Growth hormone abuse: methods of detection

Performance-enhancing substances in sport: A scientometric review of 75 years of research

The use of performance-enhancing substances not only undermines the core values of sports but also poses significant health risks to athletes. In a fast-evolving doping environment, where sport professionals are constantly seeking novel and illegal means to bypass doping tests, and new substances are regularly detected on the drug market, it is crucial to inform authorities with updated evidence emerging from scientific research. The current study aims to (i) outline the structure of knowledge in the literature on performance enhancers in sports (i.e., most active countries, main sources, most productive authors, and most frequently used keywords); (ii) identify the most impactful documents in the field; and (iii) uncover the main domains of research in the literature. To do so, we conducted a comprehensive scientometric analysis of the literature on doping, sourcing our data from Scopus. Our research involved a document co-citation analysis of 193,076 references, leading to the identification of the 51 most influential documents and seven key thematic areas within the doping literature. Our results indicate that the scientific community has extensively studied the most prevalent doping classes, such as anabolic agents and peptide hormones, and little is still known about the use of contaminated supplements or other types of enhancers identified as emergent trends. Concurrently, technological advancements contributed to the development of more sophisticated doping detection techniques, using blood or urine samples. More recently, the focus has shifted towards the athlete biological passport, with research efforts aimed at identifying biomarkers indicative of doping. The dynamic nature of doping methods underlines the necessity for more robust educational campaigns, aiming at raising awareness among sports professionals and their entourage about the dangers of doping and the intricacies of its control mechanisms.

Development of an UPLC/MS–MS method for quantification of intact IGF-I from human serum

Aim: Developing LC–MS methods for biomolecules is often challenging due to issues with molecular size and complexity, nonspecific binding, protein binding, solubility and sensitivity. As a result, complex sample preparation workflows, including immune-affinity and/or protein digestion and lengthy analysis potentially using nano-flow LC, may be needed to achieve the required sensitivity. This work aims to provide a simple, sensitive, fast and robust method for quantification of intact IGF-I from human serum using UPLC–MS/MS. Methods: IGF-I serum samples were denatured with sodium dodecyl sulfate, followed by organic protein precipitation to effectively disrupt protein binding and subsequent SPE of the resulting supernatant for sample cleanup and enrichment prior to LC–MS/MS analysis. Separation was performed on an analytical scale LC using a reversed-phase column containing <2 μm solid core particle followed by detection on a tandem quadrupole MS in multiple reaction monitoring mode. Results: Intact IGF-I was quantified from serum using the method described above at a LLOQ of 5 ng/ml with a dynamic range 5–1000 ng/ml (r2>0.99) and mean accuracy of 101.76%. Accuracies for quality control samples were between 93.9–107.7% with RSD <7%. Conclusion: The analytical sensitivity, linear dynamic range and excellent reproducibility of this method reliably measures endogenous and elevated serum IGF-I levels, demonstrating its utility in discovery, bioanalysis and clinical research.

Obese adolescents exhibit a constant ratio of GH isoforms after whole body vibration and maximal voluntary contractions

BACKGROUND

Growth hormone (GH) is a heterogeneous protein composed of several molecular isoforms, the most abundant ones being the 22 kDa- and 20 kDa-GH. Exercise-induced secretion of GH isoforms has been extensively investigated in normal-weight individuals due to antidoping purposes, particularly recombinant human GH (rhGH) abuse. On the other hand, the evaluation of exercise-induced responses in GH isoforms has never been performed in obese subjects.

METHODS

The acute effects of whole body vibration (WBV) or maximal voluntary contraction (MVC) alone and the combination of MVC with WBV (MVC + WBV) on circulating levels of 22 kDa- and 20 kDa-GH were evaluated in 8 obese male adolescents [mean age ± SD: 17.1 ± 3.3 yrs.; weight: 107.4 ± 17.8 kg; body mass index (BMI): 36.5 ± 6.6 kg/m²; BMI standard deviation score (SDS): 3.1 ± 0.6].

RESULTS

MVC (alone or combined with WBV) significantly stimulated 22 kDa- and 20 kDa-GH secretion, while WBV alone was ineffective. In particular, 22 kDa- and 20 kDa-GH peaks were significantly higher after MVC + WBV and MVC than WBV. In addition, 22 kDa-GH (but not 20 kDa-GH) peak was significantly higher after MVC + WBV than MVC. Importantly, the ratio of circulating levels of 22 kDa- to 20 kDa-GH was constant throughout the time window of evaluation after exercise and similar among the three different protocols of exercise.

