A surge of gonadotropin-releasing hormone accompanies the estradiol-induced gonadotropin surge in the rhesus monkey.

In several species, the ovulatory LH surge is preceded by a surge of GnRH. Although a role for estradiol in the initiation of the LH surge is well established in the primate, several observations in the rhesus monkey have questioned whether such an estradiol-induced neurosecretory event takes place. We report on GnRH measurements in cerebrospinal fluid (CSF) samples obtained from the third ventricle of intact and ovariectomized (OVX) conscious rhesus monkeys during control periods and throughout the estradiol-induced positive feedback phase. In the first experiment, we measured control GnRH concentrations in CSF collected at 15-min intervals uninterruptedly for a period of 1-5 days in tethered OVX monkeys (n = 4) in their cages without steroid priming. As had been demonstrated previously with the same method in restrained animals, CSF from the third ventricle contained detectable amounts of GnRH. Spontaneous GnRH secretion was pulsatile; overall mean pulse interval was 67.4 (+/- 2.2 SE) min for a total of 177 GnRH pulses. During 2 periods (8 and 6 h) when simultaneous blood and CSF samples were obtained, 14 out of 15 GnRH pulses were accompanied by an LH pulse. To evaluate the effects of an estrogen challenge on GnRH secretion, estradiol benzoate (E2B; 330 micrograms) was given to 4 intact (5 experiments) and to 2 OVX monkeys. CSF collection was initiated 8-24 h before E2B injection and continued for 72-84 h thereafter. E2B administration resulted in a surge of LH and of GnRH in all but one experiment. The mean time of onset of the GnRH surge was 22.0 (+/- 4.0) h after E2B, whereas that of the LH surge was 24.7 (+/- 3.4) h. In contrast to LH, which declined after a peak at 35.2 +/- 3.9 h, the increase in GnRH secretion persisted throughout most of the observation period. The magnitude of the GnRH response differed in the 2 groups; in the intact animals, mean peak GnRH concentration increased 8.9-fold but only 3.8-fold in the OVX monkeys. A similar GnRH surge was observed in 1 OVX monkey, receiving an iv infusion of E2, which produced more physiological concentrations of E2. In this animal, an initial suppression of GnRH concentration in the 24-48 h period after E2 (GnRH control, 14.6 +/- 1.9; post-E2, 4.0 +/- 0.5 pg/ml) preceded the initiation of the GnRH surge which occurred at 54 h after E2.(ABSTRACT TRUNCATED AT 400 WORDS)     

Progesterone treatment that either blocks or augments the estradiol-induced gonadotropin-releasing hormone surge is associated with different patterns of hypothalamic neural activation.

Progesterone can either augment or inhibit the surge of gonadotropin-releasing hormone (GnRH) that drives the preovulatory luteinizing hormone (LH) surge. This study investigated the central mechanisms through which progesterone might achieve these divergent effects by examining the effects of exogenous steroids on the activation of GnRH neurons and non-GnRH-immunopositive cells in the preoptic area/anterior hypothalamus of steroid-treated ovariectomized ewes. Fos expression (an index of cellular activation) was examined during the estradiol-induced GnRH surge in ewes treated with progesterone using regimes that have been reported to either augment (progesterone pretreatment) or inhibit (progesterone treatment at the time of the surge-inducing estradiol increment) the GnRH surge. Control groups received either no progesterone pretreatment or no surge-inducing estradiol increment. Induction of an LH surge was associated with a significant (p < 0.0001) increase in the proportion of activated GnRH neurons, irrespective of whether ewes received progesterone pretreatment. However, the number of non-GnRH-immunopositive cells activated during the surge was significantly (p < 0.0001) increased in ewes that received the progesterone pretreatment. By contrast, the proportion of GnRH neurons and non-GnRH-immunopositive cells that expressed Fos was significantly (p < 0.0001) reduced in ewes in which the surge was inhibited by progesterone compared to ewes in which a surge was stimulated. These data indicate that (1) progesterone pretreatment increases the activation of non-GnRH cells during the estradiol-induced surge, but does not affect the proportion of GnRH neurons activated and (2) when administered concurrently with a surge-inducing estradiol increment, progesterone prevents the activation of GnRH neurons and non-GnRH cells that is normally associated with the estradiol-induced surge. Therefore, progesterone does not appear to augment the GnRH surge by increasing the proportion of GnRH neurons that are activated by estradiol, whereas inhibition of the GnRH surge involves prevention of the activation of GnRH neurons. Thus, the augmentation and inhibition of the GnRH surge by progesterone appear to be regulated via different effects on the GnRH neurosecretory system.     

