Orexin A is a neuropeptide hormone released by the hypothalamus that excites neurons and affects hunger and wakefulness by modulating the dopamine, norepinephrine, histamine, and acetylcholine systems. Lack of orexin A tends to result in narcolepsy. Potent orexin receptor agonist (Ki=20 and 38nM for OX1 and OX2, respectively).