Morphine- and ketocyclazocine-induced analgesia in the developing rat: differences due to type of noxious stimulus and body topography.
Patterns of morphine- and ketocyclazocine-induced analgesia in limb withdrawal and tail-flick tests of thermal and mechanical nociception were examined in the preweanling rat. In the forepaw test, morphine was more effective than ketocyclazocine with both thermal and mechanical stimuli. Both drugs first induced analgesia between 3 and 5 days of age. In the tail-flick test, ketocyclazocine-induced analgesia preceded morphine's effects against both thermal and mechanical stimuli by several days. Ketocyclazocine produced robust analgesia between 7 and 10 days of age, while the effects of morphine did not peak until day 14. In the hindpaw, morphine was more effective than ketocyclazocine against a higher intensity mechanical stimulus, while ketocyclazocine was more effective against a lower intensity mechanical stimulus. Morphine-induced analgesia was reversed by lower doses of naloxone than was ketocyclazocine-induced analgesia, regardless of body part tested, against all noxious stimuli. These findings demonstrate differences in morphine- and ketocyclazocine-induced analgesia that are dependent upon age, body topography, stimulus type and intensity and imply different physiologic roles of mu- and chi-opioid receptors in analgesia.
D000367:Age Factors; D000700:Analgesics / Q000494:pharmacology*; D000818:Animals; D000833:Animals, Suckling; D002490:Central Nervous System / Q000187:drug effects* / Q000254:growth & development; D003496:Cyclazocine / Q000031:analogs & derivatives* / Q000494:pharmacology; D016314:Ethylketocyclazocine / Q000031:analogs & derivatives*; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D010146:Pain / Q000188:drug therapy*; D010147:Pain Measurement; D051381:Rats; D011957:Receptors, Opioid / Q000187:drug effects; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu
D000700:Analgesics; D011957:Receptors, Opioid; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D009270:Naloxone; D016314:Ethylketocyclazocine; C012395:ketazocine; D009020:Morphine; D003496:Cyclazocine