The orexin/hypocretin peptides orexin A and orexin B (also known as hcrt-1 and hcrt-2) are 33- and 28-amino acid peptides, respectively. They are preferentially expressed in hypothalamus. The orexins have a range of physiological functions including feeding control, energy metabolism, neuroendocrine function modulation, and sleep-wake cycle regulation. The two orexin receptor subtypes OX1 and OX2 both mediate the action of orexin-A and orexin-B and couple efficiently through Gq/11 to activate phospholipase C and lead to elevation of intracellular calcium.