Nutritional status in the neuroendocrine control of growth hormone secretion: the model of anorexia nervosa

Frontiers in Neuroendocrinology Frontiers in Neuroendocrinology 24 (2003) 200–224 www.elsevier.com/locate/yfrne

Nutritional status in the neuroendocrine control of growth hormone secretion: the model of anorexia nervosa Massimo Scacchi, Angela Ida Pincelli, and Francesco Cavagnini* Chair of Endocrinology, University of Milan, Ospedale San Luca IRCCS, Istituto Auxologico Italiano, Milan, Italy

Abstract Growth hormone (GH) plays a key role not only in the promotion of linear growth but also in the regulation of intermediary metabolism, body composition, and energy expenditure. On the whole, the hormone appears to direct fuel metabolism towards the preferential oxidation of lipids instead of glucose and proteins, and to convey the energy derived from metabolic processes towards the synthesis of proteins. On the other hand, body energy stores and circulating energetic substrates take an important part in the regulation of somatotropin release. Finally, central and peripheral peptides participating in the control of food intake and energy expenditure (neuropeptide Y, leptin, and ghrelin) are also involved in the regulation of GH secretion. Altogether, nutritional status has to be regarded as a major determinant in the regulation of the somatotropin–somatomedin axis in animals and humans. In these latter, overweight is associated with marked impairment of spontaneous and stimulated GH release, while acute dietary restriction and chronic undernutrition induce an amplification of spontaneous secretion together with a clear-cut decrease in insulin-like growth factor I (IGF-I) plasma levels. Thus, over- and undernutrition represent two conditions connoted by GH hypersensitivity and GH resistance, respectively. Anorexia nervosa (AN) is a psychiatric disorder characterized by peculiar changes of the GH–IGFI axis. In these patients, low circulating IGF-I levels are associated with enhanced GH production rate, highly disordered mode of somatotropin release, and variability of GH responsiveness to different pharmacological challenges. These abnormalities are likely due not only to the lack of negative IGF-I feedback, but also to a primary hypothalamic alteration with increased frequency of growth hormone releasing hormone discharges and decreased somatostatinergic tone. Given the reversal of the above alterations following weight recovery, these abnormalities can be seen as secondary, and possibly adaptive, to nutritional deprivation. The model of AN may provide important insights into the pathophysiology of GH secretion, in particular as regards the mechanisms whereby nutritional status effects its regulation. Ó 2003 Elsevier Science (USA). All rights reserved. Keywords: Anorexia nervosa; Nutritional status; Neuroendocrine; Growth hormone; Insulin-like growth factor I; Leptin; Ghrelin; Secretagogues; Macronutrients; Malnutrition

1. Introduction Growth hormone (GH) secretion is governed by a fine regulation effected by central and peripheral signals. The complex neuroendocrine control of GH release has recently been extensively reviewed [126,199]. In essence, the chief hypothalamic regulators of GH release are growth hormone releasing hormone (GHRH) and somatostatin (somatotropin release inhibiting hormone, SRIH). GHRH is able not only to stimulate pituitary GH secretion but also to promote hypothalamic SRIH

* Corresponding author. Fax: +39-02-58216777. E-mail address: [email protected] (F. Cavagnini).

output thus starting an autoregulatory circuit, whereas SRIH inhibits both GH release from the pituitary and GHRH secretion from the hypothalamus. The finding of specific hypothalamic and pituitary receptors for the recently synthesized GH secretagogues (GHS) has led to the hypothesis of a third neuroendocrine pathway affecting GH release. Indeed, an endogenous ligand for the GHS receptor has recently been isolated from the gastric mucosa of rodents and humans, and named ghrelin. This peptide, endowed with a strong GH-releasing effect, has proved to be also an effective stimulator of food intake in humans. A number of other neuroendocrine signals represented by neurotransmitters, neuropeptides, and neurohormones may also influence hypothalamic GHRH and SRIH output,

0091-3022/$ - see front matter Ó 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0091-3022(03)00014-1

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indirectly modulating GH secretion. Among the peripheral factors affecting GH release, a pivotal role is played by the negative feedback exerted at both hypothalamic and pituitary levels by insulin-like growth factor I (IGF-I), the chief mediator of most biological actions of GH. Somatotropin exerts a relevant physiological role in the regulation of fuel metabolism, body composition, and energy expenditure, and thus it is not surprising that metabolic substrates (glucose, amino acids, and lipids) participate in the control of GH release. This review will first describe the influence of GH on intermediary metabolism, body composition, and energy balance, and then focus on the effects exerted by macronutrients and by peptides involved in the control of food intake on GH secretion. In this context, the consequences of overand undernutrition will be illustrated, with particular reference to the model of anorexia nervosa (AN).

2. Influence of GH on intermediary metabolism, body composition, and energy expenditure Promotion of statural growth is just one of the numerous actions carried out by the so-called ‘‘growth hormone,’’ which also plays a fundamental role in the regulation of intermediary metabolism, body composition, and energy expenditure, as well as in the function of several organs and systems. GH administration to normal human subjects causes, after a transient insulin-like effect leading to a decrease in blood glucose, a rise in glycemic values. The early action, occurring within the first 2 h, is mediated by suppression of hepatic glucose output and increase in glucose clearance. The late and prevailing effect is attributable to a reduction of insulin sensitivity, with an attendant increase in post-prandial hepatic glucose output, and decrease in insulin-dependent glucose uptake by peripheral tissues [28,143]. GH increases hepatic glucose output by stimulating gluconeogenesis and glycogenolysis while reducing glucose utilization in muscle through inhibition of glycogen synthesis and glucose oxidation [44,128]. The inhibition exerted by GH on insulin-mediated activation of glycogen synthase in skeletal muscle likely implicates mechanisms distal to the binding of insulin to its own receptor [20]. More recently, GH has been shown to inhibit insulin-dependent phosphorylation of insulin receptor substrate (IRS)-1 and -2, with subsequent decreased interaction between these substrates and phosphatidylinositol-3-kinase [259]. Furthermore, a negative influence of GH on the synthesis of GLUT-1 and GLUT-4 glucose transporters may be in play [159]. Insulin resistance is also worsened by free fatty acids (FFA), whose circulating levels are increased by GH (see below). Through the above-mentioned actions GH, together with catechol-

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amines, cortisol and glucagon, antagonizes the influence of insulin on glucose and lipid metabolism. Thus, during prolonged fasting or hypoglycemia, these counterregulatory hormones prevent further reductions of blood glucose, and during physical exercise they render energetic substrates (glucose and FFA) readily available. IGF-I exerts insulin-like activities on glucose metabolism, acting on its own receptors. In particular, although IGF-I is even more effective than insulin in promoting glucose uptake by muscle tissue [167], the amount of IGF-I that is free from binding proteins and biologically active does not seem to be high enough to exert significant insulin-like activity under physiological conditions. The lipolytic effect of GH was among the first metabolic actions of the hormone to be described. GH administration to both laboratory animals and humans induces an increase in serum concentrations of glycerol and FFA [69] through the activation of hormone-sensitive lipase [92]. Furthermore, GH inhibits the activity of adipose tissue lipoprotein lipase, the enzyme that, catalysing the hydrolysis of triglycerides transported by chylomicrons and very low density lipoproteins (VLDL), makes FFA available for the synthesis of triglycerides and their storage in adipocytes [48]. Additional effects of GH on lipoprotein metabolism have been documented. The hormone favours the formation of VLDL containing Apo B48 instead of Apo B100 : the former undergo a more rapid clearance from plasma, so that fewer VLDL are metabolized to low density lipoproteins (LDL). Furthermore, GH increases the expression of liver LDL receptors, leading to enhanced clearance of LDL-cholesterol. Finally, GH promotes lipid oxidation, favouring the use of FFA as an energy source. Of particular interest, experimental data suggest a stimulatory influence of GH on fat cell formation in clonal preadipocyte cell lines [94,140,197], with an action likely mediated by locally produced IGF-I [93,258]. While acute i.v. injection of high doses of IGF-I is followed by an insulin-like antilipolytic effect, prolonged s.c. administration of the growth factor leads to increased circulating concentrations of FFA, enhanced fat oxidation, and decreased adipose tissue lipoprotein lipase activity [150,216]. Several animal studies indicate that GH promotes protein anabolism. In rats, somatotropin administration for a few days has been demonstrated to counter the loss of body protein induced by hypophysectomy [107]. GH is known to stimulate the uptake of amino acids by muscle tissue, to increase the number and activity of ribosomes in myocytes, and to increase muscle RNA polymerase and RNA [289]. Human studies with isotopic techniques have shown that GH reduces leucine oxidation and favours its incorporation into proteins. In normal subjects, intra-arterial infusion of the hormone increases the forearm muscle protein synthesis by 66% [112]. In adult GH-deficient patients, displaying reduced

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lean body mass (LBM), long-term GH administration is followed by an increase in whole-body protein synthesis with no effects on proteolysis [242]. Likewise, in hypercatabolic states such as burns, trauma or AIDS, GH treatment has proved capable of promoting positive nitrogen balance and reducing protein oxidation [201,229]. As for IGF-I, while its acute i.v. infusion at high doses is followed by an insulin-like effect consisting in the reduction of proteolysis, s.c. administration for at least one week leads to a marked increase in protein synthesis [241,290]. In normal humans, a 6 h intra-arterial infusion of IGF-I has been reported to promote forearm tissue anabolism by a 70% increase in protein synthesis [111]. In addition, in hypercatabolic patients treatment with recombinant human IGF-I (rhIGF-I) induces an improvement in nitrogen balance similar to that observed after GH therapy [174], while the combined administration of the two drugs appears to exert synergistic effects. Thus, as far as protein metabolism is concerned, IGF-I mediates the action of GH which, however, is also capable of exerting direct effects. Indeed, somatotropin infusion stimulates protein synthesis in forearm muscle in absence of significant changes in circulating IGF-I levels [112]. Overall, through its lipolytic and protein anabolic properties, GH influences body composition. Other somatotropin effects, however, are important with respect to this issue, namely its antinatriuretic action leading to sodium and water retention and the stimulation of bone turnover with bone formation prevailing on resorption [152,210]. The above-mentioned metabolic actions of GH and IGF-I account for alterations in body composition that may be found in some disease states. Adult patients with GH deficiency present increased abdominal fat, reduced muscle mass, dehydration due to decreased extracellular water, and osteopenia with increased fracture risk [67]. Similar and even more marked abnormalities of fat and muscle mass are present in patients with Prader–Willi syndrome, a form of genetic obesity associated with an actual GH deficiency [37]. GH treatment is able to improve these abnormalities of body composition in both these diseases [67,97]. Somatotropin administration is also effective in preventing the loss of LBM in obese patients undergoing caloric restriction [65,269]. Along the same line, an increase in LBM and a decrease in adipose tissue are well documented in acromegaly [25]; once again, these abnormalities can be reversed by successful treatment of the disease. A number of experimental data indicate that GH also plays a major role in the regulation of resting energy expenditure (REE). Indeed, REE is increased in acromegaly and reduced in adult GH deficiency and Prader–Willi syndrome [49,52]. GH administration to GH-deficient patients or to obese subjects is followed by a significant increase in REE. This phenomenon, most likely mediated by GH-dependent stimulation of thyroxine (T4 ) to triiodothyronine

(T3 ) conversion, is due not only to an increase in LBM but also to an enhanced metabolic efficiency of LBM itself [249]. Finally, a reduction of respiratory quotient is well documented during GH administration to animals and humans: this finding reflects the somatotropin action addressing fuel metabolism towards the preferential oxidation of lipids instead of glucose and proteins. On the whole, the hormone appears to convey the energy derived from food intake and from enhanced lipid oxidation towards the synthesis of proteins [177].

3. Role of macronutrients and insulin in the control of GH release Given the critical role played by GH in the control of body composition, fuel metabolism, and energy expenditure, it is not surprising that metabolic substrates participate as peripheral factors in the complex regulation of GH release. Glucose is an important regulator of somatotropin secretion. In normal humans, a decrease in blood glucose is followed by a significant rise in serum GH levels, whereas hyperglycemia inhibits GH release [126,199]. Hypoglycemia is believed to release GH via a reduction of central SRIH tone: indeed, pretreatment with GHRH, which is able to prevent the somatotropin response to a subsequent GHRH application, does not affect the GH response to insulin-induced hypoglycemia, while the combined application of GHRH and hypoglycemia exerts an additive effect on GH secretion [253]. Oral glucose administration to normal subjects leads to a GH suppression lasting for 1–3 h, and followed by a subsequent rise of the hormonal levels. Acute hyperglycemia is able to block not only basal but also GHRHstimulated somatotropin release [79,191,252]. The rapid inhibitory effect of glucose on GH secretion is currently believed to be mediated by central SRIH discharge. In keeping with this contention, administration of pyridostigmine, an indirect cholinergic agonist thought to inhibit hypothalamic SRIH secretion, fully counteracts the glucose-mediated inhibition of GH release [21,86,222]. A direct effect of glucose on the pituitary is unlikely, since the macronutrient does not influence basal and GHRHstimulated GH output in vitro [46]. In contrast to what is observed in humans, hyperglycemia does not affect basal or GHRH-induced GH secretion in the rat, while hypoglycemia suppresses somatotropin release [126]. These findings disclose major differences between the two species in the responsiveness of GH to changes in blood glucose; likewise, discrepancies between rodents and humans regarding somatotropin reactivity to caloric restriction are well recognized (see below). A classic feedback relationship between GH and FFA has been convincingly demonstrated. As already men-

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tioned, GH promotes lipolysis leading to the release of glycerol and FFA from triglycerides. FFA, in turn, exert an inhibitory effect on GH secretion taking place at the pituitary [6,55,253] and possibly at the hypothalamus, likely via SRIH stimulation [153]. In normal subjects, an increase in serum FFA concentrations blunts the GH response to several stimuli [55,154], whereas the reduction of circulating FFA following the administration of acipimox, a nicotinic acid analogue able to block lipolysis, stimulates basal GH release and enhances the GH responsiveness to different challenges [220,230]. Acipimox administration has also been shown to greatly improve spontaneous and stimulated GH release in obese patients, suggesting a contribution of elevated FFA circulating levels to the pathophysiology of the hyposomatotropinism of obesity [8,70,127,172,182,207,215,231]. High protein meals as well as the amino acids arginine, lysine, ornithine, tyrosine, glycine, and tryptophan enhance GH release in humans, but not in rats [126,199]. The observations that arginine potentiates the GH response to a maximal dose of GHRH but not to pyridostigmine, a drug believed to inhibit SRIH release, and that pretreatment with GHRH does not prevent the somatotropin response to subsequent arginine infusion all point to a hypothalamic action of this amino acid, likely mediated through inhibition of SRIH release [4,117]. Arginine is the immediate biological precursor of nitric oxide (NO), a gaseous neurotransmitter involved in the control of corticotropin releasing hormone (CRH) and GHRH secretion. The question as to whether the GH-releasing activity of arginine is mediated by NO is still controversial. In vitro results are conflicting, while according to human in vivo studies this possibility seems unlikely: indeed, no effects of NO donors on serum GH levels [181] as well as no influence of NO synthase inhibitors on the GH response to arginine infusion [104,105] have been demonstrated in normal subjects. Insulin too appears to be involved in the control of GH secretion, exerting both direct and indirect inhibitory effects. In vitro, insulin is able to reduce GH mRNA content as well as GH synthesis and release in rat anterior pituitary cells [299]. A hypothalamic action has also been hypothesized, most likely by enhanced catecholamine release and subsequent stimulation of SRIH discharge via b-adrenergic receptors [62,192,246]. Furthermore, insulin might act through the inhibition of IGF binding protein-1 (IGFBP-1) with consequent increase in free IGF-1, available for negative feedback on GH secretion [66].

4. Leptin: effects on food intake, energy expenditure, and GH secretion Leptin, the protein encoded by the obese gene and discovered at the end of 1994 [301], is predominantly

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secreted by white adipose tissue. Circulating levels of this peptide are positively correlated with measures of body fat and reflect the amount of lipid stores. Indeed, leptin serum concentrations are elevated in diseases characterized by fat accumulation such as simple obesity [68], CushingÕs syndrome [171], and adult GH deficiency [106,108], and low in conditions of decreased fat depots such as AN [54,189,266]. Leptin is thought to play a critical role in the regulation of body weight, by decreasing food intake [134,221], and enhancing REE [151,261]. Both the leptin-induced decrease in appetite and stimulation of energy expenditure seem to be mediated by a reduction of hypothalamic neuropeptide Y (NPY) gene expression [76,251,262]. However, this is unlikely to be the only mechanism of action of leptin, since the obese gene product maintains its effectiveness also in NPY knock-out mice [100]. Indeed, leptin has been demonstrated to stimulate, within the arcuate nucleus, a group of neurons containing propiomelanocortin (POMC), which could mediate the anorexigenic effect of leptin itself [5]. Due to these actions, leptin is now recognized as the chief peripheral signal informing the brain of nutritional status of the body and a major player in the catabolic limb of metabolic pathways for the maintenance of energy balance. Serum leptin levels display circadian variations. In humans, the highest levels are observed at night or in the first morning hours and the nadir has been described in the morning or in the afternoon in different studies [169,256]. The membrane-spanning leptin receptor is highly expressed in the hypothalamus, but it is present also in other tissues; furthermore, several shorter forms of leptin receptor are widely expressed. While the shorter variants are present in both fetal and human pituitaries, the intact form is expressed by fetal pituitaries and by most GH, prolactin, and non-functioning pituitary adenomas [162,255]. In addition, leptin production in the human pituitary has been recently demonstrated [157,294]: the peptide is expressed by somatotropes, corticotropes, thyrotropes, and gonadotropes, but not by lactotropes. The expression of both leptin and its receptor in the rodent and human hypophysis suggests a possible paracrine/autocrine influence of the obese gene product on pituitary function. In this context, considering that both leptin and GH are involved in the regulation of body composition and energy expenditure, several studies have recently addressed the issue of reciprocal interactions of the two hormones. In vitro observations seem to exclude a relevant effect of GH on leptin release from rodent adipocytes [30,101,135]. In vivo, GH replacement therapy has been shown to normalize basally elevated leptin levels in GH-deficient adults [106,108], but this effect has been ascribed to the reduction of fat mass induced by treatment. This interpretation, however, has been challenged by the

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observation that a single s.c. injection of biosynthetic GH in normal subjects is followed by a significant increase in serum leptin after 24 h and by a subsequent decrease after 72 h. These changes, taking place in absence of modifications of body composition, suggest an effect of GH on leptin independent from fat mass reduction [176]. The same view is supported by the significant decrease in serum leptin per unit fat mass observed in obese prepubertal boys treated with GH [98]. As for the effect of leptin on GH secretion, most of the relevant information has been obtained from studies in rodents. I.c.v. administration of anti-leptin antiserum to normal fed rats induced a significant suppression of somatotropin release, while i.c.v. injection of leptin to fasted rats was followed by reversal of the inhibitory effect of food deprivation on GH secretion [50]. The stimulation of GH release effected by leptin in rodents appears to be mediated by inhibition of hypothalamic NPY, since i.c.v. administration of this peptide was shown to prevent leptin-induced somatotropin secretion in fasted rats [90]. The observation that the GH response to leptin is blocked by anti-GHRH antiserum and enhanced by anti-SRIH antiserum points to a physiological influence of leptin on the two main hypothalamic regulators of somatotropin release [90]. This hypothesis is reinforced by the findings that incubation of rat hypothalamic neurons with leptin leads to a decrease in basal SRIH secretion in the medium and SRIH mRNA within the cells [234] and that leptin administration to hypophysectomized fasting rats increases GHRH mRNA and reduces SRIH mRNA hypothalamic content [51]. In summary, in rodents leptin seems to exert a stimulatory role on GH secretion acting at suprapituitary level through inhibition of NPY and modulation of GHRH and SRIH production. However, a direct effect of leptin on the pituitary cannot be ruled out, since the peptide has been shown to increase GH secretion from pig pituitary cells, likely via stimulation of NO production [23]. In any case, the important differences existing between rodents and humans with respect to GH regulation by nutritional factors (e.g., fasting and hypoglycemia suppress somatotropin release in the former and enhance it in the latter) do not allow the immediate transfer of animal results to human physiology. In human diseases associated with high circulating leptin levels such as obesity and CushingÕs syndrome, GH secretion is markedly blunted. In obesity, the hyposomatotropinism might be explained by a resistance to leptin action or, on the contrary, by an inhibitory role of the peptide on GH release in humans [91,249]. In favour of the latter interpretation is the recent observation of a slight inhibitory effect of the peptide on GH release from human adenomatous GH-secreting cells in culture [124]. Unfortunately, the lack of human studies on the effects of leptin administration on GH secretion does not allow a final statement on this issue.

