Abstract
Orexins are neuropeptides that are postulated to play a central role in the regulation of the sleep-wake cycle, appetite, affect, and reward circuitry. The objectives of the current review are to comprehensively evaluate (1) the potential role of orexins in the pathophysiology of major depressive disorders (MDD) and (2) the orexin system as a novel target in the treatment of MDD. Dysfunction of the sleep-wake cycle is observed as a central feature of MDD pathophysiology. Orexin system disturbances produce sleep-wake dysfunction, as observed in MDD. Orexin antagonists have been shown to treat insomnia effectively without disrupting normal sleep architecture in both preclinical (e.g., animal models) and clinical studies. Orexin antagonists are generally safe, well-tolerated, and associated with an acceptable long-term adverse effect profile with relatively low propensity for tolerance or dependence. Orexin antagonists have also been shown to possess antidepressant-like properties in some animal models of MDD. Extant evidence indicates that orexin-modulating treatments exert pleiotropic effects on multiple neural systems implicated in the phenomenology of mood disorders and suggests orexins as a promising target for investigation and intervention in mood disorders. To date, no human clinical trials evaluating the antidepressant effects of orexin antagonists in MDD have been completed. Given the promising results from preclinical studies, clinical trials are merited to evaluate the antidepressant effects of orexin antagonists in MDD.