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

Functional interactions between adenosine A2A and dopamine D2 receptors have been demonstrated both at the D2 agonist-binding and second messenger levels. The present studies use a [3H]dopamine-binding assay as a sensitive measure of A2A receptor-mediated modulation of D2 receptors. Co-incubation with an A2A receptor agonist increased the Kd value of high-affinity [3H]dopamine-binding sites of the D2 receptor without changing their Bmax values in a cotransfected cell line. This interaction was shown to be subtype specific, as the A2A receptor agonist did not modulate the affinity of the D1 receptor for [3H]dopamine. The domains of the D2 receptor important for the A2A/D2 receptor interaction were studied with chimeric dopamine D2/D1 receptors. The results showed that the A2A receptor agonist still strongly reduced the affinity of a D2/D1 chimera with the sixth transmembrane (TM) domain and third extracellular loop from the D1 receptor. However, the A2A receptor agonist was not able to modulate a D2/D1 chimeric receptor containing the fifth and sixth TM domains and the third intracellular and extracellular loops from the D1 receptor, indicating that the fifth TM domain and/or the third intracellular loop may be involved in the interaction between A2A and D2 receptors.