tested, which correlated with a significant reduction in pituitary and mammary gland tumors and decreased body weight gain (both genders). Additional studies were subsequently conducted in rats and monkeys to further explore the mechanisms likely responsible for the observed effects. Results from studies in hypophysectomized and chemically castrated female rats indicated that BZA did not directly stimulate formation of ovarian cysts, but an intact pituitary was required for cyst formation. Further, BZA increased estradiol concentrations in rats and monkeys. In monkeys, BZA increased concentrations of luteinizing hormone (LH) after onset of treatment and prohibited the preovulatory surge of LH until after cessation of treatment. These hormonal changes suggest that BZA inhibited both the positive and negative feedback effects of estrogen on gonadotropins and the resulting increase in LH caused formation and persistence of ovarian cysts, which eventually transformed into benign ovarian granulosa cell tumors in the rat carcinogenicity study. These results also suggest that the reductions in pituitary and mammary gland tumors were attributed to BZA-related antagonism of endogenous estrogens at the estrogen receptors.

The neuroendocrine regulatory mechanisms of mammalian seasonal reproduction

The seasonal reproduction of mammal means the reproduction experiences an annual period from quiescence to renaissance. Studies have shown that kisspeptin and RFRP play an important role in the reproductive seasonality. The non-breeding season is characterized by an increase in the negative feedback effect of estrogen on GnRH, and this effect is transmitted by kisspeptin neurons, which may be an important factor affecting the reproduction activities. The expression of RFRP depends on melatonin secretion, and shows an apparent inhibition on reproduction in non-breeding season. In addition, thyroid hormones influence termination of the breeding season. Dopaminergic neuron A14/A15 also contributes to the seasonal changes in estrogen negative feedback. These neural systems may synergistically modulate the seasonal changes of reproductive function with the photoperiod. This review makes a systematic expatiation on the relationship between seasonal reproduction and these neural systems.

Co-localisation of neuropeptides with kisspeptin in preoptic/hypothalamic neurons of the mouse brain

Event Abstract Back to Event Co-localisation of neuropeptides with kisspeptin in preoptic/hypothalamic neurons of the mouse brain Barbara Vida1, Levente Deli1, Theodosis Kalamatianos2, Erik Hrabovszky1, Alain Caraty3, Coen W. Clive2, Zsolt Liposits1 and Imre Kalló1* 1 Hungarian Academy of Sciences, Hungary 2 King’s College London, United Kingdom 3 NRA/CNRS/University of Tours/Haras Nationaux, France Besides the classical neurotransmitters, neuropeptides also play significant roles in the regulation of gonadotropin releasing hormone (GnRH) secretion. Galanin and neurokinin B (NKB) have a modulatory function on GnRH secretion. In contrast, kisspeptin (KP) initiates a long-lasting activation of GnRH neurons. KP neurons in the rostral periventricular area of the third ventricle (RP3V) have been shown recently to convey estrogens’ positive feed-back action on GnRH secretion, whereas kisspeptin neurons located in the arcuate nucleus (Arc) have been implicated in the negative feed-back effects of estrogen. The aim of this study was to determine, whether galanin or NKB are co-expressed with kisspeptin in the mouse RP3V and/or Arc or alternatively, they are synthesized in different subsets of cells. Dual-label immunohistochemistry for KP and preproneurokinin B or galanin was carried out on sections from colchicine-treated female mice representing the full rostrocaudal extent of either the RP3V or the Arc. Maximum kisspeptin expression was detected in the RP3V of sections originating from ovariectomised (OVX) and estrogen-treated females, whereas the Arc showed the highest levels in sections derived from OVX females. Confocal microscopic analysis revealed that almost all KP neurons in the Arc, but none in the RP3V express NKB. In contrast, galanin immunoreactivity was observed in nearly every KP neurons in the RP3V, whereas in the Arc galanin immunoreactivity was detected only in 15% of KP neurons. These observations raise intriguing questions about the physiological roles of these neuropeptides potentially co-released in the vicinity of GnRH neurons. Conference: 12th Meeting of the Hungarian Neuroscience Society, Budapest, Hungary, 22 Jan - 24 Jan, 2009. Presentation Type: Poster Presentation Topic: Homeostatic regulatory mechanisms Citation: Vida B, Deli L, Kalamatianos T, Hrabovszky E, Caraty A, Clive CW, Liposits Z and Kalló I (2009). Co-localisation of neuropeptides with kisspeptin in preoptic/hypothalamic neurons of the mouse brain. Front. Syst. Neurosci. Conference Abstract: 12th Meeting of the Hungarian Neuroscience Society. doi: 10.3389/conf.neuro.01.2009.04.025 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 26 Feb 2009; Published Online: 26 Feb 2009. * Correspondence: Imre Kalló, Hungarian Academy of Sciences, Budapest, Hungary, kallo@koki.hu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Barbara Vida Levente Deli Theodosis Kalamatianos Erik Hrabovszky Alain Caraty Coen W Clive Zsolt Liposits Imre Kalló Google Barbara Vida Levente Deli Theodosis Kalamatianos Erik Hrabovszky Alain Caraty Coen W Clive Zsolt Liposits Imre Kalló Google Scholar Barbara Vida Levente Deli Theodosis Kalamatianos Erik Hrabovszky Alain Caraty Coen W Clive Zsolt Liposits Imre Kalló PubMed Barbara Vida Levente Deli Theodosis Kalamatianos Erik Hrabovszky Alain Caraty Coen W Clive Zsolt Liposits Imre Kalló Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Alterations in RFamide-Related Peptide Expression Are Coordinated with the Preovulatory Luteinizing Hormone Surge

