Effects of leptin replacement on hypothalamic-pituitary growth hormone axis function and circulating ghrelin levels in ob/ob mice

Leptin-deficient obese mice (ob/ob) have decreased circulating growth hormone (GH) and pituitary GH and ghrelin receptor (GHS-R) mRNA levels, whereas hypothalamic GH-releasing hormone (GHRH) and somatostatin (SST) expression do not differ from lean controls. Given the fact that GH is suppressed in diet-induced obesity (a state of hyperleptinemia), it remains to be determined whether the absence of leptin contributes to changes in the GH axis of ob/ob mice. Therefore, to study the impact of leptin replacement on the hypothalamic-pituitary GH axis of ob/ob mice, leptin was infused for 7 days (sc), resulting in circulating leptin levels that were similar to wild-type controls (approximately 1 ng/ml). Leptin treatment reduced food intake, body weight, and circulating insulin while elevating circulating n-octanoyl ghrelin concentrations. Leptin treatment did not alter hypothalamic GHRH, SST, or GHS-R mRNA levels compared with vehicle-treated controls. However, leptin significantly increased pituitary GH and GHRH-R expression and tended to enhance circulating GH levels, but this latter effect did not reach statistical significance. In vitro, leptin (1 ng/ml, 24 h) did not affect pituitary GH, GHRH-R, or GHS-R mRNA but did enhance GH release. The in vivo effects of leptin on circulating hormone and pituitary mRNA levels were not replicated by pair feeding ob/ob mice to match the food intake of leptin-treated mice. However, leptin did prevent the fall in hypothalamic GHRH mRNA and circulating IGF-I levels observed in pair-fed mice. These results demonstrate that leptin replacement has positive effects on multiple levels of GH axis function in ob/ob mice.     

Growth hormone receptor deficiency results in blunted ghrelin feeding response, obesity, and hypolipidemia in mice

We have previously shown that growth hormone (GH) overexpression in the brain increased food intake, accompanied with increased hypothalamic agouti-related protein (AgRP) expression. Ghrelin, which stimulates both appetite and GH secretion, was injected intracerebroventricularly to GHR-/- and littermate control (+/+) mice to determine whether ghrelin's acute effects on appetite are dependent on GHR signaling. GHR-/- mice were also analyzed with respect to serum levels of lipoproteins, apolipoprotein (apo)B, leptin, glucose, and insulin as well as body composition. Central injection of ghrelin into the third dorsal ventricle increased food consumption in +/+ mice, whereas no change was observed in GHR-/- mice. After ghrelin injection, AgRP mRNA expression in the hypothalamus was higher in +/+ littermates than in GHR-/- mice, indicating a possible importance of AgRP in the GHR-mediated effect of ghrelin. Compared with controls, GHR-/- mice had increased food intake, leptin levels, and total and intra-abdominal fat mass per body weight and deceased lean mass. Moreover, serum levels of triglycerides, LDL and HDL cholesterol, and apoB, as well as glucose and insulin levels were lower in the GHR-/- mice. In summary, ghrelin's acute central action to increase food intake requires functionally intact GHR signaling. Long-term GHR deficiency in mice is associated with high plasma leptin levels, obesity, and increased food intake but a marked decrease in all lipoprotein fractions.     

Thyroid hormone receptor α and regulation of type 3 deiodinase

Mice deficient in thyroid hormone receptor α (TRα) display hypersensitivity to thyroid hormone (TH), with normal serum TSH but diminished serum T(4). Our aim was to determine whether altered TH metabolism played a role in this hypersensitivity. TRα knockout (KO) mice have lower levels of rT(3), and lower rT(3)/T(4) ratios compared with wild-type (WT) mice. These alterations could be due to increased type 1 deiodinase (D1) or decreased type 3 deiodinase (D3). No differences in D1 mRNA expression and enzymatic activity were found between WT and TRαKO mice. We observed that T(3) treatment increased D3 mRNA in mouse embryonic fibroblasts obtained from WT or TRβKO mice, but not in those from TRαKO mice. T(3) stimulated the promoter activity of 1.5 kb 5'-flanking region of the human (h) DIO3 promoter in GH3 cells after cotransfection with hTRα but not with hTRβ. Moreover, treatment of GH3 cells with T(3) increased D3 mRNA after overexpression of TRα. The region necessary for the T(3)-TRα stimulation of the hD3 promoter (region -1200 to -1369) was identified by transfection studies in Neuro2A cells that stably overexpress either TRα or TRβ. These results indicate that TRα mediates the up-regulation of D3 by TH in vitro. TRαKO mice display impairment in the regulation of D3 by TH in both brain and pituitary and have reduced clearance rate of TH as a consequence of D3 deregulation. We conclude that the absence of TRα results in decreased clearance of TH by D3 and contributes to the TH hypersensitivity.     