Modulation of the estradiol-induced luteinizing hormone surge by progesterone or antiestrogens: effects on pituitary gonadotropin-releasing hormone receptors

The present study examined the question of whether modulation of estradiol-induced LH surges by progesterone or antiestrogens in the immature rat might be related to changes in the concentration of pituitary GnRH receptors (GnRH-R). Rats (28 days old) that received estradiol implants at 0900 h had LH surges approximately 32 h later. Administration of progesterone or nafoxidine (U-11,100 A; 1-(2-[P-(3,4-dihydro-6-methoxy-2-phenyl-1-naphthyl)phenoxy]pyrrolidine hydrochloride) concomitantly with estradiol led to blockade of these LH surges (progesterone or nafoxidine inhibition), while progesterone treatment 24 h after estradiol brought about premature and enhanced LH release (progesterone facilitation). GnRH-R-binding capacity was determined by saturation analysis in homogenates of single pituitaries from immature rats treated with estradiol and progesterone or nafoxidine and controls treated only with estradiol using [125I]iodo-(D-Ala6,Des-Gly10)GnRh ethylamide. The affinity of GnRH-R for this analog ranged from 8.2-15.1 X 10(9) M-1 and was not affected by in vivo steroid or antiestrogen treatment. The number of GnRH-R in gonadotrophs from untreated 28-day-old rats (57.2 +/- 2.6 fmol/pituitary or 177 +/- 11 fmol/mg protein) was comparable to values previously reported for 30 day-old females. GnRH-R levels were first measured 1, 8, 24, 32, and 48 h after estradiol treatment. The pituitary content of GnRH-R paralleled changes in total pituitary protein (nadir at 24 h, rebound at 32 h, continued increase at 48 h), while their concentration (femtomoles per mg protein) was highest at 8 h. Next, GnRH-R levels were examined at 1200 h and at hourly intervals (1400-1800 h) on the afternoon of the LH surge. While GnRH-R concentrations were significantly lower at 1400 and 1700 h than at 1200 or 1800 h in animals treated with estradiol in the progesterone facilitation model, they did not change over time in the other two paradigms. There was no significant difference in pituitary content or concentration of GnRH-R at any time between immature rats treated with estradiol and progesterone or nafoxidine and their respective estradiol-treated controls. These results suggest that changes in GnRH-R levels in pituitary gonadotrophs do not play a major role in enhancement of LH surges by progesterone or in their suppression by progesterone or nafoxidine in the immature rat; therefore, these compounds may affect the pituitary at a site distal to the GnRH receptor.     

Absence of an estradiol-induced surge of luteinizing hormone in pigs receiving unvarying pulsatile gonadotropin-releasing hormone stimulation

