Effect of antenatal exposure to opiates on the development of opiate receptors in rat brain.

The ontogenesis of opiate receptors in several brain regions of the developing rat was measured by specific binding of [3H]methionine-enkephalin ([3H]Met). The amount of [3H]Met bound varied significantly with brain region as well as with age: a transient peak was observed at day 4 post-partum (p.p.) in the cerebellum; a hump was observed at days 8-17 in the brain stem; while in the whole forebrain the adult level was reached at day 17 p.p. Administration of morphine or naloxone to female rats before and during pregnancy significantly altered the regional development of [3H]Met binding sites in the brains of the offspring. These data suggest that the development of [3H]Met binding sites in brain follows a caudal to rostral sequence and that antenatal exposure to opiates affects the development of opiate receptors in brain. The latter effect may be responsible for the neonatal withdrawal symptoms in infants born to addicted mothers.