## Opioid Receptor Pharmacology **Key Point:** The mu (μ) receptor is the primary target for all clinically relevant opioid analgesics and is the receptor subtype most strongly associated with addiction potential. ### Receptor Subtypes and Their Functions | Receptor | Location | Primary Effects | Addiction Relevance | |----------|----------|-----------------|--------------------| | **Mu (μ)** | Periaqueductal gray, nucleus accumbens, ventral tegmentum | Analgesia, euphoria, respiratory depression, physical dependence | **Highest** — drives reward and addiction | | **Delta (δ)** | Limbic system, spinal cord | Analgesia, anxiolysis | Minimal addiction potential | | **Kappa (κ)** | Hypothalamus, brainstem | Analgesia, dysphoria, anti-reward | **Negative** — produces aversive effects | | **Nociceptin** | Spinal cord, brain | Analgesia, anxiolysis | Not clinically significant | ### Mechanism of Mu-Mediated Addiction 1. Mu receptor activation in the ventral tegmentum → dopamine release in nucleus accumbens 2. Dopamine surge → reinforcement and reward learning 3. Repeated activation → neuroadaptation and tolerance 4. Withdrawal upon discontinuation → physical dependence **High-Yield:** All FDA-approved opioid medications (morphine, heroin, fentanyl, codeine, tramadol) exert their therapeutic AND addictive effects primarily through mu receptor agonism. **Clinical Pearl:** Kappa receptor agonists (e.g., butorphanol, nalbuphine) produce analgesia WITHOUT euphoria and have lower addiction potential, making them useful in patients with opioid use disorder who still require analgesia. **Mnemonic:** **MUD** — **M**u receptor = **U**phoria and **D**ependence (addiction).
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