The goal of this study was to pharmacologically block central nervous system (CNS) input to gonadotropes in mature ovariectomized gilts to determine the direct actions of estradiol (E2) on pituitary LH release when given at a dose sufficient to elicit a gonadotropin surge. Feeding AIMAX [N-methyl-N'-(1-methyl-2-propenyl)1,2-hydrazinedicarbothioamide; 125 mg/day] for 7 days reduced serum LH concentrations from 1.25 +/- 0.13 (mean +/- SE) to less than 0.18 ng/ml, abolished LH pulses, but did not compromise LH release in response to exogenous GnRH. Serum FSH concentrations were reduced by 27%, whereas serum concentrations of PRL, GH, thyroid hormones and cortisol were not affected after 7 days of AIMAX treatment. Behavior was not altered, aside from a slightly reduced appetite. The LH surge that peaked 48-80 h after injecting E2 benzoate (E2B) into control gilts was blocked in five of eight gilts given AIMAX. Giving GnRH pulses (1 microgram every 45 min) to AIMAX-treated gilts restored mean serum LH concentrations as well as the frequency and amplitude of LH pulses to those of untreated ovariectomized gilts. E2B suppressed the LH response to these GnRH pulses by 88% at 12 h, whereas from 24-96 h after E2B treatment, the LH response to GnRH and mean serum concentrations of LH were again similar to those of controls not given estradiol. These data indicate that induction of the gonadotropin surge by E2 in the gilt requires CNS input. The action of E2 on the pituitary in the presence of unvarying GnRH pulsation may, however, be limited to an early transient inhibition of responsiveness to GnRH, with no subsequent direct stimulation during the period of the surge.     

Control of the preovulatory gonadotropin surge in the ewe

The aims of this study were to determine the effect of ovariectomy on the release of LH and FSH during the preovulatory gonadotropin surge and to ascertain, by the use of sodium pentobarbitone (NaPb), if the secretion of these pituitary hormones requires continuous stimulation from the hypothalamus. Sheep were treated with NaPb for 2 h beginning 1) immediately before the gonadotropin surge, 2) during the ascending limb of the gonadotropin surge, and 3) during the descending limb of the gonadotropin surge. Ewes were ovariectomized (ovx) at each of the time periods listed above, and intact ewes included were at times 2 and 3. A group of intact ewes was given 100 microgram gonadotropin-releasing hormone (GnRH) in addition to NaPb at time 2, NaPb given during the ascending limb of the gonadotropin surge caused a transient fall in peripheral LH and FSH; however, the release of gonadotropins was reinitiated and the surge continued when the ewes recovered from anesthesia. Treatment with NaPb after the apex of the gonadotropin surge did not affect circulating levels of LH and FSH. Ewes given NaPb and ovx before the initiation of the gonadotropin surge released significantly less LH and FSH during the surge than the other treatment groups. The total amounts of LH and FSH released in intact and ovx ewes treated with NaPb after the surge was initiated were not different than those levels in the saline-treated controls. Intact ewes treated with 100 micrograms GnRH also released an amount of LH similar to that in the control group. We conclude that gonadotropin release from the pituitary gland requires the continual presence of GnRH during the ascending limb of the preovulatory gonadotropin surge, and that once the surge has been triggered, the ovaries do not appear to be required for further hypothalamic stimulation.     

Progesterone modulation of gonadotropin secretion by dispersed rat pituitary cells in culture. I. Basal and gonadotropin-releasing hormone-stimulated luteinizing hormone release

Dispersed, estradiol-treated, rat pituitary cells were cultured to characterize the influences of a physiologic concentration of progesterone (P, 10(-7) M) on gonadotroph responsiveness to gonadotropin-releasing hormone (GnRH). Acute (less than 6 h) P treatment enhanced and chronic (greater than 12 h) treatment suppressed both basal and GnRH-stimulated luteinizing hormone (LH) release. This modulation took place without any change in intracellular LH stores, indicating that the secretory changes are not attributable to changes in LH synthesis, and were not accompanied by similar alterations in basal or thyrotropin-releasing hormone-stimulated prolactin secretion. Moreover, the timing of these responses was fixed since a 10-fold lower P concentration produced only smaller and briefer alterations in LH release. Analyses of the temporal characteristics of effective P stimuli indicated that a brief 6 h exposure to P inhibited GnRH-stimulated LH secretion 18 h later. In contrast, P's acute actions rapidly dissipated following removal of the steroid from the culture medium. Finally, P-induced enhancement and suppression of GnRH-stimulated LH release could be blocked by appropriately timed treatments with protein synthesis inhibitors. Our findings are consistent with the hypothesis that P influences gonadotroph secretory function via the production of specific proteins.     

