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Journal of Clinical Endocrinology & Metabolism , doi:10.1210/jc.2007-2548
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The Journal of Clinical Endocrinology & Metabolism Vol. 93, No. 7 2686-2692
Copyright © 2008 by The Endocrine Society

The Relative Role of Gonadal Sex Steroids and Gonadotropin-Releasing Hormone Pulse Frequency in the Regulation of Follicle-Stimulating Hormone Secretion in Men

Nelly Pitteloud, Andrew A. Dwyer, Suzzunne DeCruz, Hang Lee, Paul A. Boepple, William F. Crowley, Jr. and Frances J. Hayes

Reproductive Endocrine Unit of the Department of Medicine (N.P., A.A.D., S.D., P.A.B., W.F.C., F.J.H.), and the Department of Biostatistics and General Clinical Research Center (H.L.), Massachusetts General Hospital, Boston, Massachusetts 02114

Address all correspondence and requests for reprints to: Frances Hayes, M.B., F.R.C.P.I., Reproductive Endocrine Unit, Massachusetts General Hospital, 55 Fruit Street, Boston, Massachusetts 02114. E-mail: Hayes.Frances{at}MGH.Harvard.edu.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Objective: Our objective was to determine the importance of testosterone (T), estradiol (E2), and GnRH pulse frequency to FSH regulation in men.

Design: This was a prospective study with four arms.

Setting: The study was performed at the General Clinical Research Center.

Patients or Other Participants: There were 20 normal (NL) men and 15 men with idiopathic hypogonadotropic hypogonadism (IHH) who completed the study.

Intervention: Medical castration and inhibition of aromatase were achieved using ketoconazole x 7 d with: 1) no sex steroid addback, 2) T addback starting on d 4, and 3) E2 addback starting on d 4. IHH men in these arms received GnRH every 120 min. In a further six IHH men receiving ketoconazole with no addback, GnRH frequency was increased to 35 min for d 4–7. Blood was drawn every 10 min x 12 h at baseline, overnight on d 3–4 and 6–7.

Main Outcome Measures: Mean FSH was calculated from the pool of each frequent sampling study.

Results: In NL men FSH levels increased from 5.1 ± 0.7 to 8.7 ± 1.3 and 9.7 ± 1.5 IU/liter (P < 0.0001). T caused no suppression of FSH. E2 reduced FSH from 12.4 ± 1.8 to 9.3 ± 1.3 IU/liter (P < 0.05). In IHH men on GnRH every 120 min, FSH levels went from 6.0 ± 1.6 to 9.0 ± 3.0 and 11.9 ± 4.3 (P = 0.07). T caused no suppression of FSH. E2 decreased FSH such that levels on d 6–7 were similar to baseline. Increasing GnRH frequency to 35 min had no impact on FSH.

Conclusions: The sex steroid component of FSH negative feedback in men is mediated by E2. Increasing GnRH frequency to castrate levels has no impact on FSH in the absence of sex steroids. When inhibin B levels are NL, sex steroids exert a modest effect on FSH.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
In male primates the major stimulus to FSH secretion is GnRH, as evidenced by the low or undetectable FSH levels seen in men with congenital GnRH deficiency (1) and in monkeys with lesions in the arcuate nucleus (2). Seminal studies in monkeys established the key requirement for pulsatile GnRH stimulation for normal (NL) gonadotrope function with the demonstration that continuous administration causes profound desensitization of LH and FSH secretion (3). Studies on the impact of changes in GnRH pulse frequency on FSH secretion have given conflicting results. Previous work by our group showed no change in mean FSH levels when the frequency of exogenous GnRH administration to GnRH-deficient men with mature gonads was either increased progressively from every 120 to every 15 min (4) or decreased from every 120 to every 240 min (5). In contrast, others have shown an increase in FSH levels when GnRH pulse frequency is decreased from every 60 to every 180 min in the setting of hypogonadal testosterone (T) levels and immature gonads in men with GnRH deficiency (6). Thus, ambient levels of sex steroids and inhibin B may be important modulators of the FSH response to GnRH pulse frequency.

In addition to stimulation by GnRH, the other major determinant of FSH levels in men is the degree of negative feedback regulation by gonadal sex steroids [T and estradiol (E2)] and inhibin B (7). Data on the relative contributions of T and E2 to FSH control are relatively sparse because many investigators have focused exclusively on LH (8, 9), and available data are conflicting. Using a variety of approaches, we (10) and others (11, 12, 13, 14, 15) have provided evidence that the negative feedback effects of T on FSH secretion are mediated largely by aromatization to E2. However, other studies suggest that androgens have direct effects on FSH that are independent of E2. For example, studies in healthy NL men show that administration of the selective androgen receptor (AR) blocker, flutamide, increases FSH (16), whereas infusing the nonaromatizable androgen, dihydrotestosterone (DHT), suppresses FSH levels (17).

