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Biology of Reproduction 67, 119-125 (2002)
© 2002 Society for the Study of Reproduction, Inc.

Regular Article

Progesterone Blocks the Estradiol-Stimulated Luteinizing Hormone Surge by Disrupting Activation in Response to a Stimulatory Estradiol Signal in the Ewe1

T.A. Richter3,,a, J.E. Robinsona, and N.P. Evans2,,b

a Laboratory of Neuroendocrinology, The Babraham Institute, Cambridge CB2 4AT, United Kingdom b Department of Veterinary Preclinical Studies, University of Glasgow Veterinary School, Glasgow G61 1QH, United Kingdom


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The preovulatory surges of GnRH and LH are activated by increased concentrations of circulating estradiol, but ovulation is blocked when progesterone concentrations are elevated. Although it is has been shown that this action of progesterone is due to a central inhibition of the GnRH surge, the mechanisms that underlie the blockade of the GnRH surge are poorly understood. In this study we investigated whether progesterone can block the estradiol-dependent activation stage of the GnRH surge induction process, and thus prevent expression of the LH surge. The results demonstrated that exposure to progesterone for half or the full duration of the activation stage can prevent the stimulation of LH surges by estradiol (experiment 1), whereas exposure to progesterone midway though a period of estradiol exposure, which in itself is sufficient to activate the surge, did not block the LH surge (experiment 2). These results suggest that progesterone 1) disrupts activation of the surge induction system in response to a stimulatory estradiol signal and 2) does not compromise the ability of animals to respond to a stimulatory estradiol signal applied immediately after progesterone exposure. Because the disruptive effects of activated progesterone in response to estradiol are rapid but transient, it may be that progesterone directly interferes with the activation of estradiol-responsive neural systems to block the GnRH/LH surge.

estradiol, luteinizing hormone, progesterone, steroid hormones


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The preovulatory LH surge in female mammals is a prerequisite for ovulation and occurs in response to a surge of GnRH released from the hypothalamus [1–3]. Expression of the GnRH/LH surge is regulated by the gonadal steroid hormones, estradiol and progesterone [4–6]. In the ewe, the rise in estradiol concentrations that occurs during the follicular phase of the estrous cycle stimulates the preovulatory GnRH/LH surge through positive feedback actions in the hypothalamus [7] and pituitary gland [8]. By contrast, during the luteal phase, progesterone secreted by the corpus luteum strongly inhibits pulsatile GnRH and LH secretion [5, 6, 9] and prevents the positive feedback effects of transient increases in estradiol that accompany waves of follicular growth in the ovary [10, 11]. Experiments have shown that the ability of progesterone to block the estradiol-induced LH surge is due to a central action, whereby progesterone blocks the stimulation of a surge of GnRH [12]. Thus, progesterone effectively controls the estrous/menstrual cycle [13, 14] by either directly inhibiting tonic GnRH secretion or limiting the positive feedback effects of estradiol to the follicular phase of the estrous cycle.

Insight into how progesterone might act to regulate expression of the GnRH surge has been gained by use of a physiological model of the follicular phase in the ewe. Use of this model has led to the proposal of an integrative model for induction of the preovulatory GnRH surge [3]. The model is based on observations that the majority of GnRH neurons do not contain estradiol receptors [15–17] (although evidence has emerged recently to suggest that at least some GnRH neurons in rodents may contain beta estrogen receptors [18]) and that the critical actions of estradiol to induce the surge occur well in advance of GnRH/LH release [3]. According to this model (Fig. 1), induction of the GnRH surge by estradiol can be differentiated into the following three stages: 1) activation, during which the stimulatory estradiol signal is read by an estradiol-receptive neuronal system; 2) transmission, during which the stimulatory estradiol signal is processed and passed from estradiol-responsive cells to the GnRH neurons, either directly or via a system of interneurons; and 3) the GnRH surge itself, which stimulates an LH surge. An important difference between these stages is that only activation is estradiol-dependent [3].