Effect of chronic hyperghrelinemia on ingestive action of ghrelin

The stomach hormone ghrelin is the endogenous ligand for the growth hormone secretagogue receptor (GHS-R). Systemic administration of ghrelin will cause elevations in growth hormone (GH) secretion, food intake, adiposity, and body growth. Ghrelin also affects insulin secretion, gastric acid secretion, and gastric motility. Several reports indicate that repeated or continuous activation of GHS-R by exogenous GHSs or ghrelin results in a diminished GH secretory response. The purpose of this study was to examine the extent to which the acute stimulation of food intake by exogenous ghrelin is altered by chronic hyperghrelinemia in transgenic mice that overexpress the human ghrelin gene. The present findings show that the orexigenic action of exogenous ghrelin is not diminished by a chronic hyperghrelinemia and indicate that the food ingestive pathway of the GHS-R is not susceptible to desensitization. In contrast, the epididymal fat pad growth response, like the GH response, to exogenous ghrelin is blunted in ghrelin transgenic mice with chronic hyperghrelinemia.     

Physiological and metabolic characteristics of novel double‐mutant female mice with targeted disruption of both growth hormone‐releasing hormone and growth hormone receptor

Mice with disruptions of growth hormone‐releasing hormone (GHRH) or growth hormone receptor (GHR) exhibit similar phenotypes of prolonged lifespan and delayed age‐related diseases. However, these two models respond differently to calorie restriction indicating that they might carry different and/or independent mechanisms for improved longevity and healthspan. In order to elucidate these mechanisms, we generated GHRH and GHR double‐knockout mice (D‐KO). In the present study, we focused specifically on the characteristics of female D‐KO mice. The D‐KO mice have reduced body weight and enhanced insulin sensitivity compared to wild‐type (WT) controls. Growth retardation in D‐KO mice is accompanied by decreased GH expression in pituitary, decreased circulating IGF‐1, increased high‐molecular‐weight (HMW) adiponectin, and leptin hormones compared to WT controls. Generalized linear model‐based regression analysis, which controls for body weight differences between D‐KO and WT groups, shows that D‐KO mice have decreased lean mass, bone mineral density, and bone mineral content, but increased adiposity. Indirect calorimetry markers including oxygen consumption, carbon dioxide production, and energy expenditure were significantly lower in D‐KO mice relative to the controls. In comparison with WT mice, the D‐KO mice displayed reduced respiratory exchange ratio (RER) values only during the light cycle, suggesting a circadian‐related metabolic shift toward fat utilization. Interestingly, to date survival data suggest extended lifespan in D‐KO female mice.     

Evidence that growth hormone exerts a feedback effect on stomach ghrelin production and secretion

Ghrelin is a recently discovered stomach hormone that stimulates pituitary growth hormone (GH) secretion potently. The purpose of these experiments was to test the hypothesis that a stomach-ghrelin-pituitary-GH axis exists in which either an elevation or reduction in systemic GH levels will exert a negative or positive feedback action, respectively, on stomach ghrelin homeostasis. In rats, GH administration decreased stomach ghrelin mRNA levels and plasma ghrelin levels significantly. In GH-releasing hormone (GHRH) transgenic mice, GHRH overexpression decreased stomach ghrelin peptide levels when compared with control mice. In aged rats (25 months) stomach ghrelin mRNA and peptide levels and plasma ghrelin levels were decreased when compared with young rats (5 months). Because GH secretion is reduced in aged rats, the elevated stomach ghrelin production and secretion may reflect a decreased GH feedback on stomach ghrelin, homeostasis, and secretion. Together, these findings suggest that endogenous pituitary GH exerts a feedback action on stomach ghrelin homeostasis and support the hypothesis that a stomach-ghrelin-pituitary GH axis exists.     

