A CLASSIFICATION OF OPIATE RECEPTORS THAT MEDIATE ANTINOCICEPTION IN ANIMALS

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Abstract
1 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. 2 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. 3 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. 4 In the writhing and paw pressure tests both μ- and κ-agonists produced steep and parallel dose-response curves. 5 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.