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  		FIG. 1. Theoretical model of the GnRH/LH surge induction process in ewes [3]. The black and gray areas represent a typical LH and GnRH surge, respectively, observed in the artificial follicular phase model. E, Estradiol; X, unknown phenotype

In the context of the aforementioned model, progesterone could act during any or each of the three stages of the surge induction process to block the discharge of GnRH. The simplest means by which progesterone could achieve this would be the result of a direct inhibition of GnRH neurons. Available evidence, however, does not support this hypothesis because GnRH neurons do not express progesterone receptors [17, 19–22]. An alternative hypothesis holds that progesterone may prevent the expression of the GnRH surge by preventing the stimulation of GnRH neurons by estradiol-responsive systems. Such an action would be predicted to occur before the surge itself, during the estradiol-dependent activation stage or the transmission stage, or both. Although the activation stage of the surge induction process does appear to be sensitive to disruption by other agents, such as endotoxin [23], the effects of progesterone on this stage of the surge induction process have not been assessed. Therefore, in the present study, we examined whether 1) progesterone exposure during the entire activation stage, or part of it, could prevent the stimulation of a GnRH surge by estradiol; and 2) administration of progesterone midway though a stimulatory estradiol signal had deleterious effects on either the preceding effects of estradiol or the ability of the surge induction system to respond to subsequent estradiol exposure.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Experimental Model

All animal procedures were conducted in accordance with license regulations (PPL 80/6629) of the United Kingdom Home Office. Experiments were conducted in the breeding season (experiment 1, August; experiment 2, October) with 35 adult Poll Dorset ewes maintained outdoors under standard husbandry conditions at the Babraham Institute, Cambridge, United Kingdom. All studies were conducted using modifications [3] of the artificial follicular phase model of Goodman et al. [24]. Briefly, all ewes were ovariectomized at least 1 mo before experimentation. At the time of ovariectomy ewes received a 1-cm Silastic capsule (s.c.) containing crystalline estradiol, and two intravaginal progesterone-releasing devices (CIDR; InterAg, Hamilton, New Zealand). This treatment paradigm produces serum concentrations (within ~1 h) of estradiol and progesterone that are similar to endogenous levels at mid to late luteal phase (estradiol, ~1 pg/ml; progesterone, 2–4 ng/ml) [1, 3, 25–28]. After 10 days, CIDRs were withdrawn to simulate luteolysis, and an artificial follicular phase was generated 24 h later by treatment with estradiol (4 x 3 cm s.c. implants), which raises serum levels to presurge levels (5–8 ng/ml) [1, 3]. The duration of treatment with incremented estradiol concentrations, hereafter referred to as the "estradiol signal," depended on the experimental designs, which for reasons of clarity, are described in Results.

Secretion of GnRH and LH is known to diverge during certain physiological situations in the ewe (e.g., periods of high-frequency pulsatile hormone secretion and the later portion of the preovulatory GnRH surge). Nevertheless, the occurrence of an LH surge is a reliable marker for the expression of a GnRH surge, because a GnRH surge is a prerequisite for an LH surge [29, 30], and the onset of the GnRH and LH surge is coincident in our model [3].

Analysis of LH Secretion

In all experiments, hourly jugular blood samples were collected over the expected time course of an LH surge (12–38 h after the start of the estradiol signal). Plasma was harvested and frozen until assayed for LH. Plasma was assayed in duplicate 100-µl aliquots using a previously described double-antibody radioimmunoassay technique [31] with LH standard NIDDK-oLH-I-3 and antiserum NIDDK-anti-oLH-1 (a gift from A.F. Parlow, National Institute for Diabetes and Digestive and Kidney Diseases [NIDDK], Torrance, CA). Samples for each experiment were assayed separately. The interassay and intraassay coefficients of variation for the combined assays were 6.0% and 4.8%, respectively. Sensitivity averaged 0.78 ng/ml.

Data Analysis and Statistics

Surges of LH were identified using statistical criteria based on those published by Harris et al. [32] and Battaglia et al. [23]. The onset of a surge was defined as the time corresponding to the first LH sample in a continuous series of at least three hourly samples that were greater than the mean plus 2 SD of the preceding LH values. The end of a surge was defined as the time corresponding to the last LH sample in a continuous series of at least 3 samples that were smaller than the mean minus 2 SD of the presurge (baseline) LH values. In some ewes (5 out of 55), LH values after the surge did not return closely enough to presurge baseline levels to fit the definition of the end of the surge. In these cases, a surge was said to have occurred, but data for the length of the surge were excluded from analyses. The peak of the LH surge was defined as the highest concentration assayed [23]. The surge peak, duration, and time of onset were compared among groups using one-way ANOVA. Incidences of LH surges in different treatment groups were compared using the Fisher exact test. Significance was established at P < 0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Pilot Study 1 Design and Results

The aim of this study was to assess the duration of the estradiol signal required to reliably stimulate an LH surge. An artificial follicular phase was created in 32 ewes with estradiol signals of 3, 6, 9, 12, or 15 h (groups 1–5, respectively).

