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Biology of Reproduction 60, 158-163 (1999)
©Copyright 1999 Society for the Study of Reproduction, Inc.

Effect of Stress-Like Concentrations of Cortisol on Gonadotroph Function in Orchidectomized Sheep1

C.A. Daley3,a, H. Sakuraia, B.M. Adamsa, and T.E. Adams2,a

a Department of Animal Science, University of California, Davis, California 95616


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The effect of stress-like concentrations of cortisol (C) on the feedback potency of estradiol (E2) was assessed using 32 orchidectomized sheep (wethers) assigned at random to 1 of 4 treatment groups in a 2 x 2 factorial design. Wethers received C (3.6 mg/50 kg per hour; groups 2 and 4) or a comparable volume of C delivery vehicle (groups 1 and 3) as a continuous infusion for 7 days. During the final 48 h of infusion, wethers received E2 (0.3 µg/50 kg/h; groups 3 and 4) or E2 delivery vehicle (groups 1 and 2). The pattern of LH secretion was assessed during a 4-h period of intensive blood collection beginning 44 h after initiation of E2 infusion. Gonadotroph responsiveness (LH secretion induced by GnRH challenge [500 ng, i.v.]) was determined 48 h after E2 delivery was begun. Although the frequency of secretory episodes of LH was not affected (p > 0.05) by infusion of C or E2 alone, LH pulse frequency was significantly decreased in wethers receiving C and E2 in combination. In contrast, neither the magnitude of basal gonadotroph responsiveness nor the extent of E2-dependent augmentation of responsiveness was significantly affected by stress-like concentrations of C. In a second experiment, the effect of C on the magnitude of E2-induced increase in pituitary concentration of GnRH receptor and GnRH receptor mRNA was assessed using 32 additional wethers. Continuous infusion of E2 for 48 h increased (p < 0.05) tissue concentrations of GnRH receptor and GnRH receptor mRNA. Concurrent delivery of C did not affect (p > 0.05) E2-induced increase in GnRH receptor mRNA but significantly reduced the magnitude of the E2-dependent increase in pituitary concentration of GnRH receptor. Collectively, these data indicate that stress-like concentrations of C enhance the negative feedback potency of E2 and reduce estrogen-dependent augmentation of the concentration of GnRH receptor in pituitary tissue.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Stressful stimuli decrease reproductive function and fertility in primates [1, 2], rodents [3], and domestic animals [4–6]. The reduced fertility associated with stress may be causally related to the augmentation of glucocorticoid secretion that is common during stressful events. Indeed, excessive glucocorticoid stimulation decreases secretion of LH in rodents [7], human [8, 9] and nonhuman [10] primates, and domestic animals [6, 11].

Although not fully characterized, the attenuation of reproductive function during glucocorticoid stimulation may be mediated by action of glucocorticoids at hypothalamic and pituitary loci. Glucocorticoid receptors have been identified in GnRH-containing neurons in the hypothalamus [12] and gonadotroph cells of the anterior pituitary [13]. Action at neural loci is suggested by reports that direct administration of glucocorticoid to hypothalamic sites blocks or delays the onset of puberty in rats [14]. In addition, glucocorticoid-induced inhibition of gonadotropin secretion in primates is reversed by circhoral delivery of GnRH [10]. Moreover, glucocorticoids decrease the transcriptional activity of the GnRH promoter in hypothalamic tissue [15]. Conversely, action at hypophyseal loci is suggested by reports that the magnitude of GnRH-induced LH secretion from pituitary cells in culture is reduced during coincubation with glucocorticoid [16–18].

