Regulation of the gonadotropin-releasing hormone neuron during stress.

The effect of stress on reproduction and gonadal function has captivated investigators for almost 100 years. Following the identification of gonadotropin-releasing hormone (GnRH) 50 years ago, a niche research field emerged fixated on how stress impairs this central node controlling downstream pituitary and gonadal function. It is now clear that both episodic GnRH secretion in males and females and surge GnRH secretion in females are inhibited during a variety of stress types. There has been considerable advancement in our understanding of numerous stress-related signaling molecules and their ability to impair reproductive neuroendocrine activity during stress. Recently, much attention has turned to the effects of stress on two populations of kisspeptin neurons: the stimulatory afferents to GnRH neurons that regulate pulsatile and surge-type gonadotropin secretion. Indeed, future work is still required to fully construct the neuroanatomical framework underlying stress effects, directly or indirectly, on GnRH neuron function. The present review evaluates and synthesizes evidence related to stress-related signaling molecules acting directly on GnRH neurons. Here, we review the evidence for and against the action of a handful of signaling molecules as inhibitors of GnRH neuron function, including corticotropin-releasing hormone, urocortins, norepinephrine, cortisol/corticosterone, calcitonin gene-related peptide and arginine-phenylalanine-amide-related peptide-3.

Neuroendocrine control of FSH secretion: IV. Hypothalamic control of pituitary FSH-regulatory proteins and their relationship to changes in FSH synthesis and secretion.

The current dogma is that the differential regulation of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) synthesis and secretion is modulated by gonadotropin-releasing hormone (GnRH) pulse frequency and by changes in inhibins, activins, and follistatins both at the pituitary and at the peripheral level. To date no studies have looked at the overlapping function of these regulators in a combined setting. We tested the hypothesis that changes in GnRH pulse frequency alter the relative abundance of these regulators at the pituitary and peripheral levels in a manner consistent with changes in pituitary and circulating concentrations of FSH; that is, an increase in FSH will be accompanied by increased stimulatory input (activin) and/or reduced follistatin and inhibin. Ovariectomized ewes were subjected to a combination hypothalamic pituitary disconnection (HPD)-hypophyseal portal blood collection procedure. Hypophyseal portal and jugular blood samples were collected for a 6-h period from non-HPD ewes, HPD ewes, or HPD ewes administered GnRH hourly or every 3 h for 4 days. In the absence of endogenous hypothalamic and ovarian hormones that regulate gonadotropin secretion, 3-hourly pulses of GnRH increased pituitary content of FSH more than hourly GnRH, although these differences were not evident in the peripheral circulation. The results failed to support the hypothesis in that the preferential increase of pituitary content of FSH by the lower GnRH pulse frequency could be explained by changes in the pituitary content of inhibin A, follistatin, or activin B. Perhaps the effects of GnRH pulse frequency on FSH is due to changes in the balance of free versus bound amounts of these FSH regulatory proteins or to the involvement of other regulators not monitored in this study.

Membrane-initiated actions of estradiol (E2) in the regulation of LH secretion in ovariectomized (OVX) ewes.

BACKGROUND: We demonstrated that E2 conjugated to BSA (E2BSA) induces a rapid membrane-initiated inhibition of LH secretion followed hours later by a slight increase in LH secretion. Whether these actions of E2BSA are restricted to the pituitary gland and whether the membrane-initiated pathway of E2BSA contributes to the up-regulation of the number of GnRH receptors during the positive feedback effect of E2 were evaluated here. We have shown that the suppression of LH secretion induced by E2 and E2BSA is the result of a decreased responsiveness of the pituitary gland to GnRH. In this study we further tested the ability of E2BSA to decrease the responsiveness of the pituitary gland to GnRH under the paradigm of the preovulatory surge of LH induced by E2. METHODS: For the first experiment GnRH and LH secretions were determined in samples of pituitary portal and jugular blood, respectively, in ewes treated with 12 mg E2BSA. In the second experiment, the number of GnRH receptors was quantified in ewes 12 h after administration of 25 micrograms E2 (the expected time for the increase in the number of GnRH receptors and the positive feedback effect of E2 in LH secretion) or 12 mg E2BSA. In the third experiment, the preovulatory-like surge of LH was characterized in ewes injected with 25 micrograms E2 alone or followed 8 h later (before the beginning of the LH surge) with 60 mg E2BSA. RESULTS: a) the decrease in LH secretion induced by E2BSA was not accompanied by changes in the pulsatile pattern of GnRH, b) E2BSA increased the number of GnRH receptors, and c) the presence of E2BSA in E2-treated ewes delayed the onset, reduced the length, and decreased the amount of LH released during the preovulatory surge of LH. CONCLUSIONS: a) the rapid suppression of LH secretion induced by E2BSA is mediated only via a direct action on the pituitary gland, b) E2 acting via a membrane-initiated pathway contributes to increase the number of GnRH receptors and, c) administration of E2BSA near the beginning of the pre-ovulatory surge of LH delays and reduces the magnitude of the surge.

