Pulsatile Growth Hormone Release Research Articles

Effect of Anamorelin, a Ghrelin Receptor Agonist, on Muscle and Bone in Adults With Osteosarcopenia.

Anamorelin, a ghrelin receptor agonist known to stimulate the pulsatile release of GH from the pituitary, has the potential to improve musculoskeletal health in adults with osteosarcopenia. To determine the effect of anamorelin treatment for 1 year on muscle mass and strength and on biochemical markers of bone turnover in adults with osteosarcopenia (OS). Randomized, placebo-controlled, 1-year anamorelin intervention trial. The Bone Metabolism Laboratory at the USDA Nutrition Center at Tufts University. 26 men and women, age 50 years and older, with OS. Muscle mass by D3-creatine dilution and lean body mass (LBM) and bone mineral density (BMD) by dual-energy X-ray absorptiometry, muscle strength, serum IGF-1, and bone turnover markers, serum procollagen 1 intact N-terminal (P1NP), and C-terminal telopeptide (CTX). Anamorelin did not have a significant effect on muscle mass or LBM; it significantly increased knee flexion torque at 240°/s by 20% (P = .013) and had a similar nonstatistically significant effect on change in knee extension; it increased bone formation (P1NP) by 75% (P = .006) and had no significant effect on bone resorption (CTX) or BMD. Serum IGF-1 increased by 50% in the anamorelin group and did not change in the placebo group (P = .0001 for group difference). In this pilot study, anamorelin did not significantly alter muscle mass; however, it may potentially improve lower extremity strength and bone formation in addition to increasing circulating IGF-1 levels in adults with OS. Further study of anamorelin in this population is warranted.

Binding Domain Characterization of Growth Hormone Secretagogue Receptor.

Background and ObjectivesActivation of ghrelin receptor growth hormone secretagogue receptor (GHS-R) by endogenous or synthetic ligands amplifies pulsatile release of growth hormone (GH) and enhances food intake, very relevant to development and growth. GHS-R is a G-protein coupled receptor that has great druggable potential. Understanding the precise ligand and receptor interactions is crucial to advance the application of GHS-R.Materials and MethodsWe used radiolabeled ligand-binding assay and growth hormone release assay to assess the binding and functional characteristics of GHS-R to synthetic agonists MK-0677 and GHS-25, as well as to endogenous peptide ligand ghrelin. We analyzed the ligand-dependent activity of GHS-R by measuring aequorin-based [Ca++]i responses. To define a ligand-binding pocket of GHS-R, we generated a series of human/puffer fish GHS-R chimeras by domain swapping, as well as a series of mutants by site-directed mutagenesis.ResultsWe found that the synthetic ligands have high binding affinity to GHS-R in the in vitro competitive binding assay. Remarkably, the in vivo GH secretagogue activity is higher with the synthetic agonists MK-0677 and GHS-25 than that of ghrelin. Importantly, the activity was completely abolished in GHS-R knockout mice. In GHS-R chimera analysis, we identified the C-terminal region, particularly the transmembrane domain 6 (TM6), to be critical for the ligand-dependent activity. Our site-directed mutagenesis study further revealed that amino acid residues D99 and W276 in GHS-R are essential for ligand binding. Interestingly, critical residues distinctively interact with different ligands, MK-0677 activation depends on E124, while ghrelin and GHS-25 preferentially interact with F279.ConclusionThe ligand-binding pocket of human GHS-R is mainly defined by interactive residues in TM6 and the adjacent region of the receptor. This novel finding in GHS-R binding domains advances the structural/ functional understanding of GHS-R, which will help to select/design better GHS-R agonists/ antagonists for future therapeutic applications.

The GH-IGF-1 Axis in Circadian Rhythm.

Organisms have developed common behavioral and physiological adaptations to the influence of the day/night cycle. The CLOCK system forms an internal circadian rhythm in the suprachiasmatic nucleus (SCN) during light/dark input. The SCN may synchronize the growth hormone (GH) secretion rhythm with the dimming cycle through somatostatin neurons, and the change of the clock system may be related to the pulsatile release of GH. The GH—insulin-like growth factor 1 (IGF-1) axis and clock system may interact further on the metabolism through regulatory pathways in peripheral organs. We have summarized the current clinical and animal evidence on the interaction of clock systems with the GH—IGF-1 axis and discussed their effects on metabolism.