Luteinizing hormone surges occurred in some animals in response to each of the estradiol signals tested. The incidence of LH surges in response to the different estradiol signals did not differ significantly among groups (group 1, 3 h, 4 out of 6; group 2, 6 h, 5 out of 6; group 3, 9 h, 4 out of 6; group 4, 12 h, 6 out of 7; group 5, 15 h, 6 out of 6 surges; P > 0.05, Fisher exact test; n = 31). All surges were qualitatively similar (P > 0.05, ANOVA; n = 31; data not shown). These data suggest that estradiol signals >=3 h were able stimulate an LH surge.

Experiment 1 design The aims of this study were to determine 1) whether administration of progesterone during the activation stage of the surge induction process could block the LH surge and 2) whether blockade of surge "activation" required progesterone exposure throughout the activation stage. Because a 3-h estradiol signal was sufficient to stimulate a surge in pilot study 1, an even shorter estradiol signal (2 h) was chosen for use in the negative controls, whereas a longer (4 h) signal was selected for use in the remaining groups to stimulate a surge.

Ewes (n = 35) were randomly allocated to 1 of 5 groups and treated as follows: negative controls (group 1, n = 7) received a 2-h estradiol signal, whereas the remaining groups (groups 2–5, n = 7 each) were treated with a 4-h estradiol signal. Animals in group 2 (the positive control) did not receive progesterone treatment during the artificial follicular phase and were therefore expected to exhibit an LH surge in response to estradiol treatment. To test whether treatment with progesterone throughout the activation stage could block the LH surge, animals in group 3 were treated with of progesterone for 4 h, concurrent with the 4-h activational estradiol signal. To assess whether progesterone treatment for a proportion of the activation stage could block the surge, progesterone was administered to groups 4 and 5 for 2 h, either during the first (early activation stage) or second (late activation stage) half of a 4-h estradiol signal, respectively. The design is summarized in Figure 2.



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  		FIG. 2. Experimental design and mean LH concentrations (±SEM) observed in experiment 1. The upper portion of each panel contains a schematic diagram of the steroid hormone treatments, shown relative to the time at which estradiol (solid lines) concentrations were increased. Treatment with progesterone (P) is depicted by shaded bars. The lower portion of each panel contains the LH secretion profiles for each group in response to the treatment depicted above, plotted relative to the start of estradiol treatment. Mean (±SEM) LH data for ewes that did (solid circles) and did not (open circles) surge in response to the experimental treatment are plotted separately. In cases in which two or fewer ewes per group exhibited an LH surge, individual data for these individuals are plotted as solid lines

Experiment 1 results All positive control ewes (7 out of 7) responded with an LH surge. Despite the use of a very short (2 h) estradiol signal, some negative controls (3 out of 7) responded with an LH surge, although the incidence of LH surges in this group was significantly (P < 0.05) smaller than positive controls (P < 0.05, Fisher exact test; n = 14).

Exposure to progesterone during the activation stage significantly reduced the proportion of animals that expressed an LH surge relative to the positive controls (group 2 vs. groups 3–5 combined; 7 out of 7 vs. 2 out of 21 ewes; P < 0.05, Fisher exact test; n = 31). The incidence of LH surges in the 3 groups that were treated with progesterone for different periods during the activation stage did not differ significantly either among these groups or compared with the negative controls (P > 0.05, Fisher exact test; n = 21; Table 1). In animals in which LH surges were not blocked by progesterone (2 out of 21 ewes), mean surge onset, duration, and peak and mean LH levels were similar to those of positive controls (P > 0.05 for each parameter, ANOVA; n = 12 LH surges; Table 1 and Fig. 2).


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  		TABLE 1. Summary of the results of experiment 1. The onset, duration, and peak and mean LH concentrations (mean ± SEM) observed in each group were calculated only for ewes in which an LH surge was detected.*

Pilot Study 2 Design and Results

Before experiment 2, a second pilot study was conducted to assess the ability of a 5-h estradiol signal to simulate an LH surge. This was considered necessary because experiment 2 took place later in the breeding season than experiment 1 and we have data (unpublished) indicating that sensitivity of the GnRH neurosecretory system to the positive feedback effects of estradiol, but not of progesterone, may change in a seasonal manner. This study was conducted in 8 ewes.

Only 2 of the 8 ewes responded with an LH surge (mean onset 17.5 ± 2.5 h; duration 14.0 ± 1.0 h; peak 16.9 ± 5.0 ng/ml; mean LH secretion 5.7 ± 1.3 ng/ml), indicating that a 5-h estradiol signal was not sufficient to stimulate an LH surge in the majority of animals at the time tested.