The estrous cycle of the ewe is resistant to disruption by repeated, acute psychosocial stress.

Five experiments were conducted to test the hypothesis that psychosocial stress interferes with the estrous cycle of sheep. In experiment 1, ewes were repeatedly isolated during the follicular phase. Timing, amplitude, and duration of the preovulatory luteinizing hormone (LH) surge were not affected. In experiment 2, follicular-phase ewes were subjected twice to a "layered stress" paradigm consisting of sequential, hourly application of isolation, restraint, blindfold, and predator cues. This reduced the LH pulse amplitude but did not affect the LH surge. In experiment 3, different acute stressors were given sequentially within the follicular phase: food denial plus unfamiliar noises and forced exercise, layered stress, exercise around midnight, and transportation. This, too, did not affect the LH surge. In experiment 4, variable acute psychosocial stress was given every 1-2 days for two entire estrous cycles; this did not disrupt any parameter of the cycle monitored. Lastly, experiment 5 examined whether the psychosocial stress paradigms of experiment 4 would disrupt the cycle and estrous behavior if sheep were metabolically stressed by chronic food restriction. Thirty percent of the food-restricted ewes exhibited deterioration of estrous cycle parameters followed by cessation of cycles and failure to express estrous behavior. However, disruption was not more evident in ewes that also encountered psychosocial stress. Collectively, these findings indicate the estrous cycle of sheep is remarkably resistant to disruption by acute bouts of psychosocial stress applied intermittently during either a single follicular phase or repeatedly over two estrous cycles.

Role of estradiol in cortisol-induced reduction of luteinizing hormone pulse frequency.

Precise control of pulsatile GnRH and LH release is imperative to ovarian cyclicity but is vulnerable to environmental perturbations, like stress. In sheep, a sustained (29 h) increase in plasma cortisol to a level observed during stress profoundly reduces GnRH pulse frequency in ovariectomized ewes treated with ovarian steroids, whereas shorter infusion (6 h) is ineffective in the absence of ovarian hormones. This study first determined whether the ovarian steroid milieu or duration of exposure is the relevant factor in determining whether cortisol reduces LH pulse frequency. Prolonged (29 h) cortisol infusion did not lower LH pulse frequency in ovariectomized ewes deprived of ovarian hormones, but it did so in ovariectomized ewes treated with estradiol and progesterone to create an artificial estrous cycle, implicating ovarian steroids as the critical factor. Importantly, this effect of cortisol was more pronounced after the simulated preovulatory estradiol rise of the artificial follicular phase. The second experiment examined which component of the ovarian steroid milieu enables cortisol to reduce LH pulse frequency in the artificial follicular phase: prior exposure to progesterone in the luteal phase, low early follicular phase estradiol levels, or the preovulatory estradiol rise. Basal estradiol enabled cortisol to decrease LH pulse frequency, but the response was potentiated by the estradiol rise. These findings lead to the conclusion that ovarian steroids, particularly estradiol, enable cortisol to inhibit LH pulse frequency. Moreover, the results provide new insight into the means by which gonadal steroids, and possibly reproductive status, modulate neuroendocrine responses to stress.

Cortisol reduces gonadotropin-releasing hormone pulse frequency in follicular phase ewes: influence of ovarian steroids.