The magnitude of the anti-gonadal response to stress [1,2] or exogenous glucocorticoid [19–21] is markedly influenced by gonadal steroids. Vreeburg and coworkers [19, 20] postulated that stress and excessive glucocorticoid stimulation compromised reproductive activity by enhancing the negative feedback potency of gonadal steroids. In the experiments presented here, we extend the postulate of Vreeburg and examine the interaction between stress-like concentrations of cortisol (C) and physiologic levels of estradiol (E2) in orchidectomized sheep. In these experiments, continuous infusion of C was used to establish a stable serum concentration of C approximating the serum concentration noted in sheep during exposure to moderate stressors, such as transportation or restraint and isolation [22, 23]. Estrogenic effects were assessed by infusing E2 at a rate that established serum concentrations of 2–3 pg/ml. During the breeding season, this concentration of E2 does not affect the mean serum concentration of LH or the episodic character of LH secretion in unstressed wethers [24, 25] or ovariectomized sheep [26], but it does increase steady-state levels of GnRH receptor and GnRH receptor mRNA [27]. We hypothesized that stress-like concentrations of C would increase the negative feedback potency of E2. We also hypothesized that C would block E2-dependent augmentation of gonadotroph responsiveness as well as compromise estrogen-dependent increase in pituitary concentrations of GnRH receptor and GnRH receptor mRNA.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals

The effect of stress-like concentrations of C on gonadotroph function was assessed using orchidectomized sheep (wethers; University of California, Davis, flock) at approximately 6–8 mo of age (body weight = 55.7 ± 1.2 kg). The spring-born lambs were castrated within 2 wk of birth and were housed in an open-sided barn under natural lighting and afforded free access to water and alfalfa pellets supplemented with cereal grains and vitamin and mineral premix. These studies were conducted during October and early November, a period of high reproductive activity in sheep at this latitude (38°N). All experimental procedures involving the use of animals were conducted in accordance with NIH Guidelines and were reviewed and approved by the Animal Use and Care Committee for the University of California, Davis. The castrated male sheep is an effective animal model for study of the effect of stress-like concentrations of C on the feedback potency of E2 at hypothalamic loci because the wether is sensitive to the negative [24], but not the confounding positive [28, 29], feedback effects of E2. E2 also acts at pituitary sites in the wether to increase GnRH receptor expression [30], and, therefore, the same animal model can be used to assess the effect of C on the magnitude of this estrogen-dependent response.

Cannulation

Before experimentation, two polyethylene cannulae (Intramedic PE 190; Clay Adams, Parsippany, NJ) were inserted into the left jugular vein to serve as hormone (C or E2) delivery cannulae. A third cannula was inserted into the contralateral vein and used for blood collection. All cannulae were passed through a protective plastic tubing sheath to the exterior of the animal holding area. Animals were freely mobile at the end of a 1-m lead. The cannulae were inserted 3 days before initiation of treatment to permit acclimation to the conditions of experimentation.

Hormone Delivery

Cannulae for the delivery of C or E2 were connected to syringes placed in Harvard infusion pumps (Model 2265; Harvard Bioscience, South Natick, MA). C (4 mg/ml; Sigma Chemical Co., St. Louis, MO) in 50% ethanol-saline (C delivery vehicle [CV]) and E2 (0.3 µg/ml; Sigma Chemical Co.) in 10% ethanol-saline (E2 delivery vehicle [EV]) were administered by continuous infusion. Controls received comparable volumes of the appropriate vehicle.

Experiment 1

The effect of C on gonadotropin secretion and gonadotroph responsiveness was assessed using 32 wethers assigned at random to 1 of 4 treatment groups (n = 8 wethers/group) in a 2 x 2 factorial design. C (3.6 mg/50 kg/h; groups 2 and 4) or CV (groups 1 and 3) was infused for 7 days. Preliminary studies demonstrated that i.v. delivery of C at this rate resulted in stable serum concentrations of C of 60–80 ng/ml. This serum concentration of C approximates that noted in sheep during exposure to moderate stressors, such as transportation or isolation and restraint [22, 23]. During the final 48 h of infusion, wethers received E2 (0.3 µg/50 kg/h; groups 3 and 4) or EV (groups 1 and 2). This rate of E2 administration establishes a serum concentration of E2 of 2–3 pg/ml [24]. This rate of estrogen delivery does not affect the pattern of LH secretion in nonstressed wethers during the breeding season [24, 25]. Daily blood samples (collection at 0900 h) were collected during the first 5 days of infusion. During the period of E2 infusion, blood samples were collected at 4-h intervals. The amplitude and frequency of secretory pulses of LH were assessed in blood samples collected during a 4-h intensive sampling period (10-min collection interval) beginning 44 h after initiation of E2 infusion. Gonadotroph responsiveness (LH release induced by GnRH challenge [500 ng, i.v.]) was assessed at the end of the infusion period. A GnRH challenge of this magnitude is sufficient to induce approximately half of maximal LH secretion in ram lambs [31] and establishes portal concentrations of GnRH that are in the physiological range [32]. Blood samples were collected at 10-min intervals for 2 h after the GnRH challenge. Blood was allowed to clot on ice, and serum was removed within 24 h of sample collection. Serum samples were rapidly frozen and stored at -20°C for later analysis.

