ABSTRACT32P-labeled oligonucleotides derived from the coding region of rat dopamine D2 receptor cDNA were used as probes to localize cells in the rat brain that contain the mRNA coding for this receptor by using in situ hybridization histochemistry. The highest level of hybridization was found in the intermediate lobe of the pituitary gland. High mRNA content was observed in the anterior lobe of the pituitary gland, the nuclei caudate-putamen and accumbens, and the olfactory tubercle. Lower levels were seen in the substantia nigra pars compacta and the ventral tegmental area, as well as in the lateral mammillary body. In these areas the distribution was comparable to that of the dopamine D2 receptor binding sites as visualized by autoradiography using ligand. However, in some areas such as the olfactory bulb, neocortex, hippocampus, superior colliculus, and cerebellum, D2 receptors have been visualized but no significant hybridization signal could be detected. The mRNA coding for these receptors in these areas could be contained in cells outside those brain regions, be different from the one recognized by our probes, or be present at levels below the detection limits of our procedure. The possibility of visualizing and quantifying the mRNA coding for dopamine D2 receptor at the microscopic level will yield more information about the in vivo regulation of the synthesis of these receptors and their alteration following selective lesions or drug treatments.Two types of dopamine receptors mediate the effects of this neurotransmitter in the central nervous system and in the periphery, the so-called D1 and D2 receptor subtypes (1). Dopamine D2 receptors are the target for drugs used in the treatment of schizophrenia and Parkinson disease and for regulation of prolactin secretion (2). Their distribution in the mammalian brain, including human, has been extensively examined by using receptor binding autoradiography (3). Dopamine D2 receptors have been directly visualized in living humans by positron-emission tomography (PET) (4). Binding and PET studies have suggested that dopamine D2 receptor levels may be elevated in schizophrenics (refs. 5, 6; see, however, ref. 7). Although dopamine receptors can be visualized at the light microscopic level by means of receptor binding autoradiography, the cellular localization of these receptors cannot be determined with this technique. The use of neurotoxin or mechanical brain lesions combined with binding studies has resulted in contradictory results (8-10). In order to establish the exact cellular localization of dopamine receptors, tools with higher anatomical resolution are necessary. The cloning and sequencing of the gene coding for the rat dopamine D2 receptor has been reported recently (11), thus opening the way to the development of these tools. On the basis of this sequence we have synthesized oligonucleotide probes and used them to visualize the regional distribution of the cells containing mRNA coding for the dopamine D2 receptor in the rat brain and pituitary gland b...