Lecithinized Cu, Zn-superoxide dismutase limits the infarct size following ischemia-reperfusion injury in rat hearts in vivo

Ghrelin is a novel gut-brain peptide that binds to the growth hormone secretagogue receptor (GHS-R), thereby functioning in the regulation of growth hormone (GH) release and food intake. Ghrelin-producing cells are most abundant in the oxyntic glands of the stomach. The regulatory mechanism that governs the biosynthesis and secretion of ghrelin has not been clarified. We report that ghrelin mRNA expression in the gastric fundus was increased, but that ghrelin peptide content decreased after a 48-h fast. Both values returned to control levels after refeeding. The ghrelin plasma concentration in the gastric vein and systemic venous blood increased after 24- and 48-h fasts. Furthermore, des-octanoylated ghrelin and n-octanoylated ghrelin were found in rat stomach, with the ratio of des-octanoylated ghrelin to n-octanoylated ghrelin markedly increased after fasting. The ghrelin mRNA level in the stomach also increased after administration of insulin and leptin. Conversely, db/db mice, which are deficient in the leptin receptor, had lower ghrelin mRNA levels than control mice. These findings suggest that this novel gastrointestinal hormone plays a role in the regulation of energy balance.     

In vitro and in vivo effects of ghrelin on luteinizing hormone and growth hormone release in goldfish

We studied the in vitro and in vivo effects of octanoylated goldfish ghrelin peptides (gGRL-19 and gGRL-12) on luteinizing hormone (LH) and growth hormone (GH) release in goldfish. gGRL-19 and gGRL-12 at picomolar doses stimulated LH and GH release from dispersed goldfish pituitary cells in perifusion and static incubation. Incubation of pituitary cells for 2 h with 10 nM gGRL-12 and 1 or 10 nM gGRL-19 increased LH-beta mRNA expression, whereas only 10 nM gGRL-19 increased GH mRNA expression. Somatostatin-14 abolished the stimulatory effects of ghrelin on GH release from dispersed pituitary cells in perifusion and static culture. The GH secretagogue receptor antagonist d-Lys(3)-GHRP-6 inhibited the ghrelin-induced LH release, whereas no effects were found on stimulation of GH release by ghrelin. Intracerebroventricular injection of 1 ng/g body wt of gGRL-19 or intraperitoneal injection of 100 ng/g body wt of gGRL-19 increased serum LH levels at 60 min after injection, whereas significant increases in GH levels were found at 15 and 30 min after these treatments. Our results indicate that, in addition to its potent stimulatory actions on GH release, goldfish ghrelin peptides have the novel function of stimulating LH release in goldfish.     

Ghrelin-induced food intake and growth hormone secretion are altered in melanocortin 3 and 4 receptor knockout mice

Ghrelin stimulates food intake in part by activating hypothalamic neuropeptide Y (NPY) neurons/agouti related peptide (AGRP) neurons. We investigated the role of AGRP/melanocortin signaling in ghrelin-induced food intake by studying melanocortin 3 and 4 receptor knockout (MC3R KO and MC4R KO) mice. We also determined whether reduced ghrelin levels and/or an altered sensitivity to the GH-stimulating effects of ghrelin accompany the obesity syndromes of MC3R KO and MC4R KO mice. Compared to wild-type (WT) mice, the effects of ghrelin on food intake were reduced in MC3R KO and MC4R KO mice and circulating ghrelin levels were reduced in female MC4R KO mice. Female MC3R KO and MC4R KO mice exhibited a diminished responsiveness to the GH-releasing effects of ghrelin. Thus, deletion of the MC3R or MC4R results in a decreased sensitivity to ghrelin and verifies the involvement in the melanocortin system in ghrelin-induced food intake.     

