Estradiol and the inhibition of hypothalamic gonadotropin-releasing hormone pulse generator activity in the rhesus monkey.

In mammals, gonadal function is controlled by a hypothalamic signal generator that directs the pulsatile release of gonadotropin-releasing hormone (GnRH) and the consequent pulsatile secretion of luteinizing hormone. In female rhesus monkeys, the electrophysiological correlates of GnRH pulse generator activity are abrupt, rhythmic increases in hypothalamic multiunit activity (MUA volleys), which represent the simultaneous increase in firing rate of individual neurons. MUA volleys are arrested by estradiol, either spontaneously at midcycle or after the administration of the steroid. Multiunit recordings, however, provide only a measure of total neuronal activity, leaving the behavior of the individual cells obscure. This study was conducted to determine the mode of action of estradiol at the level of single neurons associated with the GnRH pulse generator. Twenty-three such single units were identified by cluster analysis of multiunit recordings obtained from a total of six electrodes implanted in the mediobasal hypothalamus of three ovariectomized rhesus monkeys, and their activity was monitored before and after estradiol administration. The bursting of all 23 units was arrested within 4 h of estradiol administration although their baseline activity was maintained. The bursts of most units reappeared at the same time as the MUA volleys, the recovery of some was delayed, and one remained inhibited for the duration of the study (43 days). The results indicate that estradiol does not desynchronize the bursting of single units associated with the GnRH pulse generator but that it inhibits this phenomenon. The site and mechanism of action of estradiol in this regard remain to be determined.     

Hypoglycemic 'stress' and gonadotropin-releasing hormone pulse generator activity in the rhesus monkey: role of the ovary.

Observations of long standing have suggested that the 'stress' of chair restraint inhibits the GnRH pulse generator in normal female monkeys while this phenomenon is rarely observed in ovariectomized animals. The role of the ovary in the response of the GnRH pulse generator to the stress of insulin hypoglycemia was investigated in both intact and ovariectomized rhesus monkeys. Following an overnight fast the animals, previously habituated to restraint, were placed in primate chairs and GnRH pulse generator activity monitored electrophysiologically. Insulin-induced reductions in mean blood glucose concentrations of 10-40% of control values interrupted pulse generator activity in intact monkeys but were without effect in ovariectomized animals. With larger reductions in blood glucose, pulse generator activity was interrupted in both groups but the inhibition was twice as long in intact than in ovariectomized animals. The reduced responsiveness of ovariectomized animals to insulin hypoglycemia was significantly reversed by estradiol replacement. Naloxone administration did not prevent the hypoglycemia-induced inhibition of pulse generator activity in either intact or ovariectomized rhesus monkeys. It is concluded that hypoglycemic 'stress' inhibits the GnRH pulse generator by a nonopioidergic mechanism and that ovarian products, most probably estradiol, exacerbate this effect.     

Radiotelemetric monitoring of hypothalamic gonadotropin-releasing hormone pulse generator activity throughout the menstrual cycle of the rhesus monkey

Continuous monitoring of the electrophysiological manifestations of GnRH pulse generator activity was achieved by radiotelemetry throughout the menstrual cycles of unrestrained rhesus monkeys. The characteristic increases in hypothalamic multiunit activity (MUA volleys) associated with each LH pulse measured in the peripheral circulation were of lower frequency during the luteal phase than in the follicular phase of the cycle. Multiunit activity volley frequency increased as functional luteolysis progressed and achieved maxima of approximately one volley per hour within the first few days of the follicular phase. Unexpectedly, a dramatic decline in pulse generator frequency was observed coincidentally with the initiation of the preovulatory LH surge. Evidence is presented to support the conclusion that this deceleration of pulse generator activity is the consequence of the preovulatory rise in plasma estrogen concentration. As reported in women, a significant reduction in GnRH pulse generator frequency was observed at night during the follicular phase, but not during the luteal phase, of the menstrual cycle.     

Duration of phasic electrical activity of the hypothalamic gonadotropin-releasing hormone pulse generator and dynamics of luteinizing hormone pulses in the rhesus monkey.

