Estradiol enables cortisol to act directly upon the pituitary to suppress pituitary responsiveness to GnRH in sheep

We have shown that cortisol infusion reduced the luteinizing hormone (LH) response to fixed hourly GnRH injections in ovariectomized ewes treated with estradiol during the non-breeding season (pituitary-clamp model). In contrast, cortisol did not affect the response to 2 hourly invariant GnRH injections in hypothalamo-pituitary disconnected ovariectomized ewes during the breeding season. To understand the differing results in these animal models and to determine if cortisol can act directly at the pituitary to suppress responsiveness to GnRH, we investigated the importance of the frequency of GnRH stimulus, the presence of estradiol and stage of the circannual breeding season. In experiment 1, during the non-breeding season, ovariectomized ewes were treated with estradiol, and pulsatile LH secretion was restored with i.v. GnRH injections either hourly or 2 hourly in the presence or absence of exogenous cortisol. Experiments 2 and 3 were conducted in hypothalamo-pituitary disconnected ovariectomized ewes in which GnRH was injected i.v. every 2 h. Experiment 2 was conducted during the non-breeding season and saline or cortisol was infused for 30 h in a cross-over design. Experiment 3 was conducted during the non-breeding and breeding seasons and saline or cortisol was infused for 30 h in the absence and presence of estradiol using a cross-over design. Samples were taken from all animals to measure plasma LH. LH pulse amplitude was reduced by cortisol in the pituitary clamp model with no difference between the hourly and 2-hourly GnRH pulse mode. In the absence of estradiol, there was no effect of cortisol on LH pulse amplitude in GnRH-replaced ovariectomized hypothalamo-pituitary disconnected ewes in either season. The LH pulse amplitude was reduced in both seasons in experiment 3 when cortisol was infused during estradiol treatment. We conclude that the ability of cortisol to reduce LH secretion does not depend upon the frequency of GnRH stimulus and that estradiol enables cortisol to act directly on the pituitary of ovariectomized hypothalamo-pituitary disconnected ewes to suppress the responsiveness to GnRH; this effect occurs in the breeding and non-breeding seasons.

Psychosocial stress suppresses attractivity, proceptivity and pulsatile LH secretion in the ewe

Various stressors suppress pulsatile secretion of luteinizing hormone (LH) in ewes and cortisol has been shown to be a mediator of this effect under various conditions. In contrast, little is known about the impact of stress and cortisol on sexual behavior in the ewe. Therefore, we tested the hypothesis that both psychosocial stress and stress-like levels of cortisol will reduce the level of attractivity, proceptivity and receptivity in addition to suppressing LH secretion in the ewe. In Experiment 1, a layered stress paradigm of psychosocial stress was used, consisting of isolation for 4 h with the addition of restraint, blindfold and noise of a barking dog (predator stress) at hourly intervals. This stress paradigm reduced LH pulse amplitude in ovariectomized ewes. In Experiment 2, ovariectomized ewes were artificially induced into estrus with progesterone and estradiol benzoate treatment and the layered stress paradigm was applied. LH was measured and sexual behavior was assessed using T-mazes and mating tests. Stress reduced pulsatile LH secretion, and also reduced attractivity and proceptivity of ewes but had no effect on receptivity. In Experiment 3, ewes artificially induced into estrus were infused with cortisol for 30 h. Cortisol elevated circulating plasma concentrations of cortisol, delayed the onset of estrus and resulted in increased circling behavior of ewes (i.e. moderate avoidance) during estrus and increased investigation and courtship from rams. There was no effect of cortisol on attractivity, proceptivity or receptivity during estrus. We conclude that psychosocial stress inhibits LH secretion, the ability of ewes to attract rams (attractivity) and the motivation of ewes to seek rams and initiate mating (proceptivity), but cortisol is unlikely to be the principal mediator of these effects.

Hypothalamic, pituitary and gonadal regulation of FSH

FSH is a key reproductive hormone involved in the control of ovarian folliculogenesis and steroidogenesis. Multiple regulatory mechanisms govern the release of FSH. These regulatory mechanisms appear to work in concert to modulate the level, pattern and biological potency of circulating FSH, thereby adjusting the gonadotrophic stimulus to meet the challenge of a changing physiological need. This review (i) summarizes various neuroendocrine, autocrine and paracrine mechanisms involved in the control of FSH production and secretion; (ii) identifies possible mechanisms by which LH and FSH are differentially released from the same gonadotrophs; (iii) considers the means by which changes in the quality of the FSH signal are regulated and the implication of such changes; and (iv) emphasizes how large animal models have helped to advance our understanding of FSH control.

Mechanisms for endotoxin-induced disruption of ovarian cyclicity: observations in sheep

This review summarizes a series of experiments that address mechanisms by which endotoxin, a commonly used model of immune/inflammatory challenge, disrupts the oestrous cycle of the ewe. Initial studies in ovariectomized ewes demonstrated that endotoxin inhibits pulsatile LH secretion and that this suppression is achieved in two ways: (i) decreased episodic secretion of GnRH and (ii) reduced pituitary responsiveness to GnRH. These findings led to the hypothesis that the inhibition of pulsatile LH secretion can account for the disruptive effects of endotoxin on the oestrous cycle. Follow-up studies to test this hypothesis revealed that suppression of LH pulsatility during the follicular phase is clearly one means by which endotoxin disrupts the oestrous cycle. However, these studies also provided evidence that endotoxin can impair ovarian follicular responsiveness to gonadotrophin stimulation and inhibit the oestradiol-induced preovulatory LH surge. Collectively, these disturbances in hypothalamo-hypophyseal-ovarian function interrupt the preovulatory chain of events and thereby contribute to disruption of the ovarian cycle in response to this immune/inflammatory challenge.

Seasonal plasticity in the brain: the use of large animal models for neuroanatomical research

Seasonally breeding mammals display an annual cycle of fertility that is associated with both structural neuroplasticity and functional changes in the activity of the GnRH neurones in the brain. Sheep are valuable models for understanding the hormonal and environmental cues that regulate seasonal reproduction, as well as the brain circuitry that underlies this response. As a result of the large size of sheep, we can tightly correlate the anatomy of GnRH cells and their patterns of gene expression with direct measurements of their neurosecretory output. Tract tracing studies have begun to reveal the pathways by which seasonal changes in response to oestradiol negative feedback affect the function of the reproductive system. Electron microscopic studies have shown that synaptic inputs on to ovine GnRH cells undergo marked seasonal rearrangements that are independent of hormonal changes and may reflect the intrinsic seasonality of the brain. Recent work indicates that the polysialylated form of neural cell adhesion molecule (PSA-NCAM), a marker of neuroplasticity, is well positioned anatomically to contribute to seasonal structural and functional alterations. Applying state-of-the-art neuroanatomical techniques to this model has allowed us to delineate the neural pathways responsible for the seasonal shut down of reproduction in sheep, as well as to begin to uncover the cellular mechanisms underlying seasonal neuroplasticity in the adult mammalian brain.

Stress and the control of LH secretion in the ewe

Stress influences the activity of the reproductive system at several sites. One of the most significant effects is at level of the GnRH secretory system to reduce GnRH pulsatility and thus LH pulsatility. This in turn reduces the oestradiol signal that stimulates the GnRH-LH surge in the follicular phase. Three sequential phases have been identified in the induction of the GnRH-LH surge by oestradiol: (i) activation, (ii) transmission and (iii) surge secretion. There is evidence that administration of endotoxin prevents activation but not transmission, hypoglycaemia blocks both activation and transmission, whereas truck transport is effective during the late, but not early, transmission phase. Opioids mediate the suppressive effects of hypoglycaemia on both LH pulsatility and the delayed onset of the LH surge in ewes. The exact neurocircuitry used in sheep is yet to be identified but many of the connections that are proposed as important in rats are present in sheep. Corticotrophin-releasing hormone (CRH) neurones in the paraventricular nucleus that project axons to the median eminence probably do not directly inhibit GnRH, but either afferent or parallel central pathways are involved. New members of the CRH peptide and receptor families have been identified, but roles in the control of reproduction have yet to be determined.

Endotoxin inhibits pituitary responsiveness to gonadotropin-releasing hormone

Immune/inflammatory challenges powerfully suppress reproductive neuroendocrine activity. This inhibition is generally considered to be centrally mediated via mechanisms that regulate GnRH secretion. The present study provides two lines of evidence that bacterial endotoxin, a commonly used model of immune/inflammatory challenge, also acts to inhibit pituitary responsiveness to GNRH: In the first experiment, pulsatile secretion of GnRH into pituitary portal blood and LH into peripheral blood were monitored in ovariectomized ewes treated with a low dose of endotoxin. Although this treatment only marginally suppressed GnRH pulsatile secretion, it markedly disrupted LH pulsatility. In extreme cases, the low dose of endotoxin blocked LH pulses without inhibiting endogenous GnRH pulses, thereby uncoupling GnRH and LH pulsatile suppression. In the second experiment, we tested the hypothesis that endotoxin inhibits pituitary responsiveness to exogenous GnRH pulses. Hourly pulses of GnRH were delivered to ovariectomized ewes in which endogenous GnRH secretion was blocked. Endotoxin suppressed the amplitude of GnRH-induced LH pulses. Together, these observations support the conclusion that endotoxin inhibits pituitary responsiveness to GNRH:

Potential for polysialylated form of neural cell adhesion molecule-mediated neuroplasticity within the gonadotropin-releasing hormone neurosecretory system of the ewe

The GnRH neurosecretory system undergoes marked structural and functional changes throughout life. The initial goal of this study was to examine the neuroanatomical relationship between GnRH neurons and a glycoprotein implicated in neuroplasticity, the polysialylated form of neural cell adhesion molecule (PSA-NCAM). Using dual label immunocytochemistry in conjunction with confocal microscopy, we determined that fibers, terminals, and perikarya of GnRH neurons in adult ovariectomized ewes are intimately associated with PSA-NCAM. In the preoptic area, intense PSA-NCAM immunoreactivity was evident around the periphery of GnRH cell bodies. The second goal of this study was to determine whether PSA-NCAM expression associated with GnRH neurons varies in conjunction with seasonal changes in the activity of the GnRH neurosecretory system in ovariectomized ewes treated with constant release implants of estradiol. During the breeding season when reproductive neuroendocrine activity was enhanced, the expression of PSA-NCAM immunoreactivity associated with GnRH neurons was significantly greater than that during anestrus when GnRH secretion was reduced. This difference, which occurred despite an unchanging ovarian steroid milieu, was not observed in preoptic area structures devoid of GnRH immunoreactivity, suggesting that the seasonal change is at least partially specific to the GnRH system. The close association between PSA-NCAM and GnRH neurons and the change in this relationship in conjunction with seasonal alterations in GnRH secretion provide anatomical evidence that this molecule may contribute to seasonal remodeling of the GnRH neurosecretory system of the adult.

