Orexin neurons function in an efferent pathway of a food-entrainable circadian oscillator in eliciting food-anticipatory activity and wakefulness.

Temporal restriction of feeding can entrain circadian behavioral and physiological rhythms in mammals. Considering the critical functions of the hypothalamic orexin (hypocretin) neuropeptides in promoting wakefulness and locomotor activity, we examined the role of orexin neurons in the adaptation to restricted feeding. In orexin neuron-ablated transgenic mice, the food-entrained rhythmicity of mPer2 expression in the brain and liver, the reversal of the sleep-wake cycle, and the recovery of daily food intake were unaltered compared with wild-type littermates. In contrast, orexin neuron-ablated mice had a severe deficit in displaying the normal food-anticipatory increases in wakefulness and locomotor activity under restricted feeding. Moreover, activity of orexin neurons markedly increased during the food-anticipatory period under restricted feeding in wild-type mice. Orexin neurons thus convey an efferent signal from putative food-entrainable oscillator or oscillators to increase wakefulness and locomotor activity.