Age-related insulin resistance in hypothalamus and peripheral tissues of orexin knockout mice.
OBJECTIVE
Orexin/hypocretin is a hypothalamic neuropeptide that regulates motivated behaviours, such as feeding and arousal, and, importantly, is also involved in energy homeostasis. The aim of this study was to reveal the role of orexin in the regulation of insulin sensitivity for glucose metabolism.


METHODS
Orexin knockout mice fasted overnight underwent oral glucose tolerance testing and insulin tolerance testing. The impact of orexin deficiency on insulin signalling was studied by Western blotting to measure levels of Akt phosphorylation and its upstream and downstream molecules in the hypothalamus, muscle and liver in orexin knockout mice.


RESULTS
We found that orexin deficiency caused the age-related development of impaired glucose tolerance and insulin resistance in both male mice without obesity and female mice with mild obesity, fed a normal chow diet. When maintained on a high-fat diet, these abnormalities became more pronounced exclusively in female orexin knockout mice that developed severe obesity. Insulin signalling through Akt was disrupted in peripheral tissues of middle-aged (9-month-old) but not young adult (2-to-3-month-old) orexin knockout mice fed a normal chow diet. Moreover, basal levels of hypothalamic Akt phosphorylation were abnormally elevated in orexin knockout mice at every age studied, and insulin stimulation failed to increase the level of phosphorylation. Similar abnormalities were observed with respect to GSK3beta phosphorylation in the hypothalamus and peripheral tissues of middle-aged orexin knockout mice.


CONCLUSIONS
Our results demonstrate a novel role for orexin in hypothalamic insulin signalling, which is likely to be responsible for preventing the development of peripheral insulin resistance with age.
Tsuneki
H
H
Department of Clinical Pharmacology, University of Toyama, 2630 Sugitani, Toyama 930-0194, Japan. htsuneki@pha.u-toyama.ac.jp
Murata
S
S
Anzawa
Y
Y
Soeda
Y
Y
Tokai
E
E
Wada
T
T
Kimura
I
I
Yanagisawa
M
M
Sakurai
T
T
Sasaoka
T
T
D000375:Aging / Q000502:physiology; D000818:Animals; D001786:Blood Glucose / Q000378:metabolism; D018149:Glucose Intolerance / Q000235:genetics*; D005951:Glucose Tolerance Test; D007031:Hypothalamus / Q000502:physiology* / Q000503:physiopathology; D007333:Insulin Resistance / Q000235:genetics* / Q000502:physiology; D047908:Intracellular Signaling Peptides and Proteins / Q000172:deficiency*; D007700:Kinetics; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009479:Neuropeptides / Q000172:deficiency*; D000068797:Orexins
D001786:Blood Glucose; D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; D000068797:Orexins