Effect of ghrelin on glucose regulation in mice

Improvement of glucose metabolism after bariatric surgery appears to be from the composite effect of the alterations in multiple circulating gut hormone concentrations. However, their individual effect on glucose metabolism during different conditions is not clear. The objective of this study was to determine whether ghrelin has an impact on glycogenolysis, gluconeogenesis, and insulin sensitivity (using a mice model). Rate of appearance of glucose, glycogenolysis, and gluconeogenesis were measured in wild-type (WT), ghrelin knockout (ghrelin(-/-)), and growth hormone secretagogue receptor knockout (Ghsr(-/-)) mice in the postabsorptive state. The physiological nature of the fasting condition was ascertained by a short-term fast commenced immediately at the end of the dark cycle. Concentrations of glucose and insulin were measured, and insulin resistance and hepatic insulin sensitivity were calculated. Glucose concentrations were not different among the groups during the food-deprived period. However, plasma insulin concentrations were lower in the ghrelin(-/-) and Ghsr(-/-) than WT mice. The rates of gluconeogenesis, glycogenolysis, and indexes of insulin sensitivity were higher in the ghrelin(-/-) and Ghsr(-/-) than WT mice during the postabsorptive state. Insulin receptor substrate 1 and glucose transporter 2 gene expressions in hepatic tissues of the ghrelin(-/-) and Ghsr(-/-) were higher compared with that in WT mice. This study demonstrates that gluconeogenesis and glycogenolysis are increased and insulin sensitivity is improved by the ablation of the ghrelin or growth hormone secretagogue receptor in mice.     

Regulation of prolactin in mice with altered hypothalamic melanocortin activity

This study used two mouse models with genetic manipulation of the melanocortin system to investigate prolactin regulation. Mice with overexpression of the melanocortin receptor (MC-R) agonist, α-melanocyte-stimulating hormone (Tg-MSH) or deletion of the MC-R antagonist agouti-related protein (AgRP KO) were studied. Male Tg-MSH mice had lower blood prolactin levels at baseline (2.9±0.3 vs. 4.7±0.7ng/ml) and after restraint stress (68±6.5 vs. 117±22ng/ml) vs. WT (p<0.05); however, pituitary prolactin content was not different. Blood prolactin was also decreased in male AgRP KO mice at baseline (4.2±0.5 vs. 7.6±1.3ng/ml) and after stress (60±4.5 vs. 86.1±5.7ng/ml) vs. WT (p<0.001). Pituitary prolactin content was lower in male AgRP KO mice (4.3±0.3 vs. 6.7±0.5μg/pituitary, p<0.001) vs. WT. No differences in blood or pituitary prolactin levels were observed in female AgRP KO mice vs. WT. Hypothalamic dopamine activity was assessed as the potential mechanism responsible for changes in prolactin levels. Hypothalamic tyrosine hydroxylase mRNA was measured in both genetic models vs. WT mice and hypothalamic dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) content were measured in male AgRP KO and WT mice but neither were significantly different. However, these results do not preclude changes in dopamine activity as dopamine turnover was not directly investigated. This is the first study to show that baseline and stress-induced prolactin release and pituitary prolactin content are reduced in mice with genetic alterations of the melanocortin system and suggests that changes in hypothalamic melanocortin activity may be reflected in measurements of serum prolactin levels.     

Spontaneous sleep and homeostatic sleep regulation in ghrelin knockout mice

Ghrelin is well known for its feeding and growth hormone-releasing actions. It may also be involved in sleep regulation; intracerebroventricular administration and hypothalamic microinjections of ghrelin stimulate wakefulness in rats. Hypothalamic ghrelin, together with neuropeptide Y and orexin form a food intake-regulatory circuit. We hypothesized that this circuit also promotes arousal. To further investigate the role of ghrelin in the regulation of sleep-wakefulness, we characterized spontaneous and homeostatic sleep regulation in ghrelin knockout (KO) and wild-type (WT) mice. Both groups of mice exhibited similar diurnal rhythms with more sleep and less wakefulness during the light period. In ghrelin KO mice, spontaneous wakefulness and rapid-eye-movement sleep (REMS) were slightly elevated, and non-rapid-eye-movement sleep (NREMS) was reduced. KO mice had more fragmented NREMS than WT mice, as indicated by the shorter and greater number of NREMS episodes. Six hours of sleep deprivation induced rebound increases in NREMS and REMS and biphasic changes in electroencephalographic slow-wave activity (EEG SWA) in both genotypes. Ghrelin KO mice recovered from NREMS and REMS loss faster, and the delayed reduction in EEG SWA, occurring after sleep loss-enhanced increases in EEG SWA, was shorter-lasting compared with WT mice. These findings suggest that the basic sleep-wake regulatory mechanisms in ghrelin KO mice are not impaired and they are able to mount adequate rebound sleep in response to a homeostatic challenge. It is possible that redundancy in the arousal systems of the brain or activation of compensatory mechanisms during development allow for normal sleep-wake regulation in ghrelin KO mice.     

