Which receptor is primarily linked to mood, pain, and reward?

• A. Mu
• B. Delta
• C. Kappa
• D. Sigma

Answer: (A)

The key idea is that the mu opioid receptor is the primary mediator of analgesia, mood effects, and reward. When activated by endogenous opioids or drugs like morphine, mu receptors produce strong pain relief by inhibiting pain pathways, while also enhancing mood and reinforcing use through effects on the brain’s reward system. Mechanistically, mu receptors are Gi/o-coupled, which lowers cAMP, opens potassium channels, and closes calcium channels, dampening neuronal excitability and neurotransmitter release. In the brain’s reward circuit, mu activation disinhibits dopamine neurons in the ventral tegmental area, increasing dopamine in the nucleus accumbens, a core mechanism underlying euphoria and reinforcement. Delta receptors can contribute to analgesia and some mood effects, and kappa receptors can produce analgesia but often cause dysphoria and aversion, making them less associated with mood elevation and reward. Sigma receptors are linked to different pharmacological effects and are not the primary drivers of the mood–pain–reward triad.