A 52-year-old man with advanced pancreatic cancer is on morphine 30 mg BD for cancer pain. Regarding opioid receptor pharmacology, all of the following statements are correct EXCEPT:

## Opioid Receptor Subtypes and Their Functions ### Mu (μ) Receptors **Key Point:** Mu receptors are the PRIMARY therapeutic target for opioid analgesia and are responsible for most clinically relevant opioid effects. - Analgesia (spinal and supraspinal) - Euphoria and reward - Respiratory depression - Miosis (pinpoint pupils) - Constipation - Physical dependence ### Delta (δ) Receptors **High-Yield:** Delta receptors have minimal clinical significance in opioid analgesia. They are found in the CNS but their activation does NOT produce clinically meaningful analgesia in humans. - Modulate mu receptor signaling - Involved in mood and emotion - NOT a major contributor to therapeutic analgesia ### Kappa (κ) Receptors **Key Point:** Kappa receptor activation produces: - Analgesia (especially visceral pain) - Dysphoria and anxiety - Miosis - Diuresis - **Respiratory depression is minimal** (advantage over mu agonists) ### Sigma (σ) Receptors **Clinical Pearl:** Sigma receptors are NOT true opioid receptors (not blocked by naloxone). They mediate: - Dysphoria - Hallucinations - Psychotomimetic effects ### Comparative Table | Receptor | Analgesia | Respiratory Depression | Euphoria | Dysphoria | Clinical Use | |----------|-----------|------------------------|----------|-----------|---------------| | Mu (μ) | ✓✓✓ Strong | ✓✓✓ Severe | ✓✓ Yes | ✗ No | Primary target | | Delta (δ) | ✗ Minimal | ✗ Minimal | ✗ No | ✗ No | Not clinically used | | Kappa (κ) | ✓ Moderate | ✗ Minimal | ✗ No | ✓ Yes | Pentazocine, nalbuphine | | Sigma (σ) | ✗ No | — | ✗ No | ✓✓ Yes | Not opioid receptor | **Warning:** Delta receptors are often confused with mu receptors in exams. Remember: delta activation does NOT produce analgesia in clinical opioid therapy — morphine's analgesic effect is almost entirely mu-mediated. ## Why the Marked Answer is Wrong Option 1 (Delta receptors) is the INCORRECT statement. Delta receptors do NOT mediate the analgesic effects of morphine in clinical practice. Morphine is a **mu-selective agonist** with negligible delta affinity. All therapeutic analgesia from morphine comes from mu receptor activation.