A CLASSIFICATION OF OPIATE RECEPTORS THAT MEDIATE ANTINOCICEPTION IN ANIMALS

Abstract

To investigate the opiate receptors that mediate antinociception, the activity profiles of opioid analgesic drugs have been determined against different nociceptive stimuli in the mouse and rat. In tests that employ heat as the nociceptive stimulus, μ‐opiate receptor agonists, such as morphine, pethidine and dextropropoxyphene, had steep and parallel dose‐response curves and were capable of achieving maximum effects. In addition, the antinociceptive potency ratios of these drugs in heat tests were similar to those for analgesia in man. The κ‐agonists, such as ethylketazocine, nalorphine, Mr2034 and pentazocine, were essentially inactive against heat nociception except at doses that caused sedation and motor incapacitation. In the writhing and paw pressure tests both μ‐ and κ‐agonists produced steep and parallel dose‐response curves. It is concluded that both μ‐ and κ‐opiate receptors mediate antinociception in animals and that the interactions of analgesic drugs with these receptors may be classified in terms of their antinociceptive activities against qualitatively different nociceptive stimuli. 1980 British Pharmacological Society