The effects of systemic morphine on behavior and EEG in newborn rats

Early studies suggested that newborn animals are far more susceptible to the convulsant effect of systemic morphine than adult animals. The present study reassessed morphine's (0, 6, 12.5, 25, 50, 100 and 300 mg/kg) toxic effects, making use of electroencephalographic (EEG) recordings, behavioral observations and the specific opiate antagonist naloxone in immature rats (postnatal days 1, 3, 6, 12 and 24). Although morphine had opiate-specific effects (such as inhibition of activity at low doses), non-specific effects (such as hyperactivity) elicited by the highest doses, predominated in the 3 youngest age groups. At day 12 high doses of morphine first produced Straub tail and catatonia. At this age morphine produced EEG spikes that were not reversed by naloxone. Only at day 24 were electrographic spikes temporarily inhibited by naloxone. Behavioral convulsions were never observed, at any age. These findings indicate that morphine is less toxic in newborns than suggested previously.