Influence of ghrelin on gastric and duodenal growth and expression of digestive enzymes in young mature rats.

Ghrelin, a nature ligand for the growth hormone secretagogue receptor (GHS-R), stimulates a release of growth hormone, prolactin and adrenocorticotropic hormone. Also, ghrelin increases food intake in adult rats and humans and exhibits gastroprotective effect against experimental ulcers induced by ethanol or stress. The aim of present study was to examine the influence of ghrelin administration on gastric and duodenal growth and expression of pepsin and enterokinase in young mature rats with intact or removed pituitary. METHODS: Two week after sham operation or hypophysectomy, eight week old Wistar male rats were treated with saline (control) or ghrelin (4, 8 or 16 nmol/kg/dose) i.p. twice a day for 4 days. Expression of pepsin in the stomach and enterokinase in the duodenum was evaluated by real-time PCR. RESULTS: In animals with intact pituitary, treatment with ghrelin increased food intake, body weight gain and serum level of growth hormone and insulin-like growth factor-1 (IGF-1). These effects were accompanied with stimulation of gastric and duodenal growth. It was recognized as the significant increase in gastric and duodenal weight and mucosal DNA synthesis. In both organs, ghrelin administered at the dose of 8 nmol/kg caused maximal growth-promoting effect. In contrast to these growth-promoting effects, administration of ghrelin reduced expression of mRNA for pepsin in the stomach and was without effect on expression of mRNA for enterokinase in the duodenum. Hypophysectomy alone lowered serum concentration of growth hormone under the detection limit and reduced serum level of IGF-1 by 90%. These effects were associated with reduction in daily food intake, body weight gain and gastroduodenal growth. In hypophysectomized rats, administration of ghrelin was without significant effect on food intake, body weight gain or growth of gastroduodenal mucosa. Also, serum concentration of growth hormone or IGF-1 was not affected by ghrelin administration in rats with removed pituitary. CONCLUSION: Administration of ghrelin stimulates gastric and duodenal growth in young mature rats with intact pituitary, but inhibits expression of mRNA for pepsin in the stomach. Growth hormone and insulin-like growth factor-1 play an essential role in growth-promoting effects of ghrelin in the stomach and duodenum.     

Acylated ghrelin secretion is acutely suppressed by oral glucose load or insulin-induced hypoglycemia independently of basal growth hormone secretion in humans

BACKGROUND: Ghrelin has been reported to be the natural ligand of growth hormone (GH) secretagogue receptor, and it is known that exogenous ghrelin administration strongly stimulates GH release in humans. However, the effects of endogenous ghrelin on GH secretion and changes in ghrelin levels during dynamic changes in GH levels are not well understood. METHODS: Therefore, we measured circulating acylated ghrelin concentrations during oral glucose tolerance tests (OGTTs) in patients with active acromegaly (AA, n = 9) and in age/sex/BMI-matched group A controls (n = 12), and during insulin tolerance testing (ITT) in patients with GH deficiency (GHD, n = 10) and in group B controls (n = 10). Plasma acylated ghrelin, serum GH, insulin and glucose levels were measured during each test. RESULTS: Fasting plasma ghrelin levels correlated negatively with serum insulin levels in both group A and B controls (r = -0.665; p < 0.05) but not in patients with AA or GHD. During OGTTs, circulating ghrelin levels decreased significantly with a nadir at 30 min in both patients with AA (p < 0.05) and group A controls (p < 0.01). Also, ITTs were followed by a significant decrease in circulating ghrelin levels with a nadir at 30 min in patients with GHD (p < 0.05) and in group B controls (p < 0.05). CONCLUSION: The results of the study show that at baseline acylated ghrelin levels do not differ with respect to the GH status (GH excess or GH deficiency) and, furthermore, the suppression of acylated ghrelin levels during OGTT or ITT is independent of the GH response to the tests.     

