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Clinical Studies |
Division of Endocrinology and Metabolism, Department of Pediatrics, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908
Address all correspondence and requests for reprints to: Daniel L. Metzger, M.D., Endocrinology and Diabetes Unit, British Columbia’s Children’s Hospital, 4480 Oak Street, Room 1A46, Vancouver, British Columbia, Canada V6H 3V4. E-mail: dmetzger{at}wpog.childhosp.bc.ca
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Abstract |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Materials and Methods |
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Eleven healthy young adult men, aged 20–33 yr (mean ± SD, 24 ± 4 yr), participated in this study. All subjects were nonsmokers, nonobese (mean body mass index, 22.6 ± 2.5 kg/m2), and nonovertrained. All subjects had normal screening blood chemistry profiles, complete blood cell counts, TSH, testosterone, and 0600 h LH levels (2.9 ± 1.0 U/L). All had normal screening physical examinations. The study was reviewed and approved by the human investigation committee of the University of Virginia. Written consent was obtained after explanation of the purpose and potential risks of the study. Subjects participated as paid volunteers. All studies were conducted at the General Clinical Research Center of the University of Virginia.
Experimental design
All subjects presented on four occasions; admissions were randomized and occurred at least 2 weeks apart. A heparin lock cannula was placed in a forearm vein at least 60 min before the onset of sampling at 1800 h, and blood was then obtained for determination of serum LH concentrations at 10-min intervals for a 12-h period. From 2200–0200 h, the men received one of four infusion protocols in a blinded fashion: a normal saline (NS) bolus and NS infusion, a naloxone (NAL) bolus (4 mg) and NAL infusion (2 mg/h), a NS bolus and oCRH infusion (1 µg/kg·h; maximum, 75 µg/h), and a NAL bolus and both NAL and oCRH infusions at the above-mentioned doses. The oCRH was kindly supplied by Dr. G. P. Chrousos from the NICHHD. Subjects were fed standard meals and a bedtime snack. Normal activity was permitted during the evening, but subjects were required to be in bed with lights out from 2300–0700 h. Several subjects experienced mild facial flushing and asymptomatic hypotension (diastolic blood pressure, <50 mm Hg) associated with the oCRH infusion.
Assays
Serum LH concentrations were determined by immunoradiometric assay (Allegro LH IRMA, Nichols Institute Diagnostics, San Juan Capistrano, CA). For each subject, all LH concentrations from the first four admissions were determined in duplicate in single-run assays to minimize interassay variability. The reported intra- and interassay coefficients of variation for the LH immunoradiometric assay are 2.6% and 5.4%, respectively. Assay sensitivity is 1.0 U/L, and any LH level below this limit was assigned this value.
Statistical analysis
All data were log transformed before analysis. For each time
point, the mean serum LH values from the various experimental
conditions were compared using one-way ANOVA with repeated measures;
the two-tailed paired t test was applied post-hoc
when potentially significant differences were identified. Because of
the multiple comparisons made, P 
0.01 was considered
significant. This model is predicted to have a power of more than 90%
to identify a 1.0 U/L or greater difference in the mean LH
concentration between two experimental conditions. Statistical
calculations were performed using Systat Software (Evanston, IL).
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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0.004 for all), increase in
immunoradiometric LH levels (Fig. 1a
) compared to the NS
control value. In contrast, at no point did the mean LH levels from the
oCRH admission differ significantly from control values (Fig. 1b). The
combination of oCRH and NAL also produced significant
(P 0.002 for all) elevations in mean LH
concentrations (2240–2310 and 0020–0030 h) compared to control values
(Fig. 1c). However, when the mean LH values from the oCRH plus NAL
admission were compared with those from the NAL only admission, no
significant differences were detected (Fig. 1d).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The suppressive effects of chronic stress on the HPG axis are believed to be due primarily to the influence of elevated levels of endogenous CRH as well as of arginine vasopressin, interleukin-1, and a number of other neurotransmitters (1, 2, 3). Efforts to reproduce the phenomenon of stress-related gonadotropin suppression with exogenous CRH have produced varied results depending on the dose and route of administration of CRH as well as on the species, gonadal state, and opiatergic tone of the experimental subjects. In rats, the administration of CRH by the intracerebroventricular, but not by the iv, route leads to a prompt diminution in peripheral LH levels (16, 17) and hypophyseal-portal concentrations of GnRH (18); this effect is blocked by pretreatment with NAL (18). These results suggest that the inhibitory effect of CRH on the rat HPG axis is exerted at the hypothalamic level and is mediated at least in part via the EOP receptor. Corroborating this hypothesis is the immunocytochemical finding that CRH-secreting neurons have direct synaptic connections with GnRH-containing neurons in the medial preoptic area of the rat brain (22). Furthermore, CRH given iv produces a significant decrease in LH levels in ovariectomized (12), but not in intact male (14) or female (15), rhesus monkeys, an effect blocked by pretreatment with NAL (13). Conversely, intracerebroventricular oCRH produces an increase in plasma LH concentrations in intact ewes (19).
