In situ hybridization experiments were performed with brain sections from normal, control and haloperidol-treated rats to identify and map the cells expressing the D2 dopamine receptor gene. D2 receptor mRNA was detected with radioactive or biotinylated oligonucleotide probes. D2 receptor mRNA was present in glandular cells of the pituitary intermediate lobe and in neurons of the substantia nigra, ventral tegmental area, and forebrain, especially in caudate putamen, nucleus accumbens, olfactory tubercle, and piriform cortex. Hybridization with D2 and preproenkephalin A probes in adjacent sections, as well as combined hybridization with the two probes in the same sections, demonstrated that all detectable enkephalin neurons in the striatum contained the D2 receptor mRNA. Large neurons in caudate putamen, which were unlabeled with the preproenkephalin A probe and which may have been cholinergic, also expressed the D2 receptor gene. Haloperidol treatment (14 or 21 days) provoked an increase in mRNA content for D2 receptor and preproenkephalin A in the striatum. This suggests that the increase in D2 receptor number observed after haloperidol treatment is due to increased activity of the D2 gene. These results indicate that in the striatum, the enkephalin neurons are direct targets for dopamine liberated from mesostriatal neurons.Biochemical and pharmacological investigations have demonstrated the existence of two dopamine receptor subtypes, D1 and D2, which differentiate the signal transduction mediated by dopamine as either an activation or an inhibition of adenylate cyclase (1-4). Rat D2 receptor cDNA has been cloned and sequenced, and it appears that the D2 receptor belongs to a family of receptors that are coupled to guanine nucleotide-binding proteins (5). Its mRNA is abundantly represented in rat brain, especially in the mesencephalon and the basal ganglia (5). These areas contain, respectively, the cell bodies and terminals of the dopaminergic mesostriatal system (6). Striatal neuron activities are under the influence of the dopamine neurons of the substantia nigra and ventral tegmental area (7,8). Binding experiments with radiolabeled ligands have demonstrated the presence of striatal dopamine receptors (9-15). Drugs that interact with dopamine at receptor sites significantly affect neuronal activities, including neuropeptide gene expression (16)(17)(18)(19), in the striatum.While the existence of dopamine receptors on striatal neurons and on dopamine terminals in the striatum is commonly accepted (9-15), there is no anatomical information about the characteristics of the cells expressing the dopamine receptor gene in the striatum. To better understand how D2 receptor gene expression contributes to nigrostriatal interactions, we have used in situ hybridization to study the characteristics of cells containing D2 receptor mRNA in adult rat forebrain, under normal conditions and after blockade of dopamine transmission with a dopamine receptor antagonist, haloperidol. We report here that most cells conta...