Thus, the aim of the present study was to examine the relative contributions of T, E2, and GnRH pulse frequency to the control of FSH secretion in the human male. To address these issues, we used an ablation and replacement model comprising sex steroid suppression followed by physiological selective sex steroid addback. We studied both NL healthy men and men with idiopathic hypogonadotropic hypogonadism (IHH) due to congenital GnRH deficiency in whom T levels had been normalized with long-term GnRH therapy. The inclusion of GnRH-deficient men in whom the frequency of exogenous GnRH administration can be experimentally controlled allowed us to test the hypothesis that removing sex steroid negative feedback would unmask the dependence of FSH secretion on the frequency of GnRH stimulation.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

Healthy NL men There were 31 NL men (age 30.6 ± 1.6 yr) enrolled. All study subjects met the following criteria: 1) NL pubertal development, sexual function, and general health; 2) NL physical examination, including a testicular volume more than or equal to 15 ml; 3) NL renal and hepatic function; 4) NL serum levels of T, E2, LH, FSH, TSH, and prolactin; and 5) NL semen analysis according to World Health Organization criteria (18).

GnRH-deficient men A total of 20 men (35.7 ± 2.4 yr) with congenital GnRH deficiency participated in the study. The diagnosis of IHH was based on the following criteria: 1) failure to go through puberty by age 18 yr, 2) serum T less than or equal to 100 ng/dl (3.5 nmol/liter) in association with inappropriately low gonadotropin levels, 3) absence of NL endogenous gonadotropin pulsations during a 12- to 24-h period of frequent blood sampling, 4) otherwise NL reserve testing of anterior pituitary function, and 5) a NL hypothalamic-pituitary region by magnetic resonance imaging. At the time of study participation, all subjects had normalized their serum T, LH, and FSH for at least 3 months with pulsatile GnRH therapy delivered every 120 min (19).

The study was approved by the Human Research Committee at the Massachusetts General Hospital, and all subjects provided written informed consent before the initiation of any study related procedures.

Study design

Protocol 1: impact of sex steroid suppression and replacement on FSH in NL and IHH men A detailed neuroendocrine evaluation was performed: 1) at baseline (BL) in the presence of NL sex steroids, 2) on d 3- to 4-sex steroid ablation, and 3) again on d 6–7 with or without selective sex steroid addback from d 4–7. Subjects were admitted to the General Clinical Research Center at Massachusetts General Hospital for overnight blood sampling. Samples were drawn every 10 min for 12 h for the BL evaluation. Medical castration and inhibition of aromatase activity were then induced using high-dose ketoconazole (KC), 1 g loading dose, followed by 400 mg four times per day x 7 d. KC is a potent inhibitor of C17–20 lyase, a rate-limiting step in androgen biosynthesis (20). We previously showed that this regimen reliably suppresses sex steroid levels in men to the castrate range within 24 h (10, 21). At these high doses, KC also inhibits cortisol biosynthesis and aromatase activity (22, 23). For this reason all study participants received dexamethasone 0.5 mg twice daily for the duration of the study, having first shown that this dose does not suppress gonadotropin or T secretion in men (10).

After completing the BL evaluation, subjects participated in one or more of the following arms.

a: sex steroid ablation alone (T–, E2–) There were 12 NL and six GnRH-deficient men who received KC for 7 d and underwent repeat overnight frequent sampling studies on d 3–4 and 6–7.

b: sex steroid ablation plus T addback (T+,E2–) There were 11 NL and six GnRH-deficient men who received the same KC regimen but were replaced with T for d 4–7 using T enanthate 125 mg administered im after completing the second frequent sampling study. The fact that KC inhibits aromatase meant that concomitant administration of an aromatase inhibitor was not necessary to prevent endogenous E2 levels from increasing as a result of T addback.

c: sex steroid ablation plus E2 addback (T–, E2+) Nine NL and eight GnRH-deficient men received KC for 7 d with selective addback of E2 on d 4–7 administered as a transdermal patch (Estraderm; Novartis Pharmaceuticals Corp., East Hanover, NJ) at a dose of 37.5 µg/d.

