International Union of Basic and Clinical Pharmacology. LXXXVI. Orexin receptor function, nomenclature and pharmacology.

Orexin signaling is essential for normal regulation of arousal and behavioral state control and represents an attractive target for therapeutics combating insomnia. Alternatively termed hypocretins, these neuropeptides were named to reflect sequence similarity to incretins and their potential to promote feeding. Current nomenclature reflects these molecular and biochemical discovery approaches in which HCRT, HCRTR1, and HCRTR2 genes encode prepro-orexin, the orexin 1 receptor (OX(1)) and the orexin 2 receptor (OX(2))-gene names designated by the Human Genome Organization and receptor names designated by the International Union of Basic and Clinical Pharmacology. Orexinergic neurons are most active during wakefulness and fall silent during inactive periods, a prolonged disruption in signaling most profoundly resulting in hypersomnia and narcolepsy. Hcrtr2 mutations underlie the etiology of canine narcolepsy, deficiencies in orexin-producing neurons are observed in the human disorder, and ablation of mouse orexin neurons or the Hcrt gene results in a narcolepsy-cataplexy phenotype. The development of orexin receptor antagonists and genetic models targeting components of the orexin pathway have elucidated the OX(2) receptor-specific role in histamine-mediated arousal and the contribution of both receptors in brainstem pathways involved in vigilance state gating. Orexin receptor antagonists of varying specificity uncovered additional roles beyond sleep and feeding that include addiction, depression, anxiety, and potential influences on peripheral physiology. Combined genetic and pharmacological approaches indicate that orexin signaling may represent a confluence of sleep, feeding, and reward pathways. Selective orexin receptor antagonism takes advantage of these properties toward the development of novel insomnia therapeutics.