The GnRH pulse generator

Control of the rhesus monkey menstrual cycle: permissive role of hypothalamic gonadotropin-releasing hormone

Knobil E et al., 1980 · Science

In rhesus monkeys with hypothalamic lesions (which appear to abolish the endogenous production of gonadotropin-releasing hormone), normal ovulatory mestrual cycles were reestablished by an unvarying, long-term replacement regimen consisting of one intravenous pulse of synthetic gonadotropic-releasing hormone per hour. This finding is in accord with the hypothesis that the pattern of pituitary gonadotropin secretion throughout the menstrual cycle (basal secretion interrupted, once every 28 days on the average, by a preovulatory surge) is not directed by alterations in hypothalamic gonadotropin-releasing hormone secretion but by the ebb and flow of ovarian estrogens acting directly on the pituitary gland.

Hypophysial responses to continuous and intermittent delivery of hypopthalamic gonadotropin-releasing hormone

Belchetz PE et al., 1978 · Science

In rhesus monkeys with hypothalamic lesions that abolish gonadotropic hormone release by the pituitary gland, the constant infusion of exogenous gonadotropin-releasing hormone (GnRH) fails to restore sustained gonadotropin secretion. In marked contrast, intermittent administration of the synthetic decapeptide once per hour, the physiological frequency of gonadotropin release in the monkeys, reestablishes pituitary gonadotropin secretion. This phenomenon is attributable to the pattern of GnRH delivery rather than to the amounts of this hormone to which the cells of the pituitary are exposed. Moreover, the initiation of continuous GnRH administration in animals with lesions and in which gonadotropin secretion is reestablished by intermittent GnRH replacement can result in a "desensitization" or "down regulation" of the processes responsible for gonadotropin release.

On the sites of the negative and positive feedback actions of estradiol in the control of gonadotropin secretion in the rhesus monkey

Nakai Y et al., 1978 · Endocrinology

Ovariectomized rhesus monkeys bearing hypothalamic lesions which had abolished endogenous LHRH production, as evidenced by a profound reduction in gonadotropin secretion, but in which LH and FSH secretion was reestablished by a chronic intermittent iv infusion of synthetic LHRH (1 microgram/min for 6 min every hour) were used to investigate the sites of the negative and positive feedback actions of estradiol in the control of gonadotropin secretion. The administration of estradiol to such animals, while continuing the LHRH replacement regimen, resulted in a decline in circulating LH and FSH levels, followed by an unambiguous discharge of these hormones. The time course of this biphasic pattern of gonadotropin secretion was remarkably similar to that observed in response to estradiol administration in otherwise intact ovariectomized animals. These results suggest that, in the rhesus monkey, estradiol can exert both its negative and positive feedback actions on gonadotropin secretion at the level of the pituitary gland.

Peripheral plasma progesterone levels during human pregnancy and labor

Csapo AI et al., 1971 · Am J Obstet Gynecol

Reduced frequency of pulsatile luteinizing hormone secretion in the luteal phase of the rhesus monkey. Involvement of endogenous opiates

Van Vugt DA et al., 1984 · Endocrinology

Pulsatile secretion of LH in women has been shown to vary during the menstrual cycle. LH pulse frequency during the luteal phase is markedly reduced compared to that in the follicular phase. The objectives of the present study were to determine if similar changes in pulsatile LH secretion occur in the monkey, and whether endogenous opiates are involved in producing these changes. In order to document if LH pulse frequency is reduced in the nonhuman primate luteal phase, serial blood samples were collected from 10 rhesus monkeys at 15-min intervals for 6 h at 3 different times of the luteal phase (early, mid-, and late). This pattern of secretion was contrasted to that observed during the ensuing early follicular phase. LH pulse frequency during the luteal phase was significantly reduced compared to the early follicular phase. Mean pulse frequency (+/- SE) was 0.84 +/- 0.16 pulses/6 h in the luteal phase vs. 2.99 +/- 0.58 pulses/6 h in the early follicular phase. When endogenous opioid activity was blocked during the luteal phase by a 5-h continuous infusion of naloxone (2 mg/h), an opiate antagonist, LH pulse frequency was increased to 2.48 +/- 0.25 pulses/5 h. This frequency was markedly different from the frequency of 0.85 +/- 0.17 pulses/5 h observed in the control period which immediately preceeded the naloxone infusion. The mean amplitude of the LH pulses in the luteal phase, which was significantly greater than that observed in the early follicular phase (20.9 +/- 1.9 ng/ml and 11.7 +/- 0.3 ng/ml) was not affected by naloxone (23.5 +/- 2.4 ng/ml vs. 25.3 +/- 1.9 ng/ml). Infusion of naloxone for longer periods (9 h) in 3 additional monkeys caused an increase in LH pulse frequency which was maintained in 2 of the monkeys, whereas the third animal exhibited only an acute response (a single pulse). These results indicate that the reduction in LH pulse frequency that occurs in the luteal phase of the rhesus menstrual cycle is an event in which endogenous opiates participate. Our previous finding that beta-endorphin release from neurons in the median eminence is stimulated during the luteal phase of the monkey, together with the present results, suggest that beta-endorphin functions as a modulator of pulsatile LH secretion in the primate menstrual cycle.

