Initial evidence from animal data has demonstrated distinct functions of the opioid receptor subtypes, MOR, DOR, KOR, and NOP receptor, in regulating mood-related processes including reward, motivation and social behaviours, with pharmacological studies suggesting antidepressant potential of MOR/DOR agonist and KOR/NOP receptor antagonist compounds in rodent models of depression. This evidence concerns the gene OPRM1 and major depressive disorder.