Experiment 2

In this experiment, we examined the effect of stress-like concentrations of C on E2-dependent increase in pituitary concentrations of GnRH receptor and GnRH receptor mRNA and tissue levels of gonadotropins and mRNA encoding the gonadotropin subunits. Thirty-two additional wethers received C and/or E2 by continuous infusion according to the 2 x 2 factorial design described above (n = 8 wethers/group). In nonstressed wethers, i.v. administration of E2 (0.3 µg/50 kg/h) significantly increases tissue concentrations of GnRH receptor and GnRH receptor mRNA [27]. At the end of the 7-day infusion period, animals were stunned by means of a captive bolt pistol and killed by exsanguination. Anterior pituitary tissue was quickly excised and halved by a midsagittal cut, and each half was immediately frozen in liquid nitrogen and stored at -80°C for later analysis.

Endocrine Analysis

Serum and tissue concentrations of LH and FSH, and serum concentrations of E2 were determined using previously validated RIA procedures [33–35]. The LH (NIAMDD-oLH-25) and FSH (NIAMDD-oFSH-RP-1) reference standards were gifts from the National Hormone and Pituitary Program (Baltimore, MD). In all cases, intra- and interassay coefficients of variation were less than 10%. The minimum sensitivity of the LH, FSH, and E2 assays was 0.1 ng/ml, 0.25 ng/ml, and 0.6 pg/ml, respectively.

Serum concentrations of C were determined by RIA using an antibody kindly provided by Dr. B. Lasley (Department of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, CA). The method of antibody development and characterization has been described previously [36]. Briefly, the assay procedure involved extraction of serum with 10 volumes of ethanol. The precipitate was isolated by centrifugation (2000 x g for 15 min), and an aliquot of the supernatant was evaporated to dryness. The aliquot was rehydrated with gel-PBS, and the content of C was determined by RIA. The intra- and interassay coefficients of variation were less than 10%. The minimum sensitivity of the assay was 1 ng/ml.

The affinity and tissue concentration of GnRH receptor were determined by means of a procedure previously described [37, 38]. Tissue concentrations of mRNA encoding the LHß and FSHß subunits and GnRH receptor mRNA were determined using RNase protection assays described previously [38, 39]. A plasmid containing a cDNA insert for the ovine GnRH receptor [40] was kindly provided by Dr. J. Brooks (MRC Reproductive Biology Unit, Edinburgh, UK). Plasmids containing cDNA inserts for the bovine LHß [41] and FSHß [42] subunits were kindly provided by Dr. R. Maurer (Department of Cell Biology&Anatomy, Oregon Health Sciences University, Portland, OR). The sense and antisense cRNAs were generated by in vitro transcription using either T7 RNA or SP6 RNA polymerase and the Riboprobe Gemini System II reagent system (Promega Corp., Madison, WI).

Statistical Analyses

Wethers were assigned to 1 of 4 cells in a 2 x 2 factorial design, with C and E2 status as main effects. Statistical significance of treatments was assessed by ANOVA. Sequential hormone measurements were evaluated using ANOVA for repeated measures [43]. Where significant treatment effects were noted, mean comparisons were made using Duncan's multiple-range procedure. Data are presented in the text, figures, and tables as mean ± SEM.