Effects of ghrelin administration on decreased growth hormone status in obese animals

Obesity is characterized by markedly decreased ghrelin and growth hormone (GH) secretion. Ghrelin is a GH-stimulating, stomach-derived peptide that also has orexigenic action. Ghrelin supplement may restore decreased GH secretion in obesity, but it may worsen obesity by its orexigenic action. To reveal effects of ghrelin administration on obese animals, we first examined acute GH and orexigenic responses to ghrelin in three different obese and/or diabetic mouse models: db/db mice, mice on a high-fat diet (HFD mice), and Akita mice for comparison. GH responses to ghrelin were significantly suppressed in db/db, HFD, and Akita mice. Food intake of db/db and Akita mice were basally higher, and further stimulation of food intake by ghrelin was suppressed. Pituitary GH secretagogue receptor mRNA levels in db/db and HFD mice were significantly decreased, which may partly contribute to decreased GH response to ghrelin in these mice. In Akita mice for comparison, decreased hypothalamic GH-releasing hormone (GHRH) mRNA levels may be responsible for decreased GH response, since maximum GH response to ghrelin needs GHRH. When ghrelin was injected into HFD mice with GHRH coadministrated, GH responses to ghrelin were significantly emphasized. HFD mice injected with low-dose ghrelin and GHRH for 10 days did not show weight gain. These results indicate that low-dose ghrelin and GHRH treatment may restore decreased GH secretion in obesity without worsening obesity.     

Reduced intake of carbohydrate prevents the development of obesity and impaired glucose metabolism in ghrelin O-acyltransferase knockout mice

A close relationship between acylated-ghrelin and sucrose intake has been reported. However, little has been examined about the physiological action of ghrelin on preference for different types of carbohydrate such as glucose, fructose, and starch. The current study was aimed to investigate the role of acylated-ghrelin in the determinants of the choice of carbohydrates, and pathogenesis of chronic disorders, including obesity and insulin resistance. In a two-bottle-drinking test, ghrelin O-acyltransferase (GOAT) knockout (KO) mice consumed a less amount of glucose and maltodextrin, and almost the same amount of fructose and saccharin solution compared to WT littermates. The increased consumption of glucose and maltodextrin was observed when acylated-ghrelin, but not unacylated-ghrelin, was exogeneously administered in normal C57BL/6J mice, suggesting an association of acylated-ghrelin with glucose-containing carbohydrate intake. When fed a diet rich in maltodextrin, starch and fat for 12 weeks, GOAT KO mice showed less food intake and weight gain, as well as improved glucose tolerance and insulin sensitivity than WT mice. Our data suggests that blockade of GOAT activity may offer a therapeutic option for treatment of obesity and its associated metabolic syndrome by preventing from overconsumption of carbohydrate-rich food.     

Severe pulmonary hypertension in aging female apolipoprotein E-deficient mice is rescued by estrogen replacement therapy