Stress-like elevations in plasma glucocorticoids suppress gonadotropin secretion and can disrupt ovarian cyclicity. In sheep, cortisol acts at the pituitary to reduce responsiveness to GnRH but does not affect GnRH pulse frequency in the absence of ovarian hormones. However, in ewes during the follicular phase of the estrous cycle, cortisol reduces LH pulse frequency. To test the hypothesis that cortisol reduces GnRH pulse frequency in the presence of ovarian steroids, the effect of cortisol on GnRH secretion was monitored directly in pituitary portal blood of follicular phase sheep in the presence and absence of a cortisol treatment that elevated plasma cortisol to a level observed during stress. An acute (6 h) cortisol increase in the midfollicular phase did not lower GnRH pulse frequency. However, a more prolonged (27 h) increase in cortisol beginning just before the decrease in progesterone reduced GnRH pulse frequency by 45% and delayed the preovulatory LH surge by 10 h. To determine whether the gonadal steroid milieu of the follicular phase enables cortisol to reduce GnRH pulse frequency, GnRH was monitored in ovariectomized ewes treated with estradiol and progesterone to create an artificial follicular phase. A sustained increment in plasma cortisol reduced GnRH pulse frequency by 70% in this artificial follicular phase, in contrast to the lack of an effect in untreated ovariectomized ewes as seen previously. Thus, a sustained stress-like level of cortisol suppresses GnRH pulse frequency in follicular phase ewes, and this appears to be dependent upon the presence of ovarian steroids.

Psychosocial stress inhibits amplitude of gonadotropin-releasing hormone pulses independent of cortisol action on the type II glucocorticoid receptor.

Our laboratory has developed a paradigm of psychosocial stress (sequential layering of isolation, blindfold, and predator cues) that robustly elevates cortisol secretion and decreases LH pulse amplitude in ovariectomized ewes. This decrease in LH pulse amplitude is due, at least in part, to a reduction in pituitary responsiveness to GnRH, caused by cortisol acting via the type II glucocorticoid receptor (GR). The first experiment of the current study aimed to determine whether this layered psychosocial stress also inhibits pulsatile GnRH release into pituitary portal blood. The stress paradigm significantly reduced GnRH pulse amplitude compared with nonstressed ovariectomized ewes. The second experiment tested if this stress-induced decrease in GnRH pulse amplitude is mediated by cortisol action on the type II GR. Ovariectomized ewes were allocated to three groups: nonstress control, stress, and stress plus the type II GR antagonist RU486. The layered psychosocial stress paradigm decreased GnRH and LH pulse amplitude compared with nonstress controls. Importantly, the stress also lowered GnRH pulse amplitude to a comparable extent in ewes in which cortisol action via the type II GR was antagonized. Therefore, we conclude that psychosocial stress reduces the amplitude of GnRH pulses independent of cortisol action on the type II GR. The present findings, combined with our recent observations, suggest that the mechanisms by which psychosocial stress inhibits reproductive neuroendocrine activity at the hypothalamic and pituitary levels are fundamentally different.

Cortisol interferes with the estradiol-induced surge of luteinizing hormone in the ewe.

Two experiments were conducted to test the hypothesis that cortisol interferes with the positive feedback action of estradiol that induces the luteinizing hormone (LH) surge. Ovariectomized sheep were treated sequentially with progesterone and estradiol to create artificial estrous cycles. Cortisol or vehicle (saline) was infused from 2 h before the estradiol stimulus through the time of the anticipated LH surge in the artificial follicular phase of two successive cycles. The plasma cortisol increment produced by infusion was approximately 1.5 times greater than maximal concentrations seen during infusion of endotoxin, which is a model of immune/inflammatory stress. In experiment 1, half of the ewes received vehicle in the first cycle and cortisol in the second; the others were treated in reverse order. All ewes responded with an LH surge. Cortisol delayed the LH surge and reduced its amplitude, but both effects were observed only in the second cycle. Experiment 2 was modified to provide better control for a cycle effect. Four treatment sequences were tested (cycle 1-cycle 2): vehicle-vehicle, cortisol-cortisol, vehicle-cortisol, cortisol-vehicle. Again, cortisol delayed but did not block the LH surge, and this delay occurred in both cycles. Thus, an elevation in plasma cortisol can interfere with the positive feedback action of estradiol by delaying and attenuating the LH surge.