5. Ghrelin: effects on food intake, energy expenditure, and GH secretion In 1999, Kojima et al. [160] identified in rat stomach an endogenous ligand for the until then orphan receptor of GHS, synthetic compounds endowed with strong GHreleasing activity. This peptide was named ghrelin. Interestingly, the sequence of events culminating with the discovery of ghrelin has been the reverse of usual, i.e., the discovery of an endogenous substance first, then the identification of its receptor, and finally the development of analogous compounds. The knowledge of GHS is prerequisite to a better understanding of ghrelin biology. GHS are synthetic peptidyl and non-peptidyl compounds capable of powerfully stimulating GH secretion in vitro and in vivo in a number of different species. The peptidyl molecules have been synthesized by structural modifications of the pentapeptide met-enkephalin, but are devoid of opioid activity and do not act through opiate receptors [195]. The non-peptidyl secretagogues have been subsequently synthesized using the hexapeptide growth hormone releasing peptide-6 (GHRP-6) as template. A specific GHS receptor, distinct from those of GHRH and SRIH, has been identified and cloned in 1996 using the non-peptidyl GHS MK-0677 as ligand [149,228]. Its sequence appears to be conserved between species [193]. GHRH and GHS receptors belong to different G-protein coupled seven transmembrane domain receptor families. GHS receptor mRNA is found in several hypothalamic nuclei, in particular in the arcuate nucleus, in other areas of the central nervous system, in the pituitary as well as in peripheral tissues such as adrenal, heart, ovary, testis, lung, skeletal muscle, kidney, epiphysis, and thyroid [132]. Binding of a GHS to its receptor activates the phospholipase C pathway [163]. This is at variance with the GHRH–receptor interaction, which leads to stimulation of the cAMP-dependent protein kinase and mitogen-activated protein kinase, in turn activating transcription of the GH gene [199] and somatotrope proliferation [226], respectively. Although the phospholipase pathway appears to be the prevailing mode of action of GHS, these compounds have also been reported to heighten GHRH signalling via cAMP, suggesting that GHS synergize with GHRH to further increase cAMP levels [59]. GHS display both endocrine and non-endocrine properties. Among the former, the GH-releasing effect is the most important but not the only activity. In vitro, GHS increase GH synthesis and release from rat pituitary cells [18,60,61,178] to a lesser extent than GHRH [244] but can potentiate the GH response to GHRH itself [32]. The in vitro GH-releasing effect of GHS is stronger in hypothalamic–pituitary preparations than in pituitary preparations; likewise, this effect is greater in vivo than in vitro [32]. The observations that immunoneutralization of GHRH reduces the activity of GHS in rats [27],

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that GHS administration is followed by an increased release of GHRH in hypophysial portal blood in sheep [133], and that GHS are not active in mice lacking pituitary GHRH receptors [89] all point to a hypothalamic GHRH-mediated action of GHS on GH secretion. However, in view of the inhibition of the GH-releasing activity of GHS caused by i.c.v. administration of octreotide [102], GHS could also act as functional antagonists of SRIH at both hypothalamic and pituitary levels. Prolonged treatment with GHS enhances the activity of GH/IGF-I axis [155], promoting body growth in rats with intact pituitary function [31]. These peptides are also able to evoke significant prolactin, corticotropin (ACTH), and cortisol release. While prolactin-releasing activity appears to be exerted through direct effects on somatomammotropes [1], the action on ACTH is likely mediated via the release of endogenous CRH [278] since GHS are unable to stimulate ACTH secretion directly from rat pituitaries [61]. In normal humans, GHS evoke a GH release which is greater in magnitude than the one observed in laboratory animals or induced by GHRH; further, GHS potentiate the GH-releasing effect of GHRH [114]. The observation that the GH response to GHS is markedly blunted by a GHRH antagonist as well as by hypothalamo-pituitary disconnection suggests a GHRH-mediated action of these compounds also in humans [218,232]. While the GH response to the first GHRP identified, GHRP-6, is increased by the cholinergic agonist pyridostigmine [223] and by the b-blocking agent propranolol [187], the GH rise elicited by hexarelin is neither potentiated by drugs thought to reduce hypothalamic SRIH tone (pyridostigmine, arginine, and atenolol) nor blocked by compounds capable of increasing central SRIH release (pirenzepine, salbutamol) [47]. The GHstimulating activity of GHS is also refractory to other influences known to abolish GHRH-induced GH release through an increase in hypothalamic SRIH tone (food ingestion, glucose, and GH auto-feedback) or a direct action at the pituitary (FFA) [14,83,180]. Furthermore, not even exogenous SRIH infusion, at doses capable of preventing the GH rise after GHRH, is able to completely abolish the GH response to GHS [88]. Conversely and worth noting, this response is strongly blunted by infusion of exogenous rhIGF-I, which denotes marked sensitivity of the GHS-induced GH release to this physiological feedback mechanism [118]. These observations support the view that GHS can act as functional SRIH antagonists at both hypothalamic and pituitary levels also in human [102]. In the latter, as in animals, GHS evoke slight but significant rises in circulating levels of prolactin, ACTH, and cortisol. While the prolactin response to GHS is significantly lower than that elicited by thyrotropin releasing hormone (TRH) [13], the ACTH response is comparable with that evoked by CRH [116] and appears to be

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physiologically modulated by glucocorticoid negative feedback: indeed, it is blunted in normal subjects by dexamethasone [15] and in patients bearing cortisol-secreting adrenal adenomas [116]. As for the non-endocrine actions of GHS, the most relevant are represented by stimulation of cardiac activity—these compounds exert anti-apoptotic properties on cardiomyocytes and inotropic effects in normal and GHdeficient subjects [114]—and by a strong orexigenic effect. This latter has been extensively investigated in animals. I.c.v. administration of GHRP-2 was shown to stimulate food intake and to increase body weight in rats [164,212]. Likewise, i.v. administration of hexarelin is followed by an acute feeding response in young beagle dogs [235]. A GH-independent increase in body fat, most likely secondary to increased food intake, has been recently described in mice treated with the GHS ipamorelin [168]. I.c.v. administration of GHRP-6 to rats was found to activate not only the arcuate nucleus, but also other brain areas involved in the control of eating behaviour, such as paraventricular nucleus, dorsomedial nucleus, lateral hypothalamus, and two regions of brainstem, the nucleus of the tractus solitarius and the area postrema [170]. The orexigenic effect of GHS in rats is known to be inhibited by antagonists of the Y1 NPY receptor [170,281], pointing to NPY as the main mediator of GHS action on feeding behaviour. However, the recent observation that GHRP2 increases food intake, body weight, and fat mass in NPY-deficient mice suggests the involvement of other mediators: GHRP-2 treatment has been shown to increase hypothalamic mRNA for agouti-related protein (AGRP), an orexigenic melanocortin receptor antagonist. The concomitant finding that competitive blockade of AGRP action by a melanocortin receptor agonist prevents the GHRP-2-induced weight gain in mice lacking NPY points to AGRP as an additional mediator of GHS effects on food consumption [286]. Ghrelin is a 28-amino acid acylated peptide characterized by n-octanoylation of its serine 3 residue, produced by the so-called X-like cells of oxyntic glands of the rat and human stomach [77,95]. The precursor of ghrelin, preproghrelin, displays elevated homology in rats and humans, pointing to a high interspecies conservation of the molecule. Gastrectomy and gastric bypass surgery are followed by a significant reduction of circulating ghrelin [12,75] confirming the stomach as chief source of the peptide. Ghrelin is also produced in other tissues, such as small intestine [77], kidney [196], placenta [131], normal pancreatic a- and b-cells, and pancreatic endocrine tumors [78,295]. There is still no complete agreement as to whether there are ghrelinsynthesizing neurons in the hypothalamus, although the weight of experimental evidence is trending that way. Indeed, ghrelin-immunoreactivity has been originally localized in the arcuate nucleus [160] and more recently in a previously uncharacterized group of neurons

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between the dorsal, ventral, paraventricular, and arcuate nuclei [73].The peptide is also expressed by normal and adenomatous pituitary tissue [161], and this finding supports the hypothesis of an autocrine/paracrine action of locally synthesized ghrelin on pituitary hormone secretion. Like synthetic GHS, the natural peptide ghrelin is endowed with endocrine and non-endocrine activities, the most relevant being stimulation of food intake and GH release. At the present time, the role of ghrelin in appetite regulation is better established than its physiological role in GH regulation. An increasing amount of experimental data is being collected on the regulation of ghrelin release. In rodents, fasting and hypoglycemia increase ghrelin mRNA expression in the stomach and its serum levels, whereas food ingestion diminishes ghrelin secretion [147,282]. Of note, circulating ghrelin is suppressed by nutrient intake and not by simple stomach filling with water [285]. Low-protein diets represent an additional stimulus for ghrelin release [225]; further, the associated GH increase and the attendant nitrogen retention favour the restoration of energetic homeostasis. On the other side, additional inhibitory signals include leptin, GH itself, oral glucose load, and high-fat diets [147,173,282]. The question of whether the suppressive effect of food intake and oral glucose on serum ghrelin observed also in human is mediated by changes of plasma insulin and glucose is still a matter of debate. Indeed, while in the experience of Caixas et al. [45], parenteral administration of insulin and glucose failed to suppress serum ghrelin in normal subjects, i.v. glucose injection and insulin infusion were followed by a significant decrease in circulating ghrelin in the hands of other investigators [204,243,254]. Of pathophysiological relevance, the suppressive effect of food on ghrelin secretion appears to be defective in obese patients [99]. The influence of SRIH on ghrelin levels is less controversial. In two recent human studies SRIH infusion suppressed circulating ghrelin to 20–45% of baseline values, with an effect persisting at least 30 min after the end of the infusion [42,208]. Thus, the activation of SRIH receptors, widely expressed in the gastric mucosa, results in marked suppression of the release of this gastric peptide as well as of that of other gastro-enteropancreatic hormones. Studies in Sprague–Dawley rats have demonstrated a pulsatile pattern of ghrelin secretion. Secretory pulses were correlated with food intake episodes, with a significant decrease in the 20 min following the end of food consumption, but not with GH secretory episodes [279]. Furthermore, fasting was shown to augment all parameters of ghrelin pulsatile release, to diminish leptin secretion, and to synchronize the pulse discharge of the two peptides [19]. Recent experiences indicate that ghrelin secretion exhibits a diurnal pattern also in human, with preprandial increases, postprandial decreases, and a maximum peak at 02:00 h [74,254,287]. The possibility that high ghrelin concen-

trations trigger feeding initiation is supported by observations in rodents. I.c.v. administration of the peptide to mice and rats was able to increase food intake and body weight; likewise, peripheral daily administration of ghrelin to the same animals was shown to cause weight gain by reducing fat utilization [285]. The eatingpromoting effect of ghrelin, like that of synthetic GHS, appears to be mediated by NPY and AGRP in the arcuate nucleus [158,170]. In agreement with this hypothesis, ghrelin fails to promote food intake in rats neonatally treated with monosodium glutamate, which destroys the arcuate nucleus neurons [273]. To reinforce the central role of this brain area in the control of eating behaviour, the natural ligand of GHS receptors has been recently found to exert an effect opposite to that played by leptin on these same neurons [283]. Finally, the orexigenic effect of ghrelin might also be mediated by inhibition of POMC-secreting neurons [5]. Very interestingly, ghrelin is the first circulating hormone for which an ability to stimulate food intake has been demonstrated in man: indeed, in a placebo-controlled study, i.v. administration of the peptide to normal subjects was shown to increase appetite and energy consumption from a free-choice buffet [297]. If promotion of food intake and reduction of fat oxidation can be considered the most important nonendocrine actions of ghrelin described so far, the most relevant endocrine effect is represented by stimulation of GH secretion. In vitro, ghrelin induces GH release from rat pituitary primary cultures in a dose-dependent manner. Accordingly, the peptide evokes a significant GH response in vivo when injected i.v. in anaesthetized rats [160]. In normal humans, i.v. ghrelin administration is a potent, dose-dependent stimulus for GH release [219,270]. Indeed, the GH-releasing activity of ghrelin is greater than those of GHRH and hexarelin [16]. Furthermore, the combined administration of ghrelin and GHRH, but not of ghrelin and hexarelin, is synergistic on GH secretion. In normal subjects, ghrelin is also endowed with significant prolactin-, ACTH-, and cortisol-releasing activity [16]. As observed for synthetic GHS and differently from what is reported for GHRH, the GH-releasing action of the natural peptide is neither abolished by glucose, FFA, and SRIH nor enhanced by arginine: these observations suggest that ghrelin, as well as synthetic GHS, acts at least in part by antagonizing SRIH activity [41,88]. Furthermore, in a recent study, ghrelin has been shown to stimulate GHRH and CRH release from hypothalamic explants in vitro [298]. The possible role of ghrelin in fasting-induced amplification of GH release in human is still controversial: Muller et al. [198] described the appearance of a diurnal rhythm in ghrelin concentrations, absent in the fed state, during a 4-day fast, whereas Nørrelund et al. [208] could not demonstrate a significant ghrelin rise following a 36h fast. The effect of the natural GH-releasing peptide on

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insulin secretion is still a matter of debate due to discordant results of in vitro studies in rodents and possible differences of action between species: in the rat, ghrelin was shown to stimulate insulin release from isolated pancreatic cells and pancreatic tissue fragments [2,78] and to increase insulin secretion in vivo [173]; on the other hand, the peptide was found to inhibit glucoseinduced insulin release from the isolated rat pancreas perifused in situ [96] and to reduce insulin secretion after acute i.v. injection in humans [40]. On the whole, ghrelin appears to be deeply involved in the neuroendocrine control of food intake functioning as a blood-borne orexigenic signal from the gut to the brain. In particular, the peptide is likely to play an important role in coordinating the behavioural and metabolic responses to situations of negative energy balance, such as fasting. In this condition, increased ghrelin levels can first stimulate GH and possibly inhibit insulin secretion, with the attendant activation of lipolysis and fat oxidation, and then contribute to meal initiation. After meal consumption, a decrease in ghrelin and an increase in insulin levels may start the mechanisms leading to energy storage. There are, however, several differences between the metabolic properties of ghrelin and those of GH: chronic administration of ghrelin induces adiposity in rodents [285], whereas GH is known to reduce fat mass through its lipolytic properties; furthermore, while GH increases REE, this parameter is reduced by ghrelin and this is one of the mechanisms invoked to explain the fat mass-increasing effect of the peptide; finally, fat oxidation is promoted by GH and inhibited by ghrelin. There is evidence to suggest that chronic administration of synthetic GHS to obese patients is capable of reducing body fat and increasing muscle mass and REE through amplification of endogenous GH release [268]. On the contrary, the studies so far available in laboratory animals suggest that the metabolic effects of the natural GHS receptor ligand ghrelin (increased adiposity, decreased REE, and fat oxidation) tend to overcome those typically exerted by GH. Further studies are certainly needed to fully clarify the physiological significance of a gastric hormone secreted in conditions of negative energy balance and endowed with apparently conflicting (adipogenic and GH-releasing) properties. The matter is even more puzzling given the recent observation that hypophysectomy, as well as adrenalectomy and thyroidectomy, prevents ghrelin-induced adiposity in rats, suggesting that an integrity of the pituitary axis is required for this effect [284].

6. Nutritional status and GH secretion: animal and human models With the exception of rodents, in which prolonged fasting decreases serum GH levels, in most mammals

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including man fasting and caloric restriction enhance basal GH secretion. Several mechanisms are likely to contribute to this fasting-induced stimulation of somatotropin release: reduced IGF-I generation with consequent lack of negative feedback on GH secretion, changes in hypothalamic output of GHRH and SRIH, and possibly increased ghrelin secretion (see previous paragraph). In ovariectomized ewes food restriction is followed by a reduction of SRIH release into hypophysial portal blood [277]. The observation that pyridostigmine, a purported inhibitor of central SRIH discharge, is able to potentiate the GH response to GHRH in fed but not in fasted dogs also points to a fasting-induced decrease in SRIH tone [200]. As an alternative interpretation, fasting could lead to an increase in hypothalamic cholinergic tone which would make pyridostigmine unable to further enhance cholinergic neurotransmission. Different from what is observed in other mammals, food deprivation inhibits spontaneous GH release in rats. In food-deprived rodents a generalized suppression of the hypothalamic–somatotropic axis has been described, with decreased GHRH and SRIH content in the median eminence and reduced hypothalamic levels of GHRH mRNA and SRIH mRNA as well as of pituitary GH mRNA [39]. An increased central release of NPY, peptide involved in the control of both food intake (stimulation) and GH secretion (inhibition), might contribute to the somatotropin hyposecretion of the fasted rat: indeed, central administration of antiNPY antiserum improves GH pulsatility in food-deprived rodents [213]. Refeeding after prolonged food restriction reinstates a normal spontaneous GH release in rats, provided that an adequate amount of dietary protein is administered [214]. Refeeding also normalizes IGF-I levels, which are reduced in calorie-restricted rats, most likely due to hyposomatotropinism and decreased GH binding or post-receptor defects in the liver [276]. Interestingly, an enhanced GH response to GHRH has been reported in fasted rats, possibly a consequence of increased somatotrope sensitivity to the releasing hormone: in agreement with this hypothesis, an increase in both GHRH receptor mRNA and GHRH binding has been shown in the pituitary of food-restricted adult rats [267]. The genetically obese Zucker rat is a model of animal obesity presenting a dramatic impairment of spontaneous GH release [103] associated with reduced hypothalamic GHRH and pituitary GH content and expression [3,275]. These profound alterations are thought to be linked to a precocious resistance to leptin, which is known to stimulate somatotropin secretion in rodents. In human, chronic undernutrition (marasmus and kwashiorkor) and acute dietary restriction (fasting) induce a clear-cut reduction of serum IGF-I associated with increased GH levels. Growth retardation due to insufficient IGF-I production is a common feature in

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children with severe protein restriction (kwashiorkor) or protein-energy deprivation (marasmus) [129,142,260]. Circulating IGF-I is low in other forms of malnutrition, namely inflammatory bowel disease, celiac disease, and AN (see below). Usually, the degree of IGF-I decrease is proportionate to the degree of nutritional deficiency, and IGF-I levels significantly increase with the restoration of adequate caloric intake. In particular, normalization of IGF-I after a period of caloric restriction requires an adequate protein content of the diet. In chronic malnutrition, low IGF-I plasma concentration in spite of normal or elevated circulating GH points to a condition of peripheral GH resistance (see below). In normal humans, fasting is followed by a marked reduction of serum IGF-I levels, which fall to 15–20% of baseline after 10 days of fast and return to normal with refeeding [64]. In fasted subjects an impaired IGF-I generation in response to exogenous GH is also present [194]. In the same subjects, an enhancement of spontaneous somatotropin secretion has been observed using computerized algorithms [138,144]: an increase in both the number of secretory episodes and the amount of hormone secreted per burst account for a rise in GH production rate. In fasted volunteers, the infusion of rhIGF-I suppresses the enhanced GH release with a time course similar to that of refeeding, suggesting that reduction of plasma IGF-I is a major, if not the chief, mediator of the effects of fasting on GH secretion [136]. The influence of nutritional status on IGF-I production is so strong that a 6-day fast has proved capable of reducing plasma somatomedin to 65% of baseline even in acromegalic patients [145]. Interestingly, the decrease in IGF-I induced by caloric restriction is smaller in obese patients than in normal subjects: thus, somatomedin production appears to be relatively independent of energy intake in obesity, provided that a sufficient protein supply is maintained [11,276]. The profound abnormalities of GH secretion in human obesity have been recently reviewed by us [249] and others [183]. In essence, obese patients display a marked impairment of both spontaneous and stimulated GH release. A reduction of half-life, frequency of secretory episodes, and daily production rate of the hormone has been documented, together with an impaired somatotropin responsiveness to all traditional pharmacological stimuli acting at the hypothalamus or directly at the pituitary. These alterations, fully reversed by weight normalization, seem to represent a secondary, probably adaptive, phenomenon. Hypothalamic, pituitary, and peripheral factors may be involved in the GH hyposecretion of obesity: central SRIH hypertone (which however seems unlikely in view of our observation of reduced liquoral SRIH levels in obese patients [43]), GHRH deficiency, functional and reversible failure of somatotropes, increased free IGF-I levels, high circulating FFA, and hyperinsulinemia have all been proposed as contributing

factors. The exciting report of low plasma ghrelin in human obesity [288] has led to the hypothesis that this likely adaptive phenomenon could contribute to the hyposomatotropinism of this clinical condition, although this possibility is challenged by the lack of correlation between GH and ghrelin plasma levels recently observed in a group of obese subjects [175].