CONCLUSIONS

The results of the present study confirm the ability of MVC, alone and in combination with WBV, to stimulate both 22 kDa- and 20 kDa-GH secretion in obese patients, these responses being related to the exercise workload. Since the ratio of 22 kDa- to 20 kDa-GH is constant after exercise and independent from the protocols of exercise as in normal-weight subjects, hyposomatotropism in obesity does not seem to depend on an unbalance of circulating GH isoforms. Since the present study was carried out in a small cohort of obese sedentary adolescents, these preliminary results should be confirmed in further future studies enrolling overweight/obese subjects with a wider age range.

Quantification of intact human insulin-like growth factor-I in serum by nano-ultrahigh-performance liquid chromatography/tandem mass spectrometry

RATIONALE

Insulin-like growth factor-I is one of the biomarkers used to detect growth hormone administration prohibited in human sport. Current testing approaches for IGF-I rely on commercial immunoassays, which may change from time to time requiring complex revalidation. Mass spectrometry (MS)-based approaches often rely on enzymatically digesting the protein and measuring specific peptide concentrations. In order to reinforce the current available methodology for IGF-I testing, a reliable and equally sensitive MS method is required for the analysis of intact protein using small sample volumes (<25 μL).

METHODS

IGF-I was extracted from human serum samples by a simple protein precipitation procedure. Separation was achieved via nano-ultrahigh-performance liquid chromatography and MS analysis was conducted by nano-electrospray ionisation triple-quadrupole mass spectrometry in the selected reaction monitoring mode using a stable-isotope-labelled internal standard.

RESULTS

A six-point calibration curve ranging from 50 to 1000 ng/mL of human IGF-I in rat serum was used to establish instrument response. The method provided a limit of quantification of 50 ng/mL, with intra- and inter-day precision ≤5% and intra- and inter-day accuracy ≥95%.

CONCLUSIONS

A quantitative method was developed for the quantification of intact IGF-I in human serum samples. The data generated provided important information for the development of a new reference method for the growth hormone biomarker test and helped create a reliable system for monitoring peptide hormones in individual athletes, a possible extension to the athlete biological passport system. Nano-electrospray has here been shown to be sufficiently robust for routine use in an analytical laboratory, allowing for the analysis of minute sample volumes.

Growth hormone doping: a review

The use of growth hormone (GH) as a performance enhancing substance was first promoted in lay publications, long before scientists fully acknowledged its benefits. It is thought athletes currently use GH to enhance their athletic performance and to accelerate the healing of sporting injuries. Over recent years, a number of high profile athletes have admitted to using GH. To date, there is only limited and weak evidence for its beneficial effects on performance. Nevertheless the “hype” around its effectiveness and the lack of a foolproof detection methodology that will detect its abuse longer than 24 hours after the last injection has encouraged its widespread use. This article reviews the current evidence of the ergogenic effects of GH along with the risks associated with its use. The review also examines methodologies, both currently available and in development for detecting its abuse.

Doping and musculoskeletal system: short-term and long-lasting effects of doping agents

Doping is a problem that has plagued the world of competition and sports for ages. Even before the dawn of Olympic history in ancient Greece, competitors have looked for artificial means to improve athletic performance. Since ancient times, athletes have attempted to gain an unfair competitive advantage through the use of doping substances. A Prohibited List of doping substances and methods banned in sports is published yearly by the World Anti-Doping Agency. Among the substances included are steroidal and peptide hormones and their modulators, stimulants, glucocorticosteroids, β₂-agonists, diuretics and masking agents, narcotics, and cannabinoids. Blood doping, tampering, infusions, and gene doping are examples of prohibited methods indicated on the List. Apart from the unethical aspect of doping, as it abrogates fair-play's principle, it is extremely important to consider the hazards it presents to the health and well-being of athletes. The referred negative effects for the athlete's health have to do, on the one hand, by the high doses of the performance-enhancing agents and on the other hand, by the relentless, superhuman strict training that the elite or amateur athletes put their muscles, bones, and joints. The purpose of this article is to highlight the early and the long-lasting consequences of the doping abuse on bone and muscle metabolism.

