Effects of exposure in utero to methadone and buprenorphine on enkephalin levels in the developing rat brain.

Rat pups were exposed in utero to the opiates methadone or buprenorphine by administering subcutaneous injections to the mothers daily for the duration of pregnancy. Met- and Leu-enkephalin in the brains of the pups were measured by specific radioimmunoassays. Methadone at 8 mg/kg but not 4 mg/kg significantly reduced enkephalin levels in the striatum. Buprenorphine (1 and 2 mg/kg) did not affect peptide levels in any brain region. The results suggest that buprenorphine has minimal effect on the endogenous opioid system compared with methadone. However, the toxicity data indicate that he survival of the newborn is adversely affected by in utero exposure to buprenorphine.