Analgesia in young and adult rats perinatally exposed to methadone.

The analgesic response to the hot-plate test was studied in 21-, 20-, 34-, 60-, 90-, and 120-day old male and female rats that were maternally exposed to methadone (5 mg/kg) during gestation and/or lactation. At 21, 30, 45, and 60 days of age, both sexes of every methadone-treated group of rats had longer latencies on the hot-plate than control animals. Female rats in the lactation and gestation lactation groups at 90 days, and female rats in the gestation and lactation groups at 120 days, were abnormally slow to respond to the hot-plate. With the exception of a diminished analgesic response for the lactation group, male rats in the methadone-treated groups were comparable to control animals in regard to performance on the hot-plate test. These results suggest that exposure to methadone early in life can have a profound influence on analgesic responsiveness that is present long after cessation of drug exposure and which continues into adulthood.