Development of motor activity in young rats following perinatal methadone exposure.

Ambulatory behaviors of 21, 45, and 60 day old rats exposed to methadone (5 mg/kg) during gestation and/or lactation were studied by assessing locomotion in an activity cage, open field, and activity wheel, and latency times to step down from an elevated platform. Methadone-exposed rats were found to be generally less active than controls at 21 days of age and more active than saline-treated pups at the 45 and 60 day test periods. In addition, behavioral responses appeared to be dependent on the timing and duration of opioid treatment. These data suggest that prenatal and/or postnatal methadone exposure affects behavior in young rats and provide a functional correlate to our earlier observations of microscopic and neurochemical changes in the brains of methadone-treated offspring.