Comparative Distribution of Dopamine D-1 and D-2 Receptors in the Basal Ganglia of Turtles, Pigeons, Rats, Cats, and Monkeys

The distribution and density of dopamine D-1 and D-2 receptors were studied in the basal ganglia of adult turtles, pigeons, rats, cats, and monkeys. Dopamine receptors were measured in vitro by quantitative autoradiography in alternate sections processed for D-1 and D-2 receptor subtypes and compared to adjacent sections stained for acetylcholinesterase (AChE) activity. [3H]-SCH 23390 and [3H]-spiroperidol were used to label the D-1 and D-2 dopamine receptor subtypes, respectively. The anatomic distribution of both D-1 and D-2 receptors in the basal ganglia was remarkably similar across all species examined. Whereas the absolute number of D-1 and D-2 receptors in the basal ganglia varied between species, the percentage of D-1 and D-2 receptors in a region was quite similar among species. The pattern of binding to the D-1 and D-2 receptor varied among the different species. The adult turtles, pigeons, and rats demonstrated nonpatchy D-1 and D-2 receptor binding in the striatum and pallidum. The adult cat and monkey caudate nucleus and putamen demonstrated mildly heterogeneous receptor binding in a pattern that differed from that seen with AChE staining, but did occasionally demonstrate similar patterns of the D-1 and D-2 receptor subtypes. The immature cat striatum was characterized by heterogeneous D-1 receptor binding that corresponded to heterogeneous AChE rich patches, whereas D-2 receptor binding was homogeneous. Heterogeneous binding was seen in other basal ganglia structures including the nucleus accumbens, olfactory tubercle, and substantia nigra pars compacta and reticulata. Complementary D-1 and D-2 receptor binding patterns were seen in the pallidum and substantia nigra of the mammals. The results of this study indicate that both D-1 and D-2 dopamine receptors are present in the basal ganglia of five different vertebrates. A common feature of dopamine receptors in the basal ganglia is their heterogeneity in distribution and density. The heterogeneity of dopamine receptors has similarities to and differences from the distribution of presynaptic dopamine and other neurotransmitter markers of the basal ganglia.