Protocol 2: impact of changes in GnRH pulse frequency on the FSH response to sex steroid suppression in GnRH-deficient men

Eight GnRH-deficient men received the same KC regimen as in protocol 1(a). However, in this arm the frequency of GnRH administration was started at the physiological interval of every 120 min. After completing the frequent sampling study on d 3–4, frequency was increased to every 35 min, the frequency at which we (21) and others (24) have shown GnRH is secreted in castrate males.

T, E2, FSH, LH, and inhibin B levels were measured in a pool comprising equal aliquots of each sample obtained during the study.

Hormone assays

Serum FSH and LH concentrations were determined by a microparticle enzyme immunoassay using an automated Abbott AxSYM system (Abbott Laboratories, Chicago, IL). The Second International Reference Preparation was used as the reference standard. The assay sensitivity for LH and FSH was 1.6 mIU/ml, with an intraassay coefficient of variation (CV) of less than 7% and an interassay CV of less than 7.4%. Serum T concentrations were measured using the DPC Coat-A-Count RIA kit (Diagnostic Products Corp., Los Angeles, CA), which has an intraassay and interassay CV of less than 10%. E2 was measured by RIA using hexane ethylacetate extraction and LH-20 chromatography (Endocrine Sciences, Calabasas Hills, CA). This E2 assay has a sensitivity of 5 pg/ml (18 pmol/liter), and based on a male serum pool, has an intraassay CV of 4.9% and an interassay CV of 15%. Inhibin B was measured using a commercially available (Serotec, Oxford, UK) double-antibody ELISA, which has a CV of 4–6% within plate and 15–18% between plates.

Statistical methods

Data are presented as mean ± SEM unless otherwise stated. The data were analyzed using repeated measures ANOVA with application of the conservative Bonferroni adjustment for the three comparisons made (BL vs. d 3–4, d 3–4 vs. d 6–7, and BL vs. d 6–7). When assay results were below the level of detection, the level of detection was used for statistical analysis. A P value less than 0.05 was considered statistically significant.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Protocol 1: impact of sex steroid suppression and selective replacement on FSH in NL and GnRH-deficient men

There were 11 NL and five GnRH-deficient men withdrawn from the study because of adverse events (abnormal liver function tests, skin rash, nausea). Therefore, 20 NL and 15 GnRH-deficient men completed this protocol. One NL volunteer participated in two arms, and five GnRH-deficient men participated in more than one arm of the study. In each case the interval between consecutive studies was at least 3 months. There were no significant differences in age or body mass index between the different groups.

Healthy subjects The KC regimen used was successful in creating the desired sex steroid milieu (Fig. 1Go, A and C) as we recently reported in a study examining the regulation of LH secretion in men (25). In all study arms, serum T levels decreased from the physiological range at BL to castrate levels on d 3–4 of KC administration (P < 0.005) (Fig. 1AGo). In the nine subjects who completed the arm with no sex steroid addback, T and E2 levels on d 6–7 remained suppressed at 37 ± 8 ng/dl (P < 0.005) (Fig. 1AGo) and 9 ± 1 pg/ml (P < 0.005) (Fig. 1CGo), respectively. T addback restored T concentrations on d 6–7 to BL values (Fig. 1AGo) without any significant increase in E2 levels (11 ± 2 pg/ml) (Fig. 1BGo). Similarly, E2 administration restored E2 levels on d 6–7 to the NL range (34 ± 6 pg/ml), although levels were higher than BL concentrations of 21 ± 4 pg/ml (P < 0.05) (Fig. 1BGo).


Figure 1
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  		FIG. 1. Serum T and E2 levels in healthy male volunteers (A and C) and GnRH-deficient men on GnRH therapy (B and D) during acute biochemical castration for 7 d with either no sex steroid addback or replacement with T or E2 from d 4–7. a, Significant difference from BL (P < 0.005). b, Significant difference from BL (P < 0.05).

 
In NL men receiving no sex steroid addback, mean FSH levels increased from 5.1 ± 0.7 at BL to 8.7 ± 1.3 IU/liter on d 3–4 and 9.7 ± 1.5 on d 6–7 (P < 0.0001) (Fig. 2AGo), with no difference between levels on d 3–4 and 6–7. After T replacement mean FSH levels were unchanged (12.1 ± 0.7 on d 3–4 vs. 13.2 ± 1.5 IU/liter on d 6–7) (P = 0.5) and remained higher than BL (P < 0.05) (Fig. 2AGo). In contrast, E2 addback caused FSH levels to decrease from 12.4 ± 1.8 to 9.3 ± 1.3 IU/liter (P < 0.05), although they remained higher than BL values of 6.9 ± 0.9 IU/liter (P < 0.05). Administration of KC was associated with a decrease in inhibin B levels from 183 ± 19 to 129 ± 12 pg/ml on d 3–4 and 136 ± 13 pg/ml on d 6–7 (P < 0.005). Sex steroid replacement caused no change in inhibin B levels between d 3–4 and 6–7 [134 ± 22 vs. 144 ± 22 pg/ml for T addback (P = 0.6) and 124 ± 10 vs. 114 ± 10 pg/ml for E2 addback (P = 0.6)].