Detection of doping with recombinant human growth hormone

Detection of doping with recombinant human growth hormone is one of the challenges for antidoping analysis. This review focuses on the most important relevant publications that provide insight into the laboratory measurement of human growth hormone (hGH), antibodies and standards, the isoform approach and the biomarker approach. The isoform approach monitors the changes of hGH molecular isoform composition in serum and was applied at the Olympic Games in Athens in 2004, Turin in 2006 and Beijing in 2008. The markers approach detects a formula score, which reflects the changes in concentration of IGF-1 and P-III-P. All these methodologies measure the concentrations of growth hormone and its isoforms for isoform approach, or the concentrations of IGF-1 and P-III-P. All factors that affect these measurements should be taken into account for the development of methods to detect doping with recombinant hGH.

Combined evaluation of resting IGF1, N-terminal propeptide of type III procollagen and C-terminal cross-linked telopeptide of type I collagen levels might be useful for detecting inappropriate GH administration in female athletes

OBJECTIVE

To detect exogenous recombinant human GH (rhGH) abuse in female athletes.

DESIGN

GH-dependent markers were assayed in serum of 100 female athletes (control group) and in a subgroup of nine female subjects treated with rhGH (0.09 IU/kg body weight, 6 days/week for 3 weeks).

METHODS

Cut-off values (mean+2 s.d.) for IGF1, N-terminal propeptide of type III procollagen (PIIINP) and C-terminal telopeptide of type I collagen (ICTP) were calculated and arbitrary scores (1.5 or 2.0) were assigned to abnormal markers. By using the sum of individual marker scores, positive (> or =3) or negative (<3) scores were obtained.

RESULTS

None of the control group obtained a positive score (> or =3). Abnormal IGF1, PIIINP and ICTP levels were found in 61.4, 54.5 and 11.4% samples of the treated group. Overall, positive cases were present in 43.2% blood samples drawn in subjects treated with rhGH and in 26% of samples after rhGH withdrawal. The sensitivity of the detection approach was 66.6% at the end of 3-week rhGH treatment and 11.1% at the 15th day of rhGH withdrawal, while the specificity was 100%.

CONCLUSION

Detection test for rhGH administration appears less sensitive in female (66.6%) than in male athletes (previous observation, 100% after 3 weeks of comparable rhGH dose), but shows a similar specificity (98.5-100%). Since athletes supposedly use very high doses and long-term administration of rhGH for doping purposes, it is foreseen that the here-in detection test would in future increase its strength.

High-sensitivity chemiluminescence immunoassays for detection of growth hormone doping in sports

BACKGROUND

Recombinant human growth hormone (rhGH) is abused in sports, but adequate routine doping tests are lacking. Analysis of serum hGH isoform composition has been shown to be effective in detecting rhGH doping. We developed and validated selective immunoassays for isoform analysis with potential utility for screening and confirmation in doping tests.

METHODS

Monoclonal antibodies with preference for pituitary hGH (phGH) or rhGH were used to establish 2 pairs of sandwich-type chemiluminescence assays with differential recognition of rhGH (recA and recB) and phGH (pitA and pitB). We analyzed specimens from volunteers before and after administration of rhGH and calculated ratios between the respective rec- and pit-assay results.

RESULTS

Functional sensitivities were <0.05 microg/L, with intra- and interassay imprecision < or =8.4% and < or =13.7%, respectively. In 2 independent cohorts of healthy subjects, rec/pit ratios (median range) were 0.84 (0.09-1.32)/0.81 (0.27-1.21) (recA/pitA) and 0.68 (0.08-1.20)/0.80 (0.25-1.36) (recB/pitB), with no sex difference. In 20 recreational athletes, ratios (median SD) increased after a single injection of rhGH, reaching 350% (73%) (recA/pitA) and 400% (93%) (recB/pitB) of baseline ratios. At a moderate dose (0.033 mg/kg), mean recA/pitA and recB/pitB ratios remained significantly increased for 18 h (men) and 26 h (women). After high-dose rhGH (0.083 mg/kg), mean rec/pit ratios remained increased for 32 h (recA/pitA) and 34 h (recB/pitB) in men and were still increased after 36 h in women.

CONCLUSIONS

Using sensitive chemiluminescence assays with preferential recognition of phGH or rhGH, detection of a single injection of rhGH was possible for up to 36 h.