Corticotropin-releasing factor overexpressing (CRF-OE) male mice showed an inhibited feeding response to a fast, and lower plasma acyl ghrelin and Fos expression in the arcuate nucleus compared to wild-type (WT) mice. We investigated whether hormones and hypothalamic feeding signals are impaired in CRF-OE mice and the influence of sex. Male and female CRF-OE mice and WT littermates (4–6 months old) fed ad libitum or overnight fasted were assessed for body, adrenal glands and perigonadal fat weights, food intake, plasma hormones, blood glucose, and mRNA hypothalamic signals. Under fed conditions, compared to WT, CRF-OE mice have increased adrenal glands and perigonadal fat weight, plasma corticosterone, leptin and insulin, and hypothalamic leptin receptor and decreased plasma acyl ghrelin. Compared to male, female WT mice have lower body and perigonadal fat and plasma leptin but higher adrenal glands weights. CRF-OE mice lost these sex differences except for the adrenals. Male CRF-OE and WT mice did not differ in hypothalamic expression of neuropeptide Y (NPY) and proopiomelanocortin (POMC), while female CRF-OE compared to female WT and male CRF-OE had higher NPY mRNA levels. After fasting, female WT mice lost more body weight and ate more food than male WT, while CRF-OE mice had reduced body weight loss and inhibited food intake without sex difference. In male WT mice, fasting reduced plasma insulin and leptin and increased acyl ghrelin and corticosterone while female WT showed only a rise in corticosterone. In CRF-OE mice, fasting reduced insulin while leptin, acyl ghrelin and corticosterone were unchanged with no sex difference. Fasting blood glucose was higher in CRF-OE with female > male. In WT mice, fasting increased hypothalamic NPY expression in both sexes and decreased POMC only in males, while in CRF-OE mice, NPY did not change, and POMC decreased in males and increased in females. These data indicate that CRF-OE mice have abnormal basal and fasting circulating hormones and hypothalamic feeding-related signals. CRF-OE also abolishes the sex difference in body weight, abdominal fat, and fasting-induced feeding and changes in plasma levels of leptin and acyl ghrelin.     

Insights into a role of GH secretagogues in reversing the age-related decline in the GH/IGF-I axis

Growth hormone (GH) secretion and serum insulin-like growth factor-I (IGF-I) decline with aging. This study addresses the role played by the hypothalamic regulators in the aging GH decline and investigates the mechanisms through which growth hormone secretagogues (GHS) activate GH secretion in the aging rats. Two groups of male Wistar rats were studied: young-adult (3 mo) and old (24 mo). Hypothalamic growth hormone-releasing hormone (GHRH) mRNA and immunoreactive (IR) GHRH dramatically decreased (P < 0.01 and P < 0.001) in the old rats, as did median eminence IR-GHRH. Decreases of hypothalamic IR-somatostatin (SS; P < 0.001) and SS mRNA (P < 0.01), and median eminence IR-SS were found in old rats as were GHS receptor and IGF-I mRNA (P < 0.01 and P < 0.05). Hypothalamic IGF-I receptor mRNA and protein were unmodified. Both young and old pituitary cells, cultured alone or cocultured with fetal hypothalamic cells, responded to ghrelin. Only in the presence of fetal hypothalamic cells did ghrelin elevate the age-related decrease of GH secretion to within normal adult range. In old rats, growth hormone-releasing peptide-6 returned the levels of GH and IGF-I secretion and liver IGF-I mRNA, and partially restored the lower pituitary IR-GH and GH mRNA levels to those of young untreated rats. These results suggest that the aging GH decline may result from decreased GHRH function rather than from increased SS action. The reduction of hypothalamic GHS-R gene expression might impair the action of ghrelin on GH release. The role of IGF-I is not altered. The aging GH/IGF-I axis decline could be rejuvenated by GHS treatment.     

Central effects of a novel acylated peptide, ghrelin, on growth hormone release in rats

Ghrelin, a novel growth-hormone-releasing acylated peptide, was recently isolated from rat stomach by the search of an endogenous ligand to an "orphan" G-protein-coupled-receptor. Ghrelin neuron is present in the arcuate nucleus of rat hypothalamus, but its central effect on growth hormone (GH) release has yet to be clarified. We determined the plasma GH concentration and GH mRNA level in the pituitary in response to central administration of ghrelin. A single intracerebroventricular (ICV) administration of ghrelin to rats increased the plasma GH concentration dose-dependently. A continuous ICV administration of ghrelin via osmotic pump for 12 days increased the plasma GH concentration on day 6, but did not keep the high GH concentration on day 12. The GH mRNA levels in both groups of single and continuous administration of ghrelin were not significantly different from those of controls. A single administration of growth-hormone secretagogue also did not stimulate GH synthesis. Central ghrelin stimulated GH release but did not augment GH synthesis. In addition to gastric ghrelin, hypothalamic ghrelin functions to regulate GH release.