The Safety and Efficacy of Growth Hormone Secretagogues.

Growth hormone (GH) increases lean body mass, decreases fat mass, increases exercise tolerance and maximum oxygen uptake, enhances muscle strength, and improves linear growth. Long-term studies of GH administration offer conflicting results on its safety, which has led to strict Food and Drug Administration criteria for GH use. The potential drawbacks of exogenous GH use are believed to be due in part to impaired regulatory feedback. To review the literature on GH secretagogues (GHSs), which include GH-releasing peptides and the orally available small-molecule drug ibutamoren mesylate. Review of clinical studies on the safety and efficacy of GHSs in human subjects. Report on the physiologic changes from GHS use in human subjects including its safety profile. GHSs promote pulsatile release of GH that is subject to negative feedback and can prevent supra-therapeutic levels of GH and their sequelae. To date, few long-term, rigorously controlled studies have examined the efficacy and safety of GHSs, although GHSs might improve growth velocity in children, stimulate appetite, improve lean mass in wasting states and in obese individuals, decrease bone turnover, increase fat-free mass, and improve sleep. Available studies indicate that GHSs are well tolerated, with some concern for increases in blood glucose because of decreases in insulin sensitivity. Further work is needed to better understand the long-term impact of GHSs on human anatomy and physiology and more specifically in the context of a diversity of clinical scenarios. Furthermore, the safety of these compounds with long-term use, including evaluation of cancer incidence and mortality, is needed. Sigalos JT, Pastuszak AW. The Safety and Efficacy of Growth Hormone Secretagogues. Sex Med Rev 2018;6:45-53.

Hyperphagia in male melanocortin 4 receptor deficient mice promotes growth independently of growth hormone.

Loss of function of the melanocortin 4 receptor (MC4R) results in hyperphagia, obesity and increased growth. Despite knowing that MC4Rs control food intake, we are yet to understand why defects in the function of the MC4R receptor contribute to rapid linear growth. We show that hyperphagia following germline loss of MC4R in male mice promotes growth while suppressing the growth hormone-insulin-like growth factor-1 (GH-IGF-1) axis. We propose that hyperinsulinaemia promotes growth while suppressing the GH-IGF-1 axis. It is argued that physiological responses essential to maintain energy flux override conventional mechanisms of pubertal growth to promote the storage of excess energy while ensuring growth. Defects in melanocortin-4-receptor (MC4R) signalling result in hyperphagia, obesity and increased growth. Clinical observations suggest that loss of MC4R function may enhance growth hormone (GH)-mediated growth, although this remains untested. Using male mice with germline loss of the MC4R, we assessed pulsatile GH release and insulin-like growth factor-1 (IGF-1) production and/or release relative to pubertal growth. We demonstrate early-onset suppression of GH release in rapidly growing MC4R deficient (MC4RKO) mice, confirming that increased linear growth in MC4RKO mice does not occur in response to enhanced activation of the GH-IGF-1 axis. The progressive suppression of GH release in MC4RKO mice occurred alongside increased adiposity and the progressive worsening of hyperphagia-associated hyperinsulinaemia. We next prevented hyperphagia in MC4RKO mice through restricting calorie intake in these mice to match that of wild-type (WT) littermates. Pair feeding of MC4RKO mice did not prevent increased adiposity, but attenuated hyperinsulinaemia, recovered GH release, and normalized linear growth rate to that seen in pair-fed WT littermate controls. We conclude that the suppression of GH release in MC4RKO mice occurs independently of increased adipose mass, and is a consequence of hyperphagia-associated hyperinsulinaemia. It is proposed that physiological responses essential to maintain energy flux (hyperinsulinaemia and the suppression of GH release) override conventional mechanisms of pubertal growth to promote the storage of excess energy while ensuring growth. Implications of these findings are likely to extend beyond individuals with defects in MC4R signalling, encompassing physiological changes central to mechanisms of growth and energy homeostasis universal to hyperphagia-associated childhood-onset obesity.

Effect of Deletion of Ghrelin-O-Acyltransferase on the Pulsatile Release of Growth Hormone in Mice.