The preovulatory LH surge is triggered when the circadian pacemaker, the bilateral suprachiasmatic nucleus (SCN), stimulates the GnRH system in the presence of high estrogen concentrations (positive feedback). Importantly, during the remainder of the estrous cycle, estradiol inhibits LH release via negative feedback. We have recently documented the presence of a novel mammalian RFamide-related peptide (RFRP), a putative gonadotropin-inhibitory hormone (GnIH), that presumably acts upstream of GnRH to modulate the negative feedback effects of estrogen. The present series of studies used female Syrian hamsters to examine the possibility that, in addition to driving the LH surge positively, the SCN concomitantly coordinates the removal of steroid-mediated RFRP inhibition of the gonadotropic axis to permit the surge. We found that the SCN forms close appositions with RFRP cells, suggesting the possibility for direct temporal control of RFRP activity. During the time of the LH surge, immediate-early gene expression is reduced in RFRP cells, and this temporal regulation is estrogen dependent. To determine whether projections from the SCN regulate the timed reduction in activation of the RFRP system, we exploited the phenomenon of splitting. In split animals in which the SCN are active in antiphase, activation of the RFRP system is asymmetrical. Importantly, this asymmetry is opposite to the state of the GnRH system. Together, these findings point to novel circadian control of the RFRP system and potential participation in the circuitry controlling ovulatory function.

Search for Neural Substrates Mediating Inhibitory Effects of Oestrogen on Pulsatile Luteinising Hormone‐Releasing Hormone Release In Vivo in Ovariectomized Female Rhesus Monkeys (Macaca mulatta)

Neural substrates mediating the negative feedback effects of oestrogen on luteinising hormone-releasing hormone (LHRH) release were studied using the in vivo push-pull perfusion method in female rhesus monkeys. Twelve long-term ovariectomized female monkeys were implanted with Silastic capsules containing 17beta-oestradiol 2 weeks before the experiments and, on the day of the experiment, oestradiol benzoate (EB, 50 microg/kg) or oil was subcutaneously injected. Push-pull perfusate samples from the stalk-median eminence were collected in 10-min fractions from 4 h before to 18-20 h after EB or oil injection. LHRH and neuropeptide Y (NPY) levels in the same perfusates were measured by radioimmunoassay, and glutamate and GABA in the same perfusates were assessed by high-performance liquid chromatography (HPLC). The results indicate that EB significantly suppressed LHRH release (P < 0.005) starting within 2 h after EB, and continued for 18 h or until the experiment was terminated. Pulse analysis suggested that oestrogen suppressed the pulse amplitude, but not pulse frequency, of LHRH release. By contrast, EB did not alter any parameters (mean release, pulse amplitude or frequency) of pulsatile NPY release throughout the experiment. HPLC analysis further suggested that neither glutamate nor GABA levels in the stalk-median eminence were changed with oestrogen-induced LHRH suppression. Oil treatment did not alter LHRH, NPY, GABA and glutamate levels. It is concluded that oestrogen induces suppression of pulsatile LHRH release within 2 h, but the inhibitory effect of oestrogen on LHRH release does not appear to be mediated by NPY, GABAergic, or glutamatergic neurones.

