ANS. APentazocine: Partial agonist and antagonist at mu receptor, and agonist at kappaBuprenorphine: Partial agonist at mu and antagonist at kappaTramadol: Affinity for mu is low, while that for kappa and delta is very low. (Tramadol also has an additional mechanism of inhibiting reuptake of NA and 5-HT, and thus activating monoaminergic spinal inhibition of pain. Its analgesic action is only partially reversed by opioid antagonist naloxone.Fentanyl: Congener of pethidine, 80-100 times more potent than morphine. It is chemically unrelated to morphine, but interacts with opioid receptors and its actions are blocked by naloxone.