Ghrelin, a gut hormone originating from the post-translational cleavage of preproghrelin, is the endogenous ligand of growth hormone secretagogue receptor 1a (GHS-R1a). Within the growth hormone (GH) axis, the biological activity of ghrelin requires octanoylation by ghrelin-O-acyltransferase (GOAT), conferring selective binding to the GHS-R1a receptor via acylated ghrelin. Complete loss of preproghrelin-derived signalling (through deletion of the Ghrl gene) contributes to a decline in peak GH release; however, the selective contribution of endogenous acyl-ghrelin to pulsatile GH release remains to be established. We assessed the pulsatile release of GH in ad lib. fed male germline goat(-/-) mice, extending measures to include mRNA for key hypothalamic regulators of GH release, and peripheral factors that are modulated relative to GH release. The amount of GH released was reduced in young goat(-/-) mice compared to age-matched wild-type mice, whereas pulse frequency and irregularity increased. Altered GH release did not coincide with alterations in hypothalamic Ghrh, Srif, Npy or Ghsr mRNA expression, or pituitary GH content, suggesting that loss of Goat does not compromise canonical mechanisms that contribute to pituitary GH production and release. Although loss of Goat resulted in an irregular pattern of GH release (characterised by an increase in the number of GH pulses observed during extended secretory events), this did not contribute to a change in the expression of sexually dimorphic GH-dependent liver genes. Of interest, circulating levels of insulin-like growth factor (IGF)-1 were elevated in goat(-/-) mice. This rise in circulating levels of IGF-1 was correlated with an increase in GH pulse frequency, suggesting that sustained or increased IGF-1 release in goat(-/-) mice may occur in response to altered GH release patterning. Our observations demonstrate that germline loss of Goat alters GH release and patterning. Although the biological relevance of altered GH secretory patterning remains unclear, we propose that this may contribute to sustained IGF-1 release and growth in goat(-/-) mice.

Actions of NPY, and its Y1 and Y2 receptors on pulsatile growth hormone secretion during the fed and fasted state.

The hypothalamic NPY system plays an important role in regulating food intake and energy expenditure. Different biological actions of NPY are assigned to NPY receptor subtypes. Recent studies demonstrated a close relationship between food intake and growth hormone (GH) secretion; however, the mechanism through which endogenous NPY modulates GH release remains unknown. Moreover, conclusive evidence demonstrating a role for NPY and Y-receptors in regulating the endogenous pulsatile release of GH does not exist. We used genetically modified mice (germline Npy, Y1, and Y2 receptor knock-out mice) to assess pulsatile GH secretion under both fed and fasting conditions. Deletion of NPY did not impact fed GH release; however, it reversed the fasting-induced suppression of pulsatile GH secretion. The recovery of GH secretion was associated with a reduction in hypothalamic somatotropin release inhibiting factor (Srif; somatostatin) mRNA expression. Moreover, observations revealed a differential role for Y1 and Y2 receptors, wherein the postsynaptic Y1 receptor suppresses GH secretion in fasting. In contrast, the presynaptic Y2 receptor maintains normal GH output under long-term ad libitum-fed conditions. These data demonstrate an integrated neural circuit that modulates GH release relative to food intake, and provide essential information to address the differential roles of Y1 and Y2 receptors in regulating the release of GH under fed and fasting states.

Repression of multiple CYP2D genes in mouse primary hepatocytes with a single siRNA construct.