Control of the preovulatory luteinizing hormone surge by gonadotropin-releasing hormone antagonists Prospects for clinical application.

The preovulatory LH surge of the primate menstrual cycle represents a number of positive influences, a major component of which is a direct action of estradiol on the anterior pituitary lobe. Whether the LH surge also requires a corresponding burst of GnRH release from the hypothalamus has been debated. After many years of investigation, there is now conclusive evidence that a midcycle GnRH surge does occur in the primate. This is supported by studies in women with normal ovulatory cycles that demonstrate that blockade of the GnRH receptor by potent GnRH antagonists administered within 1-2 days of the expected midcycle can delay the LH surge. The ability to prevent the positive feedback effects of estradiol by GnRH antagonists is being employed for the controlled induction of follicular development and ovulation in the treatment of infertility and in in vitro fertilization programs.     

Progesterone inhibits the estrogen-induced gonadotropin surge in the rhesus monkey independent of endogenous opiates.

Administration of an estrogen challenge during the luteal phase, a time when progesterone concentrations are elevated, fails to elicit a gonadotropin-positive feedback response. The purpose of the present study was to determine if endogenous opiates are involved in the mechanism by which progesterone blocks the estrogen-induced gonadotropin surge in monkeys. To this end, rhesus monkeys in the luteal phase were pretreated with either saline or various regimens of nalmefene, a long-acting opiate antagonist, before being given an estrogen challenge. Three groups of animals were given nalmefene (10 mg, iv) every 12 h beginning 24, 48, or 96 h before an estrogen challenge and continued until 48 h after the start of the estrogen challenge. A fourth group received a continuous sc infusion of nalmefene (20 mg/day) via osmotic minipumps beginning 48 h in advance of the estrogen challenge. In a second experiment, monkeys in the follicular phase received progesterone implants at the time of an estrogen challenge and iv injections of nalmefene every 12 h for 48 h. Gonadotropin and steroid levels were monitored in both experiments by collecting blood samples by saphenous venipuncture at intervals of 6-12 h. The majority of luteal phase animals that were pretreated with saline were unresponsive to the estrogen challenge. Only 2 of 16 (12.5%) had an increase in LH concentrations that could be classified as a surge. Animals pretreated with iv nalmefene every 12 h beginning 48 h before the estrogen challenge exhibited a higher incidence of positive feedback responses (8 of 12 or 66.7%). A concomitant FSH surge was observed in 3 of these instances. However, when progesterone concentrations, which declined before the estrogen challenge in the nalmefene-treated group, were supplemented with exogenous progesterone, nalmefene failed to evoke any LH surges. Six of 8 animals that received nalmefene by sc infusion exhibited LH responses. However, the amplitude and duration of these LH responses were diminished, and no FSH responses were observed. Monkeys pretreated with nalmefene for either shorter (24 h) or longer (96 h) periods before the challenge were less responsive (0 responses out of 6 trials and 1 response out of 4 trials, respectively). Nalmefene was equally ineffective in preventing progesterone inhibition of the estradiol-induced LH surge in follicular phase animals (0 of 15 animals had LH surge). These results indicate that nalmefene antagonism of endogenous opiates does not enable estrogen to exert positive feedback effects on LH release when progesterone levels are high, such as during the luteal phase or after progesterone administration.(ABSTRACT TRUNCATED AT 400 WORDS)     

Endotoxin disrupts the estradiol-induced luteinizing hormone surge: interference with estradiol signal reading, not surge release.

