Antinociceptive effects of locally administered morphine in infant rats

Opiates injected into a site of injury are analgesic in adult animals, but there are no data on the effectiveness of this route of administration in i mmature organisms. Since the biological processes that regulate the effects of locally administered opiates are in flux during the early postnatal lif e of the rat, it is not clear whether or not opiates given directly into lo cal tissue would be effective as analgesics. To test this we injected morph ine (0.12, 0.60, 3.0 mu g/injection) directly into the hindpaw (intraplanta r) of infant rats at 3, 10 and 21 days of age, and assessed the behavioral response and the induction of Fos like immunocytochemistry in the dorsal ho rn of the spinal cord in the formalin test. Controls included saline inject ions to the paw, or comparable doses of morphine injected subcutaneously. A t 3 days of age, the two higher doses were behaviorally analgesic when give n into the paw, but there was limited selectivity over the subcutaneous rou te. At both 10 and 21 days of age, intraplantar injections were effective a nalgesics, whereas subcutaneous injections were not. The number of Fos stai ned cells in the dorsal horn of the spinal cord, induced by the formalin tr eatment, was decreased significantly by the 3.0 mg dose of morphine at all three ages. The results demonstrate that local treatment with morphine is a n effective and selective analgesic in the infant rat. (C) 1999 Internation al Association for the Study of Pain. Published by Elsevier Science B.V.