Similar actions of kappa and mu agonists on spinal nociceptive reflexes in rats and their reversibility by naloxone.

The mu agonist fentanyl and the kappa agonists U-50,488 and tifluadom were tested intravenously in electrophysiological experiments on spinal nociceptive reflexes in anaesthetised, spinalised rats. Responses of single motoneurones to thermal and mechanical stimuli were reduced to a similar degree by both mu and kappa agonists and these effects were reversed by low doses of naloxone.