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Orexin, also called hypocretin, is a neuropeptide that regulates arousal, wakefulness, and appetite. The most common form of narcolepsy, in which the sufferer briefly loses muscle tone (cataplexy), is caused by a lack of orexin in the brain due to destruction of the cells that produce it.〔Stanford Center for Narcolepsy (FAQ ) (retrieved 27-Mar-2012)〕 The brain contains very few cells that produce orexin; there are about 10,000–20,000 orexin neurons in the human brain, which project out of the nucleus in the lateral hypothalamus.〔〔 However, the axons from these neurons extend throughout the entire brain and spinal cord, where there are also receptors for orexin. Orexin was discovered in 1998 almost simultaneously by two independent groups of rat-brain researchers. One group named it orexin, from ''orexis,'' meaning "appetite" in Greek; the other group named it hypocretin, because it is produced in the ''hypo''thalamus and bears a weak resemblance to ''secretin'', another peptide.〔 The scientific community has not yet settled on a consensus for which word to use. == Isoforms == There are two types of orexin: orexin-A and -B (hypocretin-1 and -2). They are excitatory neuropeptides with approximately 50% sequence identity, produced by cleavage of a single precursor protein. Orexin-A is 33 amino acid residues long and has two intrachain disulfide bonds; orexin-B is a linear 28 amino acid residue peptide. Studies suggest that orexin-A may be of greater biological importance than orexin-B. Although these peptides are produced by a very small population of cells in the lateral and posterior hypothalamus, they send projections throughout the brain. The orexin peptides bind to the two G-protein coupled orexin receptors, OX1 and OX2, with orexin-A binding to both OX1 and OX2 with approximately equal affinity while orexin-B binds mainly to OX2 and is 5 times less potent as OX1. The orexins are strongly conserved peptides, found in all major classes of vertebrates. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Orexin」の詳細全文を読む スポンサード リンク
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