Anatomical and affinity state comparisons between dopamine D1 and D2 receptors in the rat central nervous system

The anatomical distributions and affinity states of dopamine D1 and D2 receptors were compared in the rat central nervous system using quantitative autoradiography. [3H]SCH23390 and [3H]spiperone (in the presence of 100 nM mianserin) were used to label the D1 and D2 receptors, respectively. The densities of D1 and D2 receptors displayed a positive correlation among 21 brain regions (Pearson correlation coefficient, r = 0.80, P less than 0.001). The affinity states for the D1 and D2 receptors were found to be quite different from each other, and different from the results obtained by others using homogenate preparations. Both the D1 and D2 receptors were best modeled using a two-state model. In the absence of exogenous guanine nucleotides and using the nonselective agonist dopamine as the competitor, the D1 receptor was primarily in a low affinity agonist state (RH = 21 +/- 6%), whereas the D2 receptor was primarily in the high affinity agonist state (RH = 77 +/- 3%). In the presence of 10 microM guanylyl-imidodiphosphate or guanosine-5'-O-(2-thiophosphate) both the D1 and the D2 receptor were completely in a low affinity agonist state (RL = 100%). These affinity states were found both in the nucleus accumbens and olfactory tubercle using dopamine as the competitor and in the striatum using selective D1 or D2 agonists as competitors. Receptor occupancy of the D2 receptor with either an agonist or antagonist did not alter the affinity states of the D1 receptor, and conversely, receptor occupancy of the D1 receptor did not alter the affinity states of the D2 receptor. The correlation between densities of D1 and D2 receptors provides an anatomical framework for evaluating behavioral and electrophysiological evidence of an interaction between the two dopamine receptor subtypes. This interaction does not appear to be due to a sharing or coupling of G-proteins in such a way that binding to one dopamine receptor subtype alters the affinity state of the other receptor subtype. The differences between dopamine receptor distributions described by labeled agonists and antagonists may be due in part to differences in their affinity states. The low proportion of high affinity state D1 receptors may explain some of the difficulties in assigning specific behavioral roles to the D1 receptor.