Interactions of inflammatory pain and morphine in infant rats - Long-term behavioral effects

Neonatal rat pups exposed to repetitive acute pain show decreases in pain t hreshold and altered behavior during adulthood. A model using prolonged inf lammatory pain in neonatal rats may have greater clinical relevance for inv estigating the long-term behavioral effects of neonatal pain in ex-preterm neonates. Neonatal rat pups were exposed to repeated formalin injections on postnatal (P) days 1-7 (P1-P7), with or without morphine pretreatment, and were compared with untreated controls. Behavioral testing during adulthood assessed pain thresholds using hot-plate (HP) and tail-flick (TF) tests, a lcohol preference, and locomotor activity (baseline and post-amphetamine). Adult rats exposed to neonatal inflammatory pain exhibited longer HP latenc ies than controls and male rats had longer HP thresholds compared to female s. Male rats exposed to neonatal morphine alone exhibited longer TF latenci es than controls. Both neonatal morphine treatment and neonatal inflammator y pain decreased ethanol preference, but their effects were not additive. D uring adulthood, male rats exposed to neonatal inflammatory pain exhibited less locomotor activity than untreated controls. We conclude that neonatal formalin and morphine treatment have specific patterns of long-term behavio ral effects in adulthood, some of which are attenuated when the two treatme nts are combined. (C) 2001 Elsevier Science Inc. All rights reserved.