Abstract
Previous work has demonstrated that in utero cocaine exposure induces an uncoupling of brain D(1A) dopamine receptors (D(1A)DARs) from G(s)-protein. The present work is an attempt to define the mechanism underlying the uncoupling. We detected a significant elevation of phosphoserine in frontal cortical D(1A)DARs of rabbits that were exposed prenatally to cocaine compared with saline controls. This increase in phosphorylation is observed at gestational day 22 and persists to postnatal day 20. The hyperphosphorylation of the D(1A)DAR is accompanied by a 45% inhibition in frontal cortex (FCX) protein phsphatase-1 (PP1) activity that appears to be mediated via DARPP-32 (dopamine and cAMP-regulated phosphoprotein) as indicated by elevated FCX phospho-DARPP-32 (Thr(34)). Furthermore, we demonstrated in both FCX and in PC2 cells that express D(1A)DARs that PP1 is physically associated with D(1A)DARs. We also observed a dramatic decrease in D(1A)DAR-associated PP1 activity in FCX of prenatal cocaine-exposed rabbits, indicating that the reduction in PP1 activity may be responsible for the hyperphosphorylation of the receptor. Furthermore, pretreatment of cortical membranes obtained from cocaine-exposed animals with exogenous PP1 dephosphorylated the phosphorylated D(1A)DAR and significantly reversed the impaired receptor-G(alphas) coupling. This work indicates (1) that D(1A)DAR dephosphorylation via PP1 is essential for receptor resensitization or reactivation and (2) an alteration in the DARPP-32/PP1 cascade appears to be a primary event responsible for D(1A)DAR dysfunction in in utero cocaine-exposed rabbit progeny. The present finding of an altered DARPP-32/PP1 cascade in association with a dysfunction in D(1A)DAR signal transmission in the prenatal cocaine-exposed rabbit brain may implicate novel strategies for the prevention and treatment for in utero cocaine-induced developmental and behavioral abnormalities.