Energy expenditure by intracerebroventricular administration of orexin to anesthetized rats.

Orexin-A and -B are hypothalamic neuropeptides that have been implicated in stimulating food intake and maintaining arousal. Because food intake is closely related to the control of energy homeostasis, we examined the effects of intracerebroventricular administration of orexins on O2 consumption (VO2), an index of energy expenditure, body temperature, skin temperature and heart rate (HR) in urethane-anesthetized rats. VO2 increased significantly after an orexin-A injection, and this increase was accompanied by a significant tachycardiac response. Orexin-B also increased VO2 and HR, although orexin-A was approximately 30 times more potent in eliciting these responses than orexin-B. The effects of orexin-A were dose dependent over the range of 1 pmol(-1) x nmol, whereas an injection of the saline vehicle had no effect. These findings suggest that centrally acting orexin-A functions to increase energy expenditure.