Which statement correctly distinguishes Mu1, Mu2, and Mu3 receptor effects?

Which statement correctly distinguishes Mu1, Mu2, and Mu3 receptor effects?

• A. Mu2 mediates analgesia; Mu1 causes immune suppression; Mu3 causes euphoria
• B. Mu1 mediates analgesia (supraspinal and spinal) with bradycardia and miosis; Mu2 mediates spinal analgesia with bradycardia and respiratory depression; Mu3 mainly immune suppression
• C. Mu1 causes respiratory depression only; Mu2 causes immune suppression; Mu3 causes analgesia
• D. Mu1 and Mu2 are identical; Mu3 is unrelated to opioids

Answer: (B)

The key idea is that mu opioid receptors have subtypes with distinct roles in analgesia, autonomic effects, and immune modulation. The statement correctly assigns Mu1 to analgesia—both supraspinal and spinal—and notes bradycardia and miosis as accompanying autonomic effects, which fits the classic view that Mu1 mediates analgesia with those parasympathetic-like side effects. It then assigns Mu2 to spinal analgesia with bradycardia and respiratory depression, reflecting the subtype’s association with dose-limited analgesia at the spinal level and its prominent depressant effects on respiration and heart rate. Finally, Mu3 is linked to immune suppression, aligning with observations that this subset may modulate immune function rather than contribute primarily to analgesia. Other descriptions that place respiratory depression with the wrong subtype or claim identical roles for Mu1 and Mu2, or that Mu3 drives analgesia, do not align with the receptor-specific patterns described above.

The key idea is that mu opioid receptors have subtypes with distinct roles in analgesia, autonomic effects, and immune modulation. The statement correctly assigns Mu1 to analgesia—both supraspinal and spinal—and notes bradycardia and miosis as accompanying autonomic effects, which fits the classic view that Mu1 mediates analgesia with those parasympathetic-like side effects. It then assigns Mu2 to spinal analgesia with bradycardia and respiratory depression, reflecting the subtype’s association with dose-limited analgesia at the spinal level and its prominent depressant effects on respiration and heart rate. Finally, Mu3 is linked to immune suppression, aligning with observations that this subset may modulate immune function rather than contribute primarily to analgesia. Other descriptions that place respiratory depression with the wrong subtype or claim identical roles for Mu1 and Mu2, or that Mu3 drives analgesia, do not align with the receptor-specific patterns described above.