Analgesia induced by local plantar injections of opiates in the formalin test in infant rats

Morphine injected locally to the paw of an adult or an infant rat is analgesic. Opiates specific to m and k opioid receptors, and less consistently to d opioid receptors, given locally to the site of injury in adult animals are also analgesic in a variety of models of inflammatory pain. To determine which opioid receptor(s) are involved in local analgesia in the immature animal, agonists specific for m, k, and d opioid receptors were injected into the intraplantar pad in infant rats and the resultant nociceptive behavior and Fos expression assayed in the formalin test. The k opioid receptor agonist U50,488 reduced nociceptive behavior in both phases of the formalin test and reduced Fos expression in the dorsal horn of the lumbar spinal cord, at 3 and 21 days of age. Morphiceptin (m opioid agonist) was analgesic in the 21-day-old pups, but not the 3-day-old pups, measured behaviorally or by Fos expression. DPDPE (d opioid agonist) was not analgesic at either age. We also tested the effects of opioid receptor antagonists on morphine's local analgesic action. Naltrexone, and to a lesser extent the m opioid antagonist CTOP, antagonized morphine's analgesic effect. Kappa and d opioid receptor blockers were inactive. The results demonstrate the ability of the k opioid system to mediate analgesia in the neonate at the site of injury in acute and chronic pain models, that the m opioid agonists are active later in development, but that morphine is analgesic in part through m opioid receptors. 2003 Wiley Periodicals, Inc. Dev Psychobiol 42: 111-122, 2003.