Methadone during pregnancy: assessment of behavioral effects in the rat offspring.

Pregnant rats were administered methadone hydrochloride by gastric intubation. Beginning on day 8 of gestation, a drug group received 5 mg/kg/day; the daily maintenance dose was increased to 10 mg/kg after 4 days with the final dose given on day 22. An intubation control group received sterile water alone on the same gestation days and a nontreated control group was left undisturbed, All experimental and control litters were fostered at birth to untreated mothers, Body weights were low at birth among the methadone offspring but were similar to controls by weaning. During adulthood, offspring were tested on several operant tasks that included acquisition of a lever-pressing response, performance on a variable interval reinforcement schedule, acquisition and performance of an auditory-visual discrimination and response inhibition in a "punishment" paradigm. None of these measures revealed any learning or inhibitory impairments among the methadone-exposed offspring. The major behavioral effect produced by the treatment was a high response output which emerged for each sex in different phases of testing.