Influence of Hypothalamus and Ovary on Pituitary Function in Transgenic Mice Expressing the Bovine Growth Hormone Gene and in Growth Hormone-Deficient Ames Dwarf Mice1

Female transgenic mice expressing the bovine (b) growth hormone (GH) gene exhibit reduced fertility, and GH-deficient dwarf mice are sterile. However, in these animals, the influence of the hypothalamus and ovary on pituitary functions is poorly understood. To address this issue, female Ames dwarf mice received twice-daily injections of either vehicle or bGH (25 micrograms/injection per mouse) for 12 days. Vehicle-injected normal female siblings served as controls. On Day 7, these mice and adult female transgenic mice expressing the bGH gene with mouse metallothionein-I promoter, as well as their normal littermates, were ovariectomized. Two experiments were conducted with each group of mice: on Day 8 after ovariectomy of transgenic mice and their normal siblings, and on Day 4 after ovariectomy of dwarf mice and their normal siblings. In experiment I, mice were primed with 0.5 micrograms estradiol benzoate (EB) in oil and 24 h later received injections of 5 micrograms EB/100 g BW. The next day, these mice received injections of either saline or GnRH (1 ng/g BW), and 15 min later blood was obtained via heart puncture under anesthesia. In experiment II, ovariectomized mice either received injections of oil or were primed with 0.5 micrograms EB and 24 h later treated with 10 micrograms EB/100 g BW in oil. On the next day, blood was obtained as in experiment I. In transgenic mice and in their normal littermates, plasma bGH, FSH, LH, and prolactin levels were measured. In dwarf mice and in their normal siblings, plasma LH and insulin-like growth factor (IGF)-I levels were determined. Bovine GH was secreted in transgenic mice, but this hormone was not detectable in normal siblings. Expression of the GH gene resulted in attenuated (p < 0.001) FSH response to GnRH treatment, while LH response was similar to that in controls. The ovariectomy-induced increases in plasma FSH (p < 0.001) and LH (p < 0.05) levels were decreased in transgenic mice. The absolute circulating FSH levels were also decreased (p < 0.001) in estrogen-treated transgenic mice, while plasma LH level were similar in normal mice and in mice bearing the bGH gene. The ovariectomized transgenic mice were hyperprolactinemic. In ovariectomized dwarf mice, IGF-I was not detectable, and GH treatment induced its secretion. In these mice, plasma LH concentrations were reduced (p < 0.05), and GH treatment increased (p < 0.01) LH secretion. In oil-injected ovariectomized dwarf mice, the plasma LH response to GnRH treatment was reduced (p < 0.001), and GH administration normalized this response. The negative feedback effect of estrogen on LH secretion was decreased (p < 0.025) in dwarf mice. These indicate that the alterations in gonadotropin secretion are dependent on the duration of exposure of neuroendocrine system to bGH and that bGH alters the effects of GnRH and estrogen on gonadotropin secretion in these two widely divergent experimental mouse models.

The preovulatory LH surge: A case of a neuroendocrine switch

The preovulatory surge in LH is a unique endocrine event that involves a switch from the negative feedback effect of estrogen to a positive feedback effect. This occurs at both the level of the brain and that of the pituitary gland. Within the brain the mechanism appears to involve disinhibition of negative inputs to GnRH neurons, as well as stimulation of positive inputs. The positive feedback effects on the brain and the pituitary appear to be coordinated so that the effect of estrogen involves a number of time-delayed mechanisms.

Evidence that changes in the function of the subtypes of the receptors for gamma-amino butyric acid may be involved in the seasonal changes in the negative-feedback effects of estrogen on gonadotropin-releasing hormone secretion and plasma luteinizing hormone levels in the ewe

