Orexin neuronal circuitry: role in the regulation of sleep and wakefulness.
Orexin A and orexin B were initially identified as endogenous ligands for two orphan G protein-coupled receptors [104]. They were initially recognized as regulators of feeding behavior in view of their exclusive production in the lateral hypothalamic area (LHA), a region known as the feeding center, and their pharmacological activity [104,30,49,107]. Subsequently, the finding that orexin deficiency causes narcolepsy in humans and animals suggested that these hypothalamic neuropeptides play a critical role in regulating sleep/wake cycle [22,46,71,95,117]. These peptides activate waking-active monoaminergic and cholinergic neurons in the hypothalamus/brain stem regions to maintain a long, consolidated awake period. Recent studies on efferent and afferent systems of orexin neurons, and phenotypic characterization of genetically modified mice in the orexin system further suggested roles of orexin in the coordination of emotion, energy homeostasis, reward system, and arousal [3,80,106,137]. A link between the limbic system and orexin neurons might be important for increasing vigilance during emotional stimuli. Orexin neurons are also regulated by peripheral metabolic cues, including ghrelin, leptin, and glucose, suggesting that they might have important roles as a link between energy homeostasis and vigilance states [137]. Recent research has also implicated orexins in reward systems and the mechanisms of drug addiction [13,48,91]. These observations suggest that orexin neurons sense the outer and inner environment of the body, and maintain proper wakefulness of animals for survival. This review discusses the mechanism by which orexins maintain sleep/wakefulness states, and how this mechanism relates to other systems that regulate emotion, reward, and energy homeostasis.
Ohno
Kousaku
K
Department of Pharmacology, Institute of Basic Medical Science, University of Tsukuba, Tsukuba, Ibaraki 305-8575, Japan.
Sakurai
Takeshi
T
D000595:Amino Acid Sequence; D000818:Animals; D001921:Brain / Q000378:metabolism / Q000502:physiology; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D008954:Models, Biological; D008969:Molecular Sequence Data; D009290:Narcolepsy / Q000235:genetics; D009474:Neurons / Q000378:metabolism* / Q000502:physiology*; D009479:Neuropeptides / Q000235:genetics / Q000378:metabolism* / Q000502:physiology*; D064446:Orexin Receptors; D000068797:Orexins; D043562:Receptors, G-Protein-Coupled / Q000235:genetics / Q000502:physiology; D018013:Receptors, Neuropeptide / Q000235:genetics / Q000502:physiology; D017386:Sequence Homology, Amino Acid; D015398:Signal Transduction / Q000502:physiology; D012890:Sleep / Q000235:genetics* / Q000502:physiology; D014851:Wakefulness / Q000235:genetics* / Q000502:physiology
C000596229:HCRT protein, human; D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; D064446:Orexin Receptors; D000068797:Orexins; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide