Which statement best summarizes opioid receptor distribution and function?

Which statement best summarizes opioid receptor distribution and function?

• A. Brain receptors modulate pain, reward, and breathing; spinal receptors modulate pain transmission; peripheral receptors exist on sensory neurons and immune cells
• B. Brain receptors only modulate reward; spinal receptors mediate all analgesia; peripheral nerves lack receptors
• C. Opioid receptors are confined to the brain
• D. Peripheral nerves are the sole site of analgesia

Answer: (D)

Opioid receptors are found in the brain, spinal cord, and peripheral tissues, and each location contributes to analgesia and other opioid effects. In the brain, activation modulates pain perception, reward, and even respiratory drive. In the spinal cord, receptors dampen nociceptive transmission as signals ascend to the brain. In the periphery, receptors on sensory nerve endings and on immune cells influence peripheral nociception and inflammatory responses. Because analgesia arises from actions at all these sites, saying it comes only from the peripheral nerves or only from the brain is incomplete. The best summary recognizes that CNS and peripheral mechanisms work together to produce analgesia, with peripheral analgesia possible via agents that limit central effects.

Opioid receptors are found in the brain, spinal cord, and peripheral tissues, and each location contributes to analgesia and other opioid effects. In the brain, activation modulates pain perception, reward, and even respiratory drive. In the spinal cord, receptors dampen nociceptive transmission as signals ascend to the brain. In the periphery, receptors on sensory nerve endings and on immune cells influence peripheral nociception and inflammatory responses. Because analgesia arises from actions at all these sites, saying it comes only from the peripheral nerves or only from the brain is incomplete. The best summary recognizes that CNS and peripheral mechanisms work together to produce analgesia, with peripheral analgesia possible via agents that limit central effects.