To date, studies in humans have produced contradictory results.
The earliest studies employing iv boluses of oCRH (4) and human CRH
(hCRH) (5) and infrequent blood sampling found no acute effects on LH
levels in men or women. Some investigators have demonstrated that an iv
infusion of hCRH in young women with regular menses is associated with
a rapid significant reduction in plasma LH concentrations during both
the late follicular (6) and midluteal phases (6, 7, 11), but not during
the midfollicular phase (6); this suppression can be blocked by the
coadministration of either NAL (7) or 
MSH, a putative EOP antagonist
(11). In these subjects, hCRH treatment has no effect on gonadotrope
responsiveness to a maximal stimulatory dose of GnRH (7). In contrast,
other groups have found no significant effect of iv CRH on plasma LH
levels or on measures of LH pulsatility in either eumenorrheic or
amenorrheic female athletes (10), in agonadal older women (8), or in
normally cycling women in either the early follicular or midluteal
phase (9); interestingly, iv hCRH in one study did produce an
unexpected rise in LH levels in postmenopausal subjects (9). The
seemingly disparate effects of CRH in these studies in women may be
related to differences in CRH dosage, gonadal hormone (estrogen and/or
progesterone) milieu, or opiatergic tone.
The preponderance of data from human and animal studies,
therefore, supports the hypothesis that endogenous CRH, the central
initiator of the endocrine stress response, exerts its inhibitory
effects on the HPG axis primarily at the level of the hypothalamic GnRH
pulse generator. Most studies documenting this suppressive effect of
exogenous CRH on the HPG axis also demonstrate an ameliorating
influence of NAL (13, 18) or MSH (11), supporting the idea that CRH
action is at least partly mediated via the EOP receptor. Indeed, as was
also observed in the present study, NAL alone increases LH levels,
presumably by the release of tonic EOP inhibition of GnRH-producing
neurons.
We believe that the results of our study are consistent with the lack of a pituitary effect of exogenous oCRH on the HPG axis in healthy young men. Although the doses of oCRH used were similar to those employed in the above-mentioned studies, which did find a suppressive effect on the HPG axis in women (6, 7, 11), we cannot rule out the possibility that higher doses or a longer duration of the oCRH infusion might have produced different results in men. Finally, we believe that our study was designed with sufficient statistical power to prevent missing a significant effect of the chosen oCRH treatment on LH levels.
In summary, the iv administration of oCRH to a group of healthy young men is not associated with any significant acute effect on serum LH concentrations, nor is it able to dampen the NAL-stimulated rise in LH levels. We, therefore, believe that any CRH-mediated effect of stress on the HPG axis in men probably occurs at the hypothalamic level. It remains to be elucidated why some, but not all, researchers found dissimilar results in normally cycling women.
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Acknowledgments |
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Footnotes |
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2 Current address: Department of Pediatrics, East Tennessee State
University, Box 70578, Johnson City, Tennessee 37614-0578.
Received July 19, 1996.
Revised October 28, 1996.
Accepted November 8, 1996.
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References |
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-Melanocyte-stimulating hormone antagonizes the inhibitory effect of
corticotropin-releasing hormone on luteinizing hormone secretion during
the luteal phase in normal women. J Clin Endocrinol Metab. 79:571–575.[Abstract]
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  M. Ferin Stress and the Reproductive Cycle J. Clin. Endocrinol. Metab., June 1, 1999; 84(6): 1768 - 1774. [Full Text]
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