Importance of photoperiodic signal quality to entrainment of the circannual reproductive rhythm of the ewe

An endogenous circannual rhythm drives the seasonal reproductive cycle of a broad spectrum of species. This rhythm is synchronized to the seasons (i.e., entrained) by photoperiod, which acts by regulating the circadian pattern of melatonin secretion from the pineal gland. Prior work has revealed that melatonin patterns secreted in spring/summer entrain the circannual rhythm of reproductive neuroendocrine activity in sheep, whereas secretions in winter do not. The goal of this study was to determine if inability of the winter-melatonin pattern to entrain the rhythm is due to the specific melatonin pattern secreted in winter or to the stage of the circannual rhythm at that time of year. Either a summer- or a winter-melatonin pattern was infused for 70 days into pinealectomized ewes, centered around the summer solstice, when an effective stimulus readily entrains the rhythm. The ewes were ovariectomized and treated with constant-release estradiol implants, and circannual cycles of reproductive neuroendocrine activity were monitored by serum LH concentrations. Only the summer-melatonin pattern entrained the circannual reproductive rhythm. The inability of the winter pattern to do so indicates that the mere presence of a circadian melatonin pattern, in itself, is insufficient for entrainment. Rather, the characteristics of the melatonin pattern, in particular a pattern that mimics the photoperiodic signals of summer, determines entrainment of the circannual rhythm of reproductive neuroendocrine activity in the ewe.

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.

Endocrine alterations that underlie endotoxin-induced disruption of the follicular phase in ewes

Two experiments were conducted to investigate endocrine mechanisms by which the immune/inflammatory stimulus endotoxin disrupts the follicular phase of the estrous cycle of the ewe. In both studies, endotoxin was infused i.v. (300 ng/kg per hour) for 26 h beginning 12 h after withdrawal of progesterone to initiate the follicular phase. Experiment 1 sought to pinpoint which endocrine step or steps in the preovulatory sequence are compromised by endotoxin. In sham-infused controls, estradiol rose progressively from the time of progesterone withdrawal until the LH/FSH surges and estrous behavior, which began approximately 48 h after progesterone withdrawal. Endotoxin interrupted the preovulatory estradiol rise and delayed or blocked the LH/FSH surges and estrus. Experiment 2 tested the hypothesis that endotoxin suppresses the high-frequency LH pulses necessary to stimulate the preovulatory estradiol rise. All 6 controls exhibited high-frequency LH pulses typically associated with the preovulatory estradiol rise. As in the first experiment, endotoxin interrupted the estradiol rise and delayed or blocked the LH/FSH surges and estrus. LH pulse patterns, however, differed among the six endotoxin-treated ewes. Three showed markedly disrupted LH pulses compared to those of controls. The three remaining experimental ewes expressed LH pulses similar to those of controls; yet the estradiol rise and preovulatory LH surge were still disrupted. Our results demonstrate that endotoxin invariably interrupts the preovulatory estradiol rise and delays or blocks the subsequent LH and FSH surges in the ewe. Mechanistically, endotoxin can interfere with the preovulatory sequence of endocrine events via suppression of LH pulsatility, although other processes such as ovarian responsiveness to gonadotropin stimulation appear to be disrupted as well.

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.

Thyroid hormones act primarily within the brain to promote the seasonal inhibition of luteinizing hormone secretion in the ewe

In the ewe, thyroid hormones are required for the seasonal suppression of GnRH and LH secretion, thereby maintaining an annual rhythm in reproductive activity. The primary site of action of thyroid hormones is unknown; in particular, there is no evidence to distinguish a central from a peripheral action. In this study, we test the hypothesis that thyroid hormones can act directly within the brain to promote GnRH/LH seasonal inhibition. Ovariectomized estradiol-treated ewes were thyroidectomized late in the breeding season to prevent seasonal LH inhibition. T4 was then infused for 3 months, either peripherally or centrally. Neuroendocrine reproductive state was monitored by assaying the LH concentration in biweekly blood samples. Central infusion of low dose T4, which restored a physiological concentration of the hormone in cerebrospinal fluid of these thyroidectomized ewes, promoted the neuroendocrine changes that lead to anestrus. The serum LH concentration in these animals fell at the same time as the seasonal LH decline in euthyroid controls. Neither this same T4 dose infused peripherally nor vehicle infused centrally was effective; LH remained elevated, signifying blockade of the mechanism for anestrus. Our results provide strong evidence that thyroid hormones can act directly within the brain to promote seasonal inhibition of neuroendocrine reproductive function in the ewe.

Systemic challenge with endotoxin stimulates corticotropin-releasing hormone and arginine vasopressin secretion into hypophyseal portal blood: coincidence with gonadotropin-releasing hormone suppression

We tested the hypothesis that systemic immune/inflammatory challenge (endotoxin) activates the neuroendocrine stress axis centrally by stimulating the secretion of CRH and arginine vasopressin (AVP) into hypophyseal portal blood. In addition, we examined the temporal association between this stimulation of the stress neuropeptides and the inhibition of pulsatile GnRH and LH secretion. Using alert, normally behaving ewes, hypophyseal portal and peripheral blood were sampled simultaneously at 10-min intervals for 14 h. Temperature was monitored remotely by telemetry at the same interval. Endotoxin (400 ng/kg, i.v. bolus) or saline as a control was injected after a 4-h baseline period. Portal blood was assayed for CRH, AVP, and GnRH, and peripheral blood was assayed for cortisol, progesterone, and LH. In controls, hypophyseal portal CRH and AVP remained just above or at assay sensitivity, and cortisol showed a regular rhythmic pattern unaffected by saline and typical of basal secretion. In contrast, endotoxin potently stimulated CRH and AVP secretion into portal blood, and cortisol and progesterone into peripheral blood. Both CRH and AVP generally rose and fell simultaneously, although the peak of the AVP response was approximately 10-fold greater than that of CRH. The AVP in portal blood was not due to recirculation of hormone secreted into the peripheral circulation by the posterior pituitary gland, because the AVP increase in peripheral blood was negligible relative to the marked increase in portal blood. The stimulation of CRH and AVP coincided with significant suppression of GnRH and LH pulsatile secretion in these same ewes and with the generation of fever. We conclude that endotoxin induces central activation of the neuroendocrine stress axis, stimulating both CRH and AVP release into the hypophyseal portal blood of conscious, normally behaving ewes. This response is temporally coupled to inhibition of pulsatile GnRH and LH release as well as with stimulation of adrenal cortisol and progesterone secretion and generation of fever.

Evidence that the mediobasal hypothalamus is the primary site of action of estradiol in inducing the preovulatory gonadotropin releasing hormone surge in the ewe

Although a neural site of action for estradiol in inducing a LH surge via a surge of GnRH is now well established in sheep, the precise target(s) for estrogen within the brain is unknown. To address this issue, two experiments were conducted during the breeding season using an artificial model of the follicular phase. In the first experiment, bilateral 17beta-estradiol microimplants were positioned in either the medial preoptic area (MPOA) or the mediobasal hypothalamus (MBH), and LH secretion was monitored. An initial negative feedback inhibition of LH secretion was observed in ewes that had estradiol microimplants located in the MPOA (6 of 6 ewes) or caudal MBH in the vicinity of the arcuate nucleus (4 of 4). In contrast, a normal LH surge was only found in animals bearing estradiol microimplants in the MBH (5 of 10). Detailed analysis of estradiol microimplant location with respect to the estrogen receptor-alpha-immunoreactive cells of the hypothalamus revealed that 4 of the 5 ewes exhibiting a LH surge had microimplants located bilaterally within or adjacent to the area of estrogen receptor-expressing cells of the ventromedial nucleus. Two of these ewes exhibited a LH surge without showing any form of estrogen negative feedback. In the second experiment, we used the technique of hypophyseal portal blood collection to monitor GnRH secretion directly at the time of the LH surge induced by estradiol delivered either centrally or peripherally. Central estradiol implants induced the GnRH surge. The duration and mean plasma concentration of GnRH during the surge were not different between animals given peripheral or central MBH estradiol implants. Cholesterol-filled MBH microimplants did not evoke a GnRH surge. We conclude that the ventromedial nucleus is the primary site of action for estradiol in stimulating the preovulatory GnRH surge of the ewe, whereas the MPOA and possibly the caudal MBH are sites at which estrogen can act to inhibit LH secretion. These data provide evidence for the sites within the ovine hypothalamus responsible for mediating the bimodal influence of estradiol on GnRH secretion and suggest that different, and possibly independent, neuronal cell populations are responsible for the negative and positive feedback actions of estradiol.