Experiment 2 design The results of experiment 1 demonstrated that progesterone is able to disrupt the activation of the surge by estradiol. The aim of this experiment was to assess whether this disruptive effect was limited to the period during which progesterone was administered or whether disruption would persist beyond the period of progesterone exposure. The experiment was conducted using the same animals as experiment 1. Based on the results of the second pilot study, negative controls (group 1, n = 7) and positive controls (group 2, n = 8) received 4-h and 8-h estradiol signals, respectively. The experimental design was similar to experiment 1, except that the estradiol signal in the experimental groups was interrupted by a brief period of progesterone exposure (2 h) that effectively split the stimulatory estradiol signal into two 4-h periods. During the 2-h period of progesterone exposure, treatment with estradiol was either maintained (group 3, n = 7) or withdrawn (group 4, n = 7). The experimental design is illustrated in Figure 3.



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  		FIG. 3. Experimental design and LH concentrations observed in experiment 2. The upper portion of each panel contains a schematic diagram of the steroid hormone treatments, shown relative to the time at which estradiol (solid lines) concentrations were increased. Treatment with progesterone is depicted by shaded bars. The lower portion of each panel contains the LH secretion profiles for each group in response to the treatment depicted above, plotted relative to the start of estradiol treatment. Mean (±SEM) LH data for ewes that did (solid circles) and did not (open circles) surge in response to the experimental treatment are plotted separately. In cases in which two or fewer ewes per group did not exhibit an LH surge, individual data are plotted as broken lines

Experiment 2 results No LH surges were seen in negative controls (0 out of 7 ewes). By contrast, a significantly greater proportion of positive control animals (6 out of 8) responded to the 8-h estradiol signal with an LH surge (P < 0.05, Fisher exact test; n = 15; Table 2 and Fig. 3). The proportion of animals that expressed an LH surge when the 8-h estradiol signal was interrupted by progesterone exposure (groups 3 and 4 combined; 8 out of 14 ewes) was significantly greater than the negative controls (P < 0.05, Fisher exact test; n = 21; Table 2 and Fig. 3), but not significantly different from the positive controls (P > 0.05, Fisher exact test; n = 22; Table 2 and Fig. 3). All quantifiable aspects of the LH surges that were observed in the progesterone-treated animals were similar to those of positive controls (P > 0.05 for each parameter, ANOVA; n = 14 LH surges; Table 2; Fig. 3). Although the onset of the surge in progesterone-treated ewes appeared to be delayed relative to positive control ewes (Table 2 and Fig. 3), this was not statistically significant.


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  		TABLE 2. Summary of the results of experiment 2. The onset, duration, and peak and mean LH concentrations (mean ± SEM) observed in each group were calculated only for ewes in which an LH surge was detected.*


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The results of this study provide novel information about the mechanisms by which progesterone and estradiol interact to regulate expression of the preovulatory surges of GnRH and LH. Specifically, this study is the first to demonstrate that progesterone can act rapidly to block the estradiol-induced LH surge by preventing the "activation" of the GnRH surge induction system by estradiol. Furthermore, the results demonstrate that the disruptive effects of progesterone during activation are transient, because exposure to progesterone during activation does not compromise the ability of animals to respond to subsequent estradiol exposure.

The development of a model system in which it is possible to dissect the GnRH surge induction process into its 3 constituent stages (activation, transmission, and hormone release), has provided a means to investigate the interaction between estradiol and progesterone in regulating expression of the surge. The information obtained about the temporal sequence of steroid actions is of interest when the sequence of neuronal events that must occur to generate a GnRH surge are considered. For instance, Harris et al. [33] demonstrated that after the surge induction process had been initiated by estradiol, it was possible for progesterone to block the surge. This indicated that progesterone could act either to prevent the transfer of information between estradiol-responsive neuronal systems involved in surge generation and GnRH neurons or by altering the activity of neurons that are influenced by an estradiol-responsive GnRH-afferent system. The results of the present study demonstrate that in addition to the effects of progesterone on transmission of the surge induction signal, progesterone is extremely effective in preventing activation of the positive feedback system by estradiol. Indeed, the process of activation appears to be so sensitive to disruption by progesterone that exposure for only part (either the first or second half) of the activation stage is as effective in preventing the surge as exposure to progesterone throughout the activation stage.

