Low affinity binding of the classical D₁ antagonist SCH23390 in rodent brain: Potential interaction with A_2A and D₂-like receptors

Whereas structurally dissimilar D₁ antagonists competing for [³H]-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 ∼ 0.4 nM) consistent with D₁/D₅ receptors. The appearance of the low affinity site (K_D ∼ 300 nM) is dependent upon the absence of MgCl₂, but independent of D₁ expression (i.e., still present in D₁ knockout mice). Although the density of high affinity state receptor is lower in hippocampus or amygdala of D₁ knockout mice, some residual binding remains, consistent with the known expression of D₅ receptors in these regions. Remarkably, in hippocampus, the affinity of the low affinity site is shifted rightward in the presence of the D₂ antagonist domperidone and is largely absent in the hippocampus of D₂ 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 D₂ receptors in heterologous expression systems, consistent with the hypothesis that both D₂ and A_2A receptors are involved in the low affinity binding site. Therefore, we suggest that the heteromerization of D₂ and A_2A receptors reported previously in vitro also may occur in the brain of both rats and mice.