## Opioid Receptor Pharmacology **Key Point:** The μ (mu) opioid receptor is the primary mediator of both the therapeutic (analgesia) and reinforcing (euphoria, reward) effects of opioids. ### Receptor Subtypes and Functions | Receptor | Primary Effects | Role in OUD | |----------|-----------------|-------------| | μ (mu) | Analgesia, euphoria, respiratory depression, miosis, constipation | **Main target for abuse and dependence** | | δ (delta) | Modulation of μ effects, analgesia, mood regulation | Minimal role in addiction | | κ (kappa) | Dysphoria, aversion, analgesia | Protective against addiction; used in buprenorphine | | σ (sigma) | Psychotomimetic effects | Not a true opioid receptor | **High-Yield:** The μ receptor exists in three subtypes (μ1, μ2, μ3), with μ1 primarily responsible for analgesia and μ2 for respiratory depression and constipation. However, all clinically significant opioid abuse involves μ receptor activation. **Clinical Pearl:** Buprenorphine is a partial μ-agonist with κ-antagonist activity, which explains its lower abuse potential compared to full μ-agonists like heroin or morphine. **Mnemonic:** **MUD** = **M**u receptor = **U**phoria + **D**ependence (the addictive opioid receptor).
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