Ghrelin, a circulating growth hormone-releasing peptide derived from the stomach, stimulates food intake. The lowest systemically effective orexigenic dose of ghrelin was investigated and the resulting plasma ghrelin concentration was compared with that during fasting. The lowest dose of ghrelin that produced a significant stimulation of feeding after intraperitoneal injection was 1 nmol. The plasma ghrelin concentration after intraperitoneal injection of 1 nmol of ghrelin (2.83 ؎ 0.13 pmol/ml at 60 min postinjection) was not significantly different from that occurring after a 24-h fast (2.79 ؎ 0.32 pmol/ml). After microinjection into defined hypothalamic sites, ghrelin (30 pmol) stimulated food intake most markedly in the arcuate nucleus (Arc) (0 -1 h food intake, 427 ؎ 43% of control; P < 0.001 vs. control, P < 0.01 vs. all other nuclei), which is potentially accessible to the circulation. After chronic systemic or intracerebroventricular (ICV) administration of ghrelin for 7 days, cumulative food intake was increased (intraperitoneal ghrelin 13.6 ؎ 3.4 g greater than saline-treated, P < 0.01; ICV ghrelin 19.6 ؎ 5.5 g greater than saline-treated, P < 0.05). This was associated with excess weight gain (intraperitoneal ghrelin 21.7 ؎ 1.4 g vs. saline 10.6 ؎ 1.9 g, P < 0.001; ICV ghrelin 15.3 ؎ 4.3 g vs. saline 2.2 ؎ 3.8 g, P < 0.05) and adiposity. These data provide evidence that ghrelin is important in long-term control of food intake and body weight and that circulating ghrelin at fasting concentrations may stimulate food intake.

AMP-activated protein kinase (AMPK) is the downstream component of a protein kinase cascade that actsas an intracellular energy sensor maintaining the energy balance within the cell. The finding that leptin and adiponectin activate AMPK to alter metabolic pathways in muscle and liver provides direct evidence for this role in peripheral tissues. The hypothalamus is a key regulator of food intake and energy balance, coordinating body adiposity and nutritional state in response to peripheral hormones, such as leptin, peptide YY-(3-36), and ghrelin. To date the hormonal regulation of AMPK in the hypothalamus, or its potential role in the control of food intake, have not been reported. Here we demonstrate that counter-regulatory hormones involved in appetite control regulate AMPK activity and that pharmacological activation of AMPK in the hypothalamus increases food intake. In vivo administration of leptin, which leads to a reduction in food intake, decreases hypothalamic AMPK activity. By contrast, injection of ghrelin in vivo, which increases food intake, stimulates AMPK activity in the hypothalamus. Consistent with the effect of ghrelin, injection of 5-amino-4-imidazole carboxamide riboside, a pharmacological activator of AMPK, into either the third cerebral ventricle or directly into the paraventricular nucleus of the hypothalamus significantly increased food intake. These results suggest that AMPK is regulated in the hypothalamus by hormones which regulate food intake. Furthermore, direct pharmacological activation of AMPK in the hypothalamus is sufficient to increase food intake. These findings demonstrate that AMPK plays a role in the regulation of feeding and identify AMPK as a novel target for anti-obesity drugs. AMP-activated protein kinase (AMPK)1 plays a pivotal role in the regulation of energy metabolism and has been dubbed a cellular fuel gauge (1). AMPK is activated following an increase in the AMP:ATP ratio within the cell that occurs following a decrease in ATP levels (2, 3). Once activated, AMPK switches on ATP-generating (catabolic) pathways, e.g. fatty acid oxidation, and switches off ATP-using pathways (anabolic) pathways, e.g. fatty acid synthesis, allowing the cell to restore its energy balance (2, 3). In addition to acute effects on metabolism, AMPK has more long term effects, altering both gene (4) and protein expression (5, 6). Recent results have demonstrated activation of AMPK in the absence of changes in adenine nucleotide levels, indicating that there may be multiple pathways upstream of AMPK (7,8). The molecular mechanisms leading to activation of AMPK have not been fully elucidated, but it is clear that activation of AMPK requires phosphorylation of threonine 172 (Thr 172 ) within the activation loop segment of the catalytic (␣) subunit (9, 10). Very recently, LKB1, a protein kinase that is inactivated in a hereditary form of cancer termed Peutz-Jeghers syndrome, was shown to account for most of the AMPK kinase activity in cell extracts (11,12) raising the possibility that AMPK could l...

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The central nervous system and gut peptide neuromedin U (NMU) inhibits feeding after intracerebroventricular injection. This study explored the hypothalamic actions of NMU on feeding and the hypothalamo-pituitary-adrenal axis. Intraparaventricular nucleus (intra-PVN) NMU dose-dependently inhibited food intake, with a minimum effective dose of 0.1 nmol and a robust effect at 0.3 nmol. Feeding inhibition was mapped by NMU injection into eight hypothalamic areas. NMU (0.3 nmol) inhibited food intake in the PVN (0-1 h, 59 +/- 6.9% of the control value; P < 0.001) and arcuate nucleus (0-1 h, 76 +/- 10.4% of the control value; P < 0.05). Intra-PVN NMU markedly increased grooming and locomotor behavior and dose-dependently increased plasma ACTH (0.3 nmol NMU, 24.8 +/- 1.9 pg/ml; saline, 11.4 +/- 1.0; P < 0.001) and corticosterone (0.3 nmol NMU, 275.4 +/- 40.5 ng/ml; saline, 129.4 +/- 25.0; P < 0.01). Using hypothalamic explants in vitro, NMU stimulated CRH (100 nM NMU, 5.9 +/- 0.95 pmol/explant; basal, 3.8 +/- 0.39; P < 0.01) and arginine vasopressin release (100 nM NMU, 124.5 +/- 21.8 fmol/explant; basal, 74.5 +/- 7.6; P < 0.01). Leptin stimulated NMU release (141.9 +/- 20.4 fmol/explant; basal, 92.9 +/- 9.4; P < 0.01). Thus, we describe a novel role for NMU in the PVN to stimulate the hypothalamo-pituitary-adrenal axis and locomotor and grooming behavior and to inhibit feeding.