Figure 2
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  		FIG. 2. Mean FSH levels in healthy male volunteers (A) and GnRH-deficient men on GnRH therapy (B) during biochemical castration for 7 d with either no sex steroid addback or replacement with T or E2 from d 4–7. P value shown represents overall change using repeated measures ANOVA. a, Significant difference from BL (P < 0.005). b, Significant difference from BL (P < 0.05). c, Significant difference between d 3–4 and 6–7 (P < 0.05).

 
GnRH-deficient men As in NL men, the desired experimental hormonal milieu was achieved in the GnRH-deficient men with sustained suppression of T (Fig. 1BGo) and E2 levels (Fig. 1DGo) in those receiving no addback, restoration of physiological T levels on d 6–7 with T replacement (Fig. 1BGo) without a significant increase in E2 levels, and normalization of E2 levels on d 6–7 after E2 administration (Fig. 1DGo).

In the five GnRH-deficient men who received no addback, FSH levels went from 6.0 ± 1.6 at BL to 9.0 ± 3.0 IU/liter on d 3–4 and 11.9 ± 4.3 IU/liter on d 6–7 (P = 0.07) (Fig. 2BGo). After T administration, mean FSH levels went from 17.8 ± 5.2 IU/liter on d 3–4 to 22.5 ± 6.8 IU/liter on d 6–7 (P = 0.25) and remained significantly higher than BL (P < 0.05). After E2 replacement, mean FSH levels on d 6–7 were similar to BL (10.9 ± 2.4 vs. 8.5 ± 1.4 IU/liter; P = 0.3; Fig. 2BGo). As in NL men, suppression of sex steroids was accompanied by a reduction in serum inhibin B levels from 129 ± 23 to 90 ± 12 pg/ml on d 3–4 and 98 ± 11 pg/ml on d 6–7 (P < 0.05). Sex steroid replacement had no impact on inhibin B levels [88 ± 31 on d 3–4 vs. 91 ± 28 pg/ml on d 6–7 (P = 0.5) for T addback and 66 ± 12 on d 3–4 vs. 92 ± 18 pg/ml on d 6–7 (P = 0.08) for E2 addback].

Protocol 2: impact of changes in GnRH pulse frequency on the FSH response to sex steroid suppression in GnRH-deficient men

Six GnRH-deficient men completed the frequency arm change of the protocol; four of these subjects had also participated in protocol 1. In each case at least 3 months had elapsed between studies. Increasing the frequency of GnRH administration from every 120 to every 35 min on d 3–4 caused no increase in FSH levels (10.6 ± 3.0 IU/liter on d 3–4 vs. 11.2 ± 2.9 IU/liter on d 6–7; P = 0.6) (Fig. 3Go). LH levels increased from 15.4 ± 0.9 IU/liter (BL) to 18.7 ± 2.3 IU/liter on d 3–4 and 22.2 ± 2.5 IU/liter on d 6–7 (P < 0.05); however, the increase in LH levels between d 3–4 and 6–7 was not statistically significant (P = 0.2).


Figure 3
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  		FIG. 3. Mean FSH levels in GnRH-deficient men on GnRH therapy during biochemical castration for 7 d with no sex steroid addback. The GnRH pulse frequency (Freq) was increased from 1 pulse every 120 min to 1 pulse every 35 min for d 4–7. Values represent a study pool from the mean of samples drawn every 10 min for 12 h. P value shown represents overall change using repeated measures ANOVA.*Significant difference from BL (P < 0.05).