We have investigated the effects of gamma-amino butyric acid (GABA) agonists and antagonists to GABAA and GABAB receptors microinjected (1 microliter) into the medial preoptic area (MPOA) on LH secretion in ovariectomized (OVX) ewes with or without estrogen (E) treatment, during the nonbreeding season. Guide tubes (19 gauge) were placed into the MPOA of OVX ewes and injections of 1 microgram or 10 micrograms of the GABAA agonist muscimol or the GABAA antagonist bicuculline, and 10 micrograms of the GABAB agonist baclofen or the GABAB antagonist phaclofen were made into conscious animals. Jugular venous blood was collected at 10-min intervals for 3 h, the injection of GABA drug or vehicle was given and samples collected for a further 3 h. The plasma samples were assayed for LH. On completion of the experiments the brains were sectioned to locate the sites of injection. Thirty-four ewes were used, of which 30 had correct guide tube placement. In OVX sheep, both muscimol and bicuculline injection caused suppression of plasma LH concentrations, with a cessation of pulsatile release in many instances. Injection with baclofen or phaclofen into these sheep had no effect on LH secretion. When OVX sheep were treated with 0.5-cm Silastic implants (sc) of E for at least 7 days, there was a variable response to muscimol and bicuculline injection, depending on the degree of suppression of LH secretion by E. When E had little effect on plasma LH levels, muscimol and bicuculline both suppressed LH secretion in a similar manner to that observed in OVX ewes. When E treatment fully suppressed plasma LH levels, muscimol and bicuculline both stimulated LH secretion. This stimulation often took the form of a sustained, nonpulsatile secretion of LH. Baclofen injection into OVX ewes treated with E increased mean plasma LH levels through an increase in pulse amplitude, although there was only an effect on the amplitude of the first pulse after injection. Conversely, phaclofen injection in OVX ewes treated with E resulted in a reduction in LH pulse amplitude. These results provide evidence for a seasonal shift in the regulation of GnRH secretion by GABAergic neurons in the MPOA of the ewe. In contrast to the breeding season, where there was no effect of GABAB ligands, these appear to function in non-breeding season, this may be part of the mechanism for the seasonal shift in the negative-feedback effect of E on LH secretion.

Evidence that changes in the function of the subtypes of the receptors for gamma-amino butyric acid may be involved in the seasonal changes in the negative-feedback effects of estrogen on gonadotropin-releasing hormone secretion and plasma luteinizing hormone levels in the ewe.

We have investigated the effects of gamma-amino butyric acid (GABA) agonists and antagonists to GABAA and GABAB receptors microinjected (1 microliter) into the medial preoptic area (MPOA) on LH secretion in ovariectomized (OVX) ewes with or without estrogen (E) treatment, during the nonbreeding season. Guide tubes (19 gauge) were placed into the MPOA of OVX ewes and injections of 1 microgram or 10 micrograms of the GABAA agonist muscimol or the GABAA antagonist bicuculline, and 10 micrograms of the GABAB agonist baclofen or the GABAB antagonist phaclofen were made into conscious animals. Jugular venous blood was collected at 10-min intervals for 3 h, the injection of GABA drug or vehicle was given and samples collected for a further 3 h. The plasma samples were assayed for LH. On completion of the experiments the brains were sectioned to locate the sites of injection. Thirty-four ewes were used, of which 30 had correct guide tube placement. In OVX sheep, both muscimol and bicuculline injection caused suppression of plasma LH concentrations, with a cessation of pulsatile release in many instances. Injection with baclofen or phaclofen into these sheep had no effect on LH secretion. When OVX sheep were treated with 0.5-cm Silastic implants (sc) of E for at least 7 days, there was a variable response to muscimol and bicuculline injection, depending on the degree of suppression of LH secretion by E. When E had little effect on plasma LH levels, muscimol and bicuculline both suppressed LH secretion in a similar manner to that observed in OVX ewes. When E treatment fully suppressed plasma LH levels, muscimol and bicuculline both stimulated LH secretion. This stimulation often took the form of a sustained, nonpulsatile secretion of LH. Baclofen injection into OVX ewes treated with E increased mean plasma LH levels through an increase in pulse amplitude, although there was only an effect on the amplitude of the first pulse after injection. Conversely, phaclofen injection in OVX ewes treated with E resulted in a reduction in LH pulse amplitude. These results provide evidence for a seasonal shift in the regulation of GnRH secretion by GABAergic neurons in the MPOA of the ewe. In contrast to the breeding season, where there was no effect of GABAB ligands, these appear to function in non-breeding season, this may be part of the mechanism for the seasonal shift in the negative-feedback effect of E on LH secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies on the neuronal systems involved in the oestrogen-negative feedback effect on gonadotrophin releasing hormone neurons in the ewe.

