What does the μ opioid receptor do?

• A. When activated, it increases transmission of pain signals
• B. When activated, it silences transmission of pain neurotransmitters
• C. It stimulates inflammation
• D. It increases heart rate

Answer: (B)

The key idea is that the μ opioid receptor reduces pain signaling. When this receptor is activated by opioids or endogenous ligands, it is linked to Gi/o proteins, which lowers cAMP, closes voltage-gated calcium channels, and opens potassium channels. The result is hyperpolarization of neurons and a drop in the release of pain neurotransmitters like substance P and glutamate in the pain pathways. This dampens transmission of nociceptive signals to the brain, producing analgesia. It does not increase pain signal transmission, nor does it promote inflammation; those would be opposite effects. It also doesn’t primarily drive heart rate changes—those are not the main analgesic action of μ receptors.