Effects of prenatal cocaine, morphine, or both on postnatal opioid (μ) receptor development

We studied the effects of prenatal cocaine and morphine given separately and in combination on the (1) postnatal brain μ-opioid receptor development and (2) interaction of dopamine with μ receptors. Pregnant rats received single daily intraperitoneal (I.P.) injections of saline, cocaine (20 mg/kg), morphine (2 mg/kg), or the combination of both drugs from day 13 to day 20 of gestation. Postnatal days (P) 1, 7, 14, and 28, whole brains were analyzed for opioid receptor binding and μ mRNA. Prenatal cocaine administered by itself had no significant effect on the ontogeny of brain μ receptors on all the days studied when compared to controls. The morphine-treated group showed a significant increase in μ receptor binding on P1 and P7. Exposure to both cocaine and morphine showed a significant increase in μ receptor density on P1 and P7. In addition, there was also a significant increase in MOR mRNA in both the morphine alone and combination groups. Pretreatment with dopamine D2 receptor antagonist (sulpiride, 20 mg/kg) prior to drug administration showed decreased μ receptor binding on P1 and P7. These results suggest that prenatal exposure to morphine or a combination of cocaine and morphine significantly increases μ receptor density. By P14, μ-opioid receptor binding was no longer different than the control. This may suggest that the effect on receptor may be short-lived and that other key intracellular events may be activated to mediate the long-term effects. Also, the data show that dopaminergic mechanisms are (or opioid–dopamine interaction is) involved in the effects of morphine alone or morphine in combination with cocaine on μ receptor regulation.