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Using the experimental model of biochemical castration and selective sex steroid replacement in both NL and GnRH-deficient men, our data provide several unique insights into the understanding of FSH regulation in the human male. First, the demonstration that T administration causes no suppression of FSH in either NL or GnRH-deficient men when aromatization to E2 is inhibited indicates that androgens have no direct negative feedback effects on FSH secretion at either the hypothalamus or pituitary. In contrast, E2 administration does result in significant suppression of FSH levels. These observations are compatible with our previous assertion that E2 is the major sex steroid regulator of FSH secretion in men (10). Second, castrate frequencies of GnRH administration did not cause FSH levels to increase, even in the absence of sex steroid negative feedback. Third, the modest increase in FSH levels observed after concomitant ablation of sex steroids and castrate frequencies of GnRH stimulation indicates that in the presence of mature testes, other gonadal feedback factors (i.e. inhibin B) play a much greater role in the regulation of FSH secretion (21, 26).

Our data are consistent with previous reports that, using a variety of approaches, have suggested that the negative feedback effects of T on FSH secretion are mediated largely, if not totally, by its aromatization to estrogen (10, 11, 12, 13, 14, 15). Lacroix et al. (11) administered the antiestrogen clomiphene citrate to NL men and men with complete androgen insensitivity syndrome to create human models of selective estrogen insensitivity and combined estrogen and androgen insensitivity, respectively. The increase in FSH levels in response to clomiphene was identical in the two groups. However, interpretation of the data from the complete androgen insensitivity syndrome model is confounded by the fact that estrogen production rates are increased in this disorder (27, 28), and that clomiphene has elements of both estrogen agonism and antagonism. However, using a different model of selective sex steroid suppression in healthy men, we previously showed that the increase in FSH levels in response to suppressing both T and E2 to castrate levels is no greater than that observed with selective suppression of E2 alone (10).

Further support for the absence of a direct negative effect of androgens on FSH secretion is provided by studies in healthy eugonadal men showing an increase in endogenous LH, but not FSH levels, in response to administration of an AR blocker (12). Similarly, administration of the nonaromatizable androgen, DHT, caused no suppression of FSH levels in GnRH-deficient men, indicating that androgens have no direct inhibitory effects on FSH at the level of the pituitary (13).

However, not all studies support the contention that androgen feedback of FSH is mediated by E2. For example, other studies in healthy NL men have shown that FSH levels increase in response to the selective AR blocker, flutamide (16), and decrease in response to administration of DHT (17). The discrepancy between studies is likely explained by differences in both the doses of sex steroids infused and the type of subjects studied. Thus, in the study showing suppression of FSH levels, the dose of DHT administered was 7.5 mg (16), which was 15-fold higher than that used in the study showing no effect of DHT on FSH (13). Given the high affinity of DHT for SHBG, such high doses likely displace T and E2 from SHBG, thus altering the ratio of free to bound endogenous gonadal hormones in the circulation and potentially confounding interpretation of the results. Another possible explanation for the discrepant findings in results is differences in the study populations, namely whether GnRH-deficient men (13) or healthy volunteers were studied (16). Thus, an alternative interpretation of these results is that androgens have no direct negative pituitary effects on FSH (and, therefore, cannot suppress levels in GnRH-deficient men) but do have effects at the hypothalamic level on GnRH regulation of FSH (accounting for suppression of FSH in healthy men). However, other studies do not support the latter hypothesis. In a previous study by our group, coadministration of the aromatase inhibitor testolactone with pharmacological doses of T completely blocked the suppressive effects of T alone on FSH levels in both NL and GnRH-deficient men (29). Thus, the bulk of available data supports the conclusion of the present study that androgen negative feedback effects on FSH are mediated largely by E2.

Animal studies using genetic models of estrogen insensitivity have further refined our understanding of this field by exploring the roles of AR, estrogen receptor (ER) {alpha}, and ERβ in regulating gonadotropins. In a series of elegant studies, Lindzey et al. (15) compared the impact of T administration on FSH-β mRNA and serum FSH levels in castrate wild-type mice with that of castrate ER knockout mice. They showed that the suppressive effects of T administration on FSH that are observed in castrate wild-type animals are abolished in castrate ER knockout mice. These data suggest that, at least in the mouse, inhibition of FSH by T requires aromatization and, more specifically, activation of ER{alpha} signaling pathways (15).

Although high doses of KC are known to inhibit aromatase activity (22, 23), there was a small, but statistically insignificant, increase in E2 levels in both NL and GnRH-deficient men after T administration. This increase in E2 of approximately 4 pg/ml may reflect incomplete inhibition of aromatase or variability in the E2 assay at low levels. Regardless of the underlying mechanism, the increase in E2 levels does not appear to be of physiological significance given the failure of T addback to suppress FSH levels, even in the presence of these modest changes in E2.