Importance of the gonadotropin-releasing hormone (GnRH) surge for induction of the preovulatory luteinizing hormone surge of the ewe: dose-response relationship and excess of GnRH

The preovulatory LH surge in the ewe is stimulated by a large sustained surge of GnRH. We have previously demonstrated that the duration of this GnRH signal exceeds that necessary to initiate and sustain the LH surge. The objective of the present study was to determine whether a similar excess exists for amplitude of the GnRH surge. Experiments were performed using an animal model in which GnRH secretion was blocked by progesterone, which in itself does not block the pituitary response to GnRH. To assess the amplitude of the GnRH surge needed to induce the LH surge, we introduced artificial GnRH surges of normal contour and duration but varying amplitudes. Twelve ewes were run through 3 successive artificial follicular phases (total of 36). Six of these artificial follicular phases were positive controls, in which progesterone was removed, the estradiol stimulus was provided, and vehicle was infused. In these control cycles, animals generated endogenous LH surges. In the remaining artificial follicular phases, progesterone was not withdrawn, the estradiol stimulus was provided, and either vehicle (negative control) or GnRH solutions of varying concentrations (experimental) were infused. The circulating GnRH concentrations achieved by infusion were monitored. No LH surges were observed in negative controls, whereas LH surges were induced in experimental cycles provided a sufficient dose of GnRH was infused. A highly significant dose-response relationship was observed between the amplitude of the GnRH surge and both the amplitude of the LH surge and the area under the curve describing the LH response, but no such relationship existed between the amplitude of the GnRH surge and the duration of the LH response. In numerous cases, LH surges similar to those in the positive control animals resulted from infusion of amounts of GnRH estimated to be considerably less than those delivered to the pituitary during the endogenously generated GnRH/LH surge. These findings indicate that, in the ewe, increased GnRH secretion drives the preovulatory LH surge in a dose-dependent fashion, and they provide evidence that the amplitude of the GnRH surge may exceed that needed to generate the LH surge.

Characterization of endocrine events during the periestrous period in sheep after estrous synchronization with controlled internal drug release (CIDR) device

The Controlled Internal Drug Releasing (CIDR) device is an intravaginal pessary containing progesterone (P4) designed for synchronizing estrus in ruminants. To date, there has been little information available on the timing, duration, and quality of the follicular phase after CIDR removal and how those characteristics compare with natural periovulatory endocrine events. The present communication relates the results of methods we used to characterize the endocrine events that followed CIDR synchronization. Breeding-season ewes were given an injection (10 mg) of Lutalyse (PGF2 alpha), and then studied during three consecutive estrous cycles, beginning in the luteal phase after the estrus induced by PGF2 alpha. Cycle 1 estrus was synchronized with 1 CIDR (Type G) inserted for 8 d beginning 10 d after PGF2 alpha. Cycles 2 and 3 were synchronized with two CIDRs for 8 d beginning 10 d after previous CIDR removal. Cycle 1 estrous behavior and serum gonadotropins showed a follicular phase (the interval from CIDR withdrawal to gonadotropin surge [surge] peak) of 38.2 +/- 1.5 hr. Two CIDRs lengthened the interval to 46.2 +/- 1.5 hr (P < 0.0001). At CIDR removal, circulating P4 concentrations were higher in ewes treated with two CIDRs (5.1 +/- 0.3 and 6.4 +/- 0.4 ng/mL in Cycles 2 and 3 vs. 2.7 +/- 0.3 ng/mL in Cycle 1), whereas estradiol concentrations were higher in the 1 CIDR cycle (3.3 +/- 0.5 pg/mL in Cycle 1 vs. 0.5 +/- 0.1, and 0.7 +/- 0.2 pg/mL in Cycles 2 and 3), suggesting that the lower levels of P4 achieved with one CIDR was not sufficient to arrest follicular development. There were no differences in any other endocrine variable. Both one and two CIDR synchronization concentrated surges within a 24-hr period in 92% of the ewes in Cycles 1 and 2. Cycles 3 ewes were euthanized at estimated luteal, early follicular, late follicular, LH surge, and secondary FSH rise timepoints. Endocrine data and ovaries showed that 88% of the ewes synchronized with two CIDRs were in the predicted stage of the estrous cycle. These data demonstrate that the CIDR device applied during the luteal phase effectively synchronizes estrus and results in a CIDR removal-to-surge interval of similar length to a natural follicular phase.

Estradiol requirements for induction and maintenance of the gonadotropin-releasing hormone surge: implications for neuroendocrine processing of the estradiol signal

Two experiments were performed to examine the temporal requirements of the estradiol signal for the GnRH and LH surges in the ewe. Hypophyseal portal and jugular blood (to measure GnRH and LH, respectively) were sampled from ewes set up in an artificial follicular phase model. After progesterone withdrawal to simulate luteolysis, circulating estradiol was raised to a preovulatory level by inserting estradiol implants, which then were removed at different times to vary estradiol signal duration. The objective of the first experiment was to assess the effect of withdrawing estradiol at surge onset on development and maintenance of the GnRH/LH surges. Removal of estradiol, before surge onset, neither altered the LH surge in relation to that induced when the estradiol stimulus was maintained nor affected stimulation of a massive and sustained GnRH surge that outlasted the LH surge by many hours. Continued estradiol treatment, however, did prolong the GnRH surge. In the second experiment, the estradiol stimulus was shortened to test the hypothesis that estradiol need not be present for the whole presurge period to induce GnRH/LH surges. Ewes received estradiol either up to the time of surge onset (21 h) or for periods equivalent to the last 14 h, the last 7 h, or the earliest 7 h of the 21-h signal. Shortening the signal to 14 h did not reduce its ability to stimulate a full GnRH surge, but it did reduce the amplitude of the resultant LH surge. Further shortening of the signal to 7 h, however, produced a mixed response. Most animals (8 of 10 combining the two 7-h groups) did not express GnRH surges. In the two ewes that did, GnRH surge amplitude and duration were again within the range observed with the 21-h estradiol signal, but the LH response was greatly reduced. These results indicate that, once the GnRH/LH surges of the ewe have begun, elevated estradiol is not required for surge maintenance. Development of a full GnRH surge requires elevated estradiol for only a portion of the presurge period. More prolonged exposure to estradiol, however, is needed to maximize pituitary responsiveness to GnRH. Since the estradiol signal for the GnRH surge is relatively short (7-14 h) and temporally located well in advance of the surge itself, these results are consistent with the hypothesis that estradiol is required only to activate the steroid-responsive neuronal elements and not for progression of the signal from these elements to the actual surge process of GnRH release.

The GnRH system of seasonal breeders: anatomy and plasticity

Seasonal breeders, such as sheep and hamsters, by virtue of their annual cycles of reproduction, represent valuable models for the study of plasticity in the adult mammalian neuroendocrine brain. A major factor responsible for the occurrence of seasonal reproductive transitions is a striking change in the responsiveness of gonadotropin-releasing hormone (GnRH) neurons to the inhibitory effects of gonadal steroids. However, the neural circuitry mediating these seasonal changes is still relatively unexplored. In this article, we review recent findings that have begun to define that circuitry and its plasticity in a well-studied seasonal breeder, the ewe. Tract tracing studies and immunocytochemical analyses using Fos and FRAs as markers of activation point to a subset of neuroendocrine GnRH neurons in the MBH as potential mediators of pulsatile GnRH secretion. Because the vast majority of GnRH neurons lack estrogen receptors, seasonal changes in responsiveness to estradiol are most probably conveyed by afferents. Two possible mediators of this influence are dopaminergic cells in the A14/A15 cell groups of the hypothalamus, and estrogen receptor-containing cells in the arcuate nucleus that project to the median eminence. The importance of GnRH afferents in the regulation of season breeding is underscored by observations of seasonal changes in the density of synaptic inputs onto GnRH neurons. Thyroid hormones may participate in this remodeling, because they are important in seasonal reproduction, influence the morphology of other brain systems, and thyroid hormone receptors are expressed within GnRH neurons. Finally, in the hamster, neonatal hypothyroidism affects the number of caudally placed GnRH neurons in the adult brain, suggesting that thyroid hormones may influence development of the GnRH system as well as its reproductive functions in the adult brain.