It should be noted that 2 of 7 ewes that were treated with progesterone early in the activation stage in experiment 1 exhibited an LH surge. Although it could be argued that progesterone did not disrupt the activation stage in these 2 animals, an alternative explanation is that progesterone concentrations might have been below a critical level required to inhibit the activational effects of estradiol. However, significant interindividual differences in progesterone concentrations have not been seen in ewes treated with CIDRs [12, 31–34], and it is therefore more likely that the surges in these 2 animals were the result of the 2 h of estradiol exposure that followed removal of exogenous progesterone. If this were indeed the case, induction of surges in these animals would not be contrary to the contention that progesterone can disrupt activation of the surge. This explanation is supported by the observations that some negative controls treated with a 2-h estradiol signal exhibited an LH surge, although the 2-h signal appeared to be too short to induce a surge in most negative controls. Other studies in ewes have also found interindividual variation in sensitivity to steroids. For example, in a previous study in which the same physiological model was used in the same breed of sheep, it was concluded that an estradiol signal of at least 8 h was required to induce an LH surge [33], whereas the results of the present study and several additional experiments conducted in our laboratory [35] would suggest that the minimum estradiol signal required to stimulate a GnRH/LH surge in some animals might be as short as 2 h.

The results of the second experiment of the present study suggest that the disruptive action of progesterone on activation is transient, because the inhibition of activation action was limited to the period of administration. This conclusion is based on the observation that most ewes exposed to a stimulatory period of estradiol exposure that was split by a period of exposure to progesterone into 2 parts, which by themselves should not have been sufficient to stimulate a surge, were still able to respond with an LH surge. This conclusion contains important information on the mechanisms of action of both progesterone and estradiol in the surge induction process. First, it indicates that processes initiated by the 2 periods of estradiol exposure can be summated to generate a sufficient stimulus to initiate the surge induction process, suggesting that the positive feedback system may remain in an activated state for some time in the absence of estradiol. Second, it indicates that exposure to progesterone does not ablate the processes activated by estradiol before progesterone exposure, nor does it prevent the system from responding to further estradiol exposure.

These observations suggest that the disruption of activation of the positive feedback system by progesterone occurs only in the period during which progesterone is present. It should be noted, however, that not all ewes were able to respond with an LH surge to the estradiol signal that was "split" by progesterone. As discussed above, this might be explained by these ewes having a greater sensitivity to the inhibitory effects of progesterone or a lower sensitivity to the stimulatory effects of estradiol, or a combination of both. Finally, it is possible that the disruptive effects of progesterone are stronger in the absence of estradiol, because the incidence of LH surges was lower when estradiol was removed during the progesterone treatment. This would suggest that estradiol and progesterone interact centrally in some way in determining the sensitivity of the GnRH system to activation.

It is likely that during the activation stage of the surge induction process, estradiol-estradiol receptor complexes are formed in estradiol-receptive cells. This leads to the activation of estrogen-receptive neurons, which probably involves the induction of transcription of estradiol-responsive genes. If progesterone were to disrupt such processes through a direct action (that is, an action that did not require the induction of progesterone-responsive genes), its action would be expected to be rapid, and would be manifested only when the concentrations of both estradiol and progesterone are elevated. Alternatively, if the disruptive action required progesterone-induced gene products, it would be expected to have a relatively slower onset, but it may persist longer. The speed and time course of the disruptive actions of progesterone observed in the present study are consistent with the former possibility; namely, that progesterone may disrupt activation by directly inhibiting estradiol-induced cellular activation. Although we have not addressed the effects of progesterone on estradiol-responsive cells in this study, this hypothesis is supported by in vitro studies in which progesterone has been shown to prevent the transcription of estradiol-responsive genes [36, 37]. In addition, studies of the effects of progesterone on estradiol-induced expression of the protein product of the immediate-early gene, c-fos, which is believed to be involved in the regulation of transcription in several neuronal systems [38], also support this idea. For example, the increase in Fos expression in GnRH and non-GnRH cells that is normally observed when a GnRH surge is induced by estradiol, is absent from rats and ewes that have been treated with progesterone to prevent activation of the surge [39, 40].

An alternative explanation of the effects noted in the present study could be that the progesterone acts at the level of the pituitary gland to prevent LH secretion despite increased GnRH secretion. Although such an action has been shown to contribute to the inhibition of the LH surge by progesterone in rats [41], available evidence would suggest that in sheep, progesterone has only limited effects at the level of the pituitary gland, and that it acts primarily via a central (hypothalamic) mechanism to inhibit GnRH secretion [12, 33]. This is illustrated in the ability of exogenous GnRH to stimulate LH secretion in ewes in which a GnRH/LH surge is blocked by exposure to progesterone [33].