The Cyp2d subfamily is the second most abun-dant subfamily of hepatic drug-metabolizing CYPs. In mice, there are nine Cyp2d members that are believed to have redundant catalytic activity. We are testing and optimizing the ability of one short interfering RNA (siRNA) construct to knockdown the expression of multiple mouse Cyp2ds in primary hepatocytes. Expression of Cyp2d10, Cyp2d11, Cyp2d22, and Cyp2d26 was observed in the primary male mouse hepatocytes. Cyp2d9, which is male-specific and growth hormone-dependent, was not expressed in male primary hepatocytes, potentially because of its dependence on pulsatile growth hormone release from the anterior pituitary. Several different siRNAs at different concentrations and with different reagents were used to knockdown Cyp2d expression. siRNA constructs designed to repress only one construct often mildly repressed several Cyp2d isoforms. A construct designed to knockdown every Cyp2d isoform provided the best results, especially when incubated with transfection reagents designed specifically for primary cell culture. Interestingly, a construct designed to knockdown all Cyp2d isoforms, except Cyp2d10, caused a 2.5× increase in Cyp2d10 expression, presumably because of a compensatory response. However, while RNA expression is repressed 24h after siRNA treatment, associated changes in Cyp2d-mediated metabolism are tenuous. Overall, this study provides data on the expression of murine Cyp2ds in primary cell lines, valuable information on designing siRNAs for silencing multiple murine CYPs, and potential pros and cons of using siRNA as a tool for repressing Cyp2d and estimating Cyp2d's role in murine xenobiotic metabolism.

Does the dawn phenomenon have clinical relevance in normal pregnancy?

The dawn phenomenon is a transient rise in blood glucose between 4 and 6 am that is attributed to the pulsatile release of pituitary growth hormone (GH). In pregnancy, GH is suppressed by placental GH. Hence, we hypothesize that there is no evidence for the dawn phenomenon in late pregnancy in healthy women. Twenty glucose-tolerant women with singleton gestations between 28 weeks and 36 weeks 6 days' gestation were recruited. The women were admitted overnight to the Clinical Research Unit and had continuous glucose monitoring. Insulin and GH were measured at 2-hour intervals from 8 pm to 8 am. GH was grouped into times 1A (8-10 pm), 2A (12-2 am), and 3A (4-8 am) for changes over time. Further analysis was performed with time 1B (8 pm to 2 am) and 2B (4-8 am). Insulin was measured between 4 and 8 am. Plasma glucose decreased over time (P < .001). There were no significant changes in GH among times 1A, 2A, and 3A (P = .45) or times 1B and 2B (P = .12). Insulin concentrations increased after meals, but there were no changes from 4 am (8.5 ± 1.4 μU/mL) through 8 am (8.6 ± 1.1 μU/mL; P = .98). Glucose and insulin concentrations show no increase from 4-8 am; although there is variability in GH, there is no evidence for the dawn phenomenon in late pregnancy in healthy women.

Distribution and morphology of the juxtapositions between growth hormone-releasing hormone-(ghrh)-immunoreactive neuronal elements

Previous studies revealed that growth hormone-releasing hormone (GHRH)-immunoreactive (IR) neurons form a circumscribed cell group in the basal infundibulum/median eminence of the human hypothalamus. GHRH from these neurons is released into the hypothalamo–hypophyseal portal circulatory system in a pulsatile manner. It is a common consensus that the pulsatile release of GHRH is the main driving force behind the pulsatile release of growth hormone (GH) and may contribute to the regulation of other hypothalamic functions. The pulsatile release of GHRH requires synchronized activity of GHRH-IR neurons. However, the morphological basis of this synchronization between the GHRH-IR neural elements has not been elucidated yet. Since the utilization of electron microscopy combined with immunohistochemistry is virtually impossible in the human brain due to the long post mortem period, immunohistochemistry, evaluated with oil immersion light microscopy, was used in order to reveal the associations between the GHRH elements. Numerous GHRH–GHRH juxtapositions have been detected in the infundibular area/median eminence, where GHRH-IR axonal varicosities often formed multiple contacts with GHRH-IR perikarya. Examination of these associations with high magnification oil immersion light microscopy revealed (1) axonal swellings at the site of the contacts and (2) no gaps between the contacting elements suggesting that these juxtapositions may be functional synapses. The large number of GHRH–GHRH juxtapositions in the infundibular area/median eminence suggests that these synapse-like structures may represent the morphological substrate of the synchronized activity of GHRH neurons that in turn may result in the pulsatile release of GHRH in human.

