Orexin

Orexin (/ɒˈrɛksɪn/), also known as hypocretin, is a neuropeptide that regulates arousal, wakefulness, and appetite. It exists in the forms of orexin-A and orexin-B. The most common form of narcolepsy, type 1, in which the individual experiences brief losses of muscle tone ("drop attacks" or cataplexy), is caused by a lack of orexin in the brain due to destruction of the cells that produce it.

There are 50,000–80,000 orexin-producing neurons in the human brain, located predominantly in the perifornical area and lateral hypothalamus. They project widely throughout the central nervous system, regulating wakefulness, feeding, and other behaviours. There are two types of orexin peptide and two types of orexin receptor.

Orexin was discovered in 1998 almost simultaneously by two independent groups of researchers working on the rat brain. 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, another peptide. Officially, hypocretin (HCRT) is used to refer to the genes and transcripts, while orexin is used to refer to the encoded peptides. There is considerable similarity between the orexin system in the rat brain and that in the human brain.