Developmental programming: impact of prenatal testosterone excess on pre- and postnatal gonadotropin regulation in sheep.

The goal of this study was to explore mechanisms that mediate hypersecretion of LH and progressive loss of cyclicity in female sheep exposed during fetal life to excess testosterone. Our working hypothesis was that prenatal testosterone excess, by its androgenic action, amplifies GnRH-induced LH (but not FSH) secretion and, thus, hypersecretion of LH in adulthood, and that this results from altered developmental gene expression of GnRH and estradiol (E2) receptors, gonadotropin subunits, and paracrine factors that differentially regulate LH and FSH synthesis. We observed that, relative to controls, females exposed during fetal life to excess testosterone, as well as the nor-aromatizable androgen dihydrotestosterone, exhibited enhanced LH but not FSH responses to intermittent delivery of GnRH boluses under conditions in which endogenous LH (GnRH) pulses were suppressed. Luteinizing hormone hypersecretion was more evident in adults than in prepubertal females, and it was associated with development of acyclicity. Measurement of pituitary mRNA concentrations revealed that prenatal testosterone excess induced developmental changes in gene expression of pituitary GnRH and E2 receptors and paracrine modulators of LH and FSH synthesis in a manner consistent with subsequent amplification of LH release. Together, this series of studies suggests that prenatal testosterone excess, by its androgenic action, amplifies GnRH-induced LH response, leading to LH hypersecretion and acyclicity in adulthood, and that this programming involves developmental changes in expression of pituitary genes involved in LH and FSH release.

Insight into the neuroendocrine site and cellular mechanism by which cortisol suppresses pituitary responsiveness to gonadotropin-releasing hormone.

Stress-like elevations in plasma glucocorticoids rapidly inhibit pulsatile LH secretion in ovariectomized sheep by reducing pituitary responsiveness to GnRH. This effect can be blocked by a nonspecific antagonist of the type II glucocorticoid receptor (GR) RU486. A series of experiments was conducted to strengthen the evidence for a mediatory role of the type II GR and to investigate the neuroendocrine site and cellular mechanism underlying this inhibitory effect of cortisol. First, we demonstrated that a specific agonist of the type II GR, dexamethasone, mimics the suppressive action of cortisol on pituitary responsiveness to GnRH pulses in ovariectomized ewes. This effect, which became evident within 30 min, documents mediation via the type II GR. We next determined that exposure of cultured ovine pituitary cells to cortisol reduced the LH response to pulse-like delivery of GnRH by 50% within 30 min, indicating a pituitary site of action. Finally, we tested the hypothesis that suppression of pituitary responsiveness to GnRH in ovariectomized ewes is due to reduced tissue concentrations of GnRH receptor. Although cortisol blunted the amplitude of GnRH-induced LH pulses within 1-2 h, the amount of GnRH receptor mRNA or protein was not affected over this time frame. Collectively, these observations provide evidence that cortisol acts via the type II GR within the pituitary gland to elicit a rapid decrease in responsiveness to GnRH, independent of changes in expression of the GnRH receptor.

Does cortisol acting via the type II glucocorticoid receptor mediate suppression of pulsatile luteinizing hormone secretion in response to psychosocial stress?

