POMC and major depressive disorder: Studies have shown significantly increased plasma levels of cortisol (CORT), adrenocorticotropic hormone (ACTH), and corticotropin-releasing hormone (CRH) in a rat depression model, as well as significantly increased expression levels of interleukin (IL) 1β and IL-6 mRNAs in the hippocampus but reduced 5-hydroxytryptamine (5-HT) levels [5, 6].