OPRK1 and major depressive disorder: Notably, it has been suggested that the endogenous opioid system is directly involved in the regulation of mood and the dysregulation of that system may factor into depression and anxiety11,51,52; μ-opioid receptors (MOR) are widely distributed within the brain, including regions involved in emotion regulation,11 and κ-opioid receptors (KOR) are expressed in the cortex, striatum, hippocampus, amygdala, and thalamus,53 suggesting a role in reward, pain, and emotion.