Endotoxin inhibits the reproductive neuroendocrine axis while stimulating adrenal steroids: a simultaneous view from hypophyseal portal and peripheral blood

This study was designed to test the hypothesis that systemic immune challenge with endotoxin inhibits the reproductive axis centrally by suppressing GnRH pulsatile release into hypophyseal portal blood. Using alert, normally behaving, ovariectomized ewes, we sampled hypophyseal portal blood at 10-min intervals beginning 4 h before and continuing 10 h after endotoxin (400 ng/kg, iv bolus, n = 6) or saline (vehicle, iv, n = 6). Simultaneous jugular samples for measurement of LH, cortisol, and progesterone were taken, and core body temperature was monitored by telemetry. Saline had no effect on any of the parameters in control ewes. In contrast, endotoxin dramatically inhibited the reproductive neuroendocrine axis coincident with stimulating the adrenal steroids, cortisol and progesterone, and elevating body temperature. Mean GnRH collection rate and GnRH pulse amplitude were suppressed (pre- vs. 7 h postendotoxin: collection rate 0.93 +/- 0.31 vs. 0.34 +/- 0.13 pg/min; amplitude 4.13 +/- 1.33 vs. 1.30 +/- 0.41 pg/min per pulse; P < 0.05 and P = 0.01). However, endotoxin did not have a significant effect on GnRH pulse frequency. Along with inhibited GnRH secretion, endotoxin significantly suppressed mean LH concentrations (P = 0.001) and LH pulse amplitude (P < 0.05). In addition, endotoxin suppressed LH pulse frequency (P = 0.01). Coincident with reproductive inhibition, endotoxin stimulated cortisol (P < 0.001), progesterone (P < 0.01), and core body temperature (P < 0.001). We conclude that the suppressive effects of endotoxin on the reproductive axis can be mediated centrally through an inhibition of GnRH and thus LH pulsatile secretion. The coincident stimulation of cortisol, progesterone, and temperature raises the possibility that the central inhibition of the reproductive system may be a consequence of any or all of these activated parameters.

A critical period for thyroid hormone action on seasonal changes in reproductive neuroendocrine function in the ewe

Thyroid hormones are obligatory for the annually recurring termination of reproductive activity in a spectrum of seasonal breeders, including sheep. Previous studies involving thyroidectomy and T4 replacement have led to the hypothesis that, in the ewe, thyroid hormones are necessary only during a limited interval late in the breeding season for the neuroendocrine processes that cause the transition to anestrus. The present series of experiments tested this hypothesis by assessing the influence of thyroidectomy, with or without T4 replacement for specific durations and at different times of the year, on the transition to anestrus. Seasonal alterations in reproductive neuroendocrine activity were monitored by changes in serum LH concentration in ovariectomized ewes bearing s.c. SILASTIC brand silicon tubing implants containing estradiol. Thyroidectomy in mid-December, just before the putative period of thyroid hormone action, prevented the development of the neuroendocrine anestrous season (fall in LH in this animal model). T4 replacement for 90 days beginning in late December (i.e., during the postulated period of thyroid hormone action) overcame the blockade of anestrus, causing LH to fall in ewes thyroidectomized several months previously. The minimal effective duration of exposure to thyroid hormones required for the transition to anestrus was estimated to be 60-90 days. Further, exposure to T4 for 60-90 days beginning in late December was found to be the only time of the year that thyroid hormones were required to maintain seasonal changes in reproductive neuroendocrine activity. Finally, replacement of T4 for 90 days at a different time of year (beginning in August) failed to provoke development of neuroendocrine anestrus in thyroidectomized ewes. These results support the hypothesis that thyroid hormones are necessary only during a limited interval late in the breeding season to promote seasonal reproductive suppression in the ewe. Further, the reproductive neuroendocrine axis is not equally responsive to thyroid hormone at all times of the year. This suggests there is a critical period of responsiveness during which thyroid hormones must be present for anestrus to develop.

Influence of previous photoperiodic exposure on the reproductive response to a specific photoperiod signal in ewes

Two experiments were carried out to determine whether the reproductive response of ewes to a specific photoperiodic signal depends on the time of year that the signal is given, and, if so, whether this dependence can be attributed to the photoperiodic history of the ewes. The aim of experiment 1 was to expand upon previous findings that the reproductive response to a specific photoperiodic challenge in ewes previously maintained on natural photoperiod varies with time of year. Ewes were transferred at one of three times of year from natural photoperiod to photochambers and were immediately exposed to 35 long days (18L:6D) followed by continuous exposure to short days (8.5L: 15.5D); this treatment is referred to as LD-->SD. The three times of year when long days started corresponded to the beginning of the breeding season, the mid-breeding season, and early anestrus (September 21, December 21, March 21, respectively). In ewes exposed to LD-->SD beginning in September, the breeding season and subsequent anestrous season was not altered. In ewes exposed to LD-->SD beginning in December, the transition to anestrus was advanced (p < 0.05) relative to that in controls maintained in simulated natural photoperiod. Subsequently, half of these ewes resumed reproductive activity within 180 days; this occurred 131 +/- 8 days after transfer to short days. In contrast, all ewes exposed to LD-->SD beginning in March resumed reproductive activity; this began 100 +/- 3 days after transfer to short days (p < 0.05 versus December group). The purpose of experiment 2 was to assess the extent to which the difference in response to a photoperiodic challenge can be attributed to photoperiodic history. Ewes were maintained on short days from the winter solstice interrupted with 35 long days from March 21, June 21, September 21, or December 21. The majority of ewes exhibited an onset of reproductive activity after exposure to LD-->SD at the different times of year, and there was no group difference in latency to onset of reproductive activity. The duration of reproductive activity, however, was longer (p < 0.05) in ewes exposed to LD-->SD beginning in June than in the other groups. Thus we conclude that the seasonal difference in the ability of the photoperiodic challenge of long followed by short days to induce reproductive activity in ewes previously maintained outdoors can be attributed, in large measure, to photoperiodic history. Other factors, such as phase of the endogenous rhythm, however, may influence the duration of reproductive activity resulting from this photoperiodic challenge.

Effect of thyroidectomy on maintenance of seasonal reproductive suppression in the ewe

Thyroid hormones are essential to end the breeding season in sheep; however, it is not clear whether thyroid hormone action is limited to initiation of seasonal reproductive suppression in the ewe. The purpose of this study was to determine the influence of thyroid hormones on maintenance of anestrus and onset of the subsequent breeding season in the ewe. In both experiments, ewes were thyroidectomized (THX) either soon after they had completed the transition from the breeding season to anestrus or just before the transition into the breeding season (late anestrus). All ewes were ovariectomized and received constant-release silicone elastomer implants of estradiol. Circulating levels of LH were monitored as an index of seasonal changes in reproductive neuroendocrine activity. After thyroidectomy early in anestrus, timing of the subsequent LH rise, indicative of the next neuroendocrine breeding season, was the same among THX and thyroid-intact ewes. As observed previously, LH remained elevated in ewes THX late in anestrus beyond the time associated with development of anestrus. We conclude therefore that thyroid hormones are not needed to maintain suppression of the reproductive neuroendocrine axis once anestrus has been established, nor do they influence the onset of the subsequent breeding season in the ewe. Rather, thyroid hormone action on seasonal alterations of the reproductive neuroendocrine axis in the ewe is restricted to the changes that cause development of anestrus.

Nonclassical secretory dynamics of LH revealed by hypothalamo-hypophyseal portal sampling of sheep

Continuous withdrawal of hypophyseal portal blood from unrestrained sheep has permitted detailed assessments of the pulsatile secretion of gonadotrophin-releasing hormone (GnRH). To determine if this blood can also be used to characterize the sensory dynamics of pituitary hormones, patterns of luteinizing hormone (LH) in the hypophyseal portal blood of ovariectomized ewes was compared with previous patterns of GnRH and peripheral LH. Hypophyseal portal blood and jugular vein blood were collected every 5 min from six ovariectomized ewes over 6-12 h. Hypophyseal portal blood contained GnRH-associated, sharply defined LH pulses that were much larger than in the periphery. Pulses of secreted LH (hypophyseal portal LH less peripheral LH) showed much faster rates of rise and fall than peripheral and followed pulses of GnRH by an average of 1.26 min. In contrast to pulses in jugular blood, secreted LH pulses often reached a relatively unchanging interpulse nadir-plateau and thereby approached closely algorithm-estimated, extrapolated baselines. The interpulse baseline concentrations of secreted LH (99.6 ng/mL) in hypophyseal portal blood were 31-fold higher than those for jugular LH (3.23 ng/mL). These elevated concentrations also exceeded mean jugular peak concentrations (11.1 ng/mL) and, thus, primarily must represent newly secreted LH. The non-Gaussian profiles of this secreted LH were substantially more complex than the inputs predicted from jugular LH measurements by deconvolution. Furthermore, regardless of the analytical approach, estimations of the mass of secreted LH in each pulse did not correlate well with inputs predicted by deconvolution or Kushler-Brown pulsefit analysis of corresponding pulses in jugular blood (r2 ranging 0.40-0.48). Among alternative explanations is the possibility of heterogeneity in concentrations of GnRH in the portal vessels and variable distribution within the hypophysis. In summary, assay of hypophyseal portal blood obtained directly from the pituitary provides a method for direct assessment of secretory responses to hypothalamic peptides, and thereby serves as an unmatched method for studying the dynamics of LH secretion in vivo. With this approach, LH is revealed to be secreted as complex, non-Gaussian pulses that are far more sharply defined that those in the periphery, include non-GnRH-dependent, secretory components that cannot be predicted by deconvolution and are followed by periods of relatively constant, basal secretion.

