Abstract
Whereas structurally dissimilar D(1) antagonists competing for [(3)H]-SCH23390 binding recognize primarily one site in striatum, two distinct affinity states are observed in both amygdala and hippocampus. The binding profile of SCH23390 is similar in both of these regions, with the high affinity site (K(D) approximately 0.4 nM) consistent with D(1)/D(5) receptors. The appearance of the low affinity site (K(D) approximately 300 nM) is dependent upon the absence of MgCl(2), but independent of D(1) expression (i.e., still present in D(1) knockout mice). Although the density of high affinity state receptor is lower in hippocampus or amygdala of D(1) knockout mice, some residual binding remains, consistent with the known expression of D(5) receptors in these regions. Remarkably, in hippocampus, the affinity of the low affinity site is shifted rightward in the presence of the D(2) antagonist domperidone and is largely absent in the hippocampus of D(2) knockout animals. Additionally, this site is also shifted rightward in the presence of the A(2A) ligands SCH58261, CSC, or NECA, or in the absence of A(2A) receptors. The affinity of SCH23390 for this low affinity site is greater than seen for SCH23390 binding to D(2) receptors in heterologous expression systems, consistent with the hypothesis that both D(2) and A(2A) receptors are involved in the low affinity binding site. Therefore, we suggest that the heteromerization of D(2) and A(2A) receptors reported previously in vitro also may occur in the brain of both rats and mice.