Which opioid receptor subtype is primarily responsible for the analgesic and euphoric effects that drive opioid use disorder?

Explanation

## Opioid Receptor Pharmacology **Key Point:** The μ (mu) receptor is the primary target for clinically-used opioids and is responsible for both therapeutic (analgesia) and addictive (euphoria, reinforcement) effects. ### Receptor Subtypes and Their Functions | Receptor | Location | Primary Effects | Role in OUD | |----------|----------|-----------------|-------------| | **μ (mu)** | Periaqueductal gray, nucleus accumbens, ventral tegmental area | Analgesia, euphoria, respiratory depression, physical dependence | **PRIMARY — drives addiction** | | δ (delta) | Limbic system, spinal cord | Mild analgesia, dysphoria | Minor role; less reinforcing | | κ (kappa) | Hypothalamus, brainstem | Dysphoria, sedation, analgesia | Aversive; not addictive | | σ (sigma) | Not a true opioid receptor; binds PCP, dextromethorphan | Dissociative, hallucinogenic | Not involved in OUD | **High-Yield:** μ-receptor activation in the ventral tegmental area (VTA) and nucleus accumbens triggers dopamine release — the final common pathway for reward and addiction across all substance use disorders. **Clinical Pearl:** Naloxone and naltrexone are μ-receptor antagonists used to reverse opioid overdose and maintain abstinence in OUD treatment, confirming the centrality of the μ-receptor in opioid pharmacology. **Mnemonic:** **MUD** = **M**u receptor drives **U**se **D**isorder (μ → addiction).