Evidence for seasonal plasticity in the gonadotropin-releasing hormone (GnRH) system of the ewe: changes in synaptic inputs onto GnRH neurons

In the Suffolk ewe, seasonal reproductive transitions are due primarily to changes in the responsiveness of the GnRH neurosecretory system to the negative feedback influence of estradiol. As GnRH neurons in the sheep, like those in other mammals, lack estrogen receptors, the influence of estradiol on GnRH neurosecretory activity is probably conveyed via afferents. As a possible structural basis for seasonality, we examined the ultrastructure and synaptic inputs of GnRH neurons in the preoptic area of ewes during the breeding season and seasonal anestrus. GnRH neurons were examined in both ovary-intact ewes and ovariectomized ewes bearing implants that produced constant levels of estradiol to eliminate a changing hormonal milieu as a factor in any seasonal variations. We found that preoptic GnRH neurons in breeding season ewes received more than twice the mean number of synaptic inputs per unit of plasma membrane as GnRH neurons in anestrous animals. Although GnRH dendrites received more synaptic input than GnRH somas, significant seasonal differences were seen in both axodendritic and axosomatic inputs. In contrast, unidentified neurons in the preoptic area showed no significant seasonal changes in their synaptic inputs. Seasonal changes in synaptic inputs onto GnRH neurons were seen in both intact animals and ovariectomized ewes bearing estradiol implants. Consequently, these seasonal alterations are unlikely to be due to changing levels of endogenous sex steroids, but may instead reflect changes in the environmental photoperiod and/or the expression of an endogenous circannual rhythm.

Gonadotropin-releasing hormone requirements for ovulation

This article addresses the role of GnRH in ovulation in the context of two general models of GnRH action--deterministic and permissive. According to the deterministic model, increased GnRH secretion is required to induce the preovulatory LH surge and thus ovulation. The permissive model, in contrast, holds that GnRH secretion need not increase. Rather, the preovulatory LH surge results from enhanced sensitivity of the pituitary gland to GnRH. Studies in rodents and rabbits support the deterministic model whereas evidence in primates suggests that GnRH is permissive. Three lines of evidence are presented to support the conclusion that GnRH plays a deterministic role in sheep. First, a large GnRH surge is secreted together with the preovulatory LH surge. Second, the follicular phase increase in circulating estradiol concentration stimulates this GnRH surge by a positive feedback effect. Third, initiation of the LH surge requires an abrupt increase in GnRH, and maintenance of the LH surge requires continued GnRH support. Collectively, these observations document the fundamental importance of a GnRH surge to ovulation and generation of the estrous cycle of sheep.

Neuroendocrine control of follicle-stimulating hormone (FSH) secretion. I. Direct evidence for separate episodic and basal components of FSH secretion

Continuous sampling of hypophyseal portal blood from unrestrained sheep is providing an unprecedented means for measuring and defining the characteristics of the secretory profile of GnRH. With this method, GnRH has been shown to be released in discrete pulses lasting 5-8 min, with the amplitude of some pulses exceeding 50-fold. Although the relationship between these pulses and the accompanying pulses of LH measured in the jugular vein are unambiguous, the relationship of GnRH pulses to the release of FSH has not been well defined due to the longer clearance of FSH. In previous studies we have shown that hypophyseal portal blood, in addition to serving as a source material for hypothalamic secretions, provides a means to define secretory patterns of pituitary hormones. Because of this we hypothesized that the GnRH-FSH secretory relationship would be easier to define in hypophyseal portal than in jugular vein blood before the secretory products are subjected to dispersion and clearance in circulation. To test this possibility, we monitored hormonal patterns in blood collected at 5-min intervals for 6-12 h from the peripheral and hypophyseal portal circulation of six ovariectomized ewes from a previous study. In contrast to the nonpulsatile pattern of FSH in the peripheral blood, 93% of the GnRH pulses were associated with essentially coincident, discrete pulses of FSH in the portal plasma. Of potentially even greater interest, additional episodes of FSH release were clearly discernible between the GnRH-associated pulses of FSH. As concentrations of peripheral plasma FSH did not reach those in hypophyseal portal plasma, the inter-GnRH episodes of FSH secretion could not result from contaminating peripheral blood. In addition to the episodic mode of secretion, substantial amounts of FSH were found between FSH pulses. This basal component of FSH appeared to be the dominant mode of secretion rather than pulses. The results of this study not only confirm that GnRH pulses lead to pulsatile release of FSH, they also suggest that some other mechanism or factor may be controlling the non-GnRH-associated episodes as well as the basal components of FSH secretion.

How much of the gonadotropin-releasing hormone (GnRH) surge is required for generation of the luteinizing hormone surge in the ewe? Duration of the endogenous GnRH signal

The preovulatory LH surge in the sheep is accompanied by a massive and sustained surge of GnRH. The objective of this study was to examine the duration of the endogenous GnRH signal required to induce and maintain a LH surge of full amplitude and duration. For this purpose, we assessed the effect of a competitive GnRH receptor antagonist (Nal-Glu), administered at various times relative to the LH surge, on the development and progression of the surge pattern of LH release. All studies were conducted in a physiological model for the follicular phase of the estrous cycle (artificial follicular phase). In this model, as during the natural follicular phase, the onset of the LH surge is coincident with the initiation of a massive and sustained rise in GnRH secretion. The experimental approach was validated in a preliminary study by determination that the GnRH antagonist could block the LH surge without compromising GnRH release, as measured in pituitary portal blood. In the main experiment, 25 ewes were run through five successive artificial follicular phases, during which the antagonist was not given (control) or was administered before the LH surge, during its ascending limb, or during the descending limb. Treatment with antagonist before the expected time of the surge prevented the LH surge. Treatment during the ascending limb of the LH surge interrupted the rise in LH and caused a prompt cessation of the surge. Treatment during the descending limb of the LH surge resulted in a faster decline in circulating LH concentrations than in control cycles and caused premature termination of the LH surge. Our results are consistent with the conclusion that development and progression of the preovulatory LH surge in sheep depend upon GnRH stimulation throughout its entire time course.

Time-course of thyroid hormone involvement in the development of anestrus in the ewe

It is well established that the thyroid gland is essential for termination of seasonal reproductive activity in a variety of birds and mammals. In the present study, we examined when during the breeding season the thyroid exerts this effect in female sheep. Previous results suggest that the presence of thyroid hormones during the first 4-6 wk (20-25%) of the breeding season is not sufficient for the neuroendocrine changes that lead to anestrus. We therefore hypothesized that thyroid hormone action is exerted at some point during the latter 75-80% of the breeding season. To test this hypothesis, ewes thyroidectomized early in the breeding season received replacement of thyroxine at various times to create gaps during the mid- to late breeding season when thyroid hormones were absent. We then examined the effect, if any, of this absence on development of seasonal neuroendocrine anestrus. Each ewe was ovariectomized and treated with a constant-release Silastic capsule containing estradiol. Serum concentrations of LH were used as an index of seasonal changes in reproductive neuroendocrine activity. We found that when thyroid hormones were removed for a 60-day period in mid- to late breeding season (from mid-Oct. to late Dec., which is approximately 40% of the entire breeding season), anestrus still developed at the normal time. We conclude, therefore, that thyroid hormones need not be present for much of the breeding season (mid-Sept. through late Dec.) for anestrus to develop in the ewe. Rather, we postulate that thyroid hormones need to be present for only a brief period of time near the end of the breeding season for the neuroendocrine changes that lead to anestrus.

Feedback actions of estradiol on GnRH secretion during the follicular phase of the estrous cycle

The pattern of GnRH secretion during the follicular phase of the estrous cycle of sheep is characterized by an initial marked change in episodic secretion (increased frequency and decreased amplitude) followed by a massive and sustained discharge-the preovulatory GnRH surge. Studies employing a physiological model for the follicular phase have revealed that estradiol has profound and complex feedback effects on GnRH release during the preovulatory period. These include both quantitative effects on pulses (stimulation of frequency, inhibition of amplitude) and qualitative effects (altering pulse shape, stimulating interpulse secretion), in addition to inducing a preovulatory GnRH surge. In stimulating the surge, estradiol causes a highly characteristic change in the minute-to-minute pattern of GnRH in hypophyseal portal blood. Initially, a strictly episodic pattern gives way to one in which GnRH is consistently elevated between pulses. Then, following enhancement of both pulsatile and interpulse components, GnRh becomes extremely high and variable for the majority of the surge. From this point, a regular and well organized pulse pattern is not apparent. The characteristic time course of GnRH at surge onset provides insight into possible mechanistic changes in the GnRH neurosecretory system. Such changes include quantitative and qualitative alterations in the pulse generating mechanism, recruitment of a surge specific population of GnRH neurones, morphologic alterations in GnRH neurones and neighboring cells, and changes in efficiency or route of delivery of GnRH from its site of release to the portal vasculature. These possibilities, while untested and speculative, provide a conceptual framework for future research.

Does estradiol induce the preovulatory gonadotropin-releasing hormone (GnRH) surge in the ewe by inducing a progressive change in the mode of operation of the GnRH neurosecretory system

Estradiol profoundly influences GnRH secretion during the follicular phase of the estrous cycle of the sheep. Estradiol not only regulates the frequency and amplitude of GnRH pulses, but also produces qualitative changes in its pattern of release and induces a sustained GnRH surge during which discrete pulses are not readily evident. In this study, we tested the hypothesis that qualitative changes in GnRH secretion are an integral part of an estradiol-induced change in the mode of operation of the GnRH neurosecretory system that leads to generation of the GnRH surge. This was achieved by the measurement of GnRH in samples of pituitary portal blood collected at 1-min intervals for an 11-h period encompassing the pre- and early surge periods in an artificial follicular phase model. In each of the seven ewes studied, a highly characteristic alteration in the moment to moment pattern of GnRH was observed. This consisted of a progressive change from a strictly episodic pattern of GnRH release to one containing both episodic and nonepisodic components and, after amplification of both components, a period of extremely high values during which individual episodic increases were no longer readily recognizable. Preliminary mathematical modeling of the data suggested that these patterns could be produced by a change in GnRH from a predominantly low to a mixture of low and high amplitude inputs. Similar changes in minute to minute patterns of GnRH secretion were observed during the natural follicular phase. These findings are consistent with the hypothesis that estradiol induces the GnRH surge by altering the mode of neurosecretion, rather than by merely causing quantitative changes in the episodic pattern of release.

