Chronic Central Infusion of Ghrelin Increases Hypothalamic Neuropeptide Y and Agouti-Related Protein mRNA Levels and Body Weight in Rats

Diabetes, Nov 2001

Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor (GHS-R), was originally purified from the rat stomach. Like the synthetic growth hormone secretagogues (GHSs), ghrelin specifically releases growth hormone (GH) after intravenous administration. Also consistent with the central actions of GHSs, ghrelin-immunoreactive cells were shown to be located in the hypothalamic arcuate nucleus as well as the stomach. Recently, we showed that a single central administration of ghrelin increased food intake and hypothalamic agouti-related protein (AGRP) gene expression in rodents, and the orexigenic effect of this peptide seems to be independent of its GH-releasing activity. However, the effect of chronic infusion of ghrelin on food consumption and body weight and their possible mechanisms have not been elucidated. In this study, we determined the effects of chronic intracerebroventricular treatment with ghrelin on metabolic factors and on neuropeptide genes that are expressed in hypothalamic neurons that have been previously shown to express the GHS-R and to regulate food consumption. Chronic central administration of rat ghrelin (1 μg/rat every 12 h for 72 h) significantly increased food intake and body weight. However, it did not affect plasma insulin, glucose, leptin, or GH concentrations. We also found that chronic central administration of ghrelin increased both neuropeptide Y (NPY) mRNA levels (151.0 ± 10.1% of saline-treated controls; P < 0.05) and AGRP mRNA levels (160.0 ± 22.5% of saline-treated controls; P < 0.05) in the arcuate nucleus. Thus, the primary hypothalamic targets of ghrelin are NPY/AGRP-containing neurons, and ghrelin is a newly discovered orexigenic peptide in the brain and stomach.

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Chronic Central Infusion of Ghrelin Increases Hypothalamic Neuropeptide Y and Agouti-Related Protein mRNA Levels and Body Weight in Rats

Jun Kamegai 0 Hideki Tamura 0 Takako Shimizu 0 Shinya Ishii 0 Hitoshi Sugihara 0 Ichiji Wakabayashi 0 0 From the Department of Medicine, Nippon Medical School , Tokyo , Japan. of Medicine, Nippon Medical School , 1-1-5 Sendagi, Bunkyo-Ku, Tokyo 113- 8603 , Japan Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor (GHS-R), was originally purified from the rat stomach. Like the synthetic growth hormone secretagogues (GHSs), ghrelin specifically releases growth hormone (GH) after intravenous administration. Also consistent with the central actions of GHSs, ghrelin-immunoreactive cells were shown to be located in the hypothalamic arcuate nucleus as well as the stomach. Recently, we showed that a single central administration of ghrelin increased food intake and hypothalamic agouti-related protein (AGRP) gene expression in rodents, and the orexigenic effect of this peptide seems to be independent of its GH-releasing activity. However, the effect of chronic infusion of ghrelin on food consumption and body weight and their possible mechanisms have not been elucidated. In this study, we determined the effects of chronic intracerebroventricular treatment with ghrelin on metabolic factors and on neuropeptide genes that are expressed in hypothalamic neurons that have been previously shown to express the GHS-R and to regulate food consumption. Chronic central administration of rat ghrelin (1 g/rat every 12 h for 72 h) significantly increased food intake and body weight. However, it did not affect plasma insulin, glucose, leptin, or GH concentrations. We also found that chronic central administration of ghrelin increased both neuropeptide Y (NPY) mRNA levels (151.0 10.1% of saline-treated controls; P < 0.05) and AGRP mRNA levels (160.0 22.5% of saline-treated controls; P < 0.05) in the arcuate nucleus. Thus, the primary hypothalamic targets of ghrelin are NPY/AGRPcontaining neurons, and ghrelin is a newly discovered orexigenic peptide in the brain and stomach. Diabetes 50:2438 -2443, 2001 - nergy intake and expenditure are tightly regulated in mammals (1). Several neuronal populations, particularly in the hypothalamic arcuate nucleus (ARC), are involved in the regulation of energy homeostasis and have been implicated as possible targets of orexigenic peptides (1). These include neuropeptide Y (NPY)/agouti-related protein (AGRP) coexpressing neurons, pro-opiomelanocortin (POMC) neurons, and growth hormonereleasing hormone (GHRH) neurons, which are known to be stimulated (NPY and AGRP) or suppressed (POMC and GHRH) by starvation (210). However, central administration of NPY, AGRP, and GHRH increased food intake in rats, whereas -melanocytestimulating hormone (-MSH), an end product of POMC processing, decreased food intake when injected centrally (1113). The growth hormone secretagogues (GHSs) are synthetic peptide and nonpeptide compounds that stimulate the pulsatile release of growth hormone (GH) after intravenous administration (14). Central administration of GHSs also stimulates feeding behavior (15). GHS receptor (GHS-R) mRNA is expressed in the pituitary gland and in several areas of the brain, including the hypothalamus (16). In the hypothalamus, GHS-R mRNA is expressed in NPY/AGRP-, POMC-, and GHRH-containing neurons (17 19). Ghrelin, an endogenous ligand for the GHS-R, was purified from the rat stomach (20). Similar to the synthetic GHSs, ghrelin specifically releases GH after intravenous administration. Recently, we showed that a single central administration of ghrelin increased food intake and hypothalamic AGRP gene expression in rodents, and the orexigenic effect of this peptide seems to be independent of its GH-releasing activity (21). Thus, ghrelin has an alternative role in stimulating food intake in addition to releasing GH from the pituitary. It has been reported that previous exposure to GHS can induce desensitization of the subsequent GH responses to GHS, indicating that homologous desensitization is observed after continuous exposure to GHS with respect to GH release (22,23). However, the effect of chronic infusion of ghrelin on food consumption and body weight and their possible mechanisms has not been elucidated. In this study, we determined the effects of chronic intracerebroventricular treatment with ghrelin on metabolic factors and on the hypothalamic NPY, AGRP, POMC, and GHRH mRNA levels using radioactive in situ hybridization histochemistry. RESEARCH DESIGN AND METHODS Animals. Male Sprague-Dawley rats (250 280 g) were housed in air-conditioned animal quarters, with lights on between 08:00 and 20:00 h, and were given food and water ad libitum. The animals were anesthetized with ether, and a 23-gauge stainless-steel cannula was implanted into the right lateral ventricle using a stereotaxic apparatus, as previously described (24). The upper incisor bar was set 3.3 mm below the interauricular line, and the bregma was taken as A-P zero. The cannula tip was placed at A 0 (...truncated)


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Jun Kamegai, Hideki Tamura, Takako Shimizu, Shinya Ishii, Hitoshi Sugihara, Ichiji Wakabayashi. Chronic Central Infusion of Ghrelin Increases Hypothalamic Neuropeptide Y and Agouti-Related Protein mRNA Levels and Body Weight in Rats, Diabetes, 2001, pp. 2438-2443, 50/11, DOI: 10.2337/diabetes.50.11.2438