Concentration and Preservation of Very Low Abundance Biomarkers in Urine, such as Human Growth Hormone (hGH), by Cibacron Blue F3G-A Loaded Hydrogel Particles

Urine is a potential source of diagnostic biomarkers for detection of diseases, and is a very attractive means of non-invasive biospecimen collection. Nonetheless, proteomic measurement in urine is very challenging because diagnostic biomarkers exist in very low concentration (usually below the sensitivity of common immunoassays) and may be subject to rapid degradation. Hydrogel nanoparticles functionalized with Cibacron Blue F3G-A (CB) have been applied to address these challenges for urine biomarker measurement. We chose one of the most difficult low abundance, but medically relevant, hormones in the urine: human growth hormone (hGH). The normal range of hGH in serum is 1 to 10 ng/mL but the urine concentration is suspected to be a thousand times less, well below the detection limit (50 pg/mL) of sensitive clinical hGH immunoassays. We demonstrate that CB particles can capture, preserve and concentrate hGH in urine at physiological salt and urea concentrations, so that hGH can be measured in the linear range of a clinical immunometric assay. Recombinant and cadaveric hGH were captured from synthetic and human urine, concentrated and measured with an Immulite chemiluminescent immunoassay. Values of hGH less than 0.05 ng/mL (the Immulite detection limit) were concentrated to 2 ng/mL, with a urine volume of 1 mL. Dose response studies using 10 mL of urine demonstrated that the concentration of hGH in the particle eluate was linearly dependent on the concentration of hGH in the starting solution, and that all hGH was removed from solution. Thus if the starting urine volume is 100 mL, the detection limit will be 0.1 pg/mL. Urine from a healthy donor whose serum hGH concentration was 1.34 ng/mL was studied in order detect endogenous hGH. Starting from a volume of 33 mL, the particle eluate had an hGH concentration of 58 pg/mL, giving an estimated initial concentration of hGH in urine of 0.175 pg/mL. The nanotechnology described here appears to have the desired precision, accuracy and sensitivity to support large scale clinical studies of urine hGH levels.

Regulation of muscle mass by growth hormone and IGF-I

Growth hormone (GH) is widely used as a performance-enhancing drug. One of its best-characterized effects is increasing levels of circulating insulin-like growth factor I (IGF-I), which is primarily of hepatic origin. It also induces synthesis of IGF-I in most non-hepatic tissues. The effects of GH in promoting postnatal body growth are IGF-I dependent, but IGF-I-independent functions are beginning to be elucidated. Although benefits of GH administration have been reported for those who suffer from GH deficiency, there is currently very little evidence to support an anabolic role for supraphysiological levels of systemic GH or IGF-I in skeletal muscle of healthy individuals. There may be other performance-enhancing effects of GH. In contrast, the hypertrophic effects of muscle-specific IGF-I infusion are well documented in animal models and muscle cell culture systems. Studies examining the molecular responses to hypertrophic stimuli in animals and humans frequently cite upregulation of IGF-I messenger RNA or immunoreactivity. The circulatory/systemic (endocrine) and local (autocrine/paracrine) effects of GH and IGF-I may have distinct effects on muscle mass regulation.

Abuse of growth hormone among young athletes

The underground abuse of growth hormone (GH) among young athletes presents a challenge to medical professionals. Health care professionals providing knowledgeable guidance regarding healthy ways to improve performance and appearance, as well as accurate information regarding substances' perceived benefits, risks, and unknown qualities, is invaluable to the young athlete. Further research focused on the profile and motivation of young people who use GH is essential to understanding and intervening better with those who use these substances.

Performance-enhancing drugs in sport

This discussion reviews the concept and history of ergogenic aides, the penetration of use in society, some benefit/risk information, drug sources in our society, detection and regulation of these agents, and provides a look to the future. It also examines the role of the clinician/endocrinologist for these patients and uses some cases as examples of drug use among adolescents/teens.

Increased serum concentration of IGFBP-4 and IGFBP-5 in healthy adults during one month's treatment with supraphysiological doses of growth hormone

OBJECTIVES

To study the effects on insulin-like growth factor binding proteins (IGFBP)-4 and -5 after one month's treatment with supraphysiological doses of growth hormone (GH) in healthy, active young adults with a normal GH-IGF-I axis. Furthermore, the possible use of IGFBP-4 and IGFBP-5 as markers of GH doping is discussed.

