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 immature organisms. Since the biological processes that regulate the effects of locally administered opiates are in flux during the early postnatal life of the rat, it is not clear whether or not opiates given directly into local tissue would be effective as analgesics. To test this we injected morphine (0.12, 0.60, 3.0 microg/injection) directly into the hindpaw (intraplantar) 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 horn of the spinal cord in the formalin test. Controls included saline injections to the paw, or comparable doses of morphine injected subcutaneously. At 3 days of age, the two higher doses were behaviorally analgesic when given into the paw, but there was limited selectivity over the subcutaneous route. At both 10 and 21 days of age, intraplantar injections were effective analgesics, whereas subcutaneous injections were not. The number of Fos stained cells in the dorsal horn of the spinal cord, induced by the formalin treatment, was decreased significantly by the 3.0 mg dose of morphine at all three ages. The results demonstrate that local treatment with morphine is an effective and selective analgesic in the infant rat.