Regarding opioid receptors and their pharmacology, all of the following statements are correct EXCEPT:

Explanation

## Opioid Receptor Classification and Functions ### Mu (μ) Receptors **Key Point:** μ-receptors are the primary targets for most clinically used opioids and mediate: - Supraspinal analgesia - Euphoria and reward - Respiratory depression - Physical dependence - Miosis (pinpoint pupils) ### Delta (δ) Receptors **Key Point:** δ-receptors contribute to: - Spinal analgesia (synergistic with μ) - Minimal supraspinal analgesic effects - Dysphoria at higher doses - Modulation of μ-receptor signaling ### Kappa (κ) Receptors **Key Point:** κ-receptors produce: - Spinal and supraspinal analgesia - Dysphoria and hallucinations (unlike μ) - Visceral pain modulation - Miosis and sedation - No respiratory depression (advantage over μ-agonists) ### Sigma (σ) Receptors — The Key Distinction **High-Yield:** Sigma receptors are **NOT true opioid receptors** because: 1. They are **NOT blocked by naloxone** (the universal opioid antagonist) 2. They do **NOT mediate analgesia** from morphine or other classical opioids 3. They are involved in dysphoria, hallucinations, and psychotomimetic effects 4. They are now classified as **non-opioid receptors** (distinct from the opioid receptor family) **Warning:** Sigma receptors were historically thought to be opioid receptors, but modern pharmacology has reclassified them. This is a common NEET PG trap. ### Receptor Comparison Table | Receptor | Analgesia | Euphoria | Dysphoria | Respiratory Depression | Naloxone-Sensitive | | --- | --- | --- | --- | --- | --- | | **μ** | ✓✓✓ | ✓✓✓ | ✗ | ✓✓✓ | ✓ | | **δ** | ✓✓ | ✓ | ✓ (high dose) | ✗ | ✓ | | **κ** | ✓✓ | ✗ | ✓✓ | ✗ | ✓ | | **σ** | ✗ | ✗ | ✓✓✓ | ✗ | ✗ | **Clinical Pearl:** The distinction between true opioid receptors (μ, δ, κ) and sigma receptors is clinically important because sigma agonists (e.g., pentazocine) produce dysphoria and psychotomimetic effects that are NOT reversed by naloxone.