A super-agonist of growth hormone–releasing hormone causes rapid improvement of nutritional status in patients with chronic kidney disease

Chronic kidney disease is frequently associated with protein-energy wasting related to chronic inflammation and a resistance to anabolic hormones such as insulin and growth hormone (GH). In this study, we determined whether a new GH-releasing hormone super-agonist (AKL-0707) improved the anabolism and nutritional status of nondialyzed patients with stage 4-5 chronic kidney disease randomized to twice daily injections of the super-agonist or placebo. After 28 days, this treatment significantly increased 24-h GH secretion by almost 400%, without altering the frequency or rhythmicity of secretory bursts or fractional pulsatile GH release, and doubled the serum insulin-like growth factor-1 level. There was a significant change in the Subjective Global Assessment from 'mildly to moderately malnourished' to 'well-nourished' in 6 of 9 patients receiving AKL-0707 but in none of 10 placebo-treated patients. By dual-energy X-ray absorptiometry, both the mean fat-free mass and the body mineral content increased, but fat mass decreased, all significantly. In the AKL-0707-treated group, both serum urea and normalized protein equivalent of nitrogen appearance significantly decreased with no change in dietary protein intake, indicating a protein anabolic effect of treatment. Thus, our study shows that stimulation of endogenous GH secretion by AKL-0707 overcomes uremic catabolism of patients with advanced chronic kidney disease.

Growth Hormone Molecular Weight Variant Responses To Resistance Exercise Differ In Men Vs. Women

Growth hormone (GH) exists as a family of molecular weight (MW) variants with different biological activities and whose response patterns to exercise are poorly understood. Whereas the pulsatile release of GH is known to be sexually dimorphic, no data are available that directly compare GH MW variant exercise responses in men and women. Such information may be important in understanding sex-related differences in exercise and metabolic adaptations. PURPOSE: To compare GH MW variant responses to acute resistance exercise in men and women. METHODS: 10 men and 10 women performed an acute exercise test (AET; 6 sets of 10 RM squats), and had blood drawn pre, mid, and post (0, +15, and +30 min post) of the AET. Serum samples were fractionated using high pressure liquid chromatography into 3 MW pools (A: >60 kDa; B: 30-60 kDa; C: <30 kDa) which correspond to GH oligomers, GH dimers and GH monomers, respectively. GH concentrations were determined using a commercially-available immunoassay. Data were analyzed with repeated measures ANOVA. RESULTS: Across all time points and sex, Fraction C (GH monomer) was the highest MW variant collected (C: 14.3±1.8 > A: 9.6±1.7 > B: 5.2±0.7 ng; p<0.05). Interactions with fraction, time, and sex (p<0.01) indicated that women experienced GH increases in all 3 MW variants examined from pre to mid exercise (A: pre: 7.7±7.1 vs mid: 14.9±12.4; B: pre: 4.1±3.1 vs. mid: 6.8±5.3; C: pre: 8.9±7.2 vs. mid: 19.7±15.8), while men did not increase until post-exercise (A: pre: 2.1±2.5 vs. +15 post: 11.0±8.5; B: pre: 0.9±1.1 vs. post: 5.7±3.7; C: pre: 2.7±2.3 vs. post: 17.1±8.9). In recovery, men maintained elevations of the B (pre: 0.9±1.1 vs. +30 post: 5.5±3.9) and C fractions (pre: 2.7±2.3 vs. +30 post: 14.9±10.7) for at least 30 min (p<0.05), while all fractions in women returned to baseline within 15 min (p>0.05). CONCLUSIONS: Women experienced earlier increases in all GH variants during exercise whereas men demonstrated more sustained increases in GH monomers and dimers in the post-exercise milieu. Temporal GH molecular variant responses to exercise differ in men and women.

