Qun-Yong Zhou

A comparison of D1 receptor binding and mRNA in rat brain using receptor autoradiographic and in situ hybridization techniques

D1, a subtype of the dopamine receptors, is widely distributed in the nervous system and has been shown to be positively coupled to adenylate cyclase. Using a combination of in vitro receptor autoradiographic and in situ hybridization techniques, the present study examines the co-distribution of D1 receptor binding sites and D1 receptor mRNA in adjacent rat brain sections. D1 receptor binding sites were labeled using the selective antagonist [3H](R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzaz epin- 7-ol (SCH23390) (4.6 nM), in the presence of 1 microM ketanserin, while the D1 receptor mRNA was visualized with a 35S-labeled riboprobe corresponding to a region between transmembrane domains III and VI of the rat D1 receptor (base pairs 383-843). Analysis of serial sections suggested a good agreement between D1 receptor binding and mRNA in several brain regions, including the paleocortex, caudate-putamen, nucleus accumbens, amygdala, and suprachiasmatic nucleus. Marked discrepancies between D1 receptor binding and mRNA were observed in other brain regions including the entopeduncular and subthalamic nuclei, substantia nigra (pars reticulata), hippocampus, and cerebellum. While technical considerations may contribute to these results, much of the discordance between the distributions is probably due to the differential localization of D1 receptor mRNA in cell bodies and receptor binding sites on fibers and may provide insights into receptor synthesis, transport, and membrane insertion. In the basal ganglia, for instance, D1 receptors are synthesized in the striatum and are either transported to efferent projections in areas such as the substantia nigra, or remain localized in striatal cells bodies. Ibotenic acid lesions in the striatum are consistent with these conclusions and demonstrate a coordinate loss of D1 receptor binding and mRNA in the caudate-putamen that is accompanied by a degeneration of fibers projecting to substantia nigra and a loss of D1 binding in the pars reticulata. Neurons in the dentate gyrus and in the granular layer of the cerebellum, on the other hand, synthesize D1 receptors and transport them entirely to either their dendritic or axonal fields, respectively, in the molecular layer. This analysis provides a better understanding of dopaminergic receptor systems in the CNS and their anatomical organization.

Molecular biology of the dopamine receptors

Because of their importance in pathophysiology, the dopamine receptors have been the subjects of intense pharmacological and physiological research. Their structures have remained mostly unknown until recently with the application of molecular biological approaches. The cloning of the first dopamine receptor, the D2 receptor opened a new era in dopamine receptor research. It has led not only to new studies of its own biology but also to the characterization of the other dopamine receptors. The most striking conclusion of this fast moving research is that the dopamine receptors are more diverse than expected from their pharmacological characterizations. We discuss here the history of the cloning of the dopamine receptors and the impact that this research had on our understanding of the dopamine system.