7. The model of anorexia nervosa This psychiatric disease, characterized by refusal of food, marked weight loss, disturbances in body image, amenorrhoea, and high death rate due to complications of malnourishment or suicide, has long attracted the interest of the investigators in the field of neuroendocrinology. Indeed, patients with anorexia nervosa display extensive changes of the hypothalamic–pituitary function with activation of the GHRH–GH and CRH– ACTH axes and inhibition of the gonadotropin releasing hormone (GnRH)–gonadotropin axis. As regards the present review, this natural experimental model can provide further insights into the pathophysiology of GH secretion in relation to nutritional status in humans. 7.1. Spontaneous GH secretion The first two studies applying computer-assisted algorithms to the evaluation of spontaneous GH secretion in AN were published by us [248] and Argente et al. [10] in 1997. In our study, nocturnal GH release of anorectic patients was enhanced, with increased peak frequency and interpeak hormonal concentrations. The enhanced GH secretion was mainly due to an increase in its nonpulsatile component, which represented up to 70% of somatotropin release in our patients compared to 40% observed in control women. The pulsatile fraction of GH secretion was also increased, due to the higher number of secretory episodes and despite the normal amount of hormone released per burst. An increase in GH peak frequency and valley hormonal concentrations has also been described in acromegalic patients [139] and fasted volunteers [144]. In particular, acromegaly is characterized by a marked enhancement of nonpulsatile GH release associated with a moderate increase in the pulsatile component [137], whereas fasting is connoted by a significant enhancement of pulsatile fraction with a substantially unchanged basal secretion [138]. Interestingly, abnormalities of spontaneous GH release similar to those observed in our anorectic patients are reported in another condition of cellular fasting such as poorly controlled diabetes mellitus [17]. Among the syndromes of acquired GH resistance (see below), the pattern of spontaneous somatotropin release of AN appears to be different from that reported in the

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chronic phase of critical illness, characterized by low secretory burst amplitude and low GH amount per burst [293]. This discrepancy may reflect pathophysiological differences, since AN, even when associated with important reduction of total body protein, is not characterized by the markedly accelerated protein catabolic rate, causing vital tissue wasting with fat depot preservation, typically observed in long lasting critical illness. In their study Argente et al. [10] identified two different subpopulations of anorectic patients with respect to the GH secretion, namely hyper- and hyposecretors; further studies are needed for a better understanding of these two secretory patterns. To rule out the possibility that the elevated interpeak GH levels of our anorectic patients were due to a decrease in the hormonal metabolic clearance rate, we performed a deconvolution analysis of the results and demonstrated a comparable GH half-life in patients and controls, and a significantly increased GH basal secretion rate in anorectic women (unpublished data). This abnormality, together with the increased secretory burst frequency, accounted for the 3.5-fold higher GH production rate measured in patients compared with controls. In addition, anorectic women displayed an increased approximate entropy (ApEn), testifying a disordered pattern of GH secretion similar to that documented in acromegaly. More recently, the same alterations of spontaneous GH release in patients with AN were demonstrated by deconvolution analysis by Stoving et al. [264]. Abnormalities of IGF-I negative feedback (see below) might contribute to the highly disordered spontaneous GH secretion in AN. 7.2. Stimulated GH secretion 7.2.1. Suprapituitary stimulation In presence of elevated baseline serum GH levels, the somatotropin responsiveness to different provocative hypothalamic stimuli is variable in AN. Indeed, the GH rise in response to insulin-induced hypoglycemia [109,205], dopaminergic agents [22], and acute administration of dexamethasone [247] is impaired. On the contrary, the a2 -adrenergic agonist clonidine [34,35] and the amino acid arginine [257] elicit normal GH responses in these patients, while an even increased somatotropin responsiveness has recently been reported after the application of galanin, a peptide thought to release GH via hypothalamic GHRH stimulation and SRIH inhibition [82]. A paradoxical GH increase after glucose i.v. infusion has been described in AN by Tamai et al. [271]. 7.2.2. Direct pituitary stimulation by GHRH In patients with AN the GH response to GHRH was described as exaggerated [22,34,119,123,224,238,239] or normal [53,81,115,272]. In any case, the GHRH-induced

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GH rise is refractory to a number of neuroendocrine and metabolic factors effective in normal subjects. Thus, in these patients pharmacological blockade of cholinergic muscarinic receptor by pirenzepine fails to suppress the GH response to GHRH [238,272]; likewise, this response is not potentiated by pretreatment with the cholinesterase inhibitor, and hence indirect cholinergic agonist, pyridostigmine [115,263]. Interestingly, failure of this pharmacological cholinergic manipulation, thought to influence central SRIH release, to affect GHRH-stimulated GH secretion is similar to that observed in conscious male rats with anterolateral deafferentation of the mediobasal hypothalamus or treated with cysteamine, two experimental models of hypothalamic SRIH depletion [179]. In the experience of Gianotti et al. [119], the b1 -adrenergic antagonist atenolol, again an inhibitor of central SRIH release, failed to potentiate the somatotropin response to the specific releasing hormone in group of anorectic women, whereas the b2 -adrenergic agonist salbutamol abolished the GH response. Blood glucose, a metabolic factor able to inhibit GH secretion in normal humans by increasing hypothalamic SRIH release, is not effective in AN: indeed, Rolla et al. [239] reported a failure of i.v. glucose infusion to blunt the exaggerated GH response to GHRH in adolescent girls with acutestage AN. On the contrary, the physiological role of other metabolic factors in the control of GH release seems to be preserved in this clinical condition. In fact, the amino acid arginine potentiates, whereas FFA inhibit, the somatotropin response to GHRH [115,121] as it occurs in normal subjects. Of note, in these patients i.v. infusion of SRIH at physiological doses normalizes the exaggerated response of GH to GHRH [123]. Another physiological mechanism apparently conserved in AN is the SRIH-mediated negative GH auto-feedback. In these patients, GHRH pretreatment inhibits the GH response to a subsequent application of the neurohormone just like in normal subjects, most likely due to the release of central SRIH triggered by the GH rise induced by the first GHRH bolus [115]. Additional abnormalities of stimulated GH secretion have been reported in AN. Unlike normal subjects, anorectic patients do not display a post-prandial blunting of GHRH-induced GH release, which may even be paradoxically increased [80]. Interestingly, in the same patients this post-prandial response is not potentiated by pyridostigmine [188] but it is partially blunted by the opioid antagonist naloxone [84]. These findings, in sharp contrast with those observed in normal subjects, suggest profound alterations of the physiological actions exerted by food ingestion and the opioid system on GH release. Of note, a similar failure of food to suppress GHRHinduced GH secretion has been reported in human obesity [80], a condition in which also ghrelin levels are not suppressed by the meal [99]. Data regarding the

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effect of food consumption on ghrelin in AN are so far lacking. The possible influence of CRH, a neuropeptide endowed with anorexigenic properties which is increased in cerebrospinal fluid of patients with AN [148], on GHRH-induced GH release has been investigated with discordant results. While Corsello and co-workers [24,71] have reported that CRH inhibits this response in normal subjects but not in anorectic patients, opposite results have been published by Rolla and coworkers [240]. Patients affected by AN often display GH responses to TRH and, less frequently, to GnRH [184], which are notably absent in normal subjects. This behaviour, which lacks as yet a clear explanation, is common to other conditions of tumoral and nontumoral hypersomatotropinism such as acromegaly, renal failure, and diabetes mellitus. As regards the GH response to TRH, two reports are noteworthy. Pretreatment with GHRH pulses has been shown to instate a GH responsiveness to TRH in normal humans [245]. More recently, data were presented suggesting that low circulating T3 might give rise to this abnormal GH responsiveness: indeed, administration of T3 for a week to a group of female and male anorectic patients with a low T3 syndrome was followed by decrease in basal serum GH levels and disappearance of the GH response to TRH [292]. In a collaborative study, we recently tried to obtain indirect information on GHRH function in AN using the model of SRIH infusion withdrawal [224]. Recent studies in animals and humans [57,85] have provided evidence that the rebound GH rise following withdrawal of SRIH infusion is due, at least in part, to activation of GHRH neurons. Using this approach, Alvarez et al. [7] had reported a marked impairment of the somatotropin response to the combination of SRIH withdrawal and GHRH administration in obese patients. In our hands, SRIH withdrawal induced a greater GH rebound rise in anorectic than in control subjects. A highly positive correlation between the magnitude of this response and the one elicited by GHRH was observed. In the same study we could also confirm the impairment of GH response to this new stimulus in a group of obese patients. 7.2.3. GHS and ghrelin Studies aimed at establishing GH responsiveness to the synthetic peptide hexarelin in AN have yielded variable results. Giusti et al. [125] reported an impaired somatotropin response to the GHS in these patients, while Popovic and co-workers [233] failed to confirm this observation, describing superimposable GH responses to hexarelin in anorectic and control subjects. The latter investigators, however, disclosed an abnormal action of GHS in AN, since hexarelin failed to suppress the somatotropin response to subsequent GHRH application in these patients, but not in normal weight women, women with secondary amenorrhoea following

voluntary weight loss and fasted women. To further clarify these issues, we tested the GH responsiveness to GHRH alone (1 lg/kg body weight), to hexarelin alone (1 lg/kg body weight), and to the combined administration of the two peptides in a group of eight anorectic women (age 16–28 years; mean body mass index, BMI, 14.0
0.66 kg/m2 ) and six normal weight healthy women (age 20–28 years, BMI 20.8
1.33 kg/m2 ). In the same subjects the ACTH response to hexarelin was also evaluated. While in normal women the GH response to hexarelin was greater than the one evoked by GHRH (Fig. 1a), the opposite behaviour was observed in anorectic patients (Fig. 1b). Furthermore, in normal weight subjects the GH response to GHRH was potentiated by the concomitant administration of hexarelin (Fig. 1a), whereas the GHS was unable to significantly increase the GHRH-induced GH release in anorectic women (Fig. 1b). Finally, the ACTH rise induced by hexarelin application was significantly lower in patients than in controls (DAUC 130.6
173.85 vs 760.0
186.10 ng/L/ min, p < 0:05). In interpreting these data in 1998, we hypothesized that the impaired GH and ACTH response to the GHS observed in AN was dependent on a hypersecretion of the putative endogenous ligand for GHS receptor. This interpretation was prompted by the well-

Fig. 1. Mean GH incremental areas in response to hexarelin (HEX) alone, GHRH alone and hexarelin plus GHRH in normalweight women (a) and in anorectic patients (b).

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documented impairment of ACTH response to CRH in AN [56], a condition characterized by elevated liquoral concentrations of CRH [148]. The discovery of ghrelin as the endogenous ligand for the GHS receptor 1 year later and the subsequent demonstration by Otto et al. [217] of significantly increased fasting plasma ghrelin levels in a large group of anorectic women fully confirmed our hypothesis. In a collaborative study [236] we recently confirmed the finding of high circulating ghrelin levels in AN and low plasma concentrations of the peptide in obesity. Further, a negative correlation was present between ghrelin concentrations and BMI in both groups of patients. According to a recent study by Tanaka et al. [274], bulimic girls also display higher serum ghrelin levels than healthly women, in spite of comparable BMI. The availability of ghrelin for in vivo human studies has recently allowed Gianotti et al. [120] to evaluate the effect of the natural peptide on GH secretion in AN: in agreement with our data, they observed an impaired somatotropin response to ghrelin in patients compared with controls. 7.3. Metabolic substrates, insulin, leptin, and cytokines As mentioned above, GH secretion appears to be refractory to the influence of blood glucose changes in AN, since hypoglycemia fails to increase somatotropin release, while hyperglycemia is unable to blunt the response of GH to GHRH. Conversely, the influence of amino acids on GH release appears to be conserved in these patients. Accordingly, i.v. infusion of arginine further increases the already elevated basal GH levels and enhances the GHRH-induced GH rise. As for FFA, their physiological inhibitory role on GH secretion also seems to be preserved in AN. In fact, the infusion of a lipid-heparin emulsion leads to increased serum FFA levels and inhibits the GH response to the specific releasing hormone, while the administration of acipimox, with the attendant decrease in circulating FFA, enhances the same response [121]. Increased FFA levels have been assigned a pathogenetic role in the impairment of GH release observed in obesity, but abnormalities of FFA concentrations are unlikely to participate in the hypersomatotropinism of AN, since, reportedly, circulating FFA are essentially normal in this condition [121]. In spite of the documented inhibitory effect of insulin on GH release, the role of hyperinsulinemia in the pathogenesis of the GH hyposecretion of obesity is not universally accepted, since normalization of serum insulin is not followed by restoration of normal GH release in these patients [58]. A possible involvement of malnourishment-induced hypoinsulinemia in the GH hypersecretion of AN is not clearly defined. As previously mentioned, leptin is likely to exert an inhibitory action on GH release in human. Along this line, hyperleptinemia might contribute to the GH hy-

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posecretion of obese patients. The question of whether the malnutrition-dependent reduction of leptin levels may play a role in the hypersomatotropinism of AN remains speculative, due to the lack of specific studies to this regard. To our knowledge, the only data available have shown an inverse correlation between serum leptin levels and pulsatile, but not basal, GH secretion as established by deconvolution analysis [265]. Given the known stimulatory role of leptin on gonadotropin secretion [9], the hypothesis that hypoleptinemia might play a role in the hypogonadotropic hypogonadism of AN (amenorrhoea is one of the diagnostic criteria for this disease) is intriguing but as yet not verified. Furthermore, this hypothesis is challenged by the clinical observation that amenorrhoea may precede weight loss. Cytokines such as tumor necrosis factor (TNF)-a, interleukin (IL)-1, and IL-6 are known to induce anorexia; based on this consideration and on the increased serum concentrations of cytokines reported by several, but not all, studies in anorectic patients, a possible pathophysiological role of these molecules in AN has been investigated. Furthermore, the growing evidence of reciprocal relations between the immune and endocrine systems has prompted studies on possible effects of cytokines on GH secretion. IL-1a and IL-1b have been shown to stimulate GH release from cultured ovine pituitary cells [110], while IL-1b and IL-6 enhanced GH output from pig pituitary cells [186]. Conversely, in the same experimental setting, TNF-a was shown to inhibit GHRH- and galanin-induced GH release; likewise, long-term administration of recombinant bovine TNF-a was able to reduce GHRH-stimulated GH secretion in dairy heifers [166] and steers [165]. In humans, chronic inflammatory diseases associated with high serum levels of IL-1, IL-6, and TNF-a, such as rheumatoid arthritis, are characterized by low GH and IGF-I concentrations, and by impaired GH responses to insulin-induced hypoglycemia [63,87]. Patients treated with interferon (IFN)-b for chronic hepatitis C display a significant increase in GH levels following each drug injection. The magnitude of this response is not correlated with the simultaneous increase in IL-6 concentrations, suggesting a direct action of IFN [211]. Furthermore, the possible contribution of cytokines to GH insensitivity (high GH with low IGF-I serum levels) accompanying critical illnesses, and also AN, has been investigated. Recent experimental data in rodents suggest that IL-6 and TNF-a may mediate GH resistance at receptor and post-receptor (inhibition of JAK-2/STAT5 signaling) levels during sepsis [296,300]. Thus, elevation of circulating cytokines due to different clinical conditions may be associated with different alterations of the GH–IGF-I axis. On the other hand, the increase in TNF-a production and TNF-a serum levels reported by several studies in AN [206,250,291] has not been confirmed by all investigators [33,227]. Likewise, serum levels of IL-6,

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have been reported to be both elevated [227] or normal [33] in this disease. On the whole, the possibility that elevated circulating concentrations of cytokines, and particularly those secreted by adipose tissue (TNF-a, IL1, and IL-6), play a role in the GH hypersecretion and resistance of AN is far from being convincingly demonstrated. This uncertainty is further increased by the discordant results reported by the most recent studies on girls affected by AN, describing normal concentrations of IL-1b, IL-6, and TNF-a, and reduced production of TNF-a and IL-6 in presence of increased production of IL-1b [36,209]. 7.4. Peripheral limb of the GH–IGF-I axis Like in other clinical conditions characterized by malnourishment [276], plasma IGF-I concentrations are low in AN [10,72]. Conversely, IGF-II circulating levels are apparently comparable to those of normal weight healthy subjects. As for IGFBPs, IGFBP-3, notably stimulated by GH and IGF-I, is definitely low in AN, whereas IGFBP-1 and IGFBP-2, whose production is negatively regulated by insulin, are significantly elevated, in keeping with the persistent hypoinsulinemia of these patients [10]. In the experience of Argente et al. [10], likely because of the reduction of IGFBP-3, the main binding protein for IGFs, serum concentrations of free IGF-I were not lowered in anorectic patients, suggesting that the decrease in total IGF-I was due to a reduction of the bound fraction of the growth factor. In contrast, Stoving et al. [264] observed low free IGF-I levels in their patients, whereas an increase in IGFBP-3 proteolysis, common to the catabolic states, was not demonstrated. In a study by Counts et al. [72] IGF-I and IGF-II were positively correlated with IGFBP-3 and negatively correlated with IGFBP-1 and IGFBP-2; furthermore, the two IGFs and IGFBP-3 were positively correlated with BMI, whereas IGFBP-1 and IGFBP-2 were negatively correlated with the same parameter. In our experience [248] no negative correlation was present between IGF-I levels and parameters of spontaneous GH release in anorectic women. Along the same line, Masuda et al. [190] failed to detect a negative correlation between circulating IGF-I and the magnitude of GHRH-induced GH release in their patients. Interestingly, according to the data of Argente et al. [10], these abnormalities of the peripheral limb of the somatotropic axis, i.e., low IGF-I and IGFBP-3 serum levels, are common to all patients with AN, independently from the pattern of spontaneous GH release (hyper- and hyposecretion): this observation reinforces the concept that nutritional status prevails over GH secretory pattern in determining IGF and IGFBP production in AN. From a clinical point of view, IGF-I deficiency may be in part responsible for the impaired statural growth

observed in anorectic patients with early onset of the disease. Together with nutritional deficiency [156] and other endocrine abnormalities (hypogonadism, hypercortisolism), IGF-I insufficiency may also contribute to the osteopenia displayed by most anorectic patients [202]. Indeed, s.c. administration of rhIGF-I, alone or in combination with oral contraceptives, for 9 months has recently been shown to significantly increase spinal bone density in a large group of osteopenic women with AN [130]. The low IGF-I levels in spite of elevated somatotropin concentrations in AN point to a peripheral GH resistance. This is due to a marked reduction in the number and/or activity of GH receptors, as reflected by the consistent decrease in circulating levels of the high affinity GH binding protein (GHBP)—corresponding to the extracellular domain of the GH receptor—which has been demonstrated in these patients [10,72,203]. In this respect AN resembles Laron syndrome, a condition of congenital GH insensitivity leading to severe dwarfism, associated with low serum GHBP levels. In agreement with the above-mentioned concepts, undernutrition causes a decrease in GH binding to hepatic membranes in several animal models [38,185]. Thus, AN can be considered a condition of acquired GH resistance, resembling other diseases characterized by malnourishment and hypercatabolic states such as sepsis, surgery, and critical illness [26]. However, since visceral fat is likely to be involved in the regulation of plasma GHBP levels [237], the extreme reduction of this tissue in AN could contribute to low GHBP concentrations. We have investigated the IGF-I responsiveness to exogenous GH in AN by somatomedin generation test: in our hands, s.c. administration of biosynthetic GH, 4 IU daily for 4 consecutive days, was not followed by any significant increase in IGF-I plasma levels in a group of anorectic women, consistent with a resistance to GH action (unpublished data). Recently, in a collaborative study, we tested the integrity of IGF-I negative feedback on GH secretion in AN [122]: s.c. administration of rhIGF-I at doses capable of restoring normal IGF-I levels was able to reduce, though not to normalize, spontaneous and GHRH-stimulated GH release. This observation is noteworthy considering that central IGFI hypersensitivity induced by food restriction [29] should have enhanced IGF-I feedback in AN, a condition of GH resistance but IGF-I hypersensitivity. Indeed, elevated IGF-I binding, expression of increased IGF-I receptors on red blood cells, has been described in anorectic patients [146]. It appears therefore that other factors besides the lack of normal IGF-I negative feedback are involved in the enhanced GH secretion of patients with AN. As mentioned previously, high circulating ghrelin concentrations might play a significant role (see also below). Interestingly, it follows that, as far as the GH–IGF-I axis is concerned, obesity ap-