The demonstration in the present study that progesterone can disrupt activation of the GnRH surge is of interest with regard to the mechanisms by which other physiological insults disrupt the estradiol-induced GnRH surge. Previous work has shown that an immune/inflammatory challenge [23] and exposure to ethanol [42] can disrupt the activation stage of the surge-induction process. The activation stage, therefore, may be central to the integration of physiological information from diverse sources about the overall suitability of the environment (internal and external) for expression of the preovulatory surge. However, this does not mean that the induction of the preovulatory surge cannot be stopped at some point after activation, such as early in the transmission stage [33]. In this regard, progesterone appears to both inhibit activation and, if activation has occurred, to block transmission of the stimulatory signal from estradiol-responsive cells to GnRH neurons, hereby providing a 2-tiered protection system that prevents ovulation occurring at inappropriate times, such as during the luteal phase and pregnancy.

In conclusion, the results of this study demonstrate that progesterone blocks the LH surge in ewes by disrupting the reading of the estradiol signal during the activation stage of the GnRH surge induction process. Furthermore, the disruptive effect of progesterone during the activation stage appears to be rapid and transient. That exposure to progesterone at a comparable stage of the GnRH surge induction process in rats [4, 24, 43], women [44], and monkeys [45] also blocks expression of the surge, suggests that disruption of the estradiol-dependent activation of the positive feedback system by progesterone may be common to all spontaneously ovulating species. Consequently, the results of the present study could be interpreted as indicating that this action of progesterone is a fundamental mechanism for regulating the timing of ovulation in female mammals.


   ACKNOWLEDGMENTS
 
We are indebted to Martin White, Andrew Dady, Rachel Forsdike, and James Taylor for assistance with the animals, and to Arthur Davis for assisting with surgery. We are grateful to Dr. A.F. Parlow (NIDDK) for his donation of LH antiserum.


   FOOTNOTES
 
First decision: 1 September 2001.

1 Preliminary findings were presented at the 82nd Annual Meeting of the Endocrine Society (Toronto, Canada), abstract 545. This study was supported by the BBSRC, Wellcome Trust grant (060203). T.A.R. was supported by the Cambridge Commonwealth Trust, NRF (South Africa) and by the trustees of the Elsie Ballot Memorial Scholarship. Back

2 Correspondence: Neil P. Evans, Division of Veterinary Physiology, Department of Veterinary Preclinical Studies, University of Glasgow Veterinary School, Bearsden Road, Glasgow G61 1QH, U.K. FAX: 44 141 330 5797; n.evans{at}vet.gla.ac.uk Back

3 Current address: Department of Physiology and Biophysics, University of Miami Medical School, 1600 NW 10th St., Miami, FL 33136 Back

Accepted: January 30, 2002.