The secretion of luteinizing hormone (LH) by the pituitary gland is a pulsatile phenomenon. In the rhesus monkey, each pulse of LH in the peripheral circulation is associated with a characteristic increase in multiunit electrical activity (MUA) recorded from the medial basal hypothalamus. These "volleys" of electrical activity initiate the release of gonadotropin-releasing hormone (GnRH) into the pituitary portal circulation from the terminals of neurosecretory cells. Their duration varies from 1-3 min in normal, adult intact females to 10-25 min in long-term ovariectomized monkeys. A variety of pharmacological interventions also modify volley duration. The purpose of this investigation was to determine the physiological significance of alterations in volley duration. The dynamics of LH pulses in ovariectomized animals were observed in a number of experimental circumstances in which MUA volley duration was reduced from a maximum of 23 min to a minimum of 4 min without significantly altering their frequency. The magnitude of each LH pulse was assessed by calculating the area under the curve delineated by the time course of LH above baseline. In eight experiments, a linear regression of these values on volley duration failed to reveal a significant correlation between MUA volley duration and the magnitude of LH pulses. These results suggest that all of the GnRH secreted per pulse is released at the onset of each MUA volley, the remainder of the increase in electrical activity having no further action on GnRH secretion, although effects on other systems cannot be excluded.     

Electrophysiological manifestation of luteinizing hormone-releasing hormone pulse generator activity in the rhesus monkey: influence of alpha-adrenergic and dopaminergic blocking agents

Characteristic increases in neuronal electrical activity associated with the initiation of each LH pulse were recorded from ovariectomized rhesus monkeys bearing multiple chronic electrodes in the medial basal hypothalamus. These electrophysiological manifestations of hypothalamic LHRH pulse generator activity were inhibited by the alpha-adrenergic blocker phentolamine or the alpha 1-adrenoceptor blockers phenoxybenzamine and prazosin. At the dosages used, the effects of single injections of these drugs ranged from a reduction in the frequency of LHRH pulse generator activity to its complete arrest. This was faithfully reflected in the pattern of pulsatile LH discharges. The dopaminergic blocking agent metaclopramide similarly reduced the frequency of the pulse generator or arrested its activity altogether. The alpha 2-adrenoceptor blocker yohimbine had no demonstrable effect on hypothalamic electrical activity at the doses studied. These findings support the view of a central action of alpha 1-adrenergic and dopaminergic blockade on LHRH pulse generator activity and the concept that central adrenergic and dopaminergic inputs can modulate the frequency of the LHRH pulse generator.     

Corticotropin-releasing hormone inhibits gonadotropin secretion in the ovariectomized rhesus monkey

To evaluate whether the compromised gonadotropin secretion frequently occurring during stressful conditions in the primate may be related to an inhibitory action of CRH, the effects of ovine (oCRH) or human (hCRH) CRH on gonadotropin and cortisol secretion were studied in ovariectomized rhesus monkeys. LH secretion (assessed as area under the curve) decreased 35% and 21%, and cortisol increased 37% and 90%, 1-3 h after single iv injections of 200 and 500 micrograms oCRH, respectively (P less than 0.05 vs. pre-CRH control period; n = 4-7/dose). Single injections of 200 and 500 micrograms hCRH, respectively, resulted in 35% and 24% decreases in LH and 40% and 79% increases in cortisol secretion (P less than 0.05). Injections of 100 micrograms oCRH and hCRH elicited significant (P less than 0.05) increases in cortisol release (37% and 31%, respectively), but did not affect LH secretion. A 5-h infusion of hCRH (100 micrograms/h) reduced LH levels (23%, 49%, 59%, 61%, and 62% during the first through the fifth hour, respectively; P less than 0.05 for hours 2-5). FSH secretion also decreased during the hCRH infusion (26%, 33%, 42%, 46%, and 49% during the first through the fifth hour, respectively; P less than 0.05 for hours 3-5), while cortisol increased 76%. These data demonstrate that exogenous CRH administration results in inhibition of LH and FSH secretion in ovariectomized rhesus monkeys. These results are consistent with the hypothesis that elevated CRH levels could contribute to decreased LH and FSH secretion and, thus, disruption of reproduction function under conditions of stress in primates.     