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pears to be the opposite of AN, since it is characterized by GH hypersensitivity and IGF-I resistance. 7.5. Effects of weight gain According to Argente et al. [10], weight gain by only 6–8% in anorectic patients was followed by normalization of spontaneous GH release. As for stimulated GH secretion, both the potentiating effect of pyridostigmine and the inhibiting effect of pirenzepine on GHRH-induced GH release are restored by weight increase in AN [238,263], suggesting a normalization of the cholinergic control of somatotropin secretion. In our experience, the increased GH responsiveness to GHRH and SRIH infusion withdrawal observed in the acute phase of AN disappeared during the recovery phase [224]. Furthermore, the high levels of ghrelin decreased significantly after a 14% BMI increase obtained with psychotherapeutic intervention in a large group of anorectic women [217]. As for the peripheral limb of the GH–IGF-I axis, Counts and co-workers [72] observed a normalization of IGF-I, IGFBP, and GHBP levels after weight recovery in their patients. On the contrary, in the study of Argente et al. [10] only the initially low GHBP levels were normalized by weight gain; previously elevated IGFBP-1 and IGFBP-2 levels decreased without normalization after a recovery of more than 10% body

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weight, while the initially reduced IGF-I and IGFBP-3 were not changed significantly even 1 year after weight recovery. This observation is reminiscent of the findings in obese patients, who display a marked refractoriness with respect to the IGF-I reduction induced by caloric restriction. However, as in other forms of malnutrition such as celiac disease, a long period of weight recovery (at least 2 years) may be required to restore normal circulating IGF-I levels [141]. 7.6. Pathophysiology of the GH–IGF-I axis The different hypotheses put forward to explain the observed abnormalities of GH secretion in AN are summarized in Table 1. The decreased IGF-I production induced by malnutrition, leading to a defective hypothalamic and pituitary negative feedback on GH secretion, appears to be a major determinant of the hypersomatotropinism of AN. However, the lack of negative correlation between IGF-I levels and the parameters of spontaneous and GHRH-induced GH release, as well as the failure of exogenous IGF-I infusion to normalize spontaneous and GHRH-stimulated somatotropin secretion, suggest that factors other than or additional to the reduction of IGF-I are responsible for the altered GH secretion in these patients. Indeed, the extensive abnormalities of spontaneous and stimulated

Table 1 Hypothesised factors contributing to GH hypersecretion in AN Pathogenetic factors

Pros

Cons

Decreased IGF-I production

Defect in hypothalamic and pituitary negative feedback on GH secretion

Lack of negative correlation between IGF-I and both parameters of spontaneous GH release and magnitude of GHRH-induced GH rise Failure of exogenous IGF-I to normalize spontaneous and GHRH-stimulated GH secretion

Increased activity of GHRH neurones with augmented frequency of GHRH discharges

Increased GH pulse frequency

Enhanced GH response to SRIH infusion withdrawal Enhanced GH response to galanin Reduced central SRIH tone

Elevated interpulse GH levels Exaggerated GH response to GHRH Impaired GH reactivity to factors known to influence hypothalamic SRIH release (blood glucose changes, acute glucocorticoids) Failure of substances known to influence central SRIH release (pyridostigmine, pirenzepine, atenolol) to affect GHRH-induced GH secretion Normalization of GHRH-stimulated GH release by exogenous SRIH infusion Low cerebrospinal fluid levels of SRIH

Ghrelin hypersecretion

Elevated circulating ghrelin Reduced GH response to hexarelin and ghrelin

Normal GH responsiveness to arginine Lack of GH response to the second of two consecutive boli of GHRH

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GH release described in AN strongly suggest a concomitant hypothalamic dysregulation. An increased activity of GHRH neurons with augmented frequency of GHRH discharges is suggested by the increase in spontaneous GH pulse frequency as well as by the enhancement of the GH response to galanin and to SRIH infusion withdrawal, a test thought to provide indirect information on the function of GHRH-secreting neurons. Numerous observations speak in favour of a defective central SRIH tone, such as elevated interpulse GH levels, exaggerated somatotropin response to GHRH, and impaired GH reactivity to factors thought to influence hypothalamic SRIH release (blood glucose changes, acute glucocorticoid administration). The same point of view is also supported by the failure of pharmacological manipulation of cholinergic tone (pyridostigmine and pirenzepine) to affect GHRH-induced GH secretion similarly to what observed in rats with hypothalamic SRIH depletion, the failure of the b-blocking agent atenolol, able to reduce central SRIH tone, to potentiate the somatotropin response to its releasing hormone, and finally by the normalization of this response after exogenous SRIH infusion. Furthermore, according to the hypothesis that GHS may act, at least in part, as SRIH antagonists, the impaired GH response to hexarelin and ghrelin observed in AN could also point to a decreased hypothalamic SRIH tone. A more direct support to the hypothesis of a defective somatostatinergic tone is given by the detection of low SRIH levels in the cerebrospinal fluid of anorectic patients [113]. The reduction of SRIH tone, however, does not seem to be absolute, since the neuropeptide can be released by adequate challenges: this is suggested by the normal GH reactivity to arginine, which is believed to restrain SRIH release from the hypothalamus, and by the lack of GH response to the second of two consecutive GHRH boluses, indicating that the SRIH-mediated negative GH auto-feedback is preserved in AN. The hypersecretion of the orexigenic gastric peptide ghrelin, most likely reflecting a physiological attempt to compensate for the lack of nutritional intake and energy stores, might contribute to the hypersomatotropinism of AN. This hypothesis would find support in the demonstration of a positive correlation between ghrelin and GH levels, which however has not been shown either in anorectic [280] or in obese patients [175]. The impairment of GH response to exogenous ghrelin reported in anorectic women suggests a possible resistance to the peptide in this clinical condition. Studies on number/ function of GHS receptors in AN could add useful information on this issue, and help to clarify the question of whether the orexigenic effect of ghrelin, demonstrated in normal humans, is fully operative also in these patients. The question as to whether these women do not feel hunger, i.e., are really ‘‘anorectic’’ from a biological point of view, or on the contrary they feel hunger but

are able to strictly control this impulse by psychopathological mechanisms is still a matter of debate. The demonstration of a full ghrelin activity in AN would support the latter hypothesis. Regarding the peripheral limb of the somatotropin– somatomedin axis, as already discussed, AN is connoted as a condition of GH resistance and IGF-I hypersensitivity. With a different time course, all abnormalities of the GH–IGF-I axis are reversed by weight recuperation and thus appear to be secondary to malnutrition. According to Counts et al. [72], changes in the GH-IGF-I axis secondary to nutritional deprivation are protective, since ‘‘resources are used for maintenance of basic metabolic functions instead of growth, and glucose counterregulation is increased to prevent hypoglycemia.’’ Furthermore, high GH with low IGF-I and insulin levels might favour lipolysis making FFA available to peripheral tissues.

8. Conclusions A wide series of experimental data has clarified that the nutritional status, represented by stored or circulating energetic substrates, plays a major role in the regulation of GH secretion. Thus, nutritional conditions have always to be taken into account when interpreting GH secretory dynamics. This is not surprising considering that GH, besides promoting statural growth, exerts important influences on intermediary metabolism increasing the availability of energetic substrates such as glucose and FFA for oxidation, favouring the preferential utilization of lipids, and exerting a potent nitrogen sparing, i.e., protidoanabolic, effect. These metabolic actions profoundly affect body composition and indeed have their clinical correlates in the metabolic syndrome displayed by adults with GH deficiency and in the increased fat free mass and reduced fat mass observed in acromegaly. On the other hand, nutritional conditions themselves regulate GH secretion, and indeed undernutrion and fasting are associated with enhanced GH secretion in human, whereas overnutrition, exemplified by obesity, is characterized by suppressed GH release. AN is a paradigm of chronic undernutrition in which GH secretion is highly disordered and, on the whole, markedly enhanced. This GH secretory pattern, which may reflect an adaptive phenomenon, is ineffective, however, in re-establishing metabolic homeostasis, due to inability of the liver to generate IGF-I. Thus, AN represents a model of acquired GH resistance, opposite to that of obesity, connoted by GH hypersensitivity. The study of this disorder may provide important clues for a better understanding of the mechanisms whereby nutritional status, as indicated by specific signals (chiefly ghrelin and leptin), regulates GH secretion.

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References [1] E.F. Adams, B. Petersen, T. Lei, M. Buchfelder, R. Fahlbusch, The growth hormone secretagogue, L-692,429, induces phosphatidylinositol hydrolysis and hormone secretion by human pituitary tumors, Biochem. Biophys. Res. Commun. 208 (1995) 555–561. [2] E. Adeghate, A.S. Ponery, Ghrelin stimulates insulin secretion from the pancreas of normal and diabetic rats, J. Neuroendocrinol. 14 (2002) 555–560. [3] I. Ahmad, J.A. Finkelstein, T.R. Downs, L.A. Frohman, Obesity-associated decrease in growth hormone releasing-hormone gene expression: a mechanism for reduced growth hormone mRNA levels in genetically obese Zucker rats, Neuroendocrinology 58 (1993) 332–337. [4] J. Alba-Roth, O.A. Muller, J. Schopohl, K. Von Werder, Arginine stimulates growth hormone secretion by suppressing endogenous somatostatin secretion, J. Clin. Endocrinol. Metab. 67 (1988) 1186–1189. [5] J. Altman, Weight in the balance, Neuroendocrinology 76 (2002) 131–136. [6] C.V. Alvarez, F. Mallo, B. Burguera, L. Cacicedo, C. Dieguez, F.F. Casanueva, Evidence for a direct pituitary inhibition by free fatty acids of in vivo growth hormone responses to growth hormone-releasing hormone in the rat, Neuroendocrinology 53 (1991) 185–189. [7] P. Alvarez, L. Isidro, A. Leal-Cerro, F.F. Casanueva, C. Dieguez, F. Cordido, Effect of withdrawal of somatostatin plus GH-releasing hormone as a stimulus of GH secretion in obesity, Clin. Endocrinol. 56 (2002) 487–492. [8] A.C. Andreotti, R. Lanzi, M.F. Manzoni, A. Caumo, A. Moreschi, A.E. Pontiroli, Acute pharmacologic blockade of lipolysis normalizes nocturnal growth hormone levels and pulsatility in obese subjects, Metabolism 43 (1994) 1207–1213. [9] D. Apter, E. Hermanson, Update on female pubertal development, Curr. Opin. Obstet. Gynecol. 14 (2002) 475–481. [10] J. Argente, N. Caballo, V. Barrios, M.T. Munoz, J. Pozo, J.A. Chowen, G. Morande, M. Hernandez, Multiple endocrine abnormalities of the growth hormone and insulin-like growth factor axis in patients with anorexia nervosa: effect of short- and long-term weight recuperation, J. Clin. Endocrinol. Metab. 82 (1997) 2084–2092. [11] J. Argente, N. Caballo, V. Barrios, J. Pozo, M.T. Munoz, J.A. Chowen, M. Hernandez, Multiple endocrine abnormalities of the growth hormone and insulin-like growth factor axis in prepubertal children with exogenous obesity: effect of short- and longterm weight reduction, J. Clin. Endocrinol. Metab. 82 (1997) 2076–2083. [12] H. Ariyasu, K. Takaya, T. Tagami, Y. Ogawa, K. Hosoda, T. Akamizu, M. Suda, T. Koh, K. Natsui, S. Toyooka, G. Shirakami, T. Usui, A. Shimatsu, K. Doi, H. Hosoda, M. Kojima, K. Kangawa, K. Nakao, Stomach is a major source of circulating ghrelin, and feeding state determines plasma ghrelinlike immunoreactivity levels in humans, J. Clin. Endocrinol. Metab. 86 (2001) 4753–4758. [13] E. Arvat, L. Di Vito, B. Maccagno, F. Broglio, M.F. Boghen, R. Deghenghi, F. Camanni, E. Ghigo, Effects of GHRP-2 and hexarelin, two synthetic GH-releasing peptides, on GH, prolactin, ACTH and cortisol levels in man. Comparison with the effects of GHRH, TRH and hCRH, Peptides 18 (1997) 885–891. [14] E. Arvat, L. DiVito, L. Gianotti, J. Ramunni, M.F. Boghen, R. Deghenghi, F. Camanni, E. Ghigo, Mechanisms underlying the negative growth hormone (GH) autofeedback on the GHreleasing effect of hexarelin in man, Metabolism 46 (1997) 83–88. [15] E. Arvat, B. Maccagno, j. Ramunni, L. Di Vito, L. Gianotti, F. Broglio, A. Benso, R. Deghenghi, F. Camanni, E. Ghigo, Effects

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25] [26]

[27]

[28]

[29]

[30]

215

of dexamethasone and alprazolam, a benzodiazepine, on the stimulatory effect of hexarelin, a synthetic GHRP, on ACTH, cortisol and GH secretion in humans, Neuroendocrinology 67 (1998) 310–316. E. Arvat, M. Maccario, L. Di Vito, F. Broglio, A. Benso, C. Gottero, M. Papotti, G. Muccioli, C. Dieguez, F.F. Casanueva, R. Deghenghi, F. Camanni, E. Ghigo, Endocrine activities of ghrelin, a natural growth hormone secretagogue (GHS), in humans: comparison and interactions with hexarelin, a nonnatural peptidyl GHS, and GH-releasing hormone, J. Clin. Endocrinol. Metab. 86 (2001) 1169–1174. C.M. Asplin, A.C.S. Faria, E.C. Carlsen, V.A. Vaccaro, R.E. Barr, A. Iranmanesh, M.M. Lee, J.D. Veldhuis, W.S. Evans, Alterations in the pulsatile mode of growth hormone release in men and women with insulin-dependent diabetes mellitus, J. Clin. Endocrinol. Metab. 69 (1989) 239–245. T.M. Badger, W.J. Millard, G.F. McCormick, C.Y. Bowers, J.B. Martin, The effects of growth hormone (GH)-releasing peptides on GH secretion in perifused pituitary cells of adult male rats, Endocrinology 115 (1984) 1432–1438. M. Bagnasco, P.S. Kalra, S.P. Kalra, Ghrelin and leptin pulse discharge in fed and fasted rats, Endocrinology 143 (2002) 726– 729. J.F. Bak, N. Moller, O. Schmitz, Effects of growth hormone on fuel utilization and muscle glycogen synthase activity in normal humans, Am. J. Physiol. 260 (1991) E736–E742. S. Balzano, S. Loche, M.L. Murtas, T. Fanni, V. Sica, C. Pintor, E. Martino, Potentiation of cholinergic tone counteracts the suppressive effect of oral glucose administration on the GH response to GHRH in man, Horm. Metab. Res. 21 (1989) 52–53. B. Baranowska, M. Radzikowska, E. Wasilewska-Dziubinska, K. Roguski, M. Borowiec, The role of VIP and somatostatin in the control of GH and prolactin release in anorexia nervosa and in obesity, Ann. N. Y. Acad. Sci. 921 (2000) 443–455. M. Baratta, R. Saleri, G.L. Mainardi, D. Valle, A. Giustina, C. Tamanini, Leptin regulates GH gene expression and secretion and nitric oxide production in pig pituitary cells, Endocrinology 143 (2002) 551–557. A. Barbarino, S.M. Corsello, S. Della Casa, A. Tofani, R. Sciuto, C.A. Rota, L. Bollanti, A. Barini, Corticotropin-releasing hormone inhibition of growth hormone-releasing hormoneinduced growth hormone release in man, J. Clin. Endocrinol. Metab. 71 (1990) 1368–1374. B.A. Bengtsson, R.J. Brummer, S. Eden, I. Bosaeus, Body composition in acromegaly, Clin. Endocrinol. 30 (1989) 121–130. J. Bentham, J. Rodriguez-Arnao, R.J. Ross, Acquired growth hormone resistance in patients with hypercatabolism, Horm. Res. 40 (1993) 87–91. B.B. Bercu, S.W. Yang, R. Masuda, R.F. Walker, Role of selected endogenous peptides in growth hormone-releasing hexapeptide activity: analysis of growth hormone-releasing hormone, thyroid hormone-releasing hormone, and gonadotropin-releasing hormone, Endocrinology 130 (1992) 2579–2586. K. Berneis, U. Keller, Metabolic actions of growth hormone: direct and indirect, Bailli
ereÕs Clin. Endocrinol. Metab. 10 (1996) 337–352. N.J. Bohannon, E.S. Corp, B.J. Wilcox, D.P. Figlewicz, D.M. Dorsa, D.G. Baskin, Characterization of insulin-like growth factor I receptors in the median eminence of the brain and their modulation by food restriction, Endocrinology 122 (1988) 1940– 1947. M. Boni-Schnetzler, M.A. Gosteli-Peter, W. Moritz, E.R. Froesch, J. Zapf, Reduced ob mRNA in hypophysectomised rats is not restored by growth hormone (GH), but further suppressed by exogenously administered insulin-like growth factor (IGF) I, Biochem. Biophys. Res. Commun. 225 (1996) 296–301.

