## Opioid Receptor Pharmacology **Key Point:** The mu (μ) receptor is the primary opioid receptor mediating both analgesia and reward/reinforcement, making it the principal target for opioid dependence and addiction. ### Receptor Subtypes and Functions | Receptor | Primary Effects | Role in OUD | |----------|-----------------|-------------| | **Mu (μ)** | Analgesia, euphoria, respiratory depression, physical dependence | **Primary target for abuse and dependence** | | Delta (δ) | Analgesia (minor), dysphoria | Minimal role in addiction | | Kappa (κ) | Dysphoria, analgesia (spinal), aversion | Protective against reward | | Nociceptin (ORL-1) | Pain modulation, stress response | Not involved in opioid dependence | **High-Yield:** Mu receptor activation in the ventral tegmental area (VTA) and nucleus accumbens drives dopamine release, creating the reinforcing "high" that perpetuates opioid use disorder. This is why mu-selective agonists (methadone, buprenorphine) are effective for maintenance therapy — they occupy the mu receptor and prevent withdrawal while blocking euphoria from illicit opioids. **Clinical Pearl:** Naloxone and naltrexone are non-selective mu antagonists; they precipitate acute withdrawal by displacing opioids from mu receptors. Buprenorphine is a partial mu agonist, which is why it has a "ceiling effect" on respiratory depression — a safety advantage over full agonists. **Mnemonic:** **MU = MAIN UNIT** — Mu receptor is the main unit of opioid reward, analgesia, and dependence.
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