High affinity agonist binding to the dopamine D3 receptor: chimeric receptors delineate a role for intracellular domains.

The dopamine D3 receptor, although structurally similar to the dopamine D2 receptor, has 100-fold higher affinity for agonists such as dopamine and quinpirole, when these receptors are expressed in 293 cells. Additionally, the D3 receptor has generally lower affinity for several antagonists than does the D2 receptor. To determine which regions of the receptor account for these differences, chimeras between D2 and D3 receptors were constructed in which intracellular loops were exchanged between the two receptors. A D2 receptor containing the third intracellular loop (IL3) from the D3 receptor had 10-20-fold higher affinity for dopamine and quinpirole than did the wild-type D2 receptor. Conversely, the D3 receptor containing the IL3 of the D2 receptor had 15-30-fold lower affinity for agonists than did the wild-type D3 receptor. That is, in these chimeras the IL3 shifted agonist affinity in a direction consistent with the agonist affinity of the receptor from which the IL3 was derived. In contrast, antagonist binding was not significantly altered. Chimeras in which the second intracellular loop was switched between the D2 and D3 receptors had essentially unchanged affinity for both agonists and antagonists. The data presented here suggest that structural differences in the IL3 of the D2 and D3 receptors partially account for observed differences in agonist binding to these receptors.