216

M. Scacchi et al. / Frontiers in Neuroendocrinology 24 (2003) 200–224

[31] C.Y. Bowers, F.A. Momany, G.A. Reynolds, A. Hong, On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone, Endocrinology 114 (1984) 1537–1545. [32] C.Y. Bowers, A.O. Sartor, G.A. Reynolds, T.M. Badger, On the actions of the growth hormone-releasing hexapeptide, GHRP, Endocrinology 128 (1991) 2027–2035. [33] F. Brambilla, L. Bellodi, M. Brunetta, G. Perna, Plasma concentrations of interleukin-1 beta, interleukin-6 and tumor necrosis factor-alpha in anorexia and bulimia nervosa, Psychoneuroendocrinology 23 (1998) 439–447. [34] F. Brambilla, E. Ferrari, F. Cavagnini, C. Invitti, A. Zanoboni, R. Massironi, M. Catalano, D. Cocchi, E.E. Muller, a 2Adrenoceptor sensitivity in anorexia nervosa: GH response to clonidine or GHRH stimulation, Biol. Psychiatry 25 (1989) 256– 264. [35] F. Brambilla, M. Lampertico, L. Sali, F. Cavagnini, C. Invitti, M. Maggioni, C. Candolfi, A.E. Panerai, E.E. Muller, Clonidine stimulation in anorexia nervosa: growth hormone, cortisol, and beta-endorphin responses, Psychiatry Res. 20 (1987) 19–31. [36] F. Brambilla, D. Monti, C. Franceschi, Plasma concentrations of interleukin-1-beta, interleukin-6 and tumor necrosis factoralpha, and of their soluble receptors and receptor antagonist in anorexia nervosa, Psychiatry Res. 103 (2001) 107–114. [37] P. Brambilla, L. Bosio, P. Manzoni, A. Pietrobelli, L. Beccaria, G. Chiumello, Peculiar body composition in patients with Prader–Labhart–Willi syndrome, Am. J. Clin. Nutr. 65 (1997) 1369–1374. [38] B.H. Breier, P.D. Gluckman, J.J. Bass, The somatotrophic axis in young steers: influence of nutritional status and oestradiol-17 beta on hepatic high- and low-affinity somatotrophic binding sites, J. Endocrinol. 116 (1988) 169–177. [39] R.S. Brogan, S.K. Fife, L.K. Conley, A. Giustina, W.B. Wehrenberg, Effects of food deprivation on the GH axis: immunocytochemical and molecular analysis, Neuroendocrinology 65 (1997) 129–135. [40] F. Broglio, E. Arvat, A. Benso, C. Gottero, G. Muccioli, M. Papotti, A.J. Van Der Lely, R. Deghenghi, E. Ghigo, Ghrelin, a natural GH secretagogue produced by the stomach, induces hyperglycemia and reduces insulin secretion in humans, J. Clin. Endocrinol. Metab. 86 (2001) 5083–5086. [41] F. Broglio, A. Benso, C. Gottero, F. Prodam, S. Grottoli, F. Tassone, M. Maccario, F.F. Casanueva, C. Dieguez, R. Deghenghi, E. Ghigo, E. Arvat, Effects of glucose, free fatty acids or arginine load on the GH-releasing activity of ghrelin in humans, Clin. Endocrinol. 57 (2002) 265–271. [42] F. Broglio, P. van Koetsveld, A. Benso, C. Gottero, F. Prodam, M. Papotti, G. Muccioli, C. Gauna, L. Hofland, R. Deghenghi, E. Arvat, A.J. Van Der Lely, E. Ghigo, Ghrelin secretion is inhibited by either somatostatin or cortistatin in humans, J. Clin. Endocrinol. Metab. 87 (2002) 4829–4832. [43] A. Brunani, C. Invitti, A. Dubini, R. Piccoletti, P. Bendinelli, P. Maroni, G. Pezzoli, G. Ramella, A. Calogero, F. Cavagnini, Cerebrospinal fluid and plasma concentrations of SRIH, betaendorphin, CRH, NPY and GHRH in obese and normalweight subjects, Int. J. Obes. 19 (1995) 17–21. [44] P. Butler, E. Kryshak, R. Rizza, Mechanism of growth hormone-induced postprandial carbohydrate intolerance in humans, Am. J. Physiol. 260 (1991) E513–E520. [45] A. Caix as, C. Bashore, W. Nash, F. Pi-Sunyer, B. Laferr
ere, Insulin, unlike food intake, does not suppress ghrelin in human subjects, J. Clin. Endocrinol. Metab. 87 (2002) 1902–1907. [46] G. Caldwell, G. Hart, E.M. Kohner, J.M. Burrin, Growth hormone-releasing factor-induced growth hormone secretion from perifused rat anterior pituitary cells: lack of influence of glucose concentration, and normal responses in pituitary cells from diabetic animals, J. Endocrinol. 122 (1989) 657–660.

[47] F. Camanni, E. Ghigo, E. Arvat, Growth hormone-releasing peptides and their analogs, Front. Neuroendocrinol. 19 (1998) 47–72. [48] A.L. Carrel, D.B. Allen, Effects of growth hormone on adipose tissue, J. Pediatr. Endocrinol. Metab. 13 (Suppl. 2) (2000) 1003– 1009. [49] A.L. Carrel, S.E. Myers, B.Y. Whitman, D.B. Allen, Benefits of long-term GH therapy in Prader–Willi syndrome: a 4-year study, J. Clin. Endocrinol. Metab. 87 (2002) 1581–1585. [50] E. Carro, R. Senaris, R.V. Considine, F.F. Casanueva, C. Dieguez, Regulation of in vivo growth hormone secretion by leptin, Endocrinology 138 (1997) 2203–2206. [51] E. Carro, R.M. Senaris, L.M. Seoane, L.A. Frohman, A. Arimura, F.F. Casanueva, C. Dieguez, Role of growth hormone (GH)-releasing hormone and somatostatin on leptin-induced GH secretion, Neuroendocrinology 69 (1999) 3–10. [52] P.V. Carroll, E.R. Christ, B.A. Bengtsson, L. Carlsson, J.S. Christiansen, D. Clemmons, R. Hintz, K. Ho, Z. Laron, P. Sizonenko, P.H. Sonksen, T. Tanaka, M. Thorner, Growth hormone deficiency in adulthood and the effects of growth hormone replacement: a review. Growth Hormone Research Society Scientific Committee, J. Clin. Endocrinol. Metab. 83 (1998) 382–395. [53] F.F. Casanueva, C.G. Borras, B. Burguera, L. Lima, C. Muruais, J.A. Tresguerres, J. Devesa, Growth hormone and prolactin secretion after growth hormone-releasing hormone administration, in anorexia nervosa patients, normal controls and tamoxifenpretreated volunteers, Clin. Endocrinol. 27 (1987) 517–523. [54] F.F. Casanueva, C. Dieguez, V. Popovic, R. Peino, R.V. Considine, J.F. Caro, Serum immunoreactive leptin concentrations in patients with anorexia nervosa before and after partial weight recovery, Biochem. Mol. Med. 60 (1997) 116–120. [55] F.F. Casanueva, L. Villanueva, C. Dieguez, Y. Diaz, J.A. Cabranes, B. Szoke, M.F. Scanlon, A.V. Schally, A. FernandezCruz, Free fatty acids block growth hormone (GH) releasing hormone-stimulated GH secretion in man directly at the pituitary, J. Clin. Endocrinol. Metab. 65 (1987) 634–642. [56] F. Cavagnini, C. Invitti, M. Passamonti, E.E. Polli, Response of ACTH to corticotropin-releasing hormone in anorexia nervosa, N. Engl. J. Med. 314 (1986) 184–185. [57] S.G. Cella, M. Luceri, L. Cattaneo, A. Torsello, E.E. Muller, Somatostatin withdrawal as generator of pulsatile GH release in the dog: a possible tool to evaluate the endogenous GHRH tone?, Neuroendocrinology 63 (1996) 481–488. [58] S.A. Chalew, R.A. Lozano, K.M. Armour, A.A. Kowarski, Reduction of plasma insulin levels does not restore integrated concentration of growth hormone to normal in obese children, Int. J. Obes. Relat Metab. Disord. 16 (1992) 459–463. [59] K. Cheng, W.W. Chan, A.Jr. Barreto, E.M. Convey, R.G. Smith, The synergistic effects of His-D-Trp-Ala-Trp-D-Phe-LysNH2 on growth hormone (GH)-releasing factor-stimulated GH release and intracellular adenosine 30 ,50 -monophosphate accumulation in rat primary pituitary cell culture, Endocrinology 124 (1989) 2791–2798. [60] K. Cheng, W.W. Chan, B. Butler, L. Wei, W.R. Schoen, M.J. Wyvratt Jr., M.H. Fisher, R.G. Smith, Stimulation of growth hormone release from rat primary pituitary cells by L-692,429, a novel non-peptidyl GH secretagogue, Endocrinology 132 (1993) 2729–2731. [61] K. Cheng, W.W. Chan, B. Butler, L. Wei, R.G. Smith, A novel non-peptidyl growth hormone secretagogue, Horm. Res. 40 (1993) 109–115. [62] K. Chihara, H. Kodama, H. Kaji, T. Kita, Y. Kashio, Y. Okimura, H. Abe, T. Fujita, Augmentation by propranolol of growth hormone-releasing hormone-(1-44)-NH2-induced growth hormone release in normal short and normal children, J. Clin. Endocrinol. Metab. 61 (1985) 229–233.

M. Scacchi et al. / Frontiers in Neuroendocrinology 24 (2003) 200–224 [63] I.C. Chikanza, Neuroendocrine immune features of pediatric inflammatory rheumatic diseases, Ann. N. Y. Acad. Sci. 876 (1999) 71–80. [64] D.R. Clemmons, A. Klibanski, L.E. Underwood, J.W. McArthur, E.C. Ridgway, I.Z. Beitins, J.J. Van Wyk, Reduction of plasma immunoreactive somatomedin-C during fasting in humans, J. Clin. Endocrinol. Metab. 53 (1981) 1247–1250. [65] D.R. Clemmons, D.K. Snyder, R. Williams, L.E. Underwood, Growth hormone administration conserves lean body mass during dietary restriction in obese subjects, J. Clin. Endocrinol. Metab. 64 (1987) 878–883. [66] P. Cohen, R.G. Rosenfeld, The IGF axis, in: A.L. Rosenbloom (Ed.), Human Growth Hormone, Basic and Scientific Aspects, CRC Press, Boca Raton, 1995, pp. 43–58. [67] F.L. Conceicao, A. Bojensen, J.O. Jorgensen, J.S. Christiansen, Growth hormone therapy in adults, Front. Neuroendocrinol. 22 (2001) 213–246. [68] R.V. Considine, M.K. Sinha, M.L. Heiman, A. Kriauciunas, T.W. Stephens, M.R. Nyce, J.P. Ohannesian, C.C. Marco, L.J. McKee, T.L. Bauer, J.F. Caro, Serum immunoreactive-leptin concentrations in normal-weight and obese women, N. Engl. J. Med. 334 (1996) 292–295. [69] K.C. Copeland, K.S. Nair, Acute growth hormone effects on amino acid and lipid metabolism, J. Clin. Endocrinol. Metab. 78 (1994) 1040–1047. [70] F. Cordido, R. Peino, A. Penalva, C.V. Alvarez, F.F. Casanueva, C. Dieguez, Impaired growth hormone secretion in obese subjects is partially reversed by acipimox-mediated plasma free fatty acid depression, J. Clin. Endocrinol. Metab. 81 (1996) 914–918. [71] S.M. Corsello, A. Tofani, S. Della Casa, C.A. Rota, R. Sciuto, S. Colasanti, A. Bini, A. Barini, A. Barbarino, Effect of CRH and pirenzepine on GHRH-induced GH release in the acute and recovery phases of anorexia nervosa, in: Abstract No. 481 74th Annual Meeting of the Endocrine Society, San Antonio, 1992. [72] D.R. Counts, H. Gwirtsman, L.M. Carlsson, M. Lesem, G.B. Cutler, The effect of anorexia nervosa and refeeding on growth hormone-binding protein, the insulin-like growth factors (IGFs), and the IGF-binding proteins, J. Clin. Endocrinol. Metab. 75 (1992) 762–767. [73] M.A. Cowley, R.G. Smith, S. Diano, M. Tsch€ op, N. Pronchuk, K.L. Grove, C.J. Strasburger, M. Bidlingmaier, M. Esterman, M.L. Heiman, L.M. Garcia-Segura, E.A. Nillni, P. Mendez, M.J. Low, P. Sotonyi, J.M. Friedman, H. Liu, S. Pinto, W.F. Colmers, R.D. Cone, T.L. Horvath, The distribution and mechanism of action of ghrelin in the CNS demonstrates a novel hypothalamic circuit regulating energy homeostasis, Neuron 37 (2003) 649–661. [74] D.E. Cummings, J.Q. Purnell, R.S. Frayo, K. Schmidova, B.E. Wisse, D.S. Weigle, A preprandial rise in plasma ghrelin levels suggests a role in meal initiation in humans, Diabetes 50 (2001) 1714–1719. [75] D.E. Cummings, D.S. Weigle, R.S. Frayo, P.A. Breen, M.K. Ma, E.P. Dellinger, J.Q. Purnell, Plasma ghrelin levels after dietinduced weight loss or gastric bypass surgery, N. Engl. J. Med. 346 (2002) 1623–1630. [76] I. Cusin, F. Rohner-Jeanrenaud, A. Stricker-Krongrad, B. Jeanrenaud, The weight-reducing effect of an intracerebroventricular bolus injection of leptin in genetically obese fa/fa rats. Reduced sensitivity compared with lean animals, Diabetes 45 (1996) 1446–1450. [77] Y. Date, M. Kojima, H. Hosoda, A. Sawaguchi, M.S. Mondal, T. Suganuma, S. Matsukura, K. Kangawa, M. Nakazato, Ghrelin, a novel growth hormone-releasing acylated peptide, is synthesized in a distinct endocrine cell type in the gastrointestinal tracts of rats and humans, Endocrinology 141 (2000) 4255–4261. [78] Y. Date, M. Nakazato, S. Hashiguchi, K. Dezaki, M.S. Mondal, H. Hosoda, M. Kojima, K. Kangawa, T. Arima, H. Matsuo, T.

[79]

[80]

[81]

[82]

[83]

[84]

[85]

[86]

[87]

[88]

[89]

[90]

[91]

[92]

217

Yada, S. Matsukura, Ghrelin is present in pancreatic alpha-cells of humans and rats and stimulates insulin secretion, Diabetes 51 (2002) 124–129. R.R. Davies, S. Turner, D.G. Johnston, Oral glucose inhibits growth hormone secretion induced by human pancreatic growth hormone releasing factor 1–44 in normal man, Clin. Endocrinol. 21 (1984) 477–481. L. De Marinis, G. Folli, C. DÕAmico, A. Mancini, P. Sambo, A. Tofani, A. Oradei, A. Barbarino, Differential effects of feeding on the ultradian variation of the growth hormone (GH) response to GH-releasing hormone in normal subjects and patients with obesity and anorexia nervosa, J. Clin. Endocrinol. Metab. 66 (1988) 598–604. L. De Marinis, A. Mancini, C. DÕAmico, P. Zuppi, A. Tofani, S. Della Casa, A. Saporosi, P. Sambo, C. Fiumara, F. Calabr
o, A. Barbarino, Influence of naloxone infusion on prolactin and growth hormone response to growth hormone-releasing hormone in anorexia nervosa, Psychoneuroendocrinology 16 (1991) 499–504. L. De Marinis, A. Mancini, D. Valle, A. Bianchi, R. Gentilella, D. Milardi, C. Mascadri, A. Giustina, Effects of galanin on growth hormone and prolactin secretion in anorexia nervosa, Metabolism 49 (2000) 155–159. L. De Marinis, A. Mancini, D. Valle, D. Izzi, A. Bianchi, R. Gentilella, A. Giampietro, P. Desenzani, A. Giustina, Role of food intake in the modulation of hexarelin-induced growth hormone release in normal human subjects, Horm. Metab. Res. 32 (2000) 152–156. L. De Marinis, A. Mancini, P. Zuppi, C. Fiumara, M.L. Fabrizi, L. Sammartano, G. Conte, D. Valle, S. Daini, F.M. Ferro, Opioid dysregulation in anorexia nervosa: naloxone effects on preprandial and postprandial growth hormone response to growth hormone-releasing hormone, Metabolism 43 (1994) 140–143. E.C. degli Uberti, M.R. Ambrosio, S.G. Cella, A.R. Margutti, G. Trasforini, A.E. Rigamonti, E. Petrone, E.E. Muller, Defective hypothalamic growth hormone (GH)-releasing hormone activity may contribute to declining GH secretion with age in man, J. Clin. Endocrinol. Metab. 82 (1997) 2885–2888. G. Delitala, P.A. Tomasi, M. Palermo, P. Fresu, Interaction of glucose and pyridostigmine on the secretion of growth hormone (GH) induced by GH-releasing hormone (GHRH), J. Endocrinol. Invest. 13 (1990) 653–656. H. Demir, F. Kelestimur, M. Tunc, M. Kirnap, Y. Ozugul, Hypothalamo–pituitary–adrenal axis and growth hormone axis in patients with rheumatoid arthritis, Scand. J. Rheumatol. 28 (1999) 41–46. L. Di Vito, F. Broglio, A. Benso, C. Gottero, F. Prodam, M. Papotti, G. Muccioli, C. Dieguez, F.F. Casanueva, R. Deghenghi, E. Ghigo, E. Arvat, The GH-releasing effect of ghrelin, a natural GH secretagogue, is only blunted by the infusion of exogenous somatostatin in humans, Clin. Endocrinol. 56 (2002) 643–648. S.L. Dickson, O. Doutrelant-Viltart, G. Leng, GH-deficient dw/ dw rats and lit/lit mice show increased Fos expression in the hypothalamic arcuate nucleus following systemic injection of GH-releasing peptide-6, J. Endocrinol. 146 (1995) 519–526. C. Dieguez, Leptin: A New GH Secretagogue, International Symposium on Growth Hormone Basic Aspects and New Clinical Applications, Lilly Press, Marbella, 1998. C. Dieguez, E. Carro, L.M. Seoane, M. Garcia, J.P. Camina, R. Senaris, V. Popovic, F.F. Casanueva, Regulation of somatotroph cell function by the adipose tissue, Int. J. Obes. Relat. Metab. Disord. 24 (Suppl 2) (2002) S100–S103. J. Dietz, J. Schwartz, Growth hormone alters lipolysis and hormone-sensitive lipase activity in 3T3-F442A adipocytes, Metabolism 40 (1991) 800–806.