Although our data clearly demonstrate an important role for estrogen in the sex steroid regulation of FSH in men, it should be noted that restoring physiological levels of E2 did not normalize FSH levels in the healthy men. However, it is important to bear in mind that E2 was only administered for 3 d, and it is possible that longer treatment might have produced greater suppression. The fact that giving T on its own caused no suppression of FSH indicates that the failure of E2 to normalize FSH was not due to the lack of T. However, it is notable that inhibin B levels decreased by 20% in response to KC administration in this study, a finding we previously reported (21). Previous studies in which endogenous gonadotropins and intratesticular T levels were suppressed with exogenous T administration demonstrated a significant decline in inhibin B levels as early as 4 d after the first T injection (30), suggesting that intragonadal levels of sex steroids are important for NL inhibin B secretion. Although sperm counts in that study were not assessed until 4 wk later, the time course of decline in inhibin B and sperm counts was similar. Similarly, men with congenital estrogen deficiency due to mutations in the aromatase gene have elevated FSH levels, which decrease significantly, but are not normalized, after physiological estrogen replacement (31). Although thought to represent a human model of selective estrogen deficiency, men with aromatase deficiency also have disruption of spermatogenesis and low inhibin B levels, which undoubtedly contribute to their elevated FSH levels. In a study in which E2 addback normalized FSH levels after KC-induced biochemical castration, the E2 levels achieved were supraphysiological (14).

This study clearly demonstrates that T administration causes no suppression of FSH levels in men when aromatization is inhibited. Moreover, in the present and previous studies by our group (29), FSH levels in GnRH-deficient men tend to actually increase when T is administered with an aromatase inhibitor. Although small patient numbers preclude any definitive conclusions, these data are consistent with the demonstration that T stimulates FSH-β gene transcription in vivo in both castrate and GnRH antagonist-treated rats (32) and in vitro in rat pituitary cell cultures (33). From a clinical standpoint, such a stimulatory effect of T on FSH secretion may shed light on the clinical observation that men with acquired hypogonadotropic hypogonadism may become fertile during treatment with T (34).

Previous studies on the effects of changes in GnRH pulse frequency on gonadotropin levels have given conflicting results that were attributed to differences in the degree of gonadal maturation in the model under investigation. In previous work we showed no change in mean FSH levels when the frequency of exogenous GnRH administration was either progressively increased from every 120 to every 15 min (4) or decreased from every 120 to every 240 min in GnRH-deficient men with mature gonads (5). However, other investigators reported a preferential increase in FSH levels when the frequency of GnRH stimulation to GnRH-deficient men was decreased from every 60 to every 180 min in the presence of hypogonadal T levels early in their therapy (6). Our hypothesis that removing sex steroids would allow FSH levels to increase in response to faster frequencies of GnRH stimulation proved incorrect, and mean FSH levels were the same whether GnRH was administered at either physiological (every 120 min) or castrate (every 35 min) frequencies. Our FSH, but not our LH, results are consistent with data obtained from a hypothalamic-lesioned adult male rhesus monkey, in which increasing GnRH pulse frequency after castration caused a robust increase in LH levels but had no impact on FSH secretion (35). The failure of our study to confirm in a human model the differential effects of GnRH pulse frequency on LH and FSH that had been observed in the nonhuman primate in a similar castrate sex steroid milieu may be due to differences in the duration of exposure to GnRH. Thus, in our study, subjects were treated with castrate frequencies of GnRH for only 3 d compared with 8–12 d in the monkey (35). Therefore, it is possible that administering GnRH at the castrate pulse frequency for a longer period of time might have given different results.

In summary, this human investigative model using sex steroid ablation with physiological sex steroid addback in NL and GnRH-deficient men at varying frequencies of GnRH administration provides key insights into the regulation of FSH secretion in men. These data provide conclusive evidence that the sex steroid component to FSH negative feedback is mediated by E2 and that FSH is resistant to changes in GnRH pulse frequency, even in the absence of sex steroid negative feedback.


   Footnotes
 
This work was supported by National Institutes of Health Grants R01 HD15788-15 and M01-RR-01066, and by the National Center for Research Resources, General Clinical Research Centers Program.

Disclosure Summary: The authors have nothing to disclose.

First Published Online April 29, 2008

Abbreviations: AR, Androgen receptor; BL, baseline; CV, coefficient of variation; DHT, dihydrotestosterone; E2, estradiol; ER, estrogen receptor; IHH, idiopathic hypogonadotropic hypogonadism; KC, ketoconazole; NL, normal; T, testosterone.

Received November 19, 2007.

Accepted April 21, 2008.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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