Phytoestrogens and gonadotropin-releasing hormone pulse generator activity and pituitary luteinizing hormone release in the rat

Phytoestrogens can produce inhibitory effects on gonadotropin secretion in both animals and humans. The aims of this study were 2-fold: 1) to determine in vivo whether genistein and coumestrol act on the GnRH pulse generator to suppress hypothalamic multiunit electrical activity volleys and associated LH pulses and/or on the pituitary to suppress the LH response to GnRH; and 2) to examine the effect of these phytoestrogens on GnRH-induced pituitary LH release in vitro and to determine whether estrogen receptors are involved. Wistar rats were ovariectomized and chronically implanted with recording electrodes and/or indwelling cardiac catheters, and blood samples were taken every 5 min for 7--11 h. Intravenous infusion of coumestrol (1.6-mg bolus followed by 2.4 mg/h for 8.5 h) resulted in a profound inhibition of pulsatile LH secretion, a 50% reduction in the frequency of hypothalamic multiunit electrical activity volleys, and a complete suppression of the LH response to exogenous GnRH. In contrast, both genistein (1.6-mg bolus followed by 2.4 mg/h for 8.5 h) and vehicle were without effect on pulsatile LH secretion. Coumestrol (10(-5) M; over 2 or 4 h) suppressed GnRH-induced pituitary LH release in vitro, an effect blocked by the antiestrogen ICI 182,780. It is concluded that coumestrol acts centrally to reduce the frequency of the hypothalamic GnRH pulse generator. In addition, the inhibitory effects of coumestrol on LH pulses occur at the level of the pituitary by reducing responsiveness to GnRH via an estrogen receptor-mediated process.     

The effect of morphine on the electrophysiological activity of the hypothalamic luteinizing hormone-releasing hormone pulse generator in the rhesus monkey

In ovariectomized rhesus monkeys, the electrophysiological manifestation of luteinizing hormone-releasing hormone (LHRH) pulse generator activity was arrested by morphine and reinitiated by naloxone. These responses were noted within 1 and 2 min, respectively, after the intravenous injection of the drugs. Naloxone given alone had no effect. These results support the view that opioids modulate pulsatile gonadotropin release by an action on the hypothalamic LHRH pulse generator.     

Electrical activity of the pulse generator of gonadotropin-releasing hormone in 26-month-old female rats

To know whether age-related changes occur in the activity of the pulse generator of gonadotropin-releasing hormone (GnRH), old (26 months) and young (3 months) female rats were examined by recording multiunit activity (MUA) in the median eminence region of the hypothalamus, concurrently with blood samplings through an intra-atrial cannula at 6-min intervals to determine serum luteinizing hormone (LH) concentrations. We have regarded the MUA showing intermittent increases (volleys) at 20-30 min intervals, followed by LH pulses, as the electrical activity of the GnRH pulse generator. We were successful in recording MUA in 18 (26%) of 69 old ovariectomized rats and in 8 (32%) of 25 young ovariectomized rats. The overall mean (+/-SE) of the interval between MUA volleys in old ovariectomized rats was 35.1 +/- 2.0 min (n = 18), which was significantly longer than that of 22.5 +/- 1.5 min (n = 8) in young ovariectomized rats. The mean interval between LH pulses in old ovariectomized rats was 32.2 +/- 3.6 (n = 10), also being significantly longer than that of 23.3 +/- 1.0 (n = 8) in young ovariectomized rats. Further, the LH pulse amplitude in old rats (0. 95 +/- 0.07 ng/ml) was significantly smaller than in young rats (3. 40 +/- 0.36 ng/ml). The present study also confirmed that the increase in serum LH after intravenous injection of 50 ng GnRH was much smaller in old ovariectomized rats. These results show that the electrical activity of the GnRH pulse generator is certainly reduced with age. Taken together with findings suggesting an age-dependent decrease in stimulated transmitter release, attenuation in both frequency and amplitude of GnRH pulses as well as in pituitary responsiveness to GnRH pulses may account for the decreased pulsatile LH secretion observed in aging rats.     

