The influence of chronic morphine and spontaneous withdrawal on the expression of dopamine receptors and neuropeptide genes in the rat striatum was investigated. Morphine dependence was induced by subcutaneous implantation of two morphine pellets for 6 days. Rats were made abstinent by removal of the pellets 1, 2 or 3 days before they were killed. The mRNA levels coding for D1- and D2-dopamine receptors, dynorphin, preproenkephalin A and substance P were determined by quantitative in situ hybridization. The caudate putamen and the nucleus accumbens showed equivalent modifications in dopamine receptor and neuropeptide gene expression. After 6 days of morphine, a decrease in D2-dopamine receptor and neuropeptide mRNA levels was observed (-30%), but there was no change in D1-dopamine receptor mRNA. In abstinent rats, both D1- and D2-dopamine receptor mRNA levels were decreased 1 day after withdrawal (-30% compared with chronic morphine). In contrast, neuropeptide mRNA levels were unaffected when compared with those observed after 6 days of morphine. During the second and third day of withdrawal, there was a gradual return to the levels seen in the placebo-treated group, for both dopamine receptor and neuropeptide mRNAs. Phenotypical characterization of striatal neurons expressing mu and kappa opioid receptor mRNAs showed that, in striatonigral neurons, both mRNAs were colocalized with D1-receptor and Dyn mRNAs. Our results suggest that during morphine dependence, dopamine and morphine exert opposite effects on striatonigral neurons, and that effects occurring on striatopallidal neurons are under dopaminergic control. We also show that withdrawal is associated with a down regulation of the postsynaptic D1 and D2 receptors.