Oestrogen can act on the brain to exert negative and positive feedback effects on the secretion of gonadotrophin releasing hormone (GnRH), but this cannot be effected through the GnRH cells themselves because they have no oestrogen receptors. We have used intraventricular injection of agonists and antagonists of various transmitters to determine which systems are involved in the control of GnRH by steroid hormones. We have also developed a model for the injection of drugs directly into the medial pre-optic area of the sheep brain to study the action on the GnRH cell bodies. Opioid systems seem to be involved in the mediation of the feedback effects of progesterone, but not oestrogen. Noradrenaline appears to be involved in the negative feedback regulation of GnRH secretion during the anoestrous period. Thus, under the strong negative influence of oestrogen at this time GnRH/luteinizing hormone (LH) secretion is reduced but can be restored by the preoptic micro-injection of noradrenaline. Gamma-amino-butyric acid (GABA) agonists and antagonists inhibit LH secretion when injected into the medial pre-optic region of the hypothalamus of ovariectomized ewes with or without oestrogen treatment. We have no evidence that GABA is involved in the negative feedback regulation of GnRH, since neither agonists nor antagonists will reverse the effects of steroid feedback. There appears to be a shift in the GABA receptor subtype function with season; in the breeding season only GABAA ligands are effective, whereas during the non-breeding season both GABAA and GABAB type ligands will affect LH secretion. This suggests that the seasonal shift in responsiveness to the negative feedback effects of oestrogen involves a shift in the function of GABA receptor subtypes.

Negative feedback effects of estrogen on luteinizing hormone-releasing hormone release occur in pubertal, but not prepubertal, ovariectomized female rhesus monkeys.

In a previous study we found that ovariectomy resulted in an increase in both LHRH release and LH release in pubertal monkeys but not in prepubertal monkeys. To determine whether this castration-induced LHRH increase is due to the removal of estrogen, in the present study, the effects of estradiol benzoate (EB, 30 micrograms/kg body wt) on in vivo LHRH release were examined using a push-pull perfusion method in prepubertal (age 15-19 months, n = 5), early pubertal (24-29 months, n = 5), and midpubertal (36-48 months, n = 5) female rhesus monkeys that were ovariectomized 3 to 5 months earlier. LHRH in 10-min perfusate fractions from the stalk-median eminence was measured from -6 to +24 h after EB injection. Circulating LH levels were also monitored over the same period at various intervals. EB decreased LH levels in early pubertal and midpubertal monkeys, whereas it did not cause any significant effects on LH release in the prepubertal monkey. EB also resulted in suppression of LHRH release in both early and midpubertal monkeys; mean LHRH release before EB in the early and midpubertal groups was 6.6 +/- 0.6 and 7.0 +/- 0.6 pg/ml.10 min, respectively. EB decreased mean LHRH release beginning 3 h after EB with the nadir occurring at 18-21 h after EB (1.0 +/- 0.2 pg/ml.10 min) in early pubertal monkeys and 21-24 h after EB (1.2 +/- 0.1 pg/ml.10 min) in midpubertal monkeys. Decrease of mean LHRH release was due to a decrease in LHRH pulse amplitude and basal release but not pulse frequency. Oil injection alone (control) failed to suppress LHRH and LH release. In contrast to the results in pubertal monkeys, mean LHRH release in prepubertal monkeys was not altered by EB (before EB, 1.1 +/- 0.2 pg/ml.10 min; 18-21 h after EB, 1.5 +/- 0.3 pg/ml.10 min). These results suggest that the LHRH neurosecretory system in pubertal monkeys is responsive to the negative feedback effects of estrogen. However, the fact that estradiol failed to suppress LHRH release in prepubertal monkeys indicates that the LHRH neurosecretory system and/or its regulatory systems are not sensitive to estradiol before the onset of puberty. These findings are consistent with the hypothesis that the increase in pulsatile LHRH release at the onset of puberty is not dependent on changes in ovarian steroid feedback mechanisms.

Negative feedback effects of estrogen on luteinizing hormone-releasing hormone release occur in pubertal, but not prepubertal, ovariectomized female rhesus monkeys