Contraceptive action of a gonadotropin-releasing hormone II analog in the rhesus monkey

GnRH I (mammalian GnRH) was previously thought to be the only isoform of GnRH expressed in mammals, but GnRH II (chicken II GnRH) has now been identified in tissues of numerous mammalian species. Specific high-affinity receptors, which bind GnRH II and its analogs, have been identified throughout the reproductive tract, and GnRH II regulation of progesterone and human chorionic gonadotropin have been demonstrated. Thus, we hypothesized that GnRH II acts as a paracrine factor to regulate extrahypothalamic tissue functions and could be used as an effective contraceptive agent.In these studies, we examined the effect of a stable analog of GnRH II (GnRH II analog) on ovarian steroidogenesis, implantation, and maintenance of pregnancy in the rhesus monkey. GnRH II analog or saline was administered by osmotic minipumps on d 1-6, d 6-11, or d 11-17 to cycling monkeys mated with fertile males. Circulating progesterone and estradiol were determined during the luteal phase, and the cycle length before, during, and after treatment was observed. Circulating monkey chorionic gonadotropin was used to determine implantation. In animals treated with GnRH II analog on d 1-6, no pregnancies resulted; but in saline-treated controls, five of eight animals (62.5%) became pregnant. In animals treated with analog on d 6-11, two of five (40.0%); and on d 11-17, one of three (33.3%); implanted and normal pregnancies ensued. Cycle length or progesterone production was not affected by analog treatment.These data demonstrate that this GnRH II analog can act as a contraceptive agent when administered chronically around the time of ovulation or early luteal development. These findings suggest that GnRH II may play a role in reproductive physiology and that GnRH II analogs may serve as an effective, nonsteroidal, postcoital contraceptive.     

Electrophysiological studies on the hypothalamic GnRH pulse generator]

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

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)     

Frequency and amplitude of gonadotropin-releasing hormone stimulation and gonadotropin secretion in the rhesus monkey

In adult ovariectomized rhesus monkeys bearing hypothalamic lesions which reduced circulating LH and FSH to undetectable levels, sustained elevated gonadotropin concentrations were reestablished by the intermittent administration of gonadotropin-releasing hormone (GnRH) at the rate of 1 microgram/min for 6 min once every hour. The effects of varying either the frequency or the amplitude of these GnRH pulses on gonadotropin secretion were examined in such animals. Increasing the frequency of GnRH administration from the physiological one pulse per h to two, three, or five pulses h while maintaining a constant infusion rate and pulse duration resulted in gradual declines in plasma gonadotropin concentrations. These declines were most profound at the highest frequencies and the consequence of reduced pituitary responses to individual GnRH pulses. Decreasing the frequency of GnRH pulses from one per h to one every 3 h led to variable declines in plasma LH levels, but circulating FSH invariably rose. Reducing the GnRH infusion rate from 1 to 0.1 mg/min while maintaining constant frequency and pulse duration resulted in abrupt declines in plasma LH and FSH to immeasurable levels, although pulsatile increments in circulating GnRH concentrations without a concomitant reduction in plasma LH concentrations, which remained unchanged. An infusion rate of 0.5 microgram/min resulted in unstable plasma LH and FSH levels. These results demonstrate that changes in the frequency or amplitude of hypophysiotropic stimulation have profound effects on plasma gonadotropin levels as well as on FSH to LH ratios in the circulation. The physiological implications of these observations are discussed.     

Leptin减轻吗啡对罗猴促性腺激素释放激素脉冲发生器抑制作用的初步报告

目的　研究瘦素（ｌｅｐｔｉｎ） 对罗猴下丘脑促性腺激素释放激素脉冲发生器（ Ｇｎ Ｒ Ｈ Ｐ Ｇ） 的多单位电脉冲频率骤然增加（ Ｍ Ｕ Ａｖｏｌｌｅｙ） 的影响。方法　在基础及受吗啡抑制状态时脑室内输注瘦素，记录植入下丘脑内侧基底部电极的电脉冲释放，计算两次 Ｍ Ｕ Ａｖｏｌｌｅｙ 之间的间期（ Ｍ Ｖ Ｉ） 。结果瘦素可显著减轻吗啡使 Ｍ Ｖ Ｉ延长的抑制效应， Ｍ Ｖ Ｉ在注射吗啡后为对照的２９０ ％ ，如脑室内输注瘦素则可使其减少至１６９ ％ （ 瘦素１ ．５μｇ／ｋｇ） 和１５１ ％ （ 瘦素７ ．５μｇ／ｋｇ） ， Ｐ＜ ０ ．０１ 。结论　瘦素可部分保护 Ｇｎ Ｒ Ｈ Ｐ Ｇ 少受吗啡的抑制。     

