Input and output systems of orexin neurons

Orexin/hypocretin neurons are localized in the lateral hypothalamic area (LH)/perifornical area (PF) and project their fibers widely to many regions in the brain. Two types of orexin receptors, OX1 and OX2, are expressed in various brain areas, including neurons controlling fundamental functions in the brain, such as sleep/wake behavior, feeding and drinking, homeostatic regulation, neuroendocrine and autonomic responses. In this chapter, we summarize the input and output pathways of orexin neurons in the rodent brain and discuss their physiological roles from an anatomical point of view. We then introduce recent publications of mouse studies, demonstrating functional evidence of these neural pathways from/to orexin neurons especially for controlling sleep/wake behavior. These new, integrated anatomical-physiological evidences well document the important circuitry of orexin neurons in the mammalian brain.