Experimental arthritis: Effect on growth parameters and total skeletal calcium

Aim of the study was to investigate the possible mechanisms leading to stunted growth and osteoporosis in experimental arthritis. Fourty-two female rats of 7–8 weeks of age were randomly assigned to three groups of 14 animals each: (a) controls; (b) adjuvant-inoculated (AA); and (c) adjuvant-inoculated rats receiving 10 mg cyclosporin A (CsA) orally for 30 days. Biological parameters studied were: hindpaw swelling; vertebral length progression expressed as Δ increments between days 1 and 30 as a parameter of skeletal growth, and estimation of total skeletal mineral content by dual energy X-ray absorptiometry ( n = 10 each group) on day 30. Endocrine parameters measured were pulsatile release of growth hormone (rGH) on day 30 following jugular cannulation and measurement of insulin-like growth factor (IGF-1) in pooled plasma from rGH profiles. Results can be summarized as follows: Untreated AA rats exhibited local signs of inflammation in comparison with controls (hindpaw diameter 8.1–8.9 mm vs. 5.3–5.6 mm in controls). Treatment with CsA normalized this parameter (4.9–5.6 mm). Vertebral growth was significantly retarded in AA rats in comparison with controls (214 ± 32 vs. 473 ± 33 μm; p < 0.001). Administration of CsA normalized vertebral size increment with a clear tendency to overgrowth (523 ± 43 μm, n.s.). There was also a marked reduction in total skeletal mineral content in diseased (AA) rats as compared to controls (5.8 ± 0.1 vs. 7.5 ± 0.1 g [OH-apatite]; p < 0.001), and a moderate but significant increment above controls in the group receiving CsA (8.0 ± 0.1 vs. 7.5 ± 0.1 g [OH-appatite]; p < 0.04). Integrated rGH profiles exhibited a significant fall in arthritic rats and were completely restored to normal under CsA treatment. A trend toward higher rGH values was observed in the latter group (2908 ± 554 in AA vs. 8317 ± 1492 ng/ml/240 min in controls; p < 0.001, and 10940 ± 222 ng/ml/240 min, n.s. in the CsA group). There was a good correlation between skeletal growth and rGH pulsatility ( r = 0.81; p < 0.001). IGF-1 followed a similar pattern (630 ± 44 in AA vs. 752 ± 30 ng/ml in controls; p < 0.04, and 769 ± 59 ng/ml in the CsA group, n.s. vs. controls). Thus, a clear tendency to skeletal overgrowth following treatment was observed in agreement with the hormonal data. It can therefore be concluded that, in experimental arthritis, attenuated GH-spiking and reduced circulating IGF-1 appear to be causally related to growth retardation, probably mimicking signs and symptoms observed in juvenile arthritis. Therapy with CsA is followed by normalization of hormonal and biological parameters accompanied by a catch up phenomenon in skeletal growth which is also observed clinically in juvenile arthritis. Generalized osteopenia is a prominent feature seemingly connected with the growth abnormalities as they parallel each other during the evolution of the disease and respond equally to therapy.

Use of Growth Hormone Secretagogues to Prevent or Treat the Effects of Aging: Not Yet Ready for Prime Time

In this issue, Nass and colleagues report that at 1 year, an oral ghrelin mimetic increased pulsatile growth hormone secretion and morning insulin-like growth factor I concentration to that in heal...

Synthesis and Pharmacological Profile of an Orally-Active Growth Hormone Secretagogue, SM-130686

Hypothalamic hormones physiologically regulate pulsatile release of growth hormone (GH) from the anterior pituitary gland. Since the discovery of these hormones in the 1970s, several new chemically synthesized peptidyl and non-peptidyl derivatives have been proved to stimulate and amplify GH secretion, and this series of molecules has been named the growth hormone secretagogues (GHSs). One of these compounds led to the discovery of a GPCR-type receptor for GHSs (GHS-R), and subsequently the endogenous ligand for the receptor has been identified, and is referred to as ghrelin. The identification of GHSs as physiological regulators of GH secretion encouraged us to examine our GHSs pharmacologically. We previously reported that novel oxindole derivatives have been identified as GHS-R agonists from our internal chemical library. Among these derivatives, (+)-6-carbamoyl-3-(2-chlorophenyl)-(2-diethylaminoethyl)-4-trifluoromethyloxindole (SM-130686, 37S) was found to have potent activity in vitro with a good pharmacokinetic profile in rats (bioavailability of 28%). In this article, we review the synthesis and pharmacological evaluation of SM-130686. SM-130686 binds specifically to GHS-R and increases the Ca(2+) concentration in Chinese hamster ovary cells expressing recombinant GHS-R. Maximal enhancement of the intracellular Ca(2+) concentration induced by SM-130686 treatment was approximately 55% that induced by ghrelin, suggesting that SM-130686 may be a partial GHS-R agonist. Also, in in vivo studies, oral administration of SM-130686 increased body length and fat-free mass gain. We compare the pharmacological profile of SM-130686 with other GHSs, including GHRH and ghrelin, and discuss the therapeutic usefulness of GHSs against several disorders, as well as for treatment of GH deficiency.