Duration and frequency of multiunit electrical activity associated with the hypothalamic gonadotropin releasing hormone pulse generator in the rhesus monkey: differential effects of morphine

The effects of morphine on the frequency and duration of the characteristic bursts or 'volleys' of multiunit electrical activity (MUA) associated with pulsatile pituitary luteinizing hormone (LH) secretion were studied in unanesthetized ovariectomized rhesus monkeys bearing bilateral arrays of electrodes implanted in the mediobasal hypothalamus. Morphine administration resulted in a dose-dependent decrease in MUA volley duration and frequency. When morphine was infused at 10 micrograms/kg/h, the inhibiting effect on volley duration was observed without a change in volley frequency. It is concluded that the frequency and duration of hypothalamic MUA volleys associated with pulsatile LH secretion may be independently regulated.     

Effects of kisspeptin-10 on the electrophysiological manifestation of gonadotropin-releasing hormone pulse generator activity in the female rat

Kisspeptins are extraordinarily potent in stimulating gonadotropic hormone secretion via an action on the hypothalamic GnRH neural system. Because the physiological frequency of the GnRH pulse generator is a critical component of the control system that governs reproductive processes, the aim of this study was to examine the effect of kisspeptin-10 on pulsatile LH secretion and on the electrophysiological manifestation of GnRH pulse generator activity to determine frequency modulatory effects. Adult Sprague Dawley rats were ovariectomized and chronically implanted with electrodes in the arcuate nucleus to record the characteristic increases in hypothalamic multiunit electrical activity volleys coincident with the initiation of each LH pulse measured in peripheral blood and/or indwelling cardiac catheters for the collection of blood samples (25 microl) every 5 min for 6-7 h for the measurement of LH. Intravenous infusion of kisspeptin-10 (7.5, 35, and 100 nmol) induced a dose-dependent increase in LH secretion. The stimulatory effect of kisspeptin-10 (100 nmol) on LH secretion was blocked by the GnRH antagonist cetrorelix, precluding a singular action on gonadotropes. Unexpectedly, however, the marked increase in LH release in response to kisspeptin-10 (100 nmol) administration was not accompanied by any change in multiunit electrical activity volley frequency. It seem unlikely, therefore, that kisspeptin-10 has an appreciable frequency modulatory effect on GnRH pulse generator activity in the female rat.     

Sustained intermittent release of gonadotropin-releasing hormone in the prepubertal male rhesus monkey induced by N-methyl-DL-aspartic acid

The purpose of the present study was to determine whether gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus of the prepubertal monkey may be prematurely provoked into producing a sustained train of intermittent GnRH release N-methyl-DL-aspartic acid (NMA), an analog of the putative excitatory neurotransmitter aspartate, was used to stimulate the hypothalamus. In order to utilize pituitary luteinizing hormone (LH) secretion as a bioassay of hypothalamic GnRH release, juvenile males were castrated and the responsiveness of their gonadotrophs to GnRH was enhanced prior to the study with a chronic intermittent intravenous infusion of the synthetic decapeptide (0.1 microgram/min for 3 min every hour). Treatment with this regimen of GnRH, which appears to provide the pituitary gonadotrophs with a hypophysiotropic stimulus similar to that produced by the hypothalamus of castrated adults, elicited a pattern of pulsatile LH secretion in prepubertal animals similar to that observed in the open-loop situation in adults. This episodic pattern of LH release was sustained without decrement following termination of GnRH priming and initiation of an intermittent intravenous infusion of NMA (4.5-6.5 mg NMA/kg body weight/pulse, administered over 1 min) delivered at a frequency of 1 pulse/1 h for 50 h. In contrast, an intermittent infusion of the vehicle employed to administer NMA (saline) failed to maintain LH secretion. Administration of the same dose of NMA at a slower frequency of 1 pulse/2 h for 52 h, while also sustaining LH secretion without decrement, resulted in an exaggeration in the LH response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Further evidence for the role of glucose as a metabolic regulator of hypothalamic gonadotropin-releasing hormone pulse generator activity in goats