218

M. Scacchi et al. / Frontiers in Neuroendocrinology 24 (2003) 200–224

[93] A. Doglio, C. Dani, G. Fredrikson, P. Grimaldi, G. Ailhaud, Acute regulation of insulin-like growth factor-I gene expression by growth hormone during adipose cell differentiation, EMBO J. 6 (1987) 4011–4016. [94] A. Doglio, C. Dani, P. Grimaldi, G. Ailhaud, Growth hormone regulation of the expression of differentiation-dependent genes in preadipocyte Ob1771 cells, Biochem. J. 238 (1986) 123–129. [95] C. Dornonville de la Cour, M. Bjorkqvist, A.K. Sandvik, I. Bakke, C.M. Zhao, D. Chen, R. Hakanson, A-like cells in the rat stomach contain ghrelin and do not operate under gastrin control, Regul. Pept. 99 (2001) 141–150. [96] E.M. Egido, J. Rodriguez-Gallardo, R.A. Silvestre, J. Marco, Inhibitory effect of ghrelin on insulin and pancreatic somatostatin secretion, Eur. J. Endocrinol. 146 (2002) 241–244. [97] U. Eiholzer, R. Gisin, C. Weinmann, S. Kriemler, H. Steinert, T. Torresani, M. Zachmann, A. Prader, Treatment with human growth hormone in patients with Prader– Labhart–Willi syndrome reduces body fat and increases muscle mass and physical performance, Eur. J. Pediatr. 157 (1998) 368–377. [98] A. Elimam, S. Norgren, C. Marcus, Effects of growth hormone treatment on the leptin system and body composition in obese prepubertal boys, Acta Paediatr. 90 (2001) 520–525. [99] P.J. English, M.A. Ghatei, I.A. Malik, S.R. Bloom, J.P. Wilding, Food fails to suppress ghrelin levels in obese humans, J. Clin. Endocrinol. Metab. 87 (2002) 2984–2987. [100] J.C. Erickson, K.E. Clegg, R.D. Palmiter, Sensitivity to leptin and susceptibility to seizures of mice lacking neuropeptide Y, Nature 381 (1996) 415–421. [101] J.N. Fain, S.W. Bahouth, Regulation of lipolysis and leptin biosynthesis in rodent adipose tissue by growth hormone, Metabolism 49 (2000) 239–244. [102] K.M. Fairhall, A. Mynett, I.C. Robinson, Central effects of growth hormone-releasing hexapeptide (GHRP-6) on growth hormone release are inhibited by central somatostatin action, J. Endocrinol. 144 (1995) 555–560. [103] J.A. Finkelstein, P. Jervois, M. Menadue, J.O. Willoughby, Growth hormone and prolactin secretion in genetically obese Zucker rats, Endocrinology 118 (1986) 1233–1236. [104] S. Fisker, S. Nielsen, L. Ebdrup, J.N. Bech, J.S. Christiansen, E.B. Pedersen, J.O. Jorgensen, L -Arginine-induced growth hormone secretion is not influenced by co-infusion of the nitric oxide synthase inhibitor N-monomethyl-L -arginine in healthy men, Growth Horm. IGF Res. 9 (1999) 69–73. [105] S. Fisker, S. Nielsen, L. Ebdrup, J.N. Bech, J.S. Christiansen, E.B. Pedersen, J.O. Jorgensen, The role of nitric oxide in L arginine-stimulated growth hormone release, J. Clin. Invest. 22 (1999) 89–93. [106] S. Fisker, N. Vahl, T.B. Hansen, J.O.L. Jorgensen, C. Hagen, H. Orskov, J.S. Christiansen, Serum leptin is increased in growth hormone-deficient adults: relationship to body composition and effects of placebo-controlled growth hormone therapy for 1 year, Metabolism 46 (1997) 812–817. [107] K.E. Flaim, J.B. Li, L.S. Jefferson, Protein turnover in rat skeletal muscle: effects of hypophysectomy and growth hormone, Am. J. Physiol. 234 (1978) E38–E43. [108] C.M. Florkowski, G.R. Collier, P.Z. Zimmet, J.H. Livesey, E.A. Espiner, R.A. Donald, Low-dose growth hormone replacement lowers plasma leptin and fat stores without affecting body mass index in adults with growth hormone deficiency, Clin. Endocrinol. 45 (1996) 769–773. [109] R.J. Frankel, J.S. Jenkins, Hypothalamic–pituitary function in anorexia nervosa, Acta Endocrinol. 78 (1975) 209–221. [110] C. Fry, D.R. Gunter, C.D. McMahon, B. Steele, J.L. Sartin, Cytokine-mediated growth hormone release from cultured ovine pituitary cells, Neuroendocrinology 68 (1998) 192–200.

[111] D.A. Fryburg, Insulin-like growth factor I exerts growth hormone- and insulin-like actions on human muscle protein metabolism, Am. J. Physiol. 267 (1994) E331–E336. [112] D.A. Fryburg, R.A. Gelfand, E.J. Barrett, Growth hormone acutely stimulates forearm muscle protein synthesis in normal humans, Am. J. Physiol. 260 (1991) E499–E504. [113] R.H. Gerner, T. Yamada, Altered neuropeptide concentrations in cerebrospinal fluid of psychiatric patients, Brain Res. 238 (1982) 298–302. [114] E. Ghigo, E. Arvat, F. Broglio, R. Giordano, L. Gianotti, G. Muccioli, M. Papotti, A. Graziani, G. Bisi, R. Deghenghi, F. Camanni, Endocrine and non-endocrine activities of growth hormone secretagogues in humans, Horm. Res. 51 (Suppl. 3) (1999) 9–15. [115] E. Ghigo, E. Arvat, L. Gianotti, M. Nicolosi, M.R. Valetto, S. Avagnina, D. Bellitti, M. Rolla, E.E. Muller, F. Camanni, Arginine but not pyridostigmine, a cholinesterase inhibitor, enhances the GHRH-induced GH rise in patients with anorexia nervosa, Biol. Psychiatry 36 (1994) 689–695. [116] E. Ghigo, E. Arvat, J. Ramunni, A. Colao, L. Gianotti, R. Deghenghi, G. Lombardi, F. Camanni, Adrenocorticotropinand cortisol-releasing effect of hexarelin, a synthetic growth hormone-releasing peptide, in normal subjects and patients with CushingÕs syndrome, J. Clin. Endocrinol. Metab. 82 (1997) 2439– 2444. [117] E. Ghigo, J. Bellone, E. Mazza, E. Imperiale, M. Procopio, F. Valente, R. Lala, C. De Sanctis, F. Camanni, Arginine potentiates the GHRH- but not the pyridostigmine-induced GH secretion in normal short children. Further evidence for a somatostatin suppressing effect of arginine, Clin. Endocrinol. 32 (1990) 763–767. [118] E. Ghigo, L. Gianotti, E. Arvat, J. Ramunni, M.R. Valetto, F. Broglio, M. Rolla, F. Cavagnini, E.E. Muller, Effects of recombinant human insulin-like growth factor I administration on growth hormone (GH) secretion, both spontaneous and stimulated by GH-releasing hormone or hexarelin, a peptidyl GH secretagogue, in humans, J. Clin. Endocrinol. Metab. 84 (1999) 285–290. [119] L. Gianotti, E. Arvat, M.R. Valetto, J. Ramunni, L. Di Vito, B. Maccagno, F. Camanni, E. Ghigo, Effects of beta-adrenergic agonists and antagonists on the growth hormone response to growth hormone-releasing hormone in anorexia nervosa, Biol. Psychiatry 43 (1998) 181–187. [120] L. Gianotti, S. De Stefanis, V. Mondelli, G. Abbate Daga, S. Fassino, F. Camanni, Effects of ghrelin on somatotroph, lactotroph and corticotroph secretion in patients with anorexia nervosa, J. Endocrinol. Invest. 25 (Suppl. 6) (2002) 11. [121] L. Gianotti, S. Fassino, G.A. Daga, F. Lanfranco, C. De Bacco, J. Ramunni, E. Arvat, M. Maccario, E. Ghigo, Effects of free fatty acids and acipimox, a lipolysis inhibitor, on the somatotroph responsiveness to GHRH in anorexia nervosa, Clin. Endocrinol. 52 (2000) 713–720. [122] L. Gianotti, A.I. Pincelli, M. Scacchi, M. Rolla, D. Bellitti, E. Arvat, F. Lanfranco, A. Torsello, E. Ghigo, F. Cavagnini, E.E. Muller, Effects of recombinant human insulin-like growth factor I administration on spontaneous and growth hormone (GH)releasing hormone-stimulated GH secretion in anorexia nervosa, J. Clin. Endocrinol. Metab. 85 (2000) 2805–2809. [123] L. Gianotti, M. Rolla, E. Arvat, D. Belliti, M.R. Valetto, M. Ferdeghini, E. Ghigo, E.E. Muller, Effect of somatostatin infusion on the somatotrope responsiveness to growth hormone-releasing hormone in patients with anorexia nervosa, Biol. Psychiatry 45 (1999) 334–339. [124] M. Giusti, L. Bocca, T. Florio, A. Corsaro, R. Spaziante, G. Schettini, F. Minuto, In vitro effect of human recombinant leptin and expression of leptin receptors on growth hormone-secreting human pituitary adenomas, Clin. Endocrinol. 57 (2002) 449–455.

M. Scacchi et al. / Frontiers in Neuroendocrinology 24 (2003) 200–224 [125] M. Giusti, L. Foppiani, P. Ponzani, C.M. Cuttica, M.R. Falivene, S. Valenti, Hexarelin is a stronger GH-releasing peptide than GHRH in normal cycling women but not in anorexia nervosa, J. Endocrinol. Invest. 20 (1997) 257–263. [126] A. Giustina, J.D. Veldhuis, Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human, Endocr. Rev. 19 (1998) 717–797. [127] A. Golay, A.L. Swislocki, Y.D. Chen, J.B. Jaspan, G.M. Reaven, Effect of obesity on ambient plasma glucose, free fatty acid, insulin, growth hormone, and glucagon concentrations, J. Clin. Endocrinol. Metab. 63 (1986) 481–484. [128] D.C. Gore, D. Honeycutt, F. Jahoor, T. Rutan, R.R. Wolfe, D.N. Herndon, Effect of exogenous growth hormone on glucose utilization in burn patients, J. Surg. Res. 51 (1991) 518–523. [129] D.B. Grant, J. Hambley, D. Becker, B.L. Pimstone, Reduced sulphation factor in undernourished children, Arch. Dis. Child. 48 (1973) 596–600. [130] S. Grinspoon, L. Thomas, K. Miller, D. Herzog, A. Klibanski, Effects of recombinant human IGF-I and oral contraceptive administration on bone density in anorexia nervosa, J. Clin. Endocrinol. Metab. 87 (2002) 2883–2891. [131] O. Gualillo, J. Caminos, M. Blanco, T. Garcia-Caballero, M. Kojima, K. Kangawa, C. Dieguez, F.F. Casanueva, Ghrelin, a novel placental-derived hormone, Endocrinology 142 (2001) 788–794. [132] XM. Guan, H. Yu, O.C. Palyha, K.K. McKee, S.D. Feighner, D.J. Sirinathsinghji, R.G. Smith, L.H. Van der Ploeg, A.D. Howard, Distribution of mRNA encoding the growth hormone secretagogue receptor in brain and peripheral tissues, Mol. Brain Res. 48 (1997) 23–29. [133] V. Guillaume, E. Magnan, M. Cataldi, A. Dutour, N. Sauze, M. Renard, H. Razafindraibe, B. Conte-Devolx, R. Deghenghi, V. Lenaerts, et al., Growth hormone (GH)-releasing hormone secretion is stimulated by a new GH-releasing hexapeptide in sheep, Endocrinology 135 (1994) 1073–1076. [134] J.L. Halaas, K.S. Gajiwala, M. Maffei, S.L. Cohen, B.T. Chait, D. Rabinowitz, R.L. Lallone, S.K. Burley, J.M. Friedman, Weight-reducing effects of the plasma protein encoded by the obese gene, Science 269 (1995) 543–546. [135] L.F. Hardie, N. Guilhot, P. Trayhurn, Regulation of leptin production in cultured mature white adipocytes, Horm. Metab. Res. 28 (1996) 685–689. [136] M.L. Hartman, P.E. Clayton, M.L. Johnson, A. Celniker, A.J. Perlman, K.G.M.M. Alberti, M.O. Thorner, A low dose euglycemic infusion of recombinant human insulin-like growth factor I rapidly suppresses fasting-enhanced pulsatile growth hormone secretion in humans, J. Clin. Invest. 91 (1993) 2453–2462. [137] M.L. Hartman, S.M. Pincus, M.L. Johnson, D.H. Matthews, L.M. Faunt, M.L. Vance, M.O. Thorner, J.D. Veldhuis, Enhanced basal and disorderly growth hormone secretion distinguish acromegalic from normal pulsatile growth hormone release, J. Clin. Invest. 94 (1994) 1277–1288. [138] M.L. Hartman, J.D. Veldhuis, M.L. Johnson, M.M. Lee, K.G.M.M. Alberti, E. Samojlik, M.O. Thorner, Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men, J. Clin. Endocrinol. Metab. 74 (1992) 757–765. [139] M.L. Hartman, J.D. Veldhuis, M.L. Vance, A.C.S. Faria, R.W. Furlanetto, M.O. Thorner, Somatotropin pulse frequency and basal concentrations are increased in acromegaly and are reduced by successful therapy, J. Clin. Endocrinol. Metab. 70 (1990) 1375–1384. [140] H. Hauner, G. Loffler, Adipogenic factors in human serum promote the adipose conversion of 3T3-L1 fibroblasts, Int. J. Obes. 10 (1986) 323–330. [141] M. Hernandez, J. Argente, A. Navarro, N. Caballo, V. Barrios, F. Hervas, I. Polanco, Growth in malnutrition related to

[142]

[143] [144]

[145]

[146]

[147]

[148]

[149]

[150]

[151]

[152] [153]

[154]

[155]

[156]

219

gastrointestinal diseases: coeliac disease, Horm. Res. 38 (Suppl. 1) (1992) 79–84. R.L. Hintz, R. Suskind, K. Amatayakul, O. Thanangkul, R. Olson, Plasma somatomedin and growth hormone values in children with protein-calorie malnutrition, J. Pediatr. 92 (1978) 153–156. K.K.Y. Ho, A.J. OÕSullivan, D.M. Hoffman, Metabolic actions of growth hormone in man, Endocr. J. 43 (1996) S57–S63. K.Y. Ho, J.D. Veldhuis, M.L. Johnson, R. Furlanetto, W.S. Evans, K.G.M.M. Alberti, M.O. Thorner, Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man, J. Clin. Invest. 81 (1988) 968– 975. P.J. Ho, R. DeMott Friberg, A.L. Barkan, Regulation of pulsatile growth hormone secretion by fasting in normal subjects and patients with acromegaly, J. Clin. Endocrinol. Metab. 75 (1992) 812–819. Z. Hochberg, P. Hertz, V. Colin, S. Ish-Shalom, D. Yeshurun, M.B. Youdim, T. Amit, The distal axis of growth hormone (GH) in nutritional disorders: GH-binding protein, insulin-like growth factor-I (IGF-I), and IGF-I receptors in obesity and anorexia nervosa, Metabolism 41 (1992) 106–112. T.L. Horvath, S. Diano, P. Sotonyi, M. Heiman, M. Tschop, Minireview: ghrelin and the regulation of energy balance—a hypothalamic perspective, Endocrinology 142 (2001) 4163–4169. M. Hotta, T. Shibasaki, A. Masuda, T. Imaki, H. Demura, N. Ling, K. Shizume, The responses of plasma adrenocorticotropin and cortisol to corticotropin-releasing hormone (CRH) and cerebrospinal fluid immunoreactive CRH in anorexia nervosa patients, J. Clin. Endocrinol. Metab. 62 (1986) 319–324. A.D. Howard, S.D. Feighner, D.F. Cully, J.P. Arena, P.A. Liberator, C.I. Rosenblum, M. Hamelin, D.L. Hreniuk, O.C. Palyha, J. Anderson, P.S. Paress, C. Diaz, M. Chou, K.K. Liu, K.K. McKee, S.S. Pong, L.Y. Chaung, A. Elbrecht, M. Dashkevicz, R. Heavens, M. Rigby, D.J. Sirinathsinghji, D.C. Dean, D.G. Melillo, L.H. Van der Ploeg, et al., A receptor in pituitary and hypothalamus that functions in growth hormone release, Science 273 (1996) 974–977. M.A. Hussain, O. Schmitz, A. Mengel, Y. Glatz, J.S. Christiansen, J. Zapf, E.R. Froesch, Comparison of the effects of growth hormone and insulin-like growth factor I on substrate oxidation and on insulin sensitivity in growth hormone-deficient humans, J. Clin. Invest. 94 (1994) 1126–1133. J.J. Hwa, L. Ghibaudi, D. Compton, A.B. Fawzi, C.D. Strader, Intracerebroventricular injection of leptin increases thermogenesis and mobilizes fat metabolism in ob/ob mice, Horm. Metab. Res. 28 (1996) 659–663. D. Ikkos, R. Luft, B. Sjogren, Body water and sodium in patients with acromegaly, J. Clin. Invest. 33 (1954) 989–994. T. Imaki, T. Shibasaki, A. Masuda, M. Hotta, N. Yamauchi, H. Demura, K. Shizume, I. Wakabayashi, N. Ling, The effect of glucose and free fatty acids on growth hormone (GH)-releasing factor-mediated GH secretion in rats, Endocrinology 118 (1986) 2390–2394. T. Imaki, T. Shibasaki, K. Shizume, A. Masuda, M. Hotta, Y. Kiyosawa, K. Jibiki, H. Demura, T. Tsushima, N. Ling, The effect of free fatty acids on growth hormone (GH)-releasing hormone-mediated GH secretion in man, J. Clin. Endocrinol. Metab. 60 (1985) 290–293. T. Jacks, G. Hickey, F. Judith, J. Taylor, H. Chen, D. Krupa, W. Feeney, W. Schoen, D. Ok, M. Fisher, et al., Effects of acute and repeated intravenous administration of L-692,585, a novel nonpeptidyl growth hormone secretagogue, on plasma growth hormone, IGF-1, ACTH, cortisol, prolactin, insulin, and thyroxine levels in beagles, J. Endocrinol. 143 (1994) 399–406. F. Jacoangeli, A. Zoli, A. Taranto, F. Staar Mezzasalma, C. Ficoneri, S. Pierangeli, G. Menzinger, M.R. Bollea, Osteoporosis

220

[157]

[158]

[159]

[160]

[161]

[162]

[163]

[164]

[165]

[166]

[167]

[168]

[169]

[170]

[171]

M. Scacchi et al. / Frontiers in Neuroendocrinology 24 (2003) 200–224 and anorexia nervosa: relative role of endocrine alterations and malnutrition, Eat. Weight Disord. 7 (2002) 190–195. L. Jin, B.G. Burguera, M.E. Couce, B.W. Scheithauer, J. Lamsan, N.L. Eberhardt, E. Kulig, R.V. Lloyd, Leptin and leptin receptor expression in normal and neoplastic human pituitary: evidence of a regulatory role for leptin on pituitary cell proliferation, J. Clin. Endocrinol. Metab. 84 (1999) 2903–2911. J. Kamegai, H. Tamura, T. Shimizu, S. Ishii, H. Sugihara, I. Wakabayashi, Chronic central infusion of ghrelin increases hypothalamic neuropeptide Y and Agouti-related protein mRNA levels and body weight in rats, Diabetes 50 (2001) 2438–2443. E. Kilgour, S.A. Baldwin, D.J. Flint, Divergent regulation of rat adipocyte GLUT1 and GLUT4 glucose transporters by GH, J. Endocrinol. 145 (1995) 27–33. M. Kojima, H. Hosoda, Y. Date, M. Nakazato, H. Matsuo, K. Kangawa, Ghrelin is a growth-hormone-releasing acylated peptide from stomach, Nature 402 (1999) 656–660. M. Korbonits, S.A. Bustin, M. Kojima, S. Jordan, E.F. Adams, D.G. Lowe, K. Kangawa, A.B. Grossman, The expression of the growth hormone secretagogue receptor ligand ghrelin in normal and abnormal human pituitary and other neuroendocrine tumors, J. Clin. Endocrinol. Metab. 86 (2001) 881–887. M. Korbonits, M.M. Chitnis, M. Gueorguiev, D. Norman, N. Rosenfelder, M. Suliman, T.H. Jones, K. Noonan, A. Fabbri, G.M. Besser, J.M. Burrin, A.B. Grossman, The release of leptin and its effect on hormone release from human pituitary adenomas, Clin. Endocrinol. 54 (2001) 781–789. M. Korbonits, E. Ciccarelli, E. Ghigo, A.B. Grossman, The growth hormone secretagogue receptor, Growth Horm. IGF Res. 9 (Suppl. A) (1999) 93–99. H. Kuriyama, M. Hotta, I. Wakabayashi, T. Shibasaki, A 6-day intracerebroventricular infusion of the growth hormone-releasing peptide KP-102 stimulates food intake in both non-stressed and intermittently-stressed rats, Neurosci. Lett. 282 (2000) 109– 112. S. Kushibiki, K. Hodate, H. Shingu, Y. Ueda, Y. Mori, T. Itoh, Y. Yokomizo, Effects of long-term administration of recombinant bovine tumor necrosis factor-alpha on glucose metabolism and growth hormone secretion in steers, Am. J. Vet. Res. 62 (2001) 794–798. S. Kushibiki, K. Hodate, Y. Ueda, H. Shingu, Y. Mori, T. Itoh, Y. Yokomizo, Administration of recombinant bovine tumor necrosis factor-alpha affects intermediary metabolism and insulin and growth hormone secretion in dairy heifers, J. Anim. Sci. 78 (2000) 2164–2171. R. Laager, R. Ninnis, U. Keller, Comparison of the effects of recombinant human insulin-like growth factor-I and insulin on glucose and leucine kinetics in humans, J. Clin. Invest. 92 (1993) 1903–1909. S. Lall, L.Y. Tung, C. Ohlsson, J.O. Jansson, S.L. Dickson, Growth hormone (GH)-independent stimulation of adiposity by GH secretagogues, Biochem. Biophys. Res. Commun. 280 (2001) 132–138. J.G. Langendonk, H. Pijl, A.C. Toornvliet, J. Burggraaf, M. Frolich, R.C. Schoemaker, J. Doornbos, A.F. Cohen, A.E. Meinders, Circadian rhythm of plasma leptin levels in upper and lower body obese women: influence of body fat distribution and weight loss, J. Clin. Endocrinol. Metab. 83 (1998) 1706–1712. C.B. Lawrence, A.C. Snape, F.M. Baudoin, S.M. Luckman, Acute central ghrelin and GH secretagogues induce feeding and activate brain appetite centers, Endocrinology 143 (2002) 155– 162. A. Leal-Cerro, R.V. Considine, R. Peino, E. Venegas, R. Astorga, F.F. Casanueva, C. Dieguez, Serum immunoreactiveleptin levels are increased in patients with CushingÕs syndrome, Horm. Metab. Res. 28 (1996) 711–713.

