Abstract

Adenosine A_2A and dopamine D₂ receptors have been shown previously to form heteromeric complexes and interact at the level of agonist binding, G protein coupling, and trafficking. Because dopamine D₂ and D₃ receptors show a high degree of sequence homology, A_2A and D₃ receptors may also interact in a similar manner. The present studies with confocal microscopy showed that A_2A-yellow fluorescent protein (YFP) and D₃-green fluorescent protein 2 (GFP2) receptors colocalize in the plasma membrane. Furthermore, fluorescence resonance energy transfer (FRET) analysis demonstrated that A_2A-YFP and D₃-GFP2 receptors give a positive FRET efficiency and are thereby likely to exist as heteromeric A_2A/D₃ receptor complexes. Saturation experiments with [³H]dopamine demonstrated that the A_2A receptor agonist 4-[2-[[6-amino-9(N-ethyl-β-d-ribofuranuronaminoamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid (CGS-21680) reduced the affinity of the high-affinity agonist binding state of the D₃ receptor for [³H]dopamine. The A_2A and D_2A receptors seem to interact also at the level of G protein coupling, because the adenosine A_2A receptor agonist CGS-21680 fully counteracted the D₃ receptor-mediated inhibition of a forskolin-mediated increase in cAMP levels. Taken together, when coexpressed in the same neuron, A_2A and D₃ receptors seem to form A_2A/D₃ heteromeric receptor complexes in which A_2A receptors antagonistically modulate both the affinity and the signaling of the D₃ receptors. D₃ receptor is one of the therapeutic targets for treatment of schizophrenia, and therefore, the A_2A/D₃ receptor interactions could provide an alternative antischizophrenic treatment.