Growth Hormone Secretagogues

Growth hormone secretagogues (GHSs) are synthetic molecules that stimulate and amplify pulsatile pituitary growth hormone release, via a separate pathway distinct from GH releasing hormone/somatostatin. The activity of GHSs is not fully specific for GH secretion; some GHSs also have slight releasing activity on other pituitary hormones and mediate GH independent biological activities. The first GHSs were discovered in 1977. Since then, an intensive research to synthesize a potent oral GHSs has been undertaken. Although the potential applications of GHSs are numerous, long term trials are needed to evaluate the clinical efficacy and safety of these substances. In the present article we review the historic background of GHSs, their potential clinical uses, the types and the main GHSs that have been synthesized hitherto.

Expression Analysis of Hypothalamic and Pituitary Components of the Growth Hormone Axis in Fasted and Streptozotocin-Treated Neuropeptide Y (NPY)-Intact (NPY+/+) and NPY-Knockout (NPY –/–) Mice

In the fasted and the streptozotocin (STZ)-induced diabetic male rat, hypothalamic growth hormone (GH)-releasing hormone (GHRH) mRNA levels, and pulsatile GH release are decreased. These changes are believed to be due to a rise in hypothalamic neuropeptide Y (NPY) that inhibits GHRH expression. To directly test if NPY is required for metabolic regulation of hypothalamic neuropeptides important in GH secretion, NPY, GHRH and somatostatin (SRIH) mRNA levels were determined in fasted (48 h) and STZ-treated wild-type (NPY^+/+) and NPY-knockout (NPY^–/–) mice by ribonuclease protection assay. In addition, pituitary receptor mRNA levels for GHRH (GHRH-R), ghrelin (GHS-R) and SRIH (sst2) were assessed by RT-PCR. Under fed conditions the GH axis of NPY^+/+ and NPY^–/– did not differ. In the NPY^+/+ mouse, fasting resulted in a 23% weight loss and >250% increase in NPY mRNA accompanied by a significant reduction in both GHRH and SRIH mRNA. These changes were associated with increases in pituitary expression of GHRH-R and GHS-R and a concomitant suppression of sst2. In the NPY^–/– mouse, fasting also resulted in a 23% weight loss and comparable changes in GHRH-R and sst2, but failed to alter GHRH, SRIH and GHS-R mRNA levels. Fasting resulted in an overall increase in circulating GH, which reached significance in the fasted NPY^–/– mouse. Induction of diabetes in NPY^+/+ mice, using a single, high-dose, STZ injection (150 mg/kg), resulted in modest weight loss (5%), and a 158% increase NPY expression which was associated with reciprocal changes in pituitary GHS-R and sst2 expression, similar to that observed in the fasted state, but no change in hypothalamic GHRH or SRIF expression was observed. Induction of diabetes in NPY^+/+ and NPY^–/– mice, using a multiple, low-dose, STZ paradigm (5 consecutive daily injections of 40 mg/kg), did not alter body weight, hypothalamic neuropeptide expression or pituitary receptor expression, with the exception that sst2 mRNA levels were suppressed and GH levels did rise in the NPY^–/– mouse. These observations demonstrate that NPY is not required for basal regulation of the GH axis, but is required for fasting-induced suppression of GHRH and SRIH expression, as well as fasting-induced augmentation of pituitary GHS-R mRNA. In contrast to the rat, fasting clearly did not suppress circulating GH levels in mice, but resulted in an overall rise in mean GH levels, similar to that observed in other mammalian species. The fact that many of the fasting-induced changes in the GH axis were observed in the high-dose STZ-treated mice, but were not observed in the multiple, low-dose paradigm, suggests STZ-mediated modulation of GH axis function is dependent on the severity of the catabolic state and not hyperglycemia.

