OPRK1 and major depressive disorder: The activation of KOR by endogenous peptide or exogenous synthetic agonists is associated with behavioral and mood effects, including analgesia, sedation, and perceptual distortions [11,12,13], while antagonists binding at the same site block the activation of KOR; thus, they may be used for treatment of depression, anxiety, addictive disorders, and other psychiatric conditions [14,15].