Orexin, the wake-promoting peptide that was named after hunger
Orexin (hypocretin) is a brain peptide that stabilizes wakefulness. Found in 1998, lost in narcolepsy, and blocked by three prescription insomnia drugs.
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A Doberman carrying the wrong version of one receptor gene can be in the middle of an excited game, or about to eat, and collapse into sleep. Lin and colleagues traced that canine narcolepsy to a receptor gene in a 1999 Cell paper, and the receptor pointed at the molecule that fits it: orexin.
Orexin, also called hypocretin, is a pair of peptides made by a small cluster of neurons in the hypothalamus. Its main job is to keep wakefulness stable once you are up. The time you wake is set elsewhere.
Why does one molecule have two names?
Two groups found it independently in 1998. One, led by Sakurai and Yanagisawa, injected the peptides into the brains of rats, saw the animals eat more, and named them orexins after the Greek orexis, appetite. The other named them hypocretins, because they are made in the hypothalamus and resemble secretin. The appetite link is why the name sounds like a hunger hormone. The sleep link turned out to matter far more.
What happens when orexin goes missing?
In 1999 a Yanagisawa-lab paper in Cell reported that mice without the orexin gene behaved like narcoleptic animals. In 2000, Thannickal and colleagues counted hypocretin neurons in post-mortem human brains and found 85 to 95 percent fewer of them in narcolepsy. That finding is why narcolepsy type 1 is now understood as a disease of lost orexin cells. What destroys them is still debated, and an autoimmune attack is widely suspected.
Why does orexin matter outside a sleep clinic?
Because the newest insomnia drugs work by switching it off. Suvorexant (Belsomra, approved in 2014), lemborexant (Dayvigo, 2019) and daridorexant (Quviviq, 2022) are dual orexin receptor antagonists. They block both orexin receptors, which turns down the wake signal. Every one is a prescription drug and a controlled substance. The larger lesson is that “awake” is something the brain actively holds up. Adenosine, the molecule behind the opposing pressure to sleep, gets its own explainer, and neighboring terms like sleep latency and sleep inertia sit in a sleep medicine glossary.
Frequently asked questions
Are orexin and hypocretin the same thing?
Yes. Orexin-A and orexin-B are the same molecules as hypocretin-1 and hypocretin-2, and both are cut from one larger precursor protein.
How big was the 2000 narcolepsy study?
Thannickal and colleagues counted cells in 16 human brains, four from people with narcolepsy and 12 from controls. Neighboring melanin-concentrating hormone neurons were not depleted, which pointed to a loss specific to hypocretin cells.
Which receptors do the orexin-blocking drugs block?
They block the OX1 and OX2 receptors; OX2 is the receptor whose mutation caused canine narcolepsy.