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 threshold and altered behavior during adulthood. A model using prolonged inflammatory pain in neonatal rats may have greater clinical relevance for investigating 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, alcohol preference, and locomotor activity (baseline and postamphetamine). Adult rats exposed to neonatal inflammatory pain exhibited longer HP latencies than controls and male rats had longer HP thresholds compared to females. Male rats exposed to neonatal morphine alone exhibited longer TF latencies than controls. Both neonatal morphine treatment and neonatal inflammatory pain decreased ethanol preference, but their effects were not additive. During 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 behavioral effects in adulthood, some of which are attenuated when the two treatments are combined.