This study assessed the importance of cortisol in mediating inhibition of pulsatile LH secretion in sheep exposed to a psychosocial stress. First, we developed an acute psychosocial stress model that involves sequential layering of novel stressors over 3-4 h. This layered-stress paradigm robustly activated the hypothalamic-pituitary-adrenal axis and unambiguously inhibited pulsatile LH secretion. We next used this paradigm to test the hypothesis that cortisol, acting via the type II glucocorticoid receptor (GR), mediates stress-induced suppression of pulsatile LH secretion. Our approach was to determine whether an antagonist of the type II GR (RU486) reverses inhibition of LH pulsatility in response to the layered stress. We used two animal models to assess different aspects of LH pulse regulation. With the first model (ovariectomized ewe), LH pulse characteristics could vary as a function of both altered GnRH pulses and pituitary responsiveness to GnRH. In this case, antagonism of the type II GR did not prevent stress-induced inhibition of pulsatile LH secretion. With the second model (pituitary-clamped ovariectomized ewe), pulsatile GnRH input to the pituitary was fixed to enable assessment of stress effects specifically at the pituitary level. In this case, the layered stress inhibited pituitary responsiveness to GnRH and antagonism of the type II GR reversed the effect. Collectively, these findings indicate acute psychosocial stress inhibits pulsatile LH secretion, at least in part, by reducing pituitary responsiveness to GnRH. Cortisol, acting via the type II GR, is an obligatory mediator of this effect. However, under conditions in which GnRH input to the pituitary is not clamped, antagonism of the type II GR does not prevent stress-induced inhibition of LH pulsatility, implicating an additional pathway of suppression that is independent of cortisol acting via this receptor.

Sex difference in the suppressive effect of cortisol on pulsatile secretion of luteinizing hormone in sheep.

We tested the hypothesis that there are sex differences in the inhibitory effect of cortisol on pulsatile LH secretion and pituitary responsiveness to GnRH in gonadectomized sheep. In experiment 1, pulsatile LH secretion was examined in gonadectomized ewes and rams infused with either saline, a low (250 microg/kg.h) or a high (500 microg/kg.h) dose of cortisol for 30 h. In experiment 2, direct pituitary actions of cortisol were assessed by monitoring LH pulse amplitude in response to exogenous GnRH in hypothalamo-pituitary disconnected ewes and rams infused with the low dose of cortisol. In experiment 1, the mean (+/-sem) plasma LH concentration was (P<0.05) reduced significantly during cortisol infusion in both sexes, but the effect was greater in rams. In ewes, LH pulse amplitude and frequency were reduced (P<0.05) at the high, but not the low, cortisol dose, whereas total LH output (LH pulse amplitude multiplied by frequency) was reduced (P<0.05) at both doses. In rams, LH pulse frequency and amplitude and total LH output were (P<0.05) reduced significantly at both cortisol doses. In experiment 2, plasma LH concentration and pulse amplitude in response to exogenous GnRH were not affected by infusion of cortisol in either sex. We conclude that gonadectomized rams are more sensitive than gonadectomized ewes to the effects of cortisol to inhibit LH secretion and that sex differences exist in the specific actions of cortisol on LH pulses. The results of experiment 2 suggest that intact hypothalamic input to the pituitary is necessary for cortisol to inhibit pituitary responsiveness to GnRH.

Morphological plasticity in the neural circuitry responsible for seasonal breeding in the ewe.

An increase in the response of GnRH neurons to estrogen negative feedback is responsible for seasonal anestrus in the ewe, but the underlying neural mechanisms remain largely unknown. Neural plasticity may play an important role because the density of synaptic input to GnRH neurons changes with seasons. Moreover, the transition from breeding to anestrous season requires thyroid hormones, which are also required for neuronal development. In the first experiment, we examined whether the decrease in synapses on GnRH neurons is critical for the transition to anestrus by comparing synaptic input in thyroidectomized and thyroid-intact controls, using electron microscopic analysis. Thyroidectomized ewes remained in the breeding season, but the number of synaptic contacts on their GnRH cells was not different from those in thyroid-intact ewes that were anestrus. The next experiment tested whether there was a seasonal change in morphology of the A15 dopaminergic neurons that mediate estrogen negative feedback during anestrus by analyzing synapsin-positive close contacts onto A15 neurons with confocal microscopy. There was a 2-fold increase in these close contacts onto dendrites of A15 neurons in anestrus and a corresponding increase in the length of A15 dendrites at this time of year. The increase in dendritic length was blocked by thyroidectomy, but this procedure did not significantly affect synaptic input to A15 neurons. These results provide initial evidence that the seasonal change in synapses on GnRH neurons is not sufficient for the transition into anestrus but that plasticity of the A15 dopaminergic neurons mediating estrogen negative feedback may contribute to this seasonal alteration.

New insights regarding glucocorticoids, stress and gonadotropin suppression.