In a previous study we found that ovariectomy resulted in an increase in both LHRH release and LH release in pubertal monkeys but not in prepubertal monkeys. To determine whether this castration-induced LHRH increase is due to the removal of estrogen, in the present study, the effects of estradiol benzoate (EB, 30 micrograms/kg body wt) on in vivo LHRH release were examined using a push-pull perfusion method in prepubertal (age 15-19 months, n = 5), early pubertal (24-29 months, n = 5), and midpubertal (36-48 months, n = 5) female rhesus monkeys that were ovariectomized 3 to 5 months earlier. LHRH in 10-min perfusate fractions from the stalk-median eminence was measured from -6 to +24 h after EB injection. Circulating LH levels were also monitored over the same period at various intervals. EB decreased LH levels in early pubertal and midpubertal monkeys, whereas it did not cause any significant effects on LH release in the prepubertal monkey. EB also resulted in suppression of LHRH release in both early and midpubertal monkeys; mean LHRH release before EB in the early and midpubertal groups was 6.6 +/- 0.6 and 7.0 +/- 0.6 pg/ml.10 min, respectively. EB decreased mean LHRH release beginning 3 h after EB with the nadir occurring at 18-21 h after EB (1.0 +/- 0.2 pg/ml.10 min) in early pubertal monkeys and 21-24 h after EB (1.2 +/- 0.1 pg/ml.10 min) in midpubertal monkeys. Decrease of mean LHRH release was due to a decrease in LHRH pulse amplitude and basal release but not pulse frequency. Oil injection alone (control) failed to suppress LHRH and LH release. In contrast to the results in pubertal monkeys, mean LHRH release in prepubertal monkeys was not altered by EB (before EB, 1.1 +/- 0.2 pg/ml.10 min; 18-21 h after EB, 1.5 +/- 0.3 pg/ml.10 min). These results suggest that the LHRH neurosecretory system in pubertal monkeys is responsive to the negative feedback effects of estrogen. However, the fact that estradiol failed to suppress LHRH release in prepubertal monkeys indicates that the LHRH neurosecretory system and/or its regulatory systems are not sensitive to estradiol before the onset of puberty. These findings are consistent with the hypothesis that the increase in pulsatile LHRH release at the onset of puberty is not dependent on changes in ovarian steroid feedback mechanisms.

Stimulation of ovulation in ayu, Plecoglossus altivelis, by treatment with antiestrogens and luteinizing hormone-releasing hormone analog

The objectives of this study were to investigate the negative feedback effects of estrogen in ayu, Plecoglossus altivelis, by treatment with enclomiphene (cis-clomiphene), zuclomiphene (trans-clomiphene), clomiphene and LHRH analog (D-Ala 6, des-Gly 10 LHRH ethylamide) based on ovulation and plasma steroid levels. Ovulation was stimulated in some fish by enclomiphene and clomiphene. The majority of ayu injected with LHRH analog ovulated. Injection of enclomiphene or clomiphene al 20 mg/kg wt induced a better ovulation response than 2 mg/kg wt. Enclomiphene increased plasma levels of estradiol-17β (E 2) and testosterone (T) at both dosages. In contrast, plasma T and E 2 levels increased significantly at the high dose of clomiphene. Only one fish (3%) ovulated and no increase of plasma steroids was observed in the zuclomiphene group. Enclomiphene seemed to have antiestrogenic potency. Negative feedback inhibition of estrogen in mature female ayu is therefore suggested.

Acute Action of Oestrogen on Medial Preoptic Gamma‐Aminobutyric Acid Neurons: Correlation with Oestrogen Negative Feedback on Luteinizing Hormone Secretion

Abstract The acute effects of oestrogen on the medial preoptic area (MPOA) gamma-aminobutyric acid (GABA) system were examined by delivering an intravenous bolus of 17beta-oestradiol (5 mug/100 g body wt) to conscious ovariectomized rats implanted with microdialysis probes. Fifteen-min blood samples were taken to determine the time-course of negative feedback effects of oestrogen on luteinizing hormone (LH) secretion. Two h after administration of 17beta-oestradiol, GABA release from the MPOA was significantly elevated compared with vehicle-treated controls (P<0.05). The rise in GABA levels continued until the end of the experiment, 4 h after 17beta- oestradiol, at which time it was over 50% higher than controls (P<0.01). The pulsatile pattern of LH secretion was significantly depressed 2 and 3 h after administration of 17beta-oestradiol compared with controls (P<0.05). To determine the effects of the 17beta-oestradiol treatment on pituitary responsiveness to LH-releasing hormone (LHRH), a further group of rats were given exogenous LHRH (50ng/100g body wt, intravenously) before and 3 h after vehicle or 17beta-oestradiol treatment and blood samples taken to determine the effect on LH secretion. The maximal LH response to LHRH in 17beta-oestradiol-treated rats was approximately 50% that of control-treated values. This study demonstrates the acute and potent action of 17beta-oestradiol on GABA release in the MPOA and lends support to a genomic site of action for oestrogen in modulating neural elements regulating GABA release from the MPOA. These results, showing a parallel decrease in LH secretion with increased GABA levels in the MPOA, suggest a role for GABA elements within the MPOA as a site of oestrogen negative feedback on LH secretion.