Altered secretion of growth hormone and luteinizing hormone after 84 h of sustained physical exertion superimposed on caloric and sleep restriction

The pulsatile release of growth hormone (GH) and luteinizing hormone (LH) from the anterior pituitary gland is integral for signaling secretion of insulin-like growth factor (IGF)-I and testosterone, respectively. This study examined the hypothesis that 84 h of sustained physical exertion with caloric and sleep restriction alters the secretion of GH and LH. Ten male soldiers [22 yr (SD 3), 183 cm (SD 7), 87 kg (SD 8)] had blood drawn overnight from 1800 to 0600 every 20 min for GH, LH, and leptin and every 2 h for IGF-I (total and free), IGF binding proteins-1 and -3, testosterone (total and free), glucose, and free fatty acids during a control week and after 84 h of military operational stress. Time-series cluster and deconvolution analyses assessed the secretion parameters of GH and LH. Significant results (P < or = 0.05) were as follows: body mass (-3%), fat-free mass (-2.3%), and fat mass (-7.3%) declined after military operational stress. GH and LH secretion burst amplitude (approximately 50%) and overnight pulsatile secretion (approximately 50%), IGF binding protein-1 (+67%), and free fatty acids (+33%) increased, whereas leptin (-47%), total (-27%) and free IGF-I (-32%), total (-24%) and free testosterone (-30%), and IGF binding protein-3 (-6%) decreased. GH and LH pulse number were unaffected. Because GH and LH positively regulate IGF-I and testosterone, these data imply that the physiological strain induced a certain degree of peripheral resistance. During periods of energy deficiency, amplitude modulation of GH and LH pulses may precede alterations in pulse numbers.

Effects of the selective estrogen receptor modulator LY117018 on growth hormone secretion: in vitro studies

Sex steroids play an important role in modulating pulsatile growth hormone (GH) release, acting at both hypothalamic and pituitary level in both humans and experimental animals. Selective estrogen receptor modulators (SERMs) act as either estrogen receptor agonists or antagonists in a tissue-selective manner. In postmenopausal women, serum GH levels correlate positively with endogenous estradiol levels and insulin-like grwoth factor-I (IGF-I) is positively related to bone mineral density (BMD) at the spine and hip. The aim of the present study was to evaluate, for the first time, the direct effect of LY117018, an analog of raloxifene, on GH secretion from both human and rodent pituitary cells in vitro. Our results demonstrated that pharmacological concentrations of the raloxifene analog LY117018 can stimulate GH secretion through a direct action on the pituitary. LY117018 also showed an estrogen-like activity, inducing the proliferation of rat pituitary GH-secreting adenomatous cells (GH1).

Evaluation of the Role of Melanocortin 3 and 4 Receptors in Leptin-Stimulated and Spontaneous Growth Hormone Secretion in Rats

It has been reported that the melanocortin 4-receptor (MC4-R) may act downstream of leptin to mediate its effects on food intake and several neuroendocrine functions (the reproductive system, the hypothalamo-pituitary-thyroid axis, and prolactin secretion). However, no previous study examined whether MC4-R mediates leptin stimulatory actions on growth hormone (GH) secretion, or whether MC4-R signaling is involved in spontaneous pulsatile GH release in fed rats. Therefore in this study we examined the involvement of both MC3-R and MC4-R (the predominant MC-R subtypes expressed in the brain) in these two aspects of GH secretion in freely-moving male rats. In both fed and 3-day fasted rats, plasma GH levels were determined every 15 min over 5 h after single intracerebroventricular injections of the following substances or vehicle. Fasting diminished and leptin (0.3 nmol) reinstated the GH pulse amplitude without affecting the pulse frequency. Neither HS014 (1.0 nmol, a selective MC4-R antagonist) nor agouti-related peptide (1.0 nmol, a non-selective MC3/4-R antagonist) was effective in altering leptin-stimulated or spontaneous GH secretion. In addition, neither melanotan-II (1.0 nmol, a non-selective MC3/4-R agonist) nor γ₁-melanocyte-stimulating hormone (10 nmol, a selective MC3-R agonist) affected significantly GH release in fasted rats. We have previously demonstrated that stimulation or blockade of MC4-R, achieved by the same drug dosage as in this study, significantly affect luteinizing hormone and prolactin secretion in rats. The present results thus suggest that neither MC4-R nor MC3-R is involved in leptin-stimulated or spontaneous GH secretion, or at least that the level of MC4-R involvement in GH secretion is much lower than that in luteinizing hormone and prolactin release regulation.