Three experiments were conducted to investigate whether the immune/inflammatory stimulus endotoxin disrupts the estradiol-induced LH surge of the ewe. Ovariectomized sheep were set up in an artificial follicular phase model in which luteolysis is simulated by progesterone withdrawal and the follicular phase estradiol rise is reproduced experimentally. In the first experiment, we tested the hypothesis that endotoxin interferes with the estradiol-induced LH surge. Ewes were either infused with endotoxin (300 ng/kg/h, i.v.) for 30 h beginning at onset of a 48-h estradiol stimulus or sham infused as a control. Endotoxin significantly delayed the time to the LH surge (P < 0.01), but did not alter surge amplitude, duration, or incidence. The second experiment tested the hypothesis that the delaying effects of endotoxin on the LH surge depend on when endotoxin is introduced relative to the onset of the estradiol signal. Previous work in the ewe has shown that a 14-h estradiol signal is adequate to generate GnRH and LH surges, which begin 6-8 h later. Thus, we again infused endotoxin for 30 h, but began it 14 h after the onset of the estradiol signal. In contrast to the first experiment, endotoxin given later had no effect on any parameter of the LH surge. In the third experiment, we tested the hypothesis that endotoxin acts during the first 14 h to disrupt the initial activating effects of estradiol. Estradiol was delivered for just 14 h, and endotoxin was infused only during this time. Under these conditions, endotoxin blocked the LH surge in five of eight ewes. In a similar follow-up study, endotoxin again blocked the LH surge in six of seven ewes. We conclude that endotoxin can disrupt the estradiol-induced LH surge by interfering with the early activating effects of the estradiol signal during the first 14 h (reading of the signal). In contrast, endotoxin does not disrupt later stages of signal processing (i.e. events during the interval between estradiol signal delivery and surge onset), nor does it prevent actual hormonal surge output. Thus, endotoxin appears to disrupt estrogen action per se rather than the release of GnRH or LH at the time of the surge.     

Evidence that termination of the estradiol-induced luteinizing hormone surge in women is regulated by ovarian factors

BACKGROUND: The endogenous LH surge is the result of the estrogen-positive feedback effect. However, the factors that are responsible for the termination of LH surge are not known. OBJECTIVE: The objective of the study was to investigate the mechanism that terminates the LH surge in women. SUBJECTS AND METHODS: Eight normally cycling women (aged 42-48 yr) were investigated in two cycles, i.e. cycle 1 (control) and cycle 2. In cycle 2 total abdominal hysterectomy plus bilateral salpingooophorectomy was performed on d 3. In both cycles, estradiol was administered transdermally at the dose of 100 microg on d 3 and 150 microg on d 4 and 5. Blood samples were obtained every 12 h from d 3 to 5 and every 6 h thereafter until d 9. RESULTS: In both cycles, after suppression of gonadotropins, the women displayed an endogenous LH surge. The time intervals between the commencement of estradiol treatment and the LH surge onset (73.5 +/- 1.5 vs. 76.5 +/- 2.5 h) and peak LH values (11.4 +/- 1.9 vs. 12.4 +/- 3.1 IU/liter) were comparable in the two cycles (mean +/- sem). After peaking, LH values decreased gradually in cycle 1, whereas in cycle 2 they remained stable and were higher than the corresponding values in cycle 1 (P < 0.05). Before the LH surge onset, estradiol values showed in both cycles a preovulatory pattern of changes, but starting 24 h after the onset of the LH surge, they were lower in cycle 2 (P < 0.05). Progesterone levels were similar in both cycles until the day of the LH surge onset, but in cycle 2 they declined thereafter and were lower than in cycle 1 (P < 0.05). CONCLUSIONS: It is suggested that ovarian factors rather than exhaustion of pituitary reserves are important for termination of the endogenous LH surge during the normal menstrual cycle.     

Modulation of pituitary gonadotropin response to gonadotropin-releasing hormone by estradiol

The effects of 17beta-estradiol on the responsiveness of the pituitary to gonadotropin-releasing hormone (GnRH) and on the rate of disappearance of GnRH were studied in 15 healthy nulliparous women aged 18-21 years. The women were divided into 3 groups: Group 1 received no estradiol, Group 2 received the amount of estradiol needed to achieve a circulating level comparable with that in the late follicular phase, and Group 3 received enough estradiol to achieve a concentration similar to that at midcycle. Following administration of GnRH, a marked increase in both LH and FSH was seen in Group 1 subjects. A smaller increase in LH level was observed in Group 2, and virtually no LH response occurred in Group 3. There was no significant increase in FSH level in either group treated with estradiol. The infusion of estradiol did not affect the maximal plasma concentration of exogenously administered GnRH or its disappearance rate in 4 women studied.     