The frequency and amplitude of secretory episodes of LH were evaluated using the criteria of Goodman and Karsch [44]. The basal level of LH was defined as the serum concentration of LH noted immediately before a pulse of LH secretion. The amplitude of a secretory pulse was defined as the concentration of LH at the peak less the concentration at the preceding nadir. The total LH released, in excess of basal, during the 2-h period after GnRH challenge was taken as a measure of gonadotroph responsiveness.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Serum Concentrations of C

Serum concentration of C in wethers receiving vehicle alone (groups 1 and 3) did not differ from the pretreatment concentration (22 ± 2 ng/ml). Conversely, serum concentration of C was increased to 68 ± 8 ng/ml within 24 h of initiation of infusion of exogenous C (groups 2 and 4). Serum concentrations of C were maintained at this level for the remainder of C delivery, and final serum concentrations of C were 70 ± 6 ng/ml and 24 ± 2 ng/ml in wethers receiving C and CV, respectively.

Serum Concentrations of E2

Continuous infusion of E2 at 0.3 µg/50 kg per hour increased serum concentration of E2 to 2.8 ± 0.1 pg/ml within 4 h of initiation of E2 infusion. Serum concentration of E2 was maintained at this level throughout the remainder of the E2 infusion period (groups 3 and 4). E2 was not evident (< 0.6 pg/ml) in wethers receiving EV (groups 1 and 2).

Serum Concentrations of LH and FSH

Serum concentrations of LH at the beginning of E2 infusion did not differ (p > 0.05) in wethers receiving C or CV alone (4.4 ± 0.5 and 5.4 ± 0.6 ng/ml, respectively). During the final 48 h of infusion, serum concentrations of LH were not affected by administration of C or E2 alone (Fig. 1). In contrast, concurrent infusion of C and E2 decreased (p < 0.05) serum concentration of LH. Indeed, at the end of the infusion period, the serum concentration of LH in wethers receiving C and E2 (2.8 ± 0.4 ng/ml) was significantly reduced relative to the final serum concentration of LH in controls (5.5 ± 0.2 ng/ml) or wethers receiving C or E2 alone (4.9 ± 0.7 and 5.1 ± 0.2 ng/ml, respectively). Conversely, the final serum concentration of FSH noted in controls (7.1 ± 0.8 ng/ml) receiving vehicle alone did not differ (p > 0.05) from values noted in wethers receiving C (7.9 ± 0.8 ng/ml) or E2 (6.8 ± 0.5 ng/ml) alone, or in combination (6.7 ± 0.8 ng/ml) (not shown).



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  		FIG. 1. Serum concentrations of LH in wethers during the final 48 h of a 7-day infusion. Sixteen wethers received either C (circles) or a comparable volume of CV (triangles) during the 7-day infusion period. Eight wethers from each of the C and CV infusion groups received E2 (solid symbols) or EV (open symbols) during the final 48 h of infusion.

Pattern of LH Secretion

Although the frequency of secretory episodes of LH in controls did not differ (p > 0.05) from pulse frequency in wethers receiving C or E2 alone, LH pulse frequency was significantly reduced in animals receiving C and E2 concurrently (Table 1). The amplitude of secretory episodes of LH was not affected by i.v. delivery of C or E2 alone or in combination. However, concentrations of LH were depressed (p < 0.05) by infusion of C or E2 alone compared to basal levels. This reduction was significantly enhanced (p < 0.05) by concurrent administration of E2 and C.


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  		TABLE 1. Effect of continuous infusion of C or a comparable volume of CV for 7 days on the pattern of LH secretion in wethers receiving E2 or EV during the final 48 h of infusion.

Gonadotroph Responsiveness

Although the magnitude of total GnRH-induced LH secretion did not differ (p > 0.05) between controls receiving vehicle alone (0.38 ± 0.05 µg/ml/2 h) and wethers receiving stress-like concentrations of C (0.58 ± 0.13 µg/ml/2 h), gonadotroph responsiveness was significantly increased (p < 0.05) after 48 h of infusion of E2 alone (0.73 ± 0.14 µg/ml/2 h) or in combination (1.18 ± 0.17 µg/ml/2 h) with C (Fig. 2).



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  		FIG. 2. Effect of C and/or E2 on the magnitude of change in LH secretion (increase above basal) induced by exogenous GnRH (500 ng, i.v.) in wethers. Sixteen wethers received either C (circles) or a comparable volume of CV (triangles) during a 7-day infusion period. Eight wethers from each of the C and CV infusion groups received E2 (closed symbols) or EV (open symbols) during the final 48 h of infusion. Gonadotroph responsiveness (LH secretion induced by GnRH challenge) was assessed at the end of the 7-day infusion period.