This review highlights our recent work investigating the inhibitory effects of acute, physiologic stress-like increases in cortisol on reproductive neuroendocrine activity in sheep, the mechanisms responsible for this suppression, and the relevance of enhanced glucocorticoid secretion to stress-induced inhibition of gonadotropin secretion in this species. Initial studies established that cortisol rapidly suppresses pulsatile luteinizing hormone secretion. In ovariectomized ewes, this inhibition reflects the reduction of pituitary responsiveness to gonadotropin-releasing hormone mediated by the type II glucocorticoid receptor, rather than the suppression in hypothalamic gonadotropin-releasing hormone release. Studies in ovary-intact ewes, however, uncovered an alternative mode of cortisol action. During the follicular phase of the estrous cycle, cortisol reduces luteinizing hormone pulse frequency, most likely via the inhibition of gonadotropin-releasing hormone pulsatility. Recent preliminary evidence in ovariectomized ewes demonstrates increased cortisol secretion is essential for disruption of pulsatile luteinizing hormone secretion in response to a psychosocial stress. Taken together, our observations reveal diverse inhibitory actions of cortisol on gonadotropin secretion and that this glucocorticoid is not only sufficient, but necessary for suppression of reproductive neuroendocrine activity in response to certain types of stress.

Does season alter responsiveness of the reproductive neuroendocrine axis to the suppressive actions of cortisol in ovariectomized ewes?

Season can profoundly influence activity of the hypothalamic-pituitary-adrenal axis and alter reproductive neuroendocrine responsiveness to stress and gonadal steroids. Here we tested the hypothesis that the inhibitory effect of a stress-like increment in plasma concentration of the adrenal steroid cortisol on pulsatile LH secretion varies with season. LH pulse patterns were monitored prior to and during the administration of cortisol in the same seven ovariectomized ewes during three stages of the yearly breeding cycle: breeding season, transition to anestrus, and midanestrus. The elevation in cortisol mimicked the rise in plasma level of cortisol in response to an immune/inflammatory stress. During all three seasons, cortisol acutely suppressed the pulsatile release of LH. This inhibition reflected a marked reduction of LH pulse amplitude and a minimal suppression of LH pulse frequency. Of interest, the suppressive effect of this physiologic increment in cortisol did not vary across seasons. This provides initial evidence that, in ovariectomized ewes, cortisol-induced suppression of pulsatile LH secretion differs from that of gonadal steroids in that it is not profoundly influenced by season.

Endocrine basis for disruptive effects of cortisol on preovulatory events.

Stress activates the hypothalamo-pituitary-adrenal axis leading to enhanced glucocorticoid secretion and concurrently inhibits gonadotropin secretion and disrupts ovarian cyclicity. Here we tested the hypothesis that stress-like concentrations of cortisol interfere with follicular phase endocrine events of the ewe by suppressing pulsatile LH secretion, which is essential for subsequent steps in the preovulatory sequence. Cortisol was infused during the early to midfollicular phase, elevating plasma cortisol concentrations to one third, one half, or the maximal value induced by isolation, a commonly used model of psychosocial stress. All cortisol treatments compromised at least some aspect of reproductive hormone secretion in follicular phase ewes. First, cortisol significantly suppressed LH pulse frequency by as much as 35%, thus attenuating the high frequency LH pulses typical of the preovulatory period. Second, cortisol interfered with timely generation of the follicular phase estradiol rise, either preventing it or delaying the estradiol peak by as much as 20 h. Third, cortisol delayed or blocked the preovulatory LH and FSH surges. Collectively, our findings support the hypothesis that stress-like increments in plasma cortisol interfere with the follicular phase by suppressing the development of high frequency LH pulses, which compromises timely expression of the preovulatory estradiol rise and LH and FSH surges. Moreover, the suppression of LH pulse frequency provides indirect evidence that cortisol acts centrally to suppress pulsatile GnRH secretion in follicular-phase ewes.

Does the type II glucocorticoid receptor mediate cortisol-induced suppression in pituitary responsiveness to gonadotropin-releasing hormone?