DESIGN

Thirty healthy, physically active volunteers (15 men and 15 women), mean age 25.9 years (range 18-35), participated in this randomized, double-blind, placebo-controlled, parallel study with three groups (n=10; 5 men and 5 women in each group). The groups comprised the following: placebo, GH 0.1IU/kg/day [0.033mg/kg/day] and GH 0.2IU/kg/day [0.067mg/kg/day].

RESULTS

Baseline levels of IGFBP-4 were higher (+20%), while IGFBP-5 levels were lower (-37%) in women than in men. IGFBP-5 levels were positively correlated to age, but no significant correlation was found for IGFBP-4. In the pooled group with active GH treatment (n=20), both IGFBP-4 and IGFBP-5 levels were increased vs. the placebo group from day 14 until end of treatment [day 28, IGFBP-4 (+40%, p<0.01) and IGFBP-5 (+61%, p <0.001)]. After inclusion of serum IGF-I as a covariate in the linear regression analysis, the associations between GH treatment and the IGFBP-4 and IGFBP-5 levels were not significant.

CONCLUSIONS

This study shows that the levels of IGFBP-4 and IGFBP-5 are affected by supraphysiological GH treatment given to young, healthy, physically active adults of both genders. The present study, including relatively few subjects, does not support that IGFBP-4 and IGFBP-5 can be used as IGF-I independent markers in a forthcoming method for detecting GH doping, although, further studies are needed to investigate the potential use of IGFBP-4 and IGFBP-5 as markers of GH doping.

Development of an absolute quantification method targeting growth hormone biomarkers using liquid chromatography coupled to isotope dilution mass spectrometry

A method to perform absolute quantification of two biomarkers (IGF-1 and IGFBP-3) of growth hormone abuse has been developed. Isotope dilution is used with synthetically labelled peptides as internal standards. Peptide selection and multiple reaction monitoring design are discussed. A simple sample preparation based on the reduction and alkylation of cysteine residues followed by tryptic digestion provides a sufficient digestion of proteins. Serum samples fortified with increasing amounts of target proteins are analysed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) on a triple quadrupole mass spectrometer. Specificity is ensured by the selection of sequences with no homology in BLAST, as well as retention time deviation check, and ion ratio monitoring. Linearity is studied in terms of calibration curves. These curves for IGFBP-3 and IGF-1 are generated with mean slopes of 0.055 and 0.065, intercepts of 0.107 and -0.011, and with coefficients of correlation of 0.95 and 0.98, respectively. These curves result from the addition of proteins to the serum. Risks of variations related to potential matrix effects are therefore reduced, as well as probable variations related to the digestion steps. The working concentration ranges are 4-10 ng/microl for IGFBP-3 and 2-8 ng/microl for IGF-1. Preliminary data regarding repeatability show that relative standard deviations (RSDs) range between 13 and 32% for IGFBP-3 and between 7 and 29% for IGF-1.

A follow-up of GH-dependent biomarkers during a 6-month period of the sporting season of male and female athletes

In order to verify the effects of the sporting season (entailing periods of training, competition, recovery, resting) on GH-dependent parameters in male and female athletes from different sporting disciplines, 47 male and female athletes (3 rowers, 5 swimmers, 7 alpine skiers, 3 soccer players, 7 middle distance runners, 14 sprinters, 4 triathletes, 1 road walker, 3 cyclists) were followed-up for a period of 6 months. Blood samples were taken every two months for the evaluation of IGF-I, N-terminal propeptide of type III procollagen (PIIINP) and C-terminal cross-linked telopeptide of type I collagen (ICTP). Abnormal IGF-I, PIIINP and ICTP levels were observed during the follow-up period in 7/100 (7%), 9/100 (9.0%) and 8/100 (8%) samples of the male group, respectively, and in 9/88 (10.2%), 1/88 (1.1%) and 0/88 (0%) samples of the female group, respectively. Abnormal levels appeared to be randomly distributed over the different periods of the sporting season and within male and female subjects, with the large majority of abnormal values being found in the younger athletes. Taking into account all the tests done during the 6-month period (no. 564), individual markers falling outside the normal range (for age) were observed in a small number of instances (34/564 tests done, 24/300 for males and 10/264 for females). When our method for the detection of exogenous recombinant GH (rhGH) administration, based on the concomitant determination of these three peripheral GH-dependent markers and on the attribution of specific scores, was applied in the same athlete at a given time point of the 6-month period, the prevalence of a positive score was extremely low (ie, 3/188 samples or 1.6%). Total positive scores were actually recorded in only three male athletes (2 swimmers and 1 skier, aged <21 yr) at one occasion during the 6-month period considered. In contrast, no total positive scores were found in female athletes (ie, 0/88 samples). In conclusion, the concentrations of IGF-I, PIIINP and ICTP were stable and not significantly modified during 6 months of a sporting season (entailing periods of training, competition, recovery, resting) in athletes from different sporting disciplines. Therefore our method, based on the concomitant determination of three peripheral GH-dependent biomarkers appears safe, acceptable, relatively inexpensive and repeatable (in case of positive or suspected values) immediately or at different intervals of the sporting season. Further additional studies are requested to precise the cut-off values for narrower age-class subdivisions in both genders in order to improve the proposed method.