Pituitary Concentrations of GnRH Receptor and GnRH Receptor mRNA

Pituitary concentrations of GnRH receptor and GnRH receptor mRNA in wethers receiving E2 were significantly increased (p < 0.05) relative to the tissue levels of GnRH receptor and receptor mRNA noted in controls receiving vehicle alone (Fig. 3). Although continuous infusion of C did not significantly affect (p > 0.05) basal concentrations of GnRH receptor or GnRH receptor mRNA, the E2-induced augmentation of tissue concentration of GnRH receptor was significantly reduced (p < 0.05) in wethers receiving C and E2 in combination. In contrast, E2-induced augmentation of the steady-state concentration of GnRH receptor mRNA was not compromised (p > 0.05) by concurrent infusion of C.



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  		FIG. 3. Effect of C and/or E2 on steady-state concentrations of GnRH receptor and GnRH receptor mRNA in pituitary tissue of wethers. Wethers received either C (n = 16) or a comparable volume of CV (control; n = 16) during a 7-day infusion period. During the final 48 h of infusion, 8 wethers from each of the C and control infusion groups received concurrent E2 or EV (vehicle) as a continuous infusion. Anterior pituitary tissue was collected at the end of the infusion period. Tissue concentrations of GnRH receptor or GnRH receptor mRNA that do not share a common letter designation differed significantly (p < 0.05).

Pituitary Concentrations of Gonadotropins and Gonadotropin Subunit mRNA

As noted in Table 2, continuous administration of C alone, or in combination with E2, significantly increased (p < 0.05) pituitary stores of LH and tended to increase (p < 0.10) tissue concentrations of FSH and FSHß mRNA.


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  		TABLE 2. Effect of continuous infusion of C or a comparable volume of CV for 7 days on pituitary tissue concentrations of LH and FSH and steady-state concentration of LHß and FSHß mRNA in wethers receiving E2 or EV during the final 48 h of the infusion period.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The results presented here demonstrate that while neither C nor E2 alone affected LH pulse frequency, episodic release of LH was significantly reduced in animals receiving C and E2 in combination. These observations indicate that the threshold of E2 required to induce the negative feedback response is reduced in wethers during prolonged exposure to stress-like levels of C and suggests that glucocorticoids enhance the negative feedback potency of E2. The alternate hypothesis, that E2 increases the feedback potency of C, seems less likely in light of our observation that high concentrations of C alone are unable to significantly alter the pattern of LH secretion. In contrast, a modest increase in the serum concentration of E2 markedly suppresses LH secretion [24]. In addition, the feedback potency of E2 is altered by environmental stimuli such as photoperiod [24] and nutritive status [25]. The data presented here indicate that stress, or stress-like concentrations of C, may have a similar effect. Moreover, these observations are consistent with the glucocorticoid-dependent increase in the negative feedback potency of gonadal steroids noted in rodents [19, 20].

In contrast to the observations reported here, Phillips and Clarke [45] noted that LH pulse frequency in E2-treated ovariectomized sheep was not affected by twice-daily administration of dexamethasone. These disparate observations may reflect a difference in animal models. Alternatively, the nature and amount of the exogenous glucocorticoid and the pattern of administration differed between studies and may account for the difference in experimental results.

The marked reduction in LH pulse frequency during combined treatment with C and E2 suggests that the steroids are acting at hypothalamic sites to decrease the activity of the GnRH pulse-generating center. This is consistent with recent reports that endotoxin-induced stress in female sheep [46] and rats [47] leads to a marked reduction in GnRH secretion and attenuated activity of GnRH-containing neurons in hypothalamic nuclei. In addition, insulin-induced hypoglycemia is associated with a reduction in multiunit electrical activity in the mediobasal hypothalamus of intact female rhesus monkeys [1]. Interestingly, the hypoglycemia-induced decrease in this measure of the activity of the GnRH pulse generator is only evident in ovariectomized monkeys after administration of supplemental E2, indicating that E2 and stress act in concert to influence hypothalamic function. Sensitivity to physical stressors also varies through the menstrual cycle, with sensitivity most acute during the follicular phase and markedly reduced during the luteal phase [2]. Collectively, these observations indicate that the hypothalamic centers controlling GnRH secretion are influenced by stress. Moreover, this response to stress seems accentuated in an estrogen-rich endocrine environment. Our observations suggest that stress-like concentrations of C may have a similar effect.