Regulation of gonadotropin secretion: implications for polycystic ovary syndrome

McCartney CR et al., 2003 · Semin Reprod Med

Polycystic ovary syndrome (PCOS) is a disorder characterized by hyperandrogenism and chronic anovulation. Although the etiology of PCOS is unknown, perturbations of gonadotropin secretion are one of the hallmarks of this disorder. In normal menstrual physiology, the monotropic rise of plasma follicle-stimulating hormone (FSH) during the luteal-follicular transition is critical for follicular development and subsequent ovulation. One of the mechanisms by which FSH is differentially synthesized involves the luteal slowing of gonadotropin-releasing hormone (GnRH) pulse frequency by ovarian steroids. In PCOS, plasma leutinizing hormone (LH) is commonly increased, FSH is typically in the lower follicular range, and LH (and by inference GnRH) pulse frequency is persistently rapid at approximately one LH pulse per hour. The etiology of the neuroendocrine abnormalities in PCOS remain unclear; however, recent studies have revealed decreased sensitivity of the GnRH pulse generator to inhibition by ovarian steroids, particularly progesterone. This abnormality is reversed by the androgen receptor antagonist flutamide, suggesting that elevated androgen levels may alter the sensitivity of the hypothalamic GnRH pulse generator to steroid inhibition and lead to enhanced LH secretion. As such, women with PCOS require higher levels of progesterone to slow the frequency of GnRH pulse secretion, resulting in inadequate FSH synthesis and persistent LH stimulation of ovarian androgens. The decreased sensitivity of the GnRH pulse generator may help to explain the genesis of PCOS during puberty. In normal early puberty, sleep-entrained increases in LH stimulate ovarian steroids, which subsequently suppress LH frequency and amplitude during the subsequent day. In hyperandrogenemic girls destined to develop PCOS, this nocturnal increase in ovarian steroids may not be adequate to suppress the GnRH pulse generator, leading to a persistently rapid LH pulse frequency, impaired FSH production, and inadequate follicular development.

Neuroendocrine regulation of the corpus luteum in the human. Evidence for pulsatile progesterone secretion

Filicori M et al., 1984 · J Clin Invest · Free full text on PubMed Central

The pattern of episodic gonadotropin release was studied in 15 normal female volunteers during the luteal phase of the menstrual cycle with 24 h of blood sampling for follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels at 10-min intervals. Six subjects (two in the early, two in the mid-, and two in the late luteal phase) also had each of these specimens processed for progesterone levels. A progressive slowing of LH pulsations was present across the luteal phase with the mean LH pulse frequency declining from 15.2 pulses/24 h in the early to 8.4/24 h in the late luteal phase. A trend towards reduction in the amplitude of LH pulses was also observed (12.3 +/- 2.2 SD mIU/ml in the early vs. 8.6 +/- 3.4 mIU/ml in the late luteal phase; NS). In addition, LH pulses of heterogeneous amplitude were identified during the same 24-h study. The mean +/- SD of the larger and of the smaller LH pulses was 16.9 +/- 4.7 and 2.3 +/- 1.0 mIU/ml, respectively (P less than 0.001). While the slowing of the frequency of all LH pulses correlated well (r = 0.80, P less than 0.001) with the day of the luteal phase and poorly with the actual plasma progesterone levels, the incidence of the small LH pulses was highest in the mid-luteal phase and correlated well with the mean progesterone plasma levels (r = 0.63, P less than 0.01). In the early luteal phase, the pattern of progesterone secretion was stable over the 24-h studies and showed no relationship to episodic LH release. In contrast, in the midand late luteal phase, plasma progesterone concentrations rapidly fluctuated during the 24-h studies from levels as low as 2.3 to peaks of 40.1 ng/ml, often within the course of minutes. Progesterone increments closely attended episodes of LH release, as documented by the significant (P less than 0.05) cross-correlation between LH and progesterone levels, at time lags of 25-55 min. The results of this study indicate that in the human luteal phase: (a) the frequency of pulsatile release of LH declines progressively and correlates well with the duration of exposure to progressively and correlates well with the duration of exposure to progesterone; (b) the amplitude of LH pulses varies with the appearance of an increased percentage of smaller pulses correlating well with the acute level of progesterone; (c) in the early luteal phase, the pattern of progesterone secretion is stable; (d) in the midand late luteal phase, progesterone secretion is episodic, and correlates with LH pulsatile release; and (e) single progesterone estimations in the midand late luteal phase do not accurately reflect corpus luteum adequacy.

Gonadotropin-releasing hormone: role of pulsatile secretion in the regulation of reproduction

Marshall JC et al., 1986 · N Engl J Med

Reproductive function in humans changes markedly during life and is usually divided into four stages. During the initial stage, which begins early in fetal life and ends in infancy, gonadotropins and gonadal steroids are secreted at levels similar to those seen in early and mid-adolescence. In the second stage, which lasts from infancy through the first decade of life, reproductive function first regresses and then becomes quiescent. Puberty, the third stage, is heralded by a nocturnal increase in the secretion of gonadotropins and sex steroids. This nocturnal predominance gradually diminishes and disappears during the fourth stage — adulthood. In men, . . .