Decreased release of gonadotropin-releasing hormone during the preovulatory midcycle luteinizing hormone surge in normal women.

To investigate the contribution of hypothalamic gonadotropin-releasing hormone (GnRH) secretion to the midcycle gonadotropin surge in the human, the response of luteinizing hormone (LH) to competitive GnRH receptor blockade achieved by administration of a range of doses of a pure GnRH antagonist was used to provide a semiquantitative estimate of endogenous GnRH secretion. The LH response to 5, 15, 50, and 150 micrograms/kg s.c. of the NAL-GLU GnRH antagonist ([Ac-D-2Nal1,D-4ClPhe2,-D-Pal3,Arg5,D-4-p-met hoxybenzoyl-2-aminobutyric acid6,D-Ala10]GnRH, where 2Nal is 2-naphthylalanine, 4ClPhe is 4-chlorophenylalanine, and 3Pal is 3-pyridylalanine) was measured in normal women in the early and late follicular phases of the menstrual cycle, at the time of the midcycle LH surge and in the early luteal phase. LH decreased in a dose-response fashion after administration of the GnRH antagonist in all cycle phases (P < 0.0001). When this suppression was expressed as maximum percent inhibition, there was no difference in response during the early and late follicular and early luteal phases. However, at the midcycle surge, there was a leftward shift of the dose-response curve with significantly greater suppression of LH at the lower antagonist doses in comparison to the other cycle phases (P < 0.005), but no difference at the highest dose. Thus, we draw the following conclusions. (i) There is a consistently greater degree of LH inhibition by GnRH antagonism at the midcycle surge at submaximal degrees of GnRH receptor blockade than at other phases of the menstrual cycle in normal women. (ii) This leftward shift of the dose-response relationship to GnRH receptor blockade suggests that the overall amount of GnRH secreted at the midcycle surge is less than at other cycle stages. (iii) These data confirm the importance of pituitary augmentation of the GnRH signal at the time of the midcycle gonadotropin surge in the human.     

Progesterone enhances the surge of luteinizing hormone by increasing the activation of luteinizing hormone-releasing hormone neurons

The ability of progesterone (P) to enhance the surge of LH in the rat is well documented, but whether its primary site of action is on the pituitary or brain is unclear. To determine whether P can alter the activation of LHRH neurons, 1) intact female rats were treated with the P antagonist RU486 (5 mg) at 1230 h on proestrus and killed at specified times during the afternoon and evening for comparison of plasma LH levels and cFos expression in LHRH neurons with untreated proestrous rats. RU486 treatment greatly reduced both the magnitude of the LH surge and the degree of cFos induction (numbers of cells expressing cFos and intensity of cFos staining) in LHRH neurons during proestrus. 2) Ovariectomized (OVX) rats were primed with estradiol benzoate (EB, 1 microgram) and then were treated with EB alone (50 microgram) or EB plus P (5 mg). Treatment with EB without P resulted in significantly lower peak LH levels and a reduced cFos response in LHRH neurons than the EB-P treated rats. These data suggest that the actions of P eventuate in an enhanced activation of LHRH neurons that may be responsible for the increased magnitude of the LH surge.     

A mechanism for the differential regulation of gonadotropin subunit gene expression by gonadotropin-releasing hormone.

The hypothalamic hormone gonadotropin-releasing hormone (GnRH) is released in a pulsatile fashion, with its frequency varying throughout the reproductive cycle. Varying pulse frequencies and amplitudes differentially regulate the biosynthesis and secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) by pituitary gonadotropes. The mechanism by which this occurs remains a major question in reproductive physiology. Previous studies have been limited by lack of available cell lines that express the LH and FSH subunit genes and respond to GnRH. We have overcome this limitation by transfecting the rat pituitary GH3 cell line with rat GnRH receptor (GnRHR) cDNA driven by a heterologous promoter. These cells, when cotransfected with regulatory regions of the common alpha, LH beta, or FSH beta subunit gene fused to a luciferase reporter gene, respond to GnRH with an increase in luciferase activity. Using this model, we demonstrate that different cell surface densities of the GnRHR result in the differential regulation of LH and FSH subunit gene expression by GnRH. This suggests that the differential regulation of gonadotropin subunit gene expression by GnRH observed in vivo in rats may, in turn, be mediated by varying gonadotrope cell surface GnRHR concentrations. This provides a physiologic mechanism by which a single ligand can act through a single receptor to regulate differentially the production of two hormones in the same cell.     