[172] E.J. Lee, S.Y. Nam, K.R. Kim, H.C. Lee, J.H. Cho, M.S. Nam, D.Y. Song, S.K. Lim, K.B. Huh, Acipimox potentiates growth hormone (GH) response to GH-releasing hormone with or without pyridostigmine by lowering serum freee fatty acid in normal and obese subjects, J. Clin. Endocrinol. Metab. 80 (1995) 2495–2498. [173] H.M. Lee, G. Wang, E.W. Englander, M. Kojima, G.H. Greeley Jr., Ghrelin, a new gastrointestinal endocrine peptide that stimulates insulin secretion: enteric distribution, ontogeny, influence of endocrine, and dietary manipulations, Endocrinology 143 (2002) 185–190. [174] S.A. Lieberman, G.E. Butterfield, D. Harrison, A.R. Hoffman, Anabolic effects of recombinant insulin-like growth factor-I in cachectic patients with the acquired immunodeficiency syndrome, J. Clin. Endocrinol. Metab. 78 (1994) 404–410. [175] J.H. Lindeman, H. Pijl, F.M. Van Dielen, E.G. Lentjes, C. Van Leuven, T. Kooistra, Ghrelin and the hyposomatotropism of obesity, Obes. Res. 10 (2002) 1161–1166. [176] C.A. Lissett, P.E. Clayton, S.M. Shalet, The acute leptin response to GH, J. Clin. Endocrinol. Metab. 86 (2001) 4412– 4415. [177] H.C. Lo, M.D. Hirvonen, K.R. Kritsch, R.E. Keesey, D.M. Ney, Growth hormone or insulin-like growth factor I increases fat oxidation and decreases protein oxidation without altering energy expenditure in parenterally fed rats, Am. J. Clin. Nutr. 65 (1997) 1384–1390. [178] V. Locatelli, R. Grilli, A. Torsello, S.G. Cella, W.B. Wehrenberg, E.E. Muller, Growth hormone-releasing hexapeptide is a potent stimulator of growth hormone gene expression and release in the growth hormone-releasing hormone-deprived infant rat, Pediatr. Res. 36 (1994) 169–174. [179] V. Locatelli, A. Torsello, M. Redaelli, E. Ghigo, F. Massara, E.E. Muller, Cholinergic agonist and antagonist drugs modulate the growth hormone response to growth hormone-releasing hormone in the rat: evidence for mediation by somatostatin, J. Endocrinol. 111 (1986) 271–278. [180] M. Maccario, E. Arvat, M. Procopio, L. Gianotti, S. Grottoli, B.P. Imbimbo, V. Lenaerts, R. Deghenghi, F. Camanni, E. Ghigo, Metabolic modulation of growth hormone-releasing activity of hexarelin in man, Metabolism 44 (1995) 134–138. [181] M. Maccario, S.E. Oleandri, M. Procopio, S. Grottoli, E. Avogadri, F. Camanni, E. Ghigo, Comparison among the effects of arginine, a nitric oxide precursor, isosorbide dinitrate and molsidomine, two nitric oxide donors, on hormonal secretions and blood pressure in man, J. Endocrinol. Invest. 20 (1997) 488– 492. [182] M. Maccario, M. Procopio, S. Grottoli, S.E. Oleandri, G.M. Boffano, M. Taliano, F. Camanni, E. Ghigo, Effects of acipimox, an antilipolytic drug, on the growth hormone (GH) response to GH-releasing hormone alone or combined with arginine in obesity, Metabolism 45 (1996) 342–346. [183] M. Maccario, F. Tassone, S. Grottoli, R. Rossetto, C. Gauna, E. Ghigo, Neuroendocrine and metabolic determinants of the adaptation of GH/IGF-I axis to obesity, Ann. Endocrinol. 63 (2002) 140–144. [184] K. Maeda, Y. Kato, N. Yamaguchi, K. Chihara, S. Ohgo, Y. Iwasaki, Y. Yoshimoto, K. Moridera, S. Kuromaru, H. Imura, Growth hormone release following thyrotropin-releasing hormone injection into patients with anorexia nervosa, Acta Endocrinol. 81 (1976) 1–8. [185] M. Maes, L.E. Underwood, J.M. Ketelslegers, Plasma somatomedin-C in fasted and refed rats: close relationship with changes in liver somatogenic but not lactogenic binding sites, J. Endocrinol. 97 (1983) 243–252. [186] G.L. Mainardi, R. Saleri, C. Tamanini, M. Baratta, Effects of interleukin-1-beta, interleukin-6 and tumor necrosis factoralpha, alone or in association with hexarelin or galanin, on

M. Scacchi et al. / Frontiers in Neuroendocrinology 24 (2003) 200–224

[187]

[188]

[189]

[190]

[191]

[192]

[193]

[194]

[195]

[196]

[197] [198]

[199] [200]

[201]

[202]

[203]

growth hormone gene expression and growth hormone release from pig pituitary cells, Horm. Res. 58 (2002) 180–186. S. Malozowski, E.H. Hao, S.G. Ren, G. Marin, L. Liu, J.L. Southers, G.R. Merriam, Growth hormone (GH) responses to the hexapeptide GH-releasing peptide and GH-releasing hormone (GHRH) in the cynomolgus macaque: evidence for nonGHRH-mediated responses, J. Clin. Endocrinol. Metab. 73 (1991) 314–317. A. Mancini, D. Valle, G. Conte, C. Fiumara, M. Perrelli, L. Fabrizi, A. Bianchi, L. De Marinis, Pre- and postprandial pyridostigmine and oxiracetam effects on growth hormone secretion in anorexia nervosa, Psychoneuroendocrinology 21 (1996) 621–629. C. Mantzoros, J.S. Flier, M.D. Lesem, T.D. Brewerton, D.C. Jimerson, Cerebrospinal fluid leptin in anorexia nervosa: correlation with nutritional status and potential role in resistance to weight gain, J. Clin. Endocrinol. Metab. 82 (1997) 1845–1851. A. Masuda, T. Shibasaki, M. Hotta, H. Suematsu, K. Shizume, Study on the mechanism of abnormal growth hormone (GH) secretion in anorexia nervosa: no evidence of involvment of a low somatomedin-C level in the abnormal GH secretion, J. Endocrinol. Invest. 11 (1988) 297–302. A. Masuda, T. Shibasaki, M. Nakahara, T. Imaki, Y. Kiyosawa, K. Jibiki, H. Demura, K. Shizume, N. Ling, The effect of glucose on growth hormone (GH)-releasing hormone-mediated GH secretion in man, J. Clin. Endocrinol. Metab. 60 (1985) 523–526. M.L. McCaleb, R.D. Myers, 2-Deoxy-D -glucose and insulin modify release of norepinephrine from rat hypothalamus, Am. J. Physiol. 242 (1982) R596–R603. K.K. McKee, O.C. Palyha, S.D. Feighner, D.L. Hreniuk, C.P. Tan, M.S. Phillips, R.G. Smith, L.H. Van der Ploeg, A.D. Howard, Molecular analysis of rat pituitary and hypothalamic growth hormone secretagogue receptors, Mol. Endocrinol. 11 (1997) 415–423. T.J. Merimee, J. Zapf, E.R. Froesch, Insulin-like growth factors in the fed and fasted states, J. Clin. Endocrinol. Metab. 55 (1982) 999–1002. F.A. Momany, C.Y. Bowers, G.A. Reynolds, D. Chang, A. Hong, K. Newlander, Design, synthesis, and biological activity of peptides which release growth hormone in vitro, Endocrinology 108 (1981) 31–39. K. Mori, A. Yoshimoto, K. Takaya, K. Hosoda, H. Ariyasu, K. Yahata, M. Mukoyama, A. Sugawara, H. Hosoda, M. Kojima, K. Kangawa, K. Nakao, Kidney produces a novel acylated peptide, ghrelin, FEBS Lett. 486 (2002) 213–216. M. Morikawa, T. Nixon, H. Green, Growth hormone and the adipose conversion of 3T3 cells, Cell 29 (1982) 783–789. A.F. Muller, S.W. Lamberts, J.A. Janssen, L.J. Hofland, P. van Koetsveld, M. Bidlingmaier, C.J. Strasburger, E. Ghigo, A.J. Van Der Lely, Ghrelin drives growth hormone secretion during fasting in man, Eur. J. Endocrinol. 146 (2002) 203–207. E.E. Muller, V. Locatelli, D. Cocchi, Neuroendocrine control of growth hormone secretion, Physiol. Rev. 79 (1999) 511–607. E.E. Muller, M. Rolla, Aspects of the neuroendocrine control of somatotropic function in calorically restricted dogs and patients with eating disorders: studies with cholinergic drugs, Psychiatry Res. 62 (1996) 51–63. K. Mulligan, C. Grunfeld, M.K. Hellerstein, R.A. Neese, M. Schambelan, Anabolic effects of recombinant human growth hormone in patients with wasting associated with human immunodeficiency virus infection, J. Clin. Endocrinol. Metab. 77 (1993) 956–962. M.T. Munoz, J. Argente, Anorexia nervosa in female adolescents: endocrine and bone mineral density disturbances, Eur. J. Endocrinol. 147 (2002) 275–286. A. Murata, T. Yasuda, H. Niimi, Growth hormone-binding protein in patients with anorexia nervosa determined in two assay systems, Horm. Metab. Res. 24 (1992) 297–299.

221

[204] E. Nakagawa, N. Nagaya, H. Okumura, M. Enomoto, H. Oya, F. Ono, H. Hosoda, M. Kojima, K. Kangawa, Hyperglycaemia suppresses the secretion of ghrelin, a novel growth-hormonereleasing peptide: responses to the intravenous and oral administration of glucose, Clin. Sci. 103 (2002) 325–328. [205] K. Nakagawa, M. Matsubara, T. Obara, M. Kubo, K. Akikawa, Responses of pituitary and adrenal medulla to insulin-induced hypoglycemia in patients with anorexia nervosa, Endocrinol. Jpn. 32 (1985) 719–724. [206] Y. Nakai, S. Hamagaki, R. Takagi, A. Taniguchi, F. Kurimoto, Plasma concentrations of tumor necrosis factor-alpha (TNFalpha) and soluble TNF receptors in patients with anorexia nervosa, J. Clin. Endocrinol. Metab. 84 (1999) 1226–1228. [207] S.Y. Nam, E.J. Lee, K.R. Kim, H.C. Lee, M.S. Nam, J.H. Cho, K.B. Huh, Long-term administration of acipimox potentiates growth hormone response to growth hormone-releasing hormone by decreasing serum free fatty acid in obesity, Metabolism 45 (1996) 594–597. [208] H. Norrelund, T.K. Hansen, H. Orskov, H. Hosoda, M. Kojima, K. Kangawa, J. Weeke, N. Moller, J.S. Christiansen, J.O. Jorgensen, Ghrelin immunoreactivity in human plasma is suppressed by somatostatin, Clin. Endocrinol. 57 (2002) 539–546. [209] E. Nova, S. Gomez-Martinez, G. Morande, A. Marcos, Cytokine production by blood mononuclear cells from in-patients with anorexia nervosa, Br. J. Nutr. 88 (2002) 183–188. [210] C. Ohlsson, B.A. Bengtsson, O.G. Isaksson, T.T. Andreassen, M.C. Slootweg, Growth hormone and bone, Endocr. Rev. 19 (1998) 55–79. [211] Y. Ohno, M. Fujimoto, A. Nishimura, N. Aoki, Change of peripheral levels of pituitary hormones and cytokines after injection of interferon (IFN)-beta in patients with chronic hepatitis C, J. Clin. Endocrinol. Metab. 83 (1998) 3681–3687. [212] K. Okada, S. Ishii, S. Minami, H. Sugihara, T. Shibasaki, I. Wakabayashi, Intracerebroventricular administration of the growth hormone-releasing peptide KP-102 increases food intake in free-feeding rats, Endocrinology 137 (1996) 5155–5158. [213] K. Okada, H. Sugihara, S. Minami, I. Wakabayashi, Effect of parenteral administration of selected nutrients and central injection of gamma-globulin from antiserum to neuropeptide Y on growth hormone secretory pattern in food-deprived rats, Neuroendocrinology 57 (1993) 678–686. [214] K. Okada, N. Suzuki, H. Sugihara, S. Minami, I. Wakabayashi, Restoration of growth hormone secretion in prolonged fooddeprived rats depends on the level of nutritional intake and dietary protein, Neuroendocrinology 59 (1994) 380–386. [215] L.H. Opie, P.G. Walfish, Plasma free fatty acid concentration in obesity, N. Engl. J. Med. 268 (1963) 757–760. [216] J. Oscarsson, M. Ottosson, S. Eden, Effects of growth hormone on lipoprotein lipase and hepatic lipase, J. Endocrinol. Invest. 22 (5 Suppl) (1999) 2–9. [217] B. Otto, U. Cuntz, E. Fruehauf, R. Wawarta, C. Folwaczny, R.L. Riepl, M.L. Heiman, P. Lehnert, M. Fichter, M. Tschop, Weight gain decreases elevated plasma ghrelin concentrations of patients with anorexia nervosa, Eur. J. Endocrinol. 145 (2001) R5–R9. [218] N. Pandya, R. DeMott-Friberg, C.Y. Bowers, A.L. Barkan, C.A. Jaffe, Growth hormone (GH)-releasing peptide-6 requires endogenous hypothalamic GH-releasing hormone for maximal GH stimulation, J. Clin. Endocrinol. Metab. 83 (1998) 1186– 1189. [219] R. Peino, R. Baldelli, J. Rodriguez-Garcia, S. Rodriguez-Segade, M. Kojima, K. Kangawa, E. Arvat, E. Ghigo, C. Dieguez, F.F. Casanueva, Ghrelin-induced growth hormone secretion in humans, Eur. J. Endocrinol. 143 (2000) R11–R14. [220] R. Peino, F. Cordido, A. Penalva, C.V. Alvarez, C. Dieguez, F.F. Casanueva, Acipimox-mediated plasma free fatty acid depression per se stimulates growth hormone (GH) secretion in

222

[221]

[222]

[223]

[224]

[225] [226]

[227]

[228]

[229]

[230]

[231]

[232]

[233]

[234]

[235]

M. Scacchi et al. / Frontiers in Neuroendocrinology 24 (2003) 200–224 normal subjects and potentiates the response to other GHreleasing stimuli, J. Clin. Endocrinol. Metab. 81 (1996) 909–913. M.A. Pelleymounter, M.J. Cullen, M.B. Baker, R. Hecht, D. Winters, T. Boone, F. Collins, Effects of the obese gene product on body weight regulation in ob/ob mice, Science 269 (1995) 540–543. A. Pe~ nalva, B. Burguera, X. Casabiell, J.A. Tresguerres, C. Dieguez, F.F. Casanueva, Activation of cholinergic neurotransmission by pyridostigmine reverses the inhibitory effect of hyperglycemia on growth hormone (GH) releasing hormoneinduced GH secretion in man: does acute hyperglycemia act through hypothalamic release of somatostatin?, Neuroendocrinology 49 (1989) 551–554. A. Pe~ nalva, A. Carballo, M. Pombo, F.F. Casanueva, C. Dieguez, Effect of growth hormone (GH)-releasing hormone (GHRH), atropine, pyridostigmine, or hypoglycemia on GHRP6-induced GH secretion in man, J. Clin. Endocrinol. Metab. 76 (1993) 168–171. A.I. Pincelli, A.E. Rigamonti, M. Scacchi, S.G. Cella, M. Cappa, F. Cavagnini, E.E. Muller, Somatostatin infusion withdrawal: studies in the acute and recovery phase of anorexia nervosa, and in obesity, Eur. J. Endocrinol. 148 (2003) 237–243. J. Pinkney, G. Williams, Ghrelin gets hungry, Lancet 359 (2002) 1360–1361. C.M. Pombo, J. Zalvide, B.D. Gaylinn, C. Dieguez, Growth hormone-releasing hormone stimulates mitogen-activated protein kinase, Endocrinology 141 (2000) 2113–2119. C. Pomeroy, E. Eckert, S. Hu, B. Eiken, M. Mentink, R.D. Crosby, C.C. Chao, Role of interleukin-6 and transforming growth factor-beta in anorexia nervosa, Biol. Psychiatry 36 (1994) 836–839. S.S. Pong, L.Y. Chaung, D.C. Dean, R.P. Nargund, A.A. Patchett, R.G. Smith, Identification of a new G-protein-linked receptor for growth hormone secretagogues, Mol. Endocrinol. 10 (1996) 57–61. G.A. Ponting, H.C. Ward, D. Halliday, A.J. Sim, Protein and energy metabolism with biosynthetic human growth hormone in patients on full intravenous nutritional support, J. Parenter. Enteral Nutr. 14 (1990) 437–441. A.E. Pontiroli, R. Lanzi, L.D. Monti, G. Pozza, Effect of acipimox, a lipid lowering drug, on growth hormone (GH) response to GH-releasing hormone in normal subjects, J. Endocrinol. Invest. 13 (1990) 539–542. A.E. Pontiroli, M.F. Manzoni, M.E. Malighetti, R. Lanzi, Restoration of growth hormone (GH) response to GH-releasing hormone in elderly and obese subjects by acute pharmacological reduction of plasma free fatty acids, J. Clin. Endocrinol. Metab. 81 (1996) 3998–4001. V. Popovic, S. Damjanovic, D. Micic, M. Djurovic, C. Dieguez, F.F. Casanueva, Blocked growth hormone-releasing peptide (GHRP-6)-induced GH secretion and absence of the synergic action of GHRP-6 plus GH-releasing hormone in patients with hypothalamopituitary disconnection: evidence that GHRP-6 main action is exerted at the hypothalamic level, J. Clin. Endocrinol. Metab. 80 (1995) 942–947. V. Popovic, D. Micic, M. Djurovic, S. Obradovic, F.F. Casanueva, C. Dieguez, Absence of desensitization by hexarelin to subsequent GH releasing hormone-mediated GH secretion in patients with anorexia nervosa, Clin. Endocrinol. 46 (1997) 539– 543. M. Quintela, R. Senaris, H.L. Heiman, F.F. Casanueva, C. Dieguez, Leptin inhibits in vitro hypothalamic somatostatin secretion and somatostatin mRNA levels, Endocrinology 138 (1997) 5641–5644. A.E. Rigamonti, S.G. Cella, N. Marazzi, E.E. Muller, Six-week treatment with hexarelin in young dogs: evaluation of the GH responsiveness to acute hexarelin or GHRH administration, and

[236]

[237]

[238]

[239]

[240]

[241]

[242]

[243]

[244]

[245]

[246] [247]

[248]

[249] [250]

