Orexin 2 receptor regulation of the hypothalamic-pituitary-adrenal (HPA) response to acute and repeated stress

Orexins are hypothalamic neuropeptides that have a documented role in mediating the acute stress response. However, their role in habituation to repeated stress, and the role of orexin receptors (OX₁R and OX₂R) in the stress response, has yet to be defined. Orexin neuronal activation and levels in the cerebrospinal fluid (CSF) were found to be stimulated with acute restraint, but were significantly reduced by day five of repeated restraint. As certain disease states such as panic disorder are associated with increased central orexin levels and failure to habituate to repeated stress, the effect of activating orexin signaling via Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) on the hypothalamic-pituitary-adrenal (HPA) response was evaluated after repeated restraint. While vehicle-treated rats displayed habituation of Adrenocorticotropic Hormone (ACTH) from day 1 to day 5 of restraint, stimulating orexins did not further increase ACTH beyond vehicle levels for either acute or repeated restraint. We delineated the roles of orexin receptors in acute and repeated stress using a selective OX₂R antagonist (MK-1064). Pretreatment with MK-1064 reduced day 1 ACTH levels, but did not allow further habituation on day 5 compared with vehicle-treated rats, indicating that endogenous OX₂R activity plays a role in acute stress, but not in habituation to repeated stress. However, in restrained rats with further stimulated orexins by DREADDs, MK-1064 decreased ACTH levels on day 5. Collectively, these results indicate that the OX₂R plays a role in acute stress, and can prevent habituation to repeated stress under conditions of high orexin release.