Fentanyl primarily activates which receptor to produce analgesia?

• A. Delta opioid receptor
• B. Kappa opioid receptor
• C. Mu-opioid receptor
• D. Sigma receptor

Answer: (C)

Fentanyl’s analgesic effect mainly comes from activating mu-opioid receptors in the central nervous system. When fentanyl binds these receptors, it triggers G-protein signaling that inhibits adenylyl cyclase, lowers cAMP, opens potassium channels to hyperpolarize neurons, and closes voltage-gated calcium channels to reduce neurotransmitter release. This dampens pain signaling at both the brain (supraspinal) and spinal (dorsal horn) levels, altering pain perception and increasing analgesia. Fentanyl is highly lipophilic, so it crosses the blood-brain barrier quickly, producing rapid and potent mu-mediated analgesia.

Delta and kappa receptors can contribute to analgesia, but mu receptors are the primary mediators for fentanyl’s strong analgesic effects. Sigma receptors are not the main opioid targets responsible for analgesia and are linked more with non-analgesic effects like some psychotomimetic actions. Blocking mu receptors, as with naloxone, reverses fentanyl’s analgesia and other mu-mediated effects, underscoring why this receptor is the central player.