Where are opioid receptors located?

• A. Peripheral nerves only
• B. Spinal cord only
• C. Brain only
• D. Peripheral nerves, spinal cord, and brain

Answer: (D)

Opioid receptors are distributed across the brain, spinal cord, and peripheral nerves, with additional presence in peripheral tissues and certain immune cells. In the brain, they reside in areas that shape pain perception and emotional response, such as pain-modulation circuits, where activating them reduces how pain is experienced. In the spinal cord, receptors in the dorsal horn modulate the transmission of nociceptive signals from the body up to the brain, dampening pain at its entry point. At peripheral terminals and in nearby tissues, receptor activation can lessen nociceptor excitability and curb the release of inflammatory mediators, contributing to analgesia at the site of injury. Because these receptors are found in all three locations, opioid effects come from central, spinal, and peripheral actions rather than a single site.