Medial Preoptic Microimplants of the Antiestrogen, Keoxifene, Affect Luteinizing Hormone‐Releasing Hormone mRNA Levels, Median Eminence Luteinizing Hormone‐Releasing Hormone Concentrations and Luteinizing Hormone Release in Ovariectomized, Estrogen‐Treated Rats

Abstract We examined the temporal changes in plasma luteinizing hormone (LH) levels, median eminence luteinizing hormone-releasing hormone (LHRH) concentrations and LHRH mRNA levels in estrogen-treated, ovariectomized rats with empty or antiestrogen- containing microcannulae stereotaxically implanted into the medial preoptic area. Neither treatment disrupted the negative feedback effects of estrogen on LH secretion, but antiestrogen (Keoxifene) blocked the afternoon LH surges. In rats exhibiting LH surges, median eminence LHRH concentrations were similar at 0800, 1200 and 1600 h, but they were significantly elevated by 2000 h. In contrast, no alterations in LHRH concentrations occurred in the Keoxifene-treated group. LHRH mRNA levels in control rats were significantly elevated at 1200, 1600 and 2000 h compared with 0800 h, but LHRH mRNA levels in Keoxifene-treated rats did not change significantly over the time period examined. When we compared treatment effects over time we saw that serum LH levels were significantly higher in control than Keoxifene-treated rats only at 1600 and 2000 h. Median eminence LHRH concentrations did not differ between treatment groups until 2000 h when control animals had significantly higher levels than those of Keoxifene-treated animals. LHRH mRNA levels in Keoxifene-treated rats were significantly higher than those of controls at 0800 hand significantly lower at 1600 h. No differences in LHRH mRNA levels were detected between groups at either 1200 h or 2000 h. In summary, although it was not clear on which neuronal system estrogen acted, depriving medial preoptic neurons of this steroid in systemically estrogenized rats certainly disrupted the neural mechanisms involved in surge, but not basal LH release. In addition, neither LHRH mRNA levels nor median eminence LHRH concentrations showed variations within the period studied when the estrogen-sensitive mechanisms involved in LH release were disrupted. Therefore, the changes in LHRH mRNA levels and LHRH concentrations in the median eminence seen in surging animals probably resulted from the same neural events which triggered LH release.

Escape from Chronic Estrogenic Suppression of Ovarian Function in the Adult Rhesus Monkey: Evidence for Changing Sensitivity of Gonadotropin Secretion to Estrogen Inhibition*

Regularly menstruating female rhesus monkeys were implanted sc with Silastic tubing packed with estrone. The implants were replaced every 6-8 months to maintain the serum estrone and estradiol concentrations constantly elevated 1.5-2.5-fold, i.e. in the midfollicular phase range, for 4 yr. Increasing the estrone and estradiol concentrations initially led to anovulation which persisted for 6 to 15 months after which three of four of the estrogen-treated animals resumed ovulatory cycles. These animals then waxed and waned between anovulation and ovulatory cycles for the remainder of the study period. The resumption of ovulatory cycles were attributed to an escape of the central nervous system-pituitary axis from the suppressive effect of the elevated estrogen concentrations, since serum estradiol concentrations did not change. During anovulatory and ovulatory months the basal LH concentration was not significantly increased in the estrogen-treated monkeys compared to the control animals, but it was significantly reduced during anovulatory months compared to ovulatory months. Furthermore, LH secretion in response to a bolus of GnRH was attenuated in the monkeys chronically exposed to the acyclic elevation of blood estrogen levels. As the sensitivity to estrogen decreased, sufficient basal concentrations of FSH and LH were achieved to support ovulatory cycles with associated midcycle surges of LH and FSH secretion and apparently normal luteal patterns of progesterone secretion. Since the daily FSH concentrations of these ovulatory cycles of the estrone-treated animals were significantly lower than that of ovulatory cycles of the control monkeys, the ovulatory cycles of the estrogen animals may not have been normal, but this was not directly documented. These studies suggest that elevated basal estrogen levels do not lead to elevated basal LH secretion in the female primate; and also in the intact adult primate the central nervous system-pituitary axis has the potential to change its sensitivity to the negative feedback effects of estrogen on gonadotropin secretion.