Which opioid receptor subtype mediates analgesia and respiratory depression?

Explanation

## Opioid Receptor Subtypes and Their Functions **Key Point:** The μ (mu) receptor is the primary opioid receptor responsible for both the therapeutic analgesic effects AND the major adverse effects of opioids, including respiratory depression, physical dependence, and euphoria. ### Mu Receptor Characteristics - **Location:** Central nervous system (brainstem, spinal cord, cortex), peripheral tissues - **Effects mediated:** - Analgesia (supraspinal and spinal) - Respiratory depression (chemoreceptor trigger zone) - Euphoria and reward - Physical dependence and tolerance - Miosis (pinpoint pupils) - Decreased GI motility (constipation) ### Comparison of Opioid Receptors | Receptor | Primary Effects | Clinical Significance | |----------|-----------------|----------------------| | **μ (Mu)** | Analgesia, respiratory depression, euphoria, dependence | Therapeutic target; causes major toxicity | | **δ (Delta)** | Analgesia (mild), dysphoria, seizures | Minimal clinical use; dysphoric effects limit utility | | **κ (Kappa)** | Analgesia, dysphoria, diuresis | Less respiratory depression; dysphoria limits use | | **σ (Sigma)** | Dysphoria, hallucinations, psychotomimetic effects | Not a true opioid receptor; blocked by naloxone | **High-Yield:** The μ receptor is the ONLY opioid receptor that produces both the desired analgesic effect AND the dangerous respiratory depression — this is why μ-selective opioids (morphine, fentanyl, remifentanil) are potent but require careful titration and monitoring in anesthesia. **Clinical Pearl:** Respiratory depression from μ-receptor agonism occurs at the level of the chemoreceptor trigger zone and respiratory centers in the medulla, resulting in decreased CO₂ responsiveness and blunted ventilatory drive — this is the most life-threatening adverse effect in perioperative settings. **Mnemonic:** **MUD** = **M**u receptor causes **U**tility (analgesia) and **D**anger (respiratory depression).