Biochemical identification of the dopamine D2 receptor domains interacting with the adenosine A2A receptor

Functional interactions between adenosine A(2A) and dopamine D-2 receptors have been demonstrated both at the D-2 agonist-binding and second messenger levels. The present studies use a [H-3]dopamine-binding assay as a sensitive measure of A(2A) receptor-mediated modulation of D-2 receptors. Co-incubation with an A(2A) receptor agonist increased the K-d value of high-affinity [H-3]dopamine-binding sites of the D-2 receptor without changing their B-max values in a cotransfected cell line. This interaction was shown to be subtype specific, as the A(2A) receptor agonist did not modulate the affinity of the D-1 receptor for [H-3]dopamine. The domains of the D-2 receptor important for the A(2A)/D-2 receptor interaction were studied with chimeric dopamine D-2/D-1 receptors. The results showed that the A(2A) receptor agonist still strongly reduced the affinity of a D-2/D-1 chimera with the sixth transmembrane (TM) domain and third extracellular loop from the D-1 receptor. However, the A(2A) receptor agonist was not able to modulate a D-2/D-1 chimeric receptor containing the fifth and sixth TM domains and the third intracellular and extracellular loops from the D-1 receptor, indicating that the fifth TM domain and/or the third intracellular loop may be involved in the interaction between A(2A) and D-2 receptors.