Received for publication June 2, 2004.
Revised November 11, 2004.
Accepted for publication November 11, 2004.

Adenosine A_2A Receptor and Dopamine D₃ Receptor Interactions: Evidence of Functional A_2A/D₃ Heteromeric Complexes

Abstract

Adenosine A_2A and dopamine D₂ receptors have previously been shown to form heteromeric complexes and interact at the level of agonist binding, G protein coupling and trafficking. Since 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-YFP and D₃-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 CGS-21680 reduced the affinity of the high affinity agonist binