Biochemical identification of the dopamine D₂ receptor domains interacting with the adenosine A_2A receptor

Functional interactions between adenosine A_2A 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 A_2A receptor-mediated modulation of D₂ receptors. Co-incubation with an A_2A receptor agonist increased the K_d value of high-affinity [³H]dopamine-binding sites of the D₂ 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 D₁ receptor for [³H]dopamine. The domains of the D₂ receptor important for the A_2A/D₂ receptor interaction were studied with chimeric dopamine D₂/D₁ receptors. The results showed that the A_2A 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 A _2A receptor agonist was not able to modulate a D₂/D ₁ 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 A_2A and D₂ receptors.