Stress-like elevations in plasma cortisol suppress LH pulse amplitude in ovariectomized ewes by inhibiting pituitary responsiveness to GnRH. Here we sought to identify the receptor mediating this effect. In a preliminary experiment GnRH and LH pulses were monitored in ovariectomized ewes treated with cortisol plus spironolactone, which antagonizes the type I mineralocorticoid receptor (MR), or with cortisol plus RU486, which antagonizes both the type II glucocorticoid receptor (GR) and the progesterone receptor (PR). Cortisol alone reduced LH pulse amplitude, but not pulsatile GnRH secretion, indicating that it reduced pituitary responsiveness to endogenous GnRH. RU486, but not spironolactone, reversed this suppression. We next tested whether RU486 reverses the inhibitory effect of cortisol on pituitary responsiveness to exogenous GnRH pulses of fixed amplitude, frequency, and duration. Hourly GnRH pulses were delivered to ovariectomized ewes in which endogenous GnRH pulses were blocked by estradiol during seasonal anestrus. Cortisol alone reduced the amplitude of LH pulses driven by the exogenous GnRH pulses. RU486, but not an antagonist of PR (Organon 31710), prevented this suppression. Thus, the efficacy of RU486 in blocking the suppressive effect of cortisol is attributed to antagonism of GR, not PR. Together, these observations imply that the type II GR mediates cortisol-induced suppression of pituitary responsiveness to GnRH.

Endotoxin inhibits the surge secretion of gonadotropin-releasing hormone via a prostaglandin-independent pathway.

Immune/inflammatory challenges, such as bacterial endotoxin, disrupt gonadotropin secretion and ovarian cyclicity. We previously determined that endotoxin can block the estradiol-induced LH surge in the ewe. Here, we investigated mechanisms underlying this suppression. First, we tested the hypothesis that endotoxin blocks the estradiol-induced LH surge centrally, by preventing the GnRH surge. Artificial follicular phases were created in ovariectomized ewes, and either endotoxin or vehicle was administered together with a surge-inducing estradiol stimulus. In each ewe in which endotoxin blocked the LH surge, the GnRH surge was also blocked. Given this evidence that endotoxin blocks the estradiol-induced LH surge at the hypothalamic level, we began to assess underlying central mechanisms. Specifically, in view of the prior demonstration that prostaglandins mediate endotoxin-induced suppression of pulsatile GnRH secretion in ewes, we tested the hypothesis that prostaglandins also mediate endotoxin-induced blockade of the surge. The prostaglandin synthesis inhibitor flurbiprofen was delivered together with endotoxin and the estradiol stimulus. Although flurbiprofen abolished endotoxin-induced fever, which is a centrally generated, prostaglandin-mediated response, it failed to reverse blockade of the LH surge. Collectively, these results indicate endotoxin blocks the LH surge centrally, suppressing GnRH secretion via a mechanism not requiring prostaglandins. This contrasts with the suppressive effect of endotoxin on GnRH pulses, which requires prostaglandins as intermediates.

Does cortisol inhibit pulsatile luteinizing hormone secretion at the hypothalamic or pituitary level?

Elevations in glucocorticoids suppress pulsatile LH secretion in sheep, but the neuroendocrine sites and mechanisms of this disruption remain unclear. Here, we conducted two experiments in ovariectomized ewes to determine whether an acute increase in plasma cortisol inhibits pulsatile LH secretion by suppressing GnRH release into pituitary portal blood or by inhibiting pituitary responsiveness to GnRH. First, we sampled pituitary portal and peripheral blood after administration of cortisol to mimic the elevation stimulated by an immune/inflammatory stress. Within 1 h, cortisol inhibited LH pulse amplitude. LH pulse frequency, however, was unaffected. In contrast, cortisol did not suppress either parameter of GnRH secretion. Next, we assessed the effect of cortisol on pituitary responsiveness to exogenous GnRH pulses of fixed amplitude, duration, and frequency. Hourly pulses of GnRH were delivered to ewes in which endogenous GnRH secretion was blocked by estradiol. Cortisol, again, rapidly and robustly suppressed the amplitude of GnRH-induced LH pulses. We conclude that, in the ovariectomized ewe, cortisol suppresses pulsatile LH secretion by inhibiting pituitary responsiveness to GnRH rather than by suppressing hypothalamic GnRH release.