The present study examined the relative importance of blood glucose vs. free fatty acids as a metabolic signal regulating GnRH release as measured electrophysiologically by multiple-unit activity (MUA) in the arcuate nucleus/median eminence region in ovariectomized, estradiol-treated goats. MUA was recorded before, during, and after: 1) cellular glucoprivation by peripheral infusion of 2-deoxy-d-glucose (2DG; 25, 50, and 75 mg/kg.h, iv); 2) peripheral hypoglycemia in response to various doses (15-195 mU/kg.h, iv) of insulin infusion; and 3) cellular lipoprivation induced by peripheral infusion of sodium mercaptoacetate (MA; 2.4 mg/kg.h alone or combined with 25 mg/kg.h of 2DG, iv), and effects on the interval of characteristic increases in MUA (MUA volleys) were examined. Infusion of the highest dose of 2DG increased the mean interval between MUA volleys, whereas the lower doses of 2DG had no effect on volley interval. The MUA volley intervals lengthened as insulin-induced hypoglycemia became profound. There was a negative correlation between MUA volley intervals and blood glucose concentrations during insulin infusion, and coinfusion of glucose with insulin returned the MUA volley interval to a normal frequency. Infusion of MA alone or MA with 2DG did not increase MUA volley intervals. These findings demonstrate that glucose availability, but not fatty acids, regulates the GnRH pulse generator activity in the ruminant. Glucose is considered a key metabolic regulator that fine-tunes pulsatile GnRH release.     

Corticotropin-releasing factor decreases plasma luteinizing hormone levels in female rats by inhibiting gonadotropin-releasing hormone release into hypophysial-portal circulation

To evaluate whether the hypothalamus is the site of action of CRF in inhibiting LH levels in female rats, we measured hypophysial-portal blood concentrations of immunoreactive GnRH (irGnRH) after the central injection of CRF. Ovine CRF (0.1, 1.0, 2.0, and 5.0 nmol) was injected intracerebroventricularly to intact rats on the afternoon of proestrus and in long term ovariectomized (OVX) rats in the presence or in absence of estradiol benzoate (OVX + EB). CRF injection decreased the amplitude of the proestrous irGnRH surge without affecting presurge levels. CRF (0.1 nmol) attenuated the afternoon irGnRH surge in OVX + EB rats; higher doses of CRF blocked this surge and decreased nonsurge irGnRH levels. No dose-related alterations of irGnRH levels were observed in OVX rats; only the highest dose of CRF was active. For comparison, plasma LH concentrations were measured after a single dose of CRF (2 nmol) in rats under the same experimental conditions. While CRF decreased LH concentrations in anesthetized proestrous and OVX + EB rats, it was inactive in OVX rats. In contrast, CRF injection in awake rats did decrease LH concentrations in all experimental conditions, suggesting that in OVX rats, the anesthetic (Saffan) used during portal blood collection affected CRF action on LH secretion. Indeed, the observation that the LH response to opiate receptor blockade with naloxone (2.5 mg/kg) in anesthetized OVX rats was different compared to that in awake rats suggested that the ineffectiveness of CRF to decrease irGnRH and LH in OVX anesthetized rats was related to the action of the anesthetic on the opioid system. The existence of a putative CRF-opioid interaction in the inhibitory control of LH secretion was supported by the effectiveness of naloxone to reverse the CRF-induced decrease in LH levels in EB-treated and untreated OVX rats. These results indicate that CRF attenuates LH secretion by a central action to inhibit irGnRH release into the hypophysial-portal circulation and that this action is independent of basal concentrations of irGnRH and/or LH. Moreover, the present results support the involvement of endogenous opioids in mediating the effect of CRF on LH secretion.