orexin receptors, member of the class A family of seven-transmembrane G protein-coupled receptors
Orexins (OXs, also referred to as hypocretins) are neuropeptide hormones that regulate the sleep-wake cycle and potently influence homeostatic systems regulating appetite and feeding behavior or modulating emotional responses such as anxiety or panic. OXs are synthesized as prepro-orexin (PPO) in the hypothalamus and then proteolytically cleaved into two forms of isoforms: orexin-A (OX-A) and orexin-B (OX-B). OXA is a 33 amino-acid peptide with N-terminal pyroglutamyl residue and two intramolecular disulfide bonds, whereas OXB is a 28 amino-acid linear peptide with no disulfide bonds. OX-A binds orexin receptor 1 (OX1R) with high-affinity, but also binds with somewhat low-affinity to OX2R, and signals primarily to Gq coupling, whereas OX-B shows a strong preference for the orexin receptor 2 (OX2R) and signals through Gq or Gi/o coupling. Thus, activation of OX1R or OX2R will activate phospholipase activity and the phosphatidylinositol and calcium signaling pathways. Additionally, OX2R activation can also lead to inhibition of adenylate cyclase.