In contrast to the effect on feedback potency, our studies demonstrate that basal gonadotroph responsiveness is not compromised in wethers during extended infusion of stress-like concentrations of C. Although gonadotroph responsiveness is attenuated during the acute response to stress [48] or exogenous glucocorticoid stimulation [7, 49], this response seems transient. Indeed, gonadotroph responsiveness is not impaired in rats during repetitive and persistent restraint stress [50]. Similarly, extended administration of exogenous glucocorticoid does not significantly decrease gonadotroph responsiveness in pigs [11], sheep [45, 51], rats [52], or primates [53]. Moreover, the recent report of Turgeon and coworkers [54] indicates that GnRH-dependent secretion of LH from a clonal gonadotroph cell line is significantly augmented during coincubation with dexamethasone. These observations are consistent with the results noted here and, collectively, indicate that prolonged exposure to stress-like concentrations of glucocorticoid does not significantly decrease gonadotroph responsiveness.

Tissue concentration of GnRH receptor is generally considered to be one of several key determinants of gonadotroph responsiveness [55, 56]. That stress-like concentrations of C did not significantly affect gonadotroph responsiveness or the concentrations of GnRH receptor or GnRH receptor mRNA in pituitary tissue is consistent with this postulate. Although acute stress reduced the steady-state concentration of GnRH receptor mRNA in female rats [47], more extended administration of glucocorticoid did not decrease tissue concentrations of GnRH receptor [57, 58] or receptor mRNA [54]. Surprisingly, E2-induced augmentation of GnRH receptor was blocked in wethers receiving stress-like levels of C. In contrast, the estrogen-dependent increase in steady-state concentration of GnRH receptor mRNA was not impaired by concurrent administration of C. The disparity between tissue levels of GnRH receptor and GnRH receptor mRNA suggests that translation, but not transcription, is impeded in animals receiving stress-like concentrations of C. Alternatively, glucocorticoids may selectively increase the rate of degradation of the GnRH receptor.

Although the E2-dependent increase in the tissue concentration of GnRH receptor is blocked by stress-like concentrations of C, persistent exposure to C does not diminish the increase in gonadotroph responsiveness induced by E2. This dichotomy between GnRH receptor concentrations and gonadotroph responsiveness during concurrent administration of C and E2 suggests that C and/or E2 may act at loci in the secretory pathway distal to the receptor. Our observation that C tended to increase concentrations of LH and FSH and FSHß mRNA in pituitary tissue is consistent with effects of glucocorticoids in rodents [59] and suggests that glucocorticoids may augment responsiveness by enhancing gonadotropin synthesis. In addition, recent observations indicate that E2 promotes the migration of LH-containing secretory granules to the periphery of gonadotroph cells [60]. Interestingly, glucocorticoids may also facilitate formation and maturation of secretory granules in glandular tissues [61]. In anterior pituitary tissue, this effect of glucocorticoids and/or E2 may translate into an augmentation of the "readily-releasable" pool of LH-containing secretory granules. This hypothesis is consistent with the recent report that the rate of GnRH-induced exocytosis in gonadotrophs is increased during incubation with dexamethasone and E2 [62].

Collectively, the observations presented here indicate that stress-like concentrations of C enhance the negative feedback potency of E2 and reduce estrogen-dependent accumulation of GnRH receptor in pituitary tissue. The decrease in LH pulse frequency and basal LH secretion during concurrent treatment with C and E2 may account, at least in part, for the reduction in reproductive activity that commonly attends stress.


   FOOTNOTES
 
1 Supported by the USDA Grant 93-37203-9111 and the California Agriculture Experiment Station. Back

2 Correspondence. FAX: 530 752 0175; teadams{at}ucdavis.edu Back

3 Current address: School of Agriculture, California State University, Chico, CA 95929. Back

Accepted: September 1, 1998.

Received: March 23, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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