FIGURE 3.

Simplified schematic representation of the orexin and MCH systems as a sensor of metabolic changes and arousal. In negative energy balance, low extracellular glucose concentration and high circulating level of ghrelin activate the orexin system but inhibit MCH neurons leading to an increase in orexin release and a decrease in MCH release to promote wakefulness, activity, foraging, and food intake. By contrast, in positive energy balance, high extracellular glucose concentration activates MCH neurons but suppresses orexin neurons which are also inhibited by the circulating level of leptin promoting sleep and decreasing energy expenditure. Orx, orexin neurons; MCH, melanin-concentrating hormone neurons.