Growth hormone isoforms, segments/fragments: Does a link exist with multifunctionality?

Understanding the relationship between growth hormone (GH) structure (its molecular fragments) and function via interaction with single or multiple receptors is of particular importance in clinical diagnostics and physiologic biochemistry. Direct and indirect actions of GH are numerous ranging from carbohydrate and lipid metabolism to growth effects at muscle and vessels. To this end, we have focused on the influence of physical exercise on GH synthesis and release into the circulation. Physical exercise is a physiological condition to which GH multifunctionality is inextricably linked and is thus important physiologically and pathologically. This review describes the potential human GH fragments with respect to protein hormone multifunctionality and the molecular regions of potential action. The intent of the review is to highlight human GH fragments and hypothesize their potential physiologic role. GH fragmentation is also reviewed in relation to the effects of physical exercise and hormone multifunctionality.

Testosterone inhibition of growth hormone release stimulated by a growth hormone secretagogue: studies in the rat and dog

Anabolic steroids are frequently taken by athletes and bodybuilders together with recombinant human GH (rhGH), though there is some scientific evidence that the use of anabolic steroids reverses the rhGH-induced effects. Recently, we have shown that treatment with rhGH (0.2 IU/kg s.c., daily x 12 days) in the dog markedly reduced the canine GH (cGH) responses stimulated by EP51216, a GH secretagogue (GHS), evaluated after 3 and 5 daily rhGH injections, and that the inhibition was still present a few days after rhGH discontinuation. The aim of the present study was to evaluate in the dog the GH response to EP51216 (125 mug/kg i.v.) in a condition of enhanced androgenic function (i.e. acute injection or 15-day treatment with testosterone at the dose of 2 mg/kg i.m. on alternate days), and in the hypophysectomized rat the hypothalamic and hippocampal expression of ghrelin, the receptor of GHSs (GHS-R), GH-releasing hormone (GHRH) and somatostatin (SS) after specific hormonal replacement therapies (testosterone, 1 mg/kg/day s.c.; hydrocortisone, 500 mug/kg/day s.c.; rhGH, 400 mug/kg/day s.c.; 0.9% saline 0.1 ml/kg/day s.c.; x11 days). In the dog experiments, under baseline conditions, a single injection of EP51216 elicited an abrupt rise of plasma cGH. Twenty-four hours from the acute bolus injection of testosterone, C(max) and AUC(0-90) of the GHS-stimulated cGH response were significantly lower than baseline cGH response; 5 days later, there was still a significant decrease of either parameter versus the original values. Short-term treatment with testosterone markedly reduced the GHS-stimulated cGH responses evaluated during (5th bolus) and at the end (8th bolus) of testosterone treatment. Four and 8 days after testosterone withdrawal, the EP51216-stimulated cGH response was still significantly reduced when compared with that under baseline conditions. Plasma concentrations of insulin-like growth factor 1 (IGF-1) were stable until the 5th bolus of testosterone and decreased progressively in the remaining time of the testosterone treatment; 4 and 8 days from treatment withdrawal, IGF-1 levels were still suppressed. In rat studies, hypothalamic mRNA levels of GHS-R were significantly reduced by treatments with testosterone and hydrocortisone, whereas hippocampal expressions of ghrelin, GHRH and SS were reduced by rhGH replacement therapy. In conclusion, these studies show that a single administration of testosterone can abrogate the cGH response ensuing acute stimulation by a GHS; the inhibitory effect of testosterone on the cGH response to GHS is present during and even 8 days after termination of a short-lived treatment with testosterone; these events occur via a