Received: August 7, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Moenter SM, Caraty A, Karsch FJ. The estradiol-induced surge of gonadotropin-releasing hormone in the ewe. Endocrinology 1990; 127:1375-1384[Abstract]
  2. Karsch FJ, Bowen JM, Caraty A, Evans NP, Moenter SM. Gonadotropin-releasing hormone requirements for ovulation. Biol Reprod 1997; 56:303-309[Abstract]
  3. Evans NP, Dahl GE, Padmanabhan V, Thun LA, Karsch FJ. Estradiol requirements for induction and maintenance of the gonadotropin-releasing hormone surge: implications for neuroendocrine processing of the estradiol signal. Endocrinology 1997; 138:5408-5414[Abstract/Free Full Text]
  4. Freeman ME. The neuroendocrine control of the ovarian cycle of the rat. In: Knobil E, Neill JD (eds.), The Physiology of Reproduction. vol. 2. New York: Raven Press; 1994: 613–658
  5. Goodman RL. Neuroendocrine control of the ovine estrous cycle. In: Knobil E, Neill JD (eds.), The Physiology of Reproduction, vol. 2. New York: Raven Press; 1994: 659–709
  6. Karsch FJ, Cummins JT, Thomas GB, Clarke IJ. Steroid feedback inhibition of pulsatile secretion of gonadotropin-releasing hormone in the ewe. Biol Reprod 1987; 36:1207-1218[Abstract]
  7. Moenter SM, Caraty A, Lehman MN, Karsch FJ. Characterization and regulation of preovulatory secretion of gonadotropin-releasing hormone. Hum Reprod 1993; 8:51-56
  8. Clarke IJ, Cummins JT. Direct pituitary effects of estrogen and progesterone on gonadotropin secretion in the ovariectomized ewe. Neuroendocrinology 1984; 39:267-274[Medline]
  9. Robinson JE, Kendrick KM. Inhibition of luteinizing hormone (LH) secretion by progesterone in the ewe—associated changes in the release of GABA and noradrenaline in the preoptic area. J Physiol (Lond) 1991; 438:P251-P251
  10. Souza CJH, Campbell BK, Baird DT. Follicular dynamics and ovarian steroid secretion in sheep during anoestrus. J Reprod Fertil 1996; 108::101-106[Abstract]
  11. Souza CJH, Campbell BK, Baird DT. Follicular waves and concentrations of steroids and inhibin A in ovarian venous blood during the luteal phase of the oestrous cycle in ewes with an ovarian autotransplant. J Endocrinol 1998; 156:563-572[Abstract]
  12. Kasa-Vubu JZ, Dahl GE, Evans NP, Moenter SM, Padmanabhan V, Thun LA, Karsch FJ. Progesterone blocks the estradiol-induced gonadotropin discharge in the ewe by inhibiting the surge of gonadotropin-releasing hormone. Endocrinology 1992; 131:208-212[Abstract]
  13. Karsch FJ, Foster DL, Legan SJ, Ryan KD, Peter GK. Control of the preovulatory endocrine events in the ewe: interrelationship of estradiol, progesterone, and luteinizing hormone. Endocrinology 1979; 105:421-426[Medline]
  14. Karsch FJ, Moenter SM, Caraty A. The neuroendocrine control of ovulation. Anim Reprod Sci 1992; 28:329-341[CrossRef]
  15. Herbison AE, Robinson JE, Skinner DC. Distribution of estrogen receptor-immunoreactive cells in the preoptic area of the ewe: colocalization with glutamic acid decarboxylase but not luteinizing hormone-releasing hormone. Neuroendocrinology 1993; 57:751-759[Medline]
  16. Lehman MN, Karsch FJ. Do gonadotropin-releasing hormone, tyrosine hydroxylase-, and ß-endorphin-immunoreactive neurons contain estrogen receptors? A double-label immunocytochemical study in the Suffolk ewe. Endocrinology 1993; 133:887-895[Abstract]
  17. Skinner DC, Caraty A, Allingham R. Unmasking the progesterone receptor in the preoptic area and hypothalamus of the ewe: no colocalization with gonadotropin-releasing neurons. Endocrinology 2001; 142:573-579[Abstract/Free Full Text]
  18. Herbison AE, Paper JR. New evidence for estrogen receptors in gonadotropin-releasing neurons. Front Neuroendocrinol 2001; 22:292-308[CrossRef][Medline]
  19. Warembourg M, Leroy D, Peytevin J, Martinet L. Estrogen receptor and progesterone receptor-immunoreactive cells are not co-localized with gonadotropin-releasing hormone in the brain of the female mink (Mustela vison). Cell Tissue Res 1998; 291:33-41[CrossRef][Medline]
  20. Herbison AE. Neurochemical identity of neurons expressing estrogen and androgen receptors in sheep hypothalamus. J Reprod Fertil 1995; 271–283
  21. Herbison AE, Horvath TL, Naftolin F, Leranth C. Distribution of estrogen receptor-immunoreactive cells in monkey hypothalamus: relationship to neurons containing luteinizing hormone-releasing hormone and tyrosine hydroxylase. Neuroendocrinology 1995; 61:1-10[CrossRef][Medline]
  22. Herbison AE, Theodosis DT. Localization of estrogen receptors in preoptic neurons containing neurotensin but not tyrosine hydroxylase, cholecystokinin or luteinizing hormone-releasing hormone in the male and female rat. Neuroscience 1992; 50:283-298[CrossRef][Medline]
  23. Battaglia DF, Beaver AB, Harris TG, Tanhehco E, Viguie C, Karsch FJ. Endotoxin disrupts the estradiol-induced luteinizing hormone surge: interference with estradiol signal reading, not surge release. Endocrinology 1999; 140:2471-2479[Abstract/Free Full Text]
