Changes in [3H]naloxone binding in spinal cord of rats treated prenatally with morphine.

Opiate receptor binding, using [3H]naloxone, was measured in spinal cords of fetuses and postnatal offspring of pregnant rats, injected with morphine or saline or pair-fed with morphine-injected animals during the last half of gestation. Binding was decreased in the spinal cord in morphine-treated offspring on the 21st day of gestation and remained depressed for 30 days postnatally. However, by 60 days postnatally the binding in the spinal cord was greater in morphine-treated offspring than in controls. The decrease in binding on the spinal cord was greater in morphine-treated offspring than in controls. The decrease in binding on the 21st day of gestation appears to be due to a decrease in the number of binding sites, with no change in binding affinity. The KD for [3H]naloxone binding in spinal cords of saline-injected and pair-fed animals decreased by about 50% between day 21 of gestation and 60 days postnatally. The increase in binding in spinal cords of adults reflects a change in the number of binding sites as well as a decrease in [3H]naloxone affinity in the morphine-treated offspring.