Orexin, also called hypocretin, is a neurotransmitter 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.
The brain contains very few cells that produce orexin: in a human brain, about 10,000 to 20,000 neurons in the 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 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 hypothalamus and bears a weak resemblance to secretin, a hormone found in the gut. The scientific community has not yet settled on a consensus for which word to use.
Read more about Orexin: Isoforms, Function, History and Nomenclature, Selective Ligands, Interaction With Other Neurotransmitter Systems