Evidence for short or ultrashort loop negative feedback of gonadotropin-releasing hormone secretion

The present studies tested the hypothesis that either short or ultrashort loop negative feedback regulation of gonadotropin-releasing hormone (GnRH) secretion occurs in the ewe. As part of ongoing studies investigating the regulation of follicle-stimulating-hormone secretion, we obtained the unexpected result that a GnRH antagonist (Nal-Glu) may stimulate GnRH secretion. In that experiment, hypophyseal portal blood was collected from five short-term ovariectomized ewes at 5-min intervals for 6 h before and 6 h after intravenous injection of Nal-Glu (10 micrograms/kg body weight). An increase in GnRH pulse frequency in association with the blockade of luteinizing hormone (LH) release was evident in 3 of the 5 animals. To determine if an effect of Nal-Glu on episodic GnRH secretion would be more evident in an animal model in which low-frequency pulses of GnRH prevail, the study was repeated in six ewes in the midluteal phase of the estrous cycle and six ovariectomized ewes bearing estradiol and progesterone implants to suppress GnRH release (artificial luteal model). In luteal-phase ewes, administration of Nal-Glu was followed by an increase in GnRH pulse frequency, pulse size and the secretion of GnRH between pulses, and by a blockade of LH release. In ovariectomized ewes treated with estradiol and progesterone, Nal-Glu administration also stimulated GnRH and inhibited LH secretion. Our finding that the GnRH antagonist stimulated GnRH secretion is consistent with the hypothesis that endogenous GnRH may influence its own release via either a short or ultrashort loop feedback mechanism.

Nature and bioactivity of gonadotropin-releasing hormone (GnRH) secreted during the GnRH surge

Previous studies have demonstrated a neural action of estradiol in inducing a surge of GnRH in the ewe. However, although the GnRH and LH surges began concurrently, the GnRH surge consistently continued well beyond the surge of LH. Three experiments were conducted to test the hypothesis that the termination of the LH surge results from the secretion of a relatively inactive variant of GnRH during the later phases of the GnRH surge. In the first experiment, hypophyseal portal blood collected during an estrogen-induced LH surge was analyzed for GnRH immunoreactivity using two antibodies having specificity for the N- or C-terminal portion of the GnRH molecule. The duration, amplitude, and time course of the GnRH surge were found to be similar irrespective of the antisera used. In a second experiment, a competitive GnRH antagonist was administered at the beginning of the estrogen-induced GnRH/LH surge at a dose capable of blocking pituitary responsiveness for approximately half the duration of the GnRH surge. Antagonist treatment did not result in any change in the time of onset of the GnRH surge, but there was no increase in LH that naturally occurs coincident with onset of the GnRH surge. Rather, a persistent increase in LH secretion was observed during the latter stages of the GnRH surge, indicating that the GnRH molecules secreted at this time were biologically active. Finally, a sensitive and specific ovine pituitary cell bioassay was used to test bioactivity of GnRH in hypophyseal portal blood during different phases of the GnRH surge. GnRH bioactivity in samples collected early in the GnRH surge was greater than that before the onset of the GnRH surge but no greater than that collected during the descending limb of the surge. The results of all three experiments fail to support the hypothesis that the LH surge ends because of a change in the nature of the GnRH secreted. Rather they show that GnRH secreted throughout the surge is biologically active. Thus, the termination of the LH surge before that of the GnRH surge occurs for reasons other than lack of a bioactive GnRH signal.

Endogenous opioid peptides control the amplitude and shape of gonadotropin-releasing hormone pulses in the ewe

This study was designed to test the hypothesis that endogenous opioid peptides (EOP) mediate the negative feedback action of estradiol on GnRH pulse size in breeding season ewes. If this hypothesis is correct, one would predict that an EOP antagonist should increase GnRH pulse size in estradiol-treated ovariectomized (OVX+E), but not in OVX, ewes. We, therefore, examined the effects of naloxone on GnRH pulse profiles in the hypophyseal portal blood of OVX and OVX+E ewes (n = 6/group). Samples were collected every 10 min for 6 h before, 6 h during, and 4 h after naloxone infusion. Estradiol treatment decreased GnRH pulse size and increased GnRH pulse frequency. Naloxone treatment had no effect on GnRH pulse frequency, but significantly increased GnRH pulse size. However, this stimulatory action of naloxone on GnRH pulse size was evident in both OVX and OVX+E ewes. These results are thus not consistent with the hypothesis that EOP mediate the negative feedback action of estradiol. Interestingly, naloxone not only increased GnRH pulse amplitude, but also prolonged the duration of GnRH release during a pulse. To obtain a more precise characterization of the effects of naloxone on the dynamics of GnRH release, pulse profiles in six OVX ewes were examined in hypophyseal portal blood sampled every minute for 4 h before and 4 h during naloxone infusion. Naloxone again increased both the amplitude and duration of GnRH pulses. The increase in GnRH pulse duration was caused by a prolongation of both the plateau and declining phases of the GnRH pulse. In addition to these effects on GnRH release during a pulse, naloxone increased the amount of GnRH collected between pulses in both experiments. The stimulatory effects of naloxone on GnRH release in OVX ewes indicate that the role of EOP in the control of GnRH is not limited to mediating the feedback actions of steroids. In particular, the dramatic effects of naloxone on GnRH pulse shape and interpulse GnRH levels raise the possibility that EOP play an important role in synchronizing the activity of the GnRH neurons involved in episodic GnRH secretion.

Estradiol induces both qualitative and quantitative changes in the pattern of gonadotropin-releasing hormone secretion during the presurge period in the ewe

The follicular phase rise in circulating estradiol causes a suppression of GnRH pulse amplitude and an increase in pulse frequency before stimulation of the preovulatory GnRH surge in the ewe. Studies in which samples were collected at 10-min intervals were found unsuitable to determine whether GnRH secretion is exclusively pulsatile at this time or whether estradiol induces additional qualitative changes in the pattern of GnRH secretion that, combined with the quantitative changes, result in generation of the surge. To address these issues, GnRH was measured in pituitary portal blood and LH in peripheral blood collected at 1- and 10-min intervals, respectively, in ewes receiving no estradiol (n = 5), a luteal phase level of circulating estradiol (n = 5), or a peak follicular phase concentration of estradiol (n = 5). The results provide evidence that, in a dose-dependent manner, estradiol induces the secretion of significant amounts of GnRH between pulses and alters the shape of GnRH pulses by reducing the slope of the rising and falling phases of each pulse. These findings lead to the conclusion that during the presurge period in the ewe, estradiol induces qualitative change in the pattern of GnRH release in addition to stimulating GnRH pulse frequency and reducing pulse amplitude.

Thyroxine is permissive to seasonal transitions in reproductive neuroendocrine activity in the ewe

The observation that circulating thyroxine concentration increases during the breeding season of the ewe, coupled with the finding that thyroid hormones are required for the transition from the breeding season to anestrus in this species, led us to test the hypothesis that the transition to anestrus is driven by a rise in circulating thyroxine. Suffolk ewes were thyroidectomized (THX) late in the anestrous season. Thyroxine was then either not replaced or provided at doses that produced nadir, incremental (simulating the seasonal rise), or mildly hyperthyroid concentrations in serum. Additional ewes remained thyroid-intact. To monitor seasonal changes in reproductive neuroendocrine activity, the ewes were ovariectomized and received implants of constant-release Silastic capsules containing estradiol. Serum concentrations of LH and thyroxine were determined in samples collected twice weekly. In all groups, LH increased in mid-September, signifying that manipulation of thyroid status did not influence onset of the neuroendocrine breeding season. In thyroid-intact controls, LH decreased to low concentrations in mid-January, marking the neuroendocrine transition to anestrus. As expected, LH remained elevated through the end of the study (April) in THX controls not receiving thyroxine, confirming that the neuroendocrine transition to anestrus is dependent on thyroid hormones. The seasonal decrease in LH was seen in all ewes treated with thyroxine. This decrease in LH was neither advanced in mildly hyperthyroid ewes nor delayed in ewes exposed to low serum concentrations of thyroxine. These results lead to the conclusion that the seasonal increase in circulating thyroid hormone in the ewe does not drive the transition from the breeding season to anestrus.(ABSTRACT TRUNCATED AT 250 WORDS)

Circannual alterations in the circadian rhythm of melatonin secretion

To determine if a circadian rhythm known to be functionally related to the reproductive axis varies on a circannual basis, we monitored the circadian secretion of melatonin at monthly intervals for 2 years in four ovariectomized, estradiol-implanted ewes held in a constant short-day photoperiod. Prior to the study, ewes had been housed in a short-day (8L:16D) photoperiod for 4 years and were exhibiting circannual reproductive rhythms as assessed by serum luteinizing hormone (LH) levels. Three of the four sheep showed unambiguous deviations from the expected nocturnal melatonin secretion at two different times approximately 1 year apart. Nocturnal rises in melatonin, which usually last the duration of the dark phase, were delayed by 3-14 h or were missing. Altogether, five of the seven melatonin alterations observed in these three ewes occurred during the nadir of the circannual LH cycle. In the remaining ewe, we did not observe an altered melatonin secretory pattern during this period, and this ewe also failed to show a high amplitude circannual cycle of LH. The results provide evidence for a circannual change in the circadian rhythm of melatonin secretion. This alteration in melatonin secretion may serve as a "functional" change in daylength, and thereby may influence the expression of the circannual reproductive rhythm of sheep held in a fixed photoperiod for an extended time.

A central negative feedback action of thyroid hormones on thyrotropin-releasing hormone secretion

Two experiments were conducted to test the hypothesis that thyroid hormones exert central negative feedback effects on the secretion of TRH from the hypothalamus in the ewe. In the first experiment, we examined the effects of thyroidectomy on the secretion of TRH and TSH. Thyroidectomy was followed by an unambiguous increase in TRH in pituitary portal plasma and TSH in the peripheral circulation. In the second experiment, we tested the effects of T4 replacement to thyroidectomized ewes. T4 replacement reversed the effects of thyroidectomy on TRH and TSH release. Of interest, TRH secretion in thyroidectomized ewes was continuously elevated during the collection, raising the possibility that TRH is secreted continuously, rather than exclusively in a strictly pulsatile manner indicative of phasic discharges synchronized among TRH neurosecretory elements. Collectively, these results suggest that thyroid hormones can act centrally to inhibit TRH (and thus TSH) release in the ewe, and they support the concept that at least part of the negative feedback action of thyroid hormones is exerted at the hypothalamic level.

The thyroid gland is required for reproductive neuroendocrine responses to photoperiod in the ewe

A study was conducted to determine the role of the thyroid gland in three neuroendocrine responses to photoperiod; secretion of melatonin, PRL, and LH. Ewes were thyroidectomized (THX) in midsummer or left thyroid intact, and both groups were moved indoors to artificial short days (8 h of light, 16 h of darkness) for 90 days. Thereafter, a subset of both THX and thyroid-intact ewes was challenged with long days (16 h of light, 8 h of darkness) for 120 days. The other ewes remained in short days so that neuroendocrine responses to the photoperiodic shift could be distinguished from hormonal changes that occur spontaneously. Blood was sampled twice weekly for determination of serum concentrations of LH and PRL and hourly for 48 h surrounding the photoperiodic switch for assay of melatonin. All ewes were ovariectomized and treated with constant release implants of estradiol, so that PRL and LH secretion would not be influenced by alterations in gonadal steroid secretion. There was no effect of thyroidectomy on the circadian pattern of circulating melatonin or on the change in this pattern after the shift from short to long days. Similarly, thyroidectomy did not alter the PRL response to this photoperiodic shift; long days caused PRL to increase whether the thyroid was present or absent. In marked contrast, thyroidectomy blocked the effect of long days on circulating LH, a hormone indicative of reproductive neuroendocrine activity. Specifically, long days induced a precipitous drop in LH in thyroid-intact ewes, but not in THX ewes. Thus, although the thyroid plays an obligatory role in photoperiodic inhibition of the reproductive neuroendocrine axis of ewes, it may not be required for photoneuroendocrine responses in terms of melatonin and PRL secretion. Our findings suggest that in the absence of the thyroid, the reproductive neuroendocrine axis is uncoupled from the photoperiodic influence between the pineal and the GnRH neurosecretory system.

Central regulation of pulsatile gonadotropin-releasing hormone (GnRH) secretion by estradiol during the period leading up to the preovulatory GnRH surge in the ewe

An experiment was conducted to investigate the central regulatory effects of estradiol on GnRH secretion leading up to the preovulatory LH surge in the ewe. Midluteal phase ewes were ovariectomized, treated with steroid implants to maintain luteal phase concentrations of progesterone and estradiol, and fitted with an apparatus for collection of hypophyseal portal blood. After simulated luteolysis (removal of progesterone implants), the ewes were allocated to one of three groups: estradiol withdrawn, estradiol maintained at a luteal phase level, or estradiol raised from a luteal phase level to a peak follicular phase level in two increments. The results demonstrated that during the interval between luteolysis and the preovulatory gonadotropin surge, estradiol exerts a dose-dependent suppression of GnRH secretion from the hypothalamus. This effect reflects a suppression of GnRH pulse size and occurs despite a stimulatory action of estradiol on GnRH pulse frequency. The suppressive effect of estradiol on GnRH secretion, however, was delayed relative to that on LH. We conclude that during the period leading up to the preovulatory surge in the ewe, estradiol acts centrally, reducing GnRH secretion by suppressing GnRH pulse size.

Photoperiodic synchronization of a circannual reproductive rhythm in sheep: identification of season-specific time cues

Seasonal reproduction in the ewe is generated by an endogenous circannual rhythm of reproductive neuroendocrine activity. Exposure to as few as 70 days of photoperiodic information a year is sufficient to synchronize the rhythm. The present study was conducted to identify which portions of the photoperiodic cycle are utilized for synchronization. For this purpose, we used pinealectomized ewes that could not respond reproductively to changes in day length. Selected photoperiodic information was provided via infusion of melatonin, a hormone that provides the neuroendocrine code for day length in this species. Melatonin was delivered according to circadian patterns. The infusion patterns were tailored to mimic those of melatonin secretion in pineal-intact ewes during one of the four seasons: winter, spring, summer, or autumn. The infusions were provided for 90 days a year during each of the three years following pinealectomy. The ewes were ovariectomized and treated with constant-release Silastic capsules containing estradiol; reproductive neuroendocrine activity was monitored by measurement of serum concentrations of LH. In the absence of exogenous melatonin, most (19 of 24) pinealectomized controls exhibited circannual LH cycles that were not in synchrony, indicating that the rhythm was free-running. Melatonin synchronized the rhythm (such that the period was 365 days and the stages of the rhythm were both concurrent among animals and in appropriate phase with the geophysical year), but not all melatonin patterns were equally effective in this regard. The most effective melatonin patterns mimicked those of secretion during summer. Spring and autumn melatonin patterns were less effective, and winter melatonin patterns were ineffective. These results support the concept that there is a seasonal specificity with regard to the photoperiodic cues that synchronize the circannual rhythm of reproductive neuroendocrine activity in the ewe. The rhythm is synchronized most effectively by long-day photoperiodic cues perceived on or around the summer solstice.

Seasonal changes in gonadotropin-releasing hormone secretion in the ewe: alteration in response to the negative feedback action of estradiol

Two experiments were performed to test the hypothesis that there is a seasonal change in the negative feedback effect of estradiol on episodic secretion of GnRH in the ewe. The first experiment identified a specific estradiol treatment (delivered by s.c. Silastic implant) that produced a 50% decrease in the frequency of pulsatile secretion of LH in ovariectomized ewes during the anestrous season. In the second experiment, this estradiol treatment was administered to ovariectomized ewes during the mid-breeding and anestrous seasons. Separate groups of ovariectomized ewes not treated with estradiol were included during each season to test for a seasonal difference in the effect of estradiol on episodic GnRH and LH secretion. Samples of hypophyseal portal blood (for GnRH) and jugular blood (for LH) were obtained at 5-min intervals approximately one month after placement of the estradiol implants. During the breeding season, no effect of estradiol was observed on either the frequency or size of GnRH and LH pulses. During anestrus, however, estradiol produced a profound suppression of the frequency of GnRH and LH pulses, and an increase in GnRH pulse size. No significant seasonal change was observed in the characteristics of GnRH and LH pulses in ovariectomized ewes in the absence of estradiol treatment. These findings lead to the conclusion that there is a marked seasonal change in the negative feedback effect of estradiol on episodic GnRH secretion in the ewe, with the steroid being maximally effective during anestrus.

Characterization and regulation of pre-ovulatory secretion of gonadotrophin-releasing hormone

Using an elegant method for sampling of pituitary portal blood the secretory characteristics of gonadotrophin-releasing hormone (GnRH) during the oestradiol induced surge are studied. It is demonstrated that the neuroendocrine signal for ovulation in the ewe is a surge of GnRH released into the portal blood.

Fos expression during the estradiol-induced gonadotropin-releasing hormone (GnRH) surge of the ewe: induction in GnRH and other neurons

The protein product of the protooncogene c-fos was used as a marker of cellular activation in an attempt to identify those neurons in the preoptic area and hypothalamus that participate in generation of the estradiol-induced surge of GnRH in the ewe. GnRH- and Fos-expressing cells were identified immunocytochemically, and the percent of coexpression was determined in three states: mid-luteal phase (low GnRH release, n = 6); short-term ovariectomy (high episodic GnRH release, n = 6); and induced GnRH surge (high sustained release, n = 8). To induce the GnRH surge, a follicular phase rise in circulating estradiol was simulated in a physiological model for the estrous cycle. Serum LH was measured as an indicator of GnRH release. In the luteal phase, LH was basal, indicating low GnRH secretion. Few cells expressed Fos; these were not GnRH cells. Despite high intermittent GnRH release in short-term ovariectomized ewes, GnRH cells did not express Fos. During the surge (sustained high GnRH release), 41 +/- 8% of GnRH cells expressed Fos; these cells were dispersed throughout the field of distribution of GnRH neurons. In addition to Fos in GnRH-positive cells, many more non-GnRH cells in the preoptic area, anterior hypothalamus, and ventrolateral hypothalamus expressed Fos during the surge than in the luteal phase or after ovariectomy. We suggest that Fos expression in GnRH cells is markedly increased by the positive feedback action of estradiol (surge), whereas short-term removal of negative feedback (ovariectomy) has little, if any, effect, despite increased GnRH release in both states. Since estradiol induces Fos expression in far more than GnRH neurons, our results also suggest that estradiol activates other cells, some of which may be part of a neuronal chain leading to GnRH surge generation, and some of which may be related to other neural actions of estradiol, such as estrous behavior.

