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
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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
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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)