  24. Goodman R. A quantitative analysis of the physiological role of estradiol and progesterone in the control of tonic and surge secretion of luteinizing hormone in the rat. Endocrinology 1978; 102:142-150[Medline]
  25. Goodman RL, Bittman EL, Foster DL, Karsch FJ. The endocrine basis of the synergistic suppression of luteinizing hormone by estradiol and progesterone. Endocrinology 1981; 109:1414-1417[Abstract]
  26. Goodman RL, Reichert LE, Legan SJ, Ryan KD, Foster DL, Karsch FJ. Role of gonadotropins and progesterone in determining the preovulatory estradiol rise in the ewe. Biol Reprod 1981; 25:134-142[Abstract]
  27. van Cleeff J, Karsch FJ, Padmanabhan V. Characterization of endocrine events during the periestrous period in sheep after estrous synchronization with controlled internal drug release (CIDR) device. Domest Anim Endocrinol 1998; 15:23-34[CrossRef][Medline]
  28. van Cleeff J, Karsch FJ, Padmanabhan V. Use of controlled internal drug release (CIDR) for estrous synchronization to control timing of endocrine events during the periestrous period in sheep. Biol Reprod 1996; 54:384-393
  29. Evans NP, Dahl GE, Caraty A, Padmanabhan V, Thun LA, Karsch FJ. 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. Endocrinology 1996; 137::4730-4737[Abstract]
  30. Evans NP, Dahl GE, Caraty A, Padmanabhan V, Karsch FJ. How much of the GnRH surge is required for generation of the LH surge in the ewe?. Biol Reprod 1994; 50:185-185
  31. Niswender GD, Reichert LE Jr, Midgley AR Jr, Nalbandov AV. Radioimmunoassay for bovine and ovine luteinizing hormone. Endocrinology 1969; 84:1166-1173[Medline]
  32. Harris TG, Robinson JE, Evans NP, Skinner DC, Herbison AE. Gonadotropin-releasing hormone messenger ribonucleic acid expression changes before the onset of the estradiol-induced luteinizing hormone surge in the ewe. Endocrinology 1998; 139:57-64[Abstract/Free Full Text]
  33. Harris TG, Dye S, Robinson JE, Skinner DC, Evans NP. Progesterone can block transmission of the estradiol-induced signal for luteinizing hormone surge generation during a specific period of time immediately after activation of the gonadotropin-releasing hormone surge-generating system. Endocrinology 1999; 140:827-834[Abstract/Free Full Text]
  34. Skinner DC, Harris TG, Evans NP. Duration and amplitude of the luteal phase progesterone increment times the estradiol-induced luteinizing: hormone surge in ewes. Biol Reprod 2000; 63:1135-1142[Abstract/Free Full Text]
  35. Robinson JE, Evans NP. Progesterone (P) delays the luteinizing hormone (LH) surge in the ewe by increasing the time taken to ‘read’ a stimulatory estrogen (E) signal. In: Program of the 28th Annual Meeting of the Society for Neuroscience; 1998; Los Angeles, CA. Abstract 244.2
  36. Vignon F, Bardon S, Chalbos D, Rochefort H. Antiestrogenic effect of R5020, a synthetic progestin in human breast cancer cells in culture. J Clin Endocrinol Metab 1983; 56:1124-1130[Abstract]
  37. Meyer ME, Gronemeyer H, Turcotte B, Bocquel MT, Tasset D, Chambon P. Steroid hormone receptors compete for factors that mediate their enhancer function. Cell 1989; 57:433-442[CrossRef][Medline]
  38. Hoffman GE, Smith MS, Verbalis JG. c-Fos and related immediate-early gene products as markers of activity in neuroendocrine systems. Front Neuroendocrinol 1993; 14:173-213[CrossRef][Medline]
  39. Le WW, Attardi B, Berghorn KA, Blaustein J, Hoffman GE. Progesterone blockade of a luteinizing hormone surge blocks luteinizing hormone-releasing hormone Fos activation and activation of its preoptic area afferents. Brain Res 1997; 778:272-280[CrossRef][Medline]
  40. Richter TA, Robinson JE, Evans NP. Progesterone treatment that either blocks or augments the estradiol-induced GnRH/LH surge is associated with different patterns of hypothalamic neural activation. Neuroendocrinology 2001; 73:378-386[CrossRef][Medline]
  41. Attardi B. Facilitation and inhibition of the estrogen-induced luteinizing hormone surge in the rat by progesterone: effects on cytoplasmic and nuclear estrogen receptors in the hypothalamus-preoptic area, pituitary and uterus. Endocrinology 1981; 108:1487-1496[Abstract]
  42. Goebbert A, Klein J, Rabil J, Advis JP. Acute ethanol ingestion and expression of synchronized LH preovulatory surges using a follicular phase model in ewes. In: Program of the 26th Annual meeting of the Society for Neuroscience; 1996; Washington, DC
  43. Brann DW, Mahesh VB. Regulation of gonadotropin secretion by steroid hormones. Front Neuroendocrinol 1991; 12:165-207
  44. Abraham GE, Klaiber EL, Broverman D. Effect of a combination of norethnyodrel and mestranol on plasma luteinizing hormone in normal women. Am J Obstet Gynecol 1969; 104:1038-1042[Medline]
  45. Dierschke DJ, Yamaji T, Karsch FJ, Weick RF, Weiss G, Knobil E. Blockade by progesterone of estrogen-induced LH and FSH release in the rhesus monkey. Endocrinology 1973; 92:1496-1501[Medline]



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