Do gonadotropin-releasing hormone, tyrosine hydroxylase-, and beta-endorphin-immunoreactive neurons contain estrogen receptors? A double-label immunocytochemical study in the Suffolk ewe

We used double label immunocytochemistry to examine the brains of ovariectomized ewes and determine whether GnRH, tyrosine hydroxylase-(TH), and beta-endorphin-immunoreactive (IR) neurons contain IR-estrogen receptors (ER). Because of their possible importance as a target for the feedback actions of estradiol, we also examined the presence of nuclear ER in LH-IR cells of the pars tuberalis of the pituitary. Although preoptic GnRH neurons were frequently in close proximity to ER-IR cells, only one out of approximately 1000 GnRH cells examined was found to coexpress ER. In contrast, in the arcuate nucleus and vicinity, 3-5% of TH cells and 15-20% of beta-endorphin cells contained ER. Virtually all LH-IR cells, seen predominantly in the ventral portion of the pars tuberalis, coexpressed ER. These results suggest that in sheep as in rodents, the influence of estradiol on the reproductive neuroendocrine system is not directly mediated by GnRH neurons, but instead is conveyed to GnRH cells via presynaptic afferents. Subsets of TH- and beta-endorphin-IR cells which coexpress ER are two candidates for relaying gonadal steroid signals to GnRH cells. At the level of the pituitary, the feedback actions of estradiol may be expressed directly upon the gonadotroph.

Distribution of estrogen receptor-immunoreactive cells in the sheep brain

The distribution of immunoreactive (IR) estrogen receptor (ER)-containing cells was studied in the brains of adult Suffolk ewes using a rat monoclonal antibody (H222) which recognizes the human estrogen receptor. IR cells were characterized by dense nuclear reaction product, and in some instances, cytoplasmic immunostaining which filled dendrite-like processes. The greatest densities of ER-IR cells were found in the medial preoptic area, the mediobasal hypothalamus, and in a number of limbic system structures (amygdala, bed nucleus of the stria terminalis, lateral septum). ER-IR cells were found at lower densities in several other subregions of the hypothalamus and limbic system, and in the periaqueductal gray of the caudal midbrain. Cytoplasmic ER immunoreactivity was most prominent among ER-IR cells in the ventrolateral-ventromedial nucleus, the bed nucleus of the stria terminalis, the midbrain periaqueductal gray, and some ER-IR cells in the substantia innominata. The distribution of ER-containing cells in the sheep brain closely parallels that seen in other mammals. ER-IR cells are found in sites such as the medial preoptic area and ventrolateral-ventromedial hypothalamus which have been implicated as targets in this species and others for the influence of estradiol on sexual behavior and reproductive neuroendocrine function.

Effect of exogenous thyroxine on timing of seasonal reproductive transitions in ewes

The thyroid gland is necessary for the transition from the breeding season to anestrus in the ewe, but the importance of seasonal changes in thyroxine in this process is not known. The objective of this experiment was to determine the effects of exogenous thyroxine during the breeding season on the time of the transition to anestrus and on the onset of the subsequent breeding season in ewes maintained on natural photoperiod. Cyclic Galway ewes were blocked on the basis of body weight and randomly allocated to the following groups: 1) untreated controls (n = 11), 2) placebo-injected controls (n = 9), and 3) ewes injected daily with thyroxine from the middle of the breeding season until the onset of anestrus (n = 10). Reproductive state was assessed from serum progesterone concentrations determined twice weekly until the breeding season had ended in placebo-injected controls, and until the onset of the subsequent breeding season in untreated and thyroxine-injected ewes. The mean (+/- SEM) date of onset of anestrus did not differ between untreated ewes (April 19 +/- 9 days) and placebo-injected controls (April 15 +/- 9 days; p > 0.05), but occurred earlier in ewes injected with thyroxine (February 28 +/- 6 days; p < 0.001). Thyroxine treatment during the breeding season, however, did not affect the onset of the subsequent breeding season. These data indicate that supplementary thyroxine during the breeding season can advance anestrus and thus shorten the duration of the breeding season.(ABSTRACT TRUNCATED AT 250 WORDS)

Variation in the timing of the reproductive season among breeds of sheep in relation to differences in photoperiodic synchronization of an endogenous rhythm

Photoperiod may regulate seasonal reproduction either by providing the primary driving force for the reproductive transitions or by synchronizing an endogenous reproductive rhythm. This study evaluated whether breed differences in timing of the reproductive seasons of Finnish Landrace (Finn) and Galway ewes are due to differences in photoperiodic drive of the reproductive transitions or to differences in photoperiodic synchronization of the endogenous rhythm of reproductive activity. The importance of decreasing photoperiod after the summer solstice in determining the onset and duration of the breeding season was tested by housing ewes from the summer solstice in either a simulated natural photoperiod or a fixed summer-solstice photoperiod (18 h light:6 h dark; summer-solstice hold). Onset of the breeding season within each breed did not differ between these photoperiodic treatments, but Galway ewes began and ended their breeding season earlier than Finn ewes. The duration of the breeding season was shorter in Galway ewes on summer-solstice hold than on simulated natural photoperiod; duration did not differ between photoperiodic treatments in Finn ewes. The requirement for increasing photoperiod after the winter solstice for initiation of anoestrus was tested by exposing ewes from the winter solstice to either a simulated natural photoperiod or a winter-solstice hold photoperiod (8.5 h light:15.5 h dark). Onset of anoestrus within each breed did not differ between these photoperiodic treatments, but the time of this transition differed between breeds. These observations suggest that genetic differences in timing of the breeding season in Galway and Finn ewes do not reflect differences in the extent to which photoperiod drives the reproductive transitions, because neither breed requires shortening days to enter the breeding season or lengthening days to end it at appropriate times. These findings are consistent with the hypothesis that photoperiod synchronizes an endogenous rhythm of reproductive activity in both breeds and that genetic differences in timing of the breeding season reflect differences in photoperiodic synchronization of this rhythm.

Progesterone blocks the estradiol-induced gonadotropin discharge in the ewe by inhibiting the surge of gonadotropin-releasing hormone

Previous studies indicate an elevation of circulating progesterone blocks the positive feedback effect of a rise in circulating estradiol. This explains the absence of gonadotropin surges in the luteal phase of the menstrual or estrous cycle despite occasional rises in circulating estradiol to a concentration sufficient for surge induction. Recent studies demonstrate estradiol initiates the LH surge in sheep by inducing a large surge of GnRH secretion, measurable in the hypophyseal portal vasculature. We tested the hypothesis that progesterone blocks the estradiol-induced surge of LH and FSH in sheep by preventing this GnRH surge. Adult Suffolk ewes were ovariectomized, treated with Silastic implants to produce and maintain midluteal phase concentrations of circulating estradiol and progesterone, and an apparatus was surgically installed for sampling of pituitary portal blood. One week later the ewes were allocated to two groups: a surge-induction group (n = 5) in which the progesterone implants were removed to simulate luteolysis, and a surge-block group (n = 5) subjected to a sham implant removal such that the elevation in progesterone was maintained. Sixteen hours after progesterone-implant removal (or sham removal), all animals were treated with additional estradiol implants to produce a rise in circulating estradiol as seen in the follicular phase of the estrous cycle. Hourly samples of pituitary portal and jugular blood were obtained for 24 h, spanning the time of the expected hormone surges, after which an iv bolus of GnRH was injected to test for pituitary responsiveness to the releasing hormone. All animals in the surge-induction group exhibited vigorous surges of GnRH, LH, and FSH, but failed to show a rise in gonadotropin secretion in response to the GnRH challenge given within hours of termination of the gonadotropin surges. The surges of GnRH, LH, and FSH were blocked in all animals in which elevated levels of progesterone were maintained. These animals in the surge-block group, however, did secrete LH in response to the GnRH challenge. We conclude progesterone blocks the estradiol-induced gonadotropin discharge in the ewe by acting centrally to inhibit the surge of GnRH secreted into the hypophyseal portal vasculature.

Seasonal changes of gonadotropin-releasing hormone secretion in the ewe

A sustained volley of high-frequency pulses of GnRH secretion is a fundamental step in the sequence of neuroendocrine events leading to ovulation during the breeding season of sheep. In the present study, the pattern of GnRH secretion into pituitary portal blood was examined in ewes during both the breeding and anestrous seasons, with a focus on determining whether the absence of ovulation during the nonbreeding season is associated with the lack of a sustained increase in pulsatile GnRH release. During the breeding season, separate groups (n = 5) of ovary-intact ewes were sampled during the midluteal phase of the estrous cycle and following the withdrawal of progesterone (removal of progesterone implants) to synchronize onset of the follicular phase. During the nonbreeding season, another two groups (n = 5) were sampled either in the absence of hormonal treatments or following withdrawal of progesterone. Pituitary portal and jugular blood for measurement of GnRH and LH, respectively, were sampled every 10 min for 6 h during the breeding season or for 12 h in anestrus. During the breeding season, mean frequency of episodic GnRH release was 1.4 pulses/6 h in luteal-phase ewes; frequency increased to 7.8 pulses/6 h during the follicular phase (following progesterone withdrawal). In marked contrast, GnRH pulse frequency was low (mean less than 1 pulse/6 h) in both groups of anestrous ewes (untreated and following progesterone withdrawal), but GnRH pulse amplitude exceeded that in both luteal and follicular phases of the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)