[251]

of the orexigenic effect of hexarelin, Eur. J. Endocrinol. 141 (1999) 313–320. A.E. Rigamonti, A.I. Pincelli, B. Corr
a, R. Viarengo, S.M. Bonomo, D. Galimberti, M. Scacchi, E. Scarpini, F. Cavagnini, E.E. Muller, Plasma ghrelin concentrations in elderly subjects: comparison with anorexic and obese patients, J. Endocrinol. 175 (2002) R1–R5. C.A. Roelen, H.P. Koppeschaar, W.R. de Vries, Y.E. Snel, M.E. Doerga, P.M. Zelissen, J.H. Thijssen, M.A. Blankenstein, Visceral adipose tissue is associated with circulating high affinity growth hormone-binding protein, J. Clin. Endocrinol. Metab. 82 (1997) 760–764. M. Rolla, A. Andreoni, D. Belliti, R. Cristofani, M. Ferdeghini, E.E. Muller, Blockade of cholinergic muscarinic receptors by pirenzepine and GHRH-induced GH secretion in the acute and recovery phase of anorexia nervosa and atypical eating disorders, Biol. Psychiatry 29 (1991) 1079–1091. M. Rolla, A. Andreoni, D. Belliti, M. Ferdeghini, E. Ferrannini, Failure of glucose infusion to suppress the exaggerated GH response to GHRH in patients with anorexia nervosa, Biol. Psychiatry 27 (1990) 215–222. M. Rolla, A. Andreoni, D. Bellitti, M. Ferdeghini, E. Ghigo, E.E. Muller, Corticotrophin-releasing hormone does not inhibit growth hormone-releasing hormone-induced release of growth hormone in control subjects but is effective in patients with eating disorders, J. Endocrinol. 140 (1994) 327–332. D.L. Russell-Jones, A.M. Umpleby, T.R. Hennessy, S.B. Bowes, F. Shojaee-Moradie, K.D. Hopkins, N.C. Jackson, J.M. Kelly, R.H. Jones, P.H. Sonksen, Use of a leucine clamp to demonstrate that IGF-I actively stimulates protein synthesis in normal humans, Am. J. Physiol. 267 (1994) E591–E598. D.L. Russell-Jones, A.J. Weissberger, S.B. Bowes, J.M. Kelly, M. Thomason, A.M. Umpleby, R.H. Jones, P.H. Sonksen, The effects of growth hormone on protein metabolism in adult growth hormone deficient patients, Clin. Endocrinol. 38 (1993) 427–431. M.F. Saad, B. Bernaba, C.M. Hwu, S. Jinagouda, S. Fahmi, E. Kogosov, R. Boyadjian, Insulin regulates plasma ghrelin concentration, J. Clin. Endocrinol. Metab. 87 (2002) 3997–4000. O. Sartor, C.Y. Bowers, D. Chang, Parallel studies of His-DTrpAla-Trp-DPhe-Lys-NH2 and human pancreatic growth hormone-releasing factor-44-NH2 in rat primary pituitary cell monolayer culture, Endocrinology 116 (1985) 952–957. A. Sartorio, A. Spada, D. Bochicchio, A. Atterrato, F. Morabito, G. Faglia, Effect of thyrotropin-releasing hormone on growth hormone release in normal subjects pretreated with human pancreatic growth hormone-releasing factor 1–44 pulsatile administration, Neuroendocrinology 44 (1986) 470–474. A. Sauter, M. Goldstein, J. Engel, K. Ueta, Effect of insulin on central catecholamines, Brain Res. 260 (1983) 330–333. M. Scacchi, C. Invitti, A.I. Pincelli, C. Pandolfi, A. Dubini, F. Cavagnini, Lack of growth hormone response to acute administration of dexamethasone in anorexia nervosa, Eur. J. Endocrinol. 132 (1995) 152–158. M. Scacchi, A.I. Pincelli, A. Caumo, P. Tomasi, G. Delitala, G. Baldi, F. Cavagnini, Spontaneous nocturnal growth hormone secretion in anorexia nervosa, J. Clin. Endocrinol. Metab. 82 (1997) 3225–3229. M. Scacchi, A.I. Pincelli, F. Cavagnini, Growth hormone in obesity, Int. J. Obes. 23 (1999) 260–271. A. Schattner, M. Steinbock, R. Tepper, A. Schonfeld, N. Vaisman, T. Hahn, Tumour necrosis factor production and cell-mediated immunity in anorexia nervosa, Clin. Exp. Immunol. 79 (1990) 62–66. M.W. Schwartz, D.G. Baskin, T.R. Bukowski, J.L. Kuijper, D. Foster, G. Lasser, D.E. Prunkard, D. Porte Jr., S.C. Woods, R.J. Seeley, D.S. Weigle, Specificity of leptin action on elevated blood

M. Scacchi et al. / Frontiers in Neuroendocrinology 24 (2003) 200–224

[252]

[253]

[254]

[255]

[256]

[257]

[258]

[259]

[260]

[261]

[262]

[263]

[264]

[265]

glucose levels and hypothalamic neuropeptide Y gene expression in ob/ob mice, Diabetes 45 (1996) 531–535. P.S. Sharp, K. Foley, P. Chahal, E.M. Kohner, The effect of plasma glucose on the growth hormone response to human pancreatic growth hormone releasing factor in normal subjects, Clin. Endocrinol. 20 (1984) 497–501. T. Shibasaki, M. Hotta, A. Masuda, T. Imaki, N. Obara, H. Demura, N. Ling, K. Shizume, Plasma GH responses to GHRH and insulin-induced hypoglycaemia in man, J. Clin. Endocrinol. Metab. 60 (1985) 1265–1267. T. Shiiya, M. Nakazato, M. Mizuta, Y. Date, M.S. Mondal, M. Tanaka, S. Nozoe, H. Hosoda, K. Kangawa, S. Matsukura, Plasma ghrelin levels in lean and obese humans and the effect of glucose on ghrelin secretion, J. Clin. Endocrinol. Metab. 87 (2002) 240–244. I. Shimon, X. Yan, D.A. Magoffin, T.C. Friedman, S. Melmed, Intact leptin receptor is selectively expressed in human fetal pituitary and pituitary adenomas and signals human fetal pituitary growth hormone secretion, J. Clin. Endocrinol. Metab. 83 (1998) 4059–4064. M.K. Sinha, J. Sturis, J. Ohannesian, S. Magosin, T. Stephens, M.L. Heiman, K.S. Polonsky, J.F. Caro, Ultradian oscillations of leptin secretion in humans, Biochem. Biophys. Res. Commun. 228 (1996) 733–738. P.C. Sizonenko, A. Rabinovitch, P. Schneider, L. Paunier, C.B. Wollheim, G. Zahnd, Plasma growth hormone, insulin, and glucagon responses to arginine infusion in children and adolescents with idiopathic short stature, isolated growth hormone deficiency, panhypopituitarism, and anorexia nervosa, Pediatr. Res. 9 (1975) 733–738. P.J. Smith, L.S. Wise, R. Berkowitz, C. Wan, C.S. Rubin, Insulin-like growth factor-I is an essential regulator of the differentiation of 3T3-L1 adipocytes, J. Biol. Chem. 263 (1988) 9402–9408. T.R. Smith, J.S. Elmendorf, T.S. David, J. Turinsky, Growth hormone-induced insulin resistance: role of the insulin receptor, IRS-1, GLUT-1, and GLUT-4, Am. J. Physiol. 272 (1997) E1071–E1079. A.T. Soliman, A.E.I. Hassan, M.K. Aref, R.L. Hintz, R.G. Rosenfeld, A.D. Rogol, Serum insulin-like growth factor I and II concentrations and growth hormone and insulin responses to arginine infusion in children with protein-energy malnutrition before and after nutritional rehabilitation, Pediatr. Res. 20 (1986) 1122–1130. O. Stehling, H. Doring, J. Ertl, G. Preibisch, I. Schmidt, Leptin reduces juvenile fat stores by altering the circadian cycle of energy expenditure, Am. J. Physiol. 271 (1996) R1770–R1774. T.W. Stephens, M. Basinski, P.K. Bristow, J.M. Bue Valleskey, S.G. Burgett, L. Craft, J. Hale, J. Hoffmann, H.M. Hsiung, A. Kriauciunas, W. MacKellar, P.R. Rostek, B. Schoner, D. Smith, F.C. Tinsley, X.Y. Zhang, M. Heiman, The role of neuropeptide Y in the antiobesity action of the obese gene product, Nature 377 (1995) 530–532. R.K. Stoving, M. Andersen, A. Flyvbjerg, J. Frystyk, J. Hangaard, J. Vinten, O.G. Koldkjaer, C. Hagen, Indirect evidence for decreased hypothalamic somatostatinergic tone in anorexia nervosa, Clin. Endocrinol. 56 (2002) 391–396. R.K. Stoving, A. Flyvbjerg, J. Frystyk, S. Fisker, J. Hangaard, M. Hansen-Nord, C. Hagen, Low serum levels of free and total insulin-like growth factor I (IGF-I) in patients with anorexia nervosa are not associated with increased IGF-binding protein-3 proteolysis, J. Clin. Endocrinol. Metab. 84 (1999) 1346–1350. R.K. Stoving, J.D. Veldhuis, A. Flyvbjerg, J. Vinten, J. Hangaard, O.G. Koldkjaer, J. Kristiansen, C. Hagen, Jointly amplified basal and pulsatile growth hormone (GH) secretion and increased process irregularity in women with anorexia nervosa: indirect evidence for disruption of feedback regulation

[266]

[267]

[268]

[269]

[270]

[271]

[272]

[273]

[274]

[275]

[276]

[277]

[278]

[279]

223

within the GH-insulin-like growth factor I axis, J. Clin. Endocrinol. Metab. 84 (1999) 2056–2063. R.K. Stoving, J. Vinten, A. Handberg, E.N. Ebbesen, J. Hangaard, M. Hansen-Nord, J. Kristiansen, C. Hagen, Diurnal variation of the serum leptin concentration in patients with anorexia nervosa, Clin. Endocrinol. 48 (1998) 761–768. H. Sugihara, N. Emoto, T. Shibasaki, S. Minami, I. Wakabayashi, Increased pituitary growth hormone-releasing factor (GRF) receptor messenger ribonucleic acid expression in fooddeprived rats, Brain Res. 742 (1996) 355–358. J. Svensson, L. Lonn, J.O. Jansson, G. Murphy, D. Wyss, D. Krupa, K. Cerchio, W. Polvino, B. Gertz, I. Boseaus, L. Sjostrom, B.A. Bengtsson, Two-month treatment of obese subjects with the oral growth hormone (GH) secretagogue MK-677 increases GH secretion, fat-free mass, and energy expenditure, J. Clin. Endocrinol. Metab. 83 (1998) 362–369. M. Tagliaferri, M. Scacchi, A.I. Pincelli, M.E. Berselli, P. Silvestri, A. Montesano, S. Ortolani, A. Dubini, F. Cavagnini, Metabolic effects of biosynthetic growth hormone treatment in severely energy-restricted obese women, Int. J. Obes. 22 (1998) 836–841. K. Takaya, H. Ariyasu, N. Kanamoto, H. Iwakura, A. Yoshimoto, M. Harada, K. Mori, Y. Komatsu, T. Usui, A. Shimatsu, Y. Ogawa, K. Hosoda, T. Akamizu, M. Kojima, K. Kangawa, K. Nakao, Ghrelin strongly stimulates growth hormone release in humans, J. Clin. Endocrinol. Metab. 85 (2000) 4908–4911. H. Tamai, K. Kiyohara, T. Mukuta, N. Kobayashi, G. Komaki, T. Nakagawa, L.F. Kumagai, T.T. Aoki, Responses of growth hormone and cortisol to intravenous glucose loading test in patients with anorexia nervosa, Metabolism 40 (1991) 31–34. H. Tamai, G. Komaki, S. Matsubayashi, N. Kobayashi, K. Mori, T. Nakagawa, M.P.M. Truong, R.M. Walter Jr., L. Kumagai, Effect of cholinergic muscarinic receptor blockade on human growth hormone (GH)-releasing hormone-(1–44)-induced GH secretion in anorexia nervosa, J. Clin. Endocrinol. Metab. 70 (1990) 738–741. H. Tamura, J. Kamegai, T. Shimizu, S. Ishii, H. Sugihara, S. Oikawa, Ghrelin stimulates GH but not food intake in arcuate nucleus ablated rats, Endocrinology 143 (2002) 3268–3275. M. Tanaka, T. Naruo, T. Muranaga, D. Yasuhara, T. Shiiya, M. Nakazato, S. Matsukura, S. Nozoe, Increased fasting plasma ghrelin levels in patients with bulimia nervosa, Eur. J. Endocrinol. 146 (2002) R1–R3. G.S. Tannenbaum, M. Lapointe, W. Gurd, J.A. Finkelstein, Mechanisms of impaired growth hormone secretion in genetically obese Zucker rats: roles of growth hormone-releasing factor and somatostatin, Endocrinology 127 (1990) 3087–3095. J.P. Thissen, J.M. Ketelslegers, L.E. Underwood, Nutritional regulation of the insulin-like growth factors, Endocr. Rev. 15 (1994) 80–101. G.B. Thomas, J.T. Cummins, H. Francis, A.W. Sudbury, P.I. McCloud, I.J. Clarke, Effect of restricted feeding on the relationship between hypophysial portal concentrations of growth hormone (GH)-releasing factor and somatostatin, and jugular concentrations of GH in ovariectomized ewes, Endocrinology 128 (1991) 1151–1158. G.B. Thomas, K.M. Fairhall, I.C. Robinson, Activation of the hypothalamo–pituitary–adrenal axis by the growth hormone (GH) secretagogue, GH-releasing peptide-6, in rats, Endocrinology 138 (1997) 1585–1591. V. Tolle, M.H. Bassant, P. Zizzari, F. Poindessous-Jazat, C. Tomasetto, J. Epelbaum, M.T. Bluet-Pajot, Ultradian rhythmicity of ghrelin secretion in relation with GH, feeding behavior, and sleep–wake patterns in rats, Endocrinology 143 (2002) 1353– 1361.

224

M. Scacchi et al. / Frontiers in Neuroendocrinology 24 (2003) 200–224

[280] V. Tolle, M. Kadem, M.T. Bluet-Pajot, D. Frere, C. Foulon, C. Bossu, R. Dardennes, C. Mounier, P. Zizzari, F. Lang, J. Epelbaum, B. Estour, Balance in ghrelin and leptin plasma levels in anorexia nervosa patients and constitutionally thin women, J. Clin. Endocrinol. Metab. 88 (2003) 109–116. [281] A. Torsello, V. Locatelli, M.R. Melis, S. Succu, M.S. Spano, R. Deghenghi, E.E. Muller, A. Argiolas, Differential orexigenic effects of hexarelin and its analogs in the rat hypothalamus: indication for multiple growth hormone secretagogue receptor subtypes, Neuroendocrinology 72 (2000) 327–332. [282] K. Toshinai, M.S. Mondal, M. Nakazato, Y. Date, N. Murakami, M. Kojima, K. Kangawa, S. Matsukura, Upregulation of ghrelin expression in the stomach upon fasting, insulin-induced hypoglycemia, and leptin administration, Biochem. Biophys. Res. Commun. 281 (2001) 1220–1225. [283] M. Traebert, T. Riediger, S. Whitebread, E. Scharrer, H.A. Schmid, Ghrelin acts on leptin-responsive neurones in the rat arcuate nucleus, J. Neuroendocrinol. 14 (2002) 580–586. [284] M. Tschop, D.B. Flora, J.P. Mayer, M.L. Heiman, Hypophysectomy prevents ghrelin-induced adiposity and increases gastric ghrelin secretion in rats, Obes. Res. 10 (2002) 991– 999. [285] M. Tschop, D.L. Smiley, M.L. Heiman, Ghrelin induces adiposity in rodents, Nature 407 (2000) 908–913. [286] M. Tschop, M.A. Statnick, T.M. Suter, M.L. Heiman, GHreleasing peptide-2 increases fat mass in mice lacking NPY: indication for a crucial mediating role of hypothalamic agoutirelated protein, Endocrinology 143 (2002) 558–568. [287] M. Tschop, R. Wawarta, R.L. Riepl, S. Friedrich, M. Bidlingmaier, R. Landgraf, C. Folwaczny, Post-prandial decrease of circulating human ghrelin levels, J. Endocrinol. Invest. 24 (2001) RC19–RC21. [288] M. Tschop, C. Weyer, P.A. Tataranni, V. Devanarayan, E. Ravussin, H.L. Heiman, Circulating ghrelin levels are decreased in human obesity, Diabetes 50 (2001) 707–709. [289] A.M. Umpleby, D.L. Russell-Jones, The hormonal control of protein metabolism, Bailli
ereÕs Clin. Endocrinol. Metab. 10 (1996) 551–570. [290] A.M. Umpleby, F. Shojaee-Moradie, M.J. Thomason, J.M. Kelly, A. Skottner, P.H. Sonksen, R.H. Jones, Effects of insulinlike growth factor-I (IGF-I), insulin and combined IGF-I-insulin

[291] [292]

[293] [294]

[295]

[296]

[297]

[298]

[299]

[300]

[301]

infusions on protein metabolism in dogs, Eur. J. Clin. Invest. 24 (1994) 337–344. N. Vaisman, T. Hahn, Tumor necrosis factor-alpha and anorexia—cause or effect? Metabolism 40 (1991) 720–723. R. Valcavi, M. Zini, I. Portioli, Triiodothyronine administration reduces serum growth hormone levels and growth hormone responses to thyrotropin-releasing hormone in patients with anorexia nervosa, Psychoneuroendocrinology 15 (1990) 287–295. G. Van den Berghe, Dynamic neuroendocrine responses to critical illness, Front. Neuroendocrinol. 23 (2002) 370–391. S. Vidal, S.M. Cohen, E. Horvath, K. Kovacs, B.W. Scheithauer, B.G. Burguera, R.V. Lloyd, Subcellular localization of leptin in non-tumorous and adenomatous human pituitaries: an immunoultrastructural study, J. Histochem. Cytochem. 48 (2000) 1147– 1152. M. Volante, E. Allia, P. Gugliotta, A. Funaro, F. Broglio, R. Deghenghi, G. Muccioli, E. Ghigo, M. Papotti, Expression of ghrelin and of the GH secretagogue receptor by pancreatic islet cells and related endocrine tumors, J. Clin. Endocrinol. Metab. 87 (2002) 1300–1308. P. Wang, N. Li, J.S. Li, W.Q. Li, The role of endotoxin, TNFalpha, and IL-6 in inducing the state of growth hormone insensitivity, World J. Gastroenterol. 8 (2002) 531–536. A.M. Wren, L.J. Seal, M.A. Cohen, A.E. Brynes, G.S. Frost, K.G. Murphy, W.S. Dhillo, M.A. Ghatei, S.R. Bloom, Ghrelin enhances appetite and increases food intake in humans, J. Clin. Endocrinol. Metab. 86 (2001) 5992–5995. A.M. Wren, C.J. Small, C.V. Fribbens, N.M. Neary, H.L. Ward, L.J. Seal, M.A. Ghatei, S.R. Bloom, The hypothalamic mechanisms of the hypophysiotropic action of ghrelin, Neuroendocrinology 76 (2002) 316–324. S. Yamashita, S. Melmed, Effects of insulin on rat anterior pituitary cells. Inhibition of growth hormone secretion and mRNA levels, Diabetes 35 (1986) 440–447. G. Yumet, M.L. Shumate, P. Bryant, C.M. Lin, C.H. Lang, R.N. Cooney, Tumor necrosis factor mediates hepatic growth hormone resistance during sepsis, Am. J. Physiol. Endocrinol. Metab. 283 (2002) E472–E481. Y. Zhang, R. Proenca, M. Maffei, M. Barone, L. Leopold, J.M. Friedman, Positional cloning of the mouse obese gene and its human homologue, Nature 372 (1994) 425–432.