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The cellular expression of adenosine A2Areceptor mRNA in the adult monkey and human striatum was examined by using single and double in situ hybridization with ribonucleotide probes. Analysis on adjacent sections demonstrated a homogeneous overlapping expression of adenosine A2Areceptor and preproenkephalin A mRNAs throughout nucleus caudatus, putamen, and nucleus accumbens. By contrast, high expression of preproenkephalin A mRNA but no expression of adenosine A2Areceptor mRNA was found in the nucleus basalis of Meynert. Double in situ hybridization demonstrated an extensive colocalization of adenosine A2Areceptor and preproenkephalin A mRNAs in approximately 50% of the medium‐sized spiny neurons of the monkey nucleus caudatus, putamen, and nucleus accumbens. A small number of neurons (4–12%) that contained adenosine A2Areceptor mRNA but not preproenkephalin A mRNA was found along the ventral borders of the striatum. Virtually all adenosine A2Areceptor mRNA‐containing neurons co‐expressed dopamine D2receptor mRNA, whereas only very few adenosine A2Areceptor mRNA containing neurons co‐expressed dopamine D1receptor or substance P mRNAs. In addition, a sub‐population of adenosine A2Areceptor mRNA‐expressing neurons that also contained preproenkephalin A mRNA was found in the septum in monkeys. These results demonstrate that there is a high expression of adenosine A2Areceptor mRNA in the primate striatum that is extensively co‐localized with dopamine D2receptor and preproenkephalin A mRNAs. It is concluded that adenosine A2Areceptors are likely to be important for the parallel organization of primate striatal neurotransmission and that these receptors could be a target for drug therapy in Parkinson's disease. J. Comp. Neurol. 399:229–240, 1998. © 1998 Wiley‐Liss, Inc.

Summary The localization of mRNA coding for the dopamine D2 receptor was studied in the rabbit carotid body usingin situ hybridization with synthetic35S-labelled oligodeoxynucleotides. Using autoradiography on cryostat or semi-thin sections, labelling was observed over the cytoplasm of glomus cells, but not over sustentacular cells. A quantitative study showed that labelling intensity (silver grain density) was increased by haloperidol treatment. These results suggest that glomus cells express the dopamine D2 receptor gene and that this expression is regulated.

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.

Gene expression within the central nervous system is regulated by complex interactions of DNA-binding proteins, among which are the POU domain-containing proteins, which are distantly related to homeobox proteins. These POU domain-containing proteins have been implicated in control of transcription and replication within the central nervous system. We used degenerate primers with the PCR to isolate another POU domain-containing cDNA, RHS2, from hypothalamic RNA. Isolation of a putative full-length cDNA was accomplished by using serial dilutions of a hypothalamic cDNA library grown on solid medium. This member of the class III POU family is expressed in rats from embryonic day 11.5 into adulthood, being especially prominent in the brain. We performed double-labeling hybridization histochemistry and determined that RHS2 is coexpressed with a variety of neuropeptides in medium-sized neurons in the caudate putamen and with dynorphin in the paraventricular and supraoptic nuclei of the hypothalamus. Expression of RHS2 in the caudate putamen was increased by elimination of its nigrostriatal dopaminergic innervation.

In situ hybridization experiments were performed in rat brain sections from normal and 6-hydroxydopamine-treated rats in order to map and identify the neurons expressing the D1 receptor gene in the striatum and the substantia nigra. Procedures of combined in situ hybridization, allowing the simultaneous detection of two mRNAs in the same section or in adjacent sections, were used to characterize the phenotypes of the neurons expressing the D1 receptor gene. D1 receptor mRNA was found in neurons all over the caudate-putamen, the accumbens nucleus, and the olfactory tubercle but not in the substantia nigra. In the caudate-putamen and accumbens nucleus, most of the neurons containing D1 receptor mRNA were characterized as medium-sized substance P neurons and distinct from those containing D2 receptor mRNA. Nevertheless, 15-20% of the substance P neurons did not contain D1 receptor mRNA. The neurons containing preproenkephalin A mRNA did not contain D1 receptor mRNA but contained D2 receptor mRNA. A small number of cholinergic and somatostatinergic neurons exhibited a weak reaction for D1 receptor mRNA. These results demonstrate that dopamine acts on efferent striatal neurons through expression of distinct receptors--namely, D1 and D2 in separate cell populations (substance P and preproenkephalin A neurons, respectively)--and can also act on nonprojecting neurons through D1 receptor expression.