Perinatal methadone exposure and its influence on the behavioral ontogeny of rats

The ontogeny of spontaneous motor and sensorimotor behaviors were evaluated daily from postnatal days 2 to 19 in rats maternally exposed to methadone (5 mg/kg) throughout gestation and/or lactation. In the methadone-treated groups, the age at which a specific behavior initially appeared for any group member and the ages at which 50% and a maximal (usually 100%) number of animals demonstrated a particular behavior was often delayed several days in comparison to controls. In addition, the time interval between the age of initial appearance and maximal achievement of a positive response was protracted. Rats subjected to methadone during either gestation or lactation exhibited the most retarded behavioral development. This study demonstrates that the timetable of behavioral maturation is altered in preweaning rats perinatally exposed to methadone, with the degree of response dependent on the timing and duration of opioid treatment. In addition, these results provide a functional correlate to our earlier observations of macroscopic and neurochemical changes in the brains of methadone-treated offspring.