Control of the estradiol-induced prolactin surge by the suprachiasmatic nucleus

In the present study we investigated how the suprachiasmatic nucleus (SCN) controls the E(2)-induced PRL surge in female rats. First, the role of vasopressin (VP), a SCN transmitter present in medial preoptic area (MPO) projections and rhythmically released by SCN neurons, as a circadian signal for the E(2)-induced PRL surge was investigated. Using a reverse microdialysis technique, VP was administered in the MPO during the PRL surge, resulting in a suppression of the surge. VP administration before the surge did not affect PRL secretion. Also, administration of a V1a receptor antagonist before the surge was ineffective. Second, lesions of the SCN were made that resulted in constant basal PRL levels, suggesting that with removal of the SCN a stimulatory factor for PRL secretion disappeared. Indeed, the PRL secretory response to blockade of pituitary dopamine receptors was significantly reduced in SCN-lesioned animals. These data suggest that the afternoon decrease of VP release in the MPO by SCN terminals enables the PRL surge to occur, and may thus be a circadian signal for the PRL surge. Simultaneously the SCN is involved in the regulation of the secretory capacity of the pituitary, possibly via specific PRL-releasing factors.     

Pattern of gonadotropin-releasing hormone (GnRH) secretion leading up to ovulation in the ewe: existence of a preovulatory GnRH surge

We have previously shown LH surges induced by physiological estradiol levels are invariably accompanied by robust and sustained GnRH surges in the ewe. Such an increase, however, has not been observed consistently during the preovulatory LH surge. In the present study, we examined GnRH secretion in Suffolk and Ile de France ewes during the preovulatory period using a method for pituitary portal blood collection which allows simultaneous portal and jugular blood samples to be taken at frequent intervals for up to 48 h. Ewes were sampled either during the mid-late luteal phase (n = 8) or follicular phase (n = 20). During the follicular phase, a robust increase in GnRH secretion occurred at the onset of the LH surge in 11 of 12 ewes sampled during the LH surge. The GnRH increase in most ewes was a massive surge, reaching values averaging 40-fold greater than baseline and extending well beyond the end of the preovulatory LH surge. In the single ewe not exhibiting a GnRH surge during the LH surge, postmortem inspection indicated blood was probably not sampled from the pituitary portal vessels. In the early follicular phase, GnRH-pulse frequency was greater than that observed in the luteal phase and, within the follicular phase, GnRH-pulse frequency increased further and amplitude decreased as the surge approached. These data demonstrate GnRH secretion leading up to ovulation in the ewe is dynamic, beginning with slow pulses during the luteal phase, progressing to higher frequency pulses during the follicular phase and invariably culminating in a robust surge of GnRH. The LH surge, however, ends despite continued elevation of GnRH.     

Antagonist of gonadotropin-releasing hormone blocks naloxone-induced elevations in serum luteinizing hormone

Administration of a gonadotropin-releasing hormone (GnRH) antagonist, [D-Phe, D-Trp3.6]-GnRH, to immature female rats blocks the equivalent elevations in serum luteinizing hormone (LH) which are provoked by exogenous, natural GnRH (8 ng/100 g BW) or naloxone (0.25 mg/100 g BW), a specific opiate antagonist. A significant inhibition of GnRH- or naloxone-induced release of LH is obtained when rats are pretreated for 0, 15, 30 or 60 min with 5,000 ng/100 g BW of GnRH antagonist but no inhibition is evident when the antagonist is injected 180 min before either stimulant of LH secretion. A similar time-course is observed for GnRH antagonist inhibition of basal LH levels. The minimally effective dose of GnRH antagonist for suppressing both GnRH- and naloxone-induced LH release is 1,000 ng/100 g BW. More than 80% of the LH response to GnRH or naloxone is blocked by the highest tested dose (10,000 ng/100 g BW) of GnRH antagonist. Since naloxone has no direct influence on pituitary release of LH, these similar influences of GnRH antagonist on the LH-releasing properties of natural GnRH and naloxone, strongly suggest that the systemic administration of the opiate antagonist, naloxone, stimulates the release of endogenous GnRH.     

Does melatonin alter pituitary responsiveness to gonadotropin-releasing hormone in the ewe?

The diurnal secretion of melatonin from the pineal gland transduces information about day length to the reproductive axis of many seasonal breeders including the ewe. In the sheep the target for melatonin is thought to be neural, such that the hormone acts through the GnRH pulse generator to produce seasonal alterations in the frequency of pulsatile LH secretion. These effects on the pulse generation mechanism take approximately 50 days to become evident. It is possible that melatonin also exerts direct effects at the level of the pituitary gland to alter responsiveness to GnRH. Such effects have been noted in other species. The site of action of melatonin to regulate pulsatile LH secretion was assessed in the ewe by determining whether the animal's endogenous melatonin acutely modifies pituitary responsiveness to sustained pulsatile administration of GnRH. Using an animal model in which endogenous GnRH was blocked, pituitary responsiveness to hourly pulses of exogenous GnRH was assessed under conditions of both high (dark period) and low (light period) melatonin. No evidence for acute effects of melatonin on pituitary response to GnRH was found. In another experiment, the amplitude and frequency of endogenously generated LH pulses in ovariectomized ewes was found not to change during the 24-hour light/dark cycle. These data lead to the conclusion that melatonin does not act at the pituitary gland to produce acute effects on LH secretion. Rather, our findings are consistent with the hypothesis that the action of melatonin, in this short-day breeder is long term, and is directed towards the neural elements of the hypothalamic pulse-generating mechanism.     

Prostaglandins mediate the endotoxin-induced suppression of pulsatile gonadotropin-releasing hormone and luteinizing hormone secretion in the ewe

Five experiments were conducted to test the hypothesis that PGs mediate the endotoxin-induced inhibition of pulsatile GnRH and LH secretion in the ewe. Our approach was to test whether the PG synthesis inhibitor, flurbiprofen, could reverse the inhibitory effects of endotoxin on pulsatile LH and GnRH secretion in ovariectomized ewes. Exp 1-4 were cross-over experiments in which ewes received either flurbiprofen or vehicle 2 weeks apart. Jugular blood samples were taken for LH analysis throughout a 9-h experimental period. Depending on the specific purpose of the experiment, flurbiprofen or vehicle was administered after 3.5 h, followed by endotoxin, vehicle, or ovarian steroids (estradiol plus progesterone) at 4 h. In Exp 1, flurbiprofen reversed the endotoxin-induced suppression of mean serum LH concentrations and the elevation of body temperature. In Exp 2, flurbiprofen prevented the endotoxin-induced inhibition of pulsatile LH secretion and stimulation of fever, reduced the stimulation of plasma cortisol and progesterone, but did not affect the rise in circulating tumor necrosis factor-alpha. In Exp 3, flurbiprofen in the absence of endotoxin had no effect on pulsatile LH secretion. In Exp 4, flurbiprofen failed to prevent suppression of pulsatile LH secretion induced by luteal phase levels of the ovarian steroids progesterone and estradiol, which produce a nonimmune suppression of gonadotropin secretion. In Exp 5, flurbiprofen prevented the endotoxin-induced inhibition of pulsatile GnRH release into pituitary portal blood. Our finding that this PG synthesis inhibitor reverses the inhibitory effect of endotoxin leads to the conclusion that PGs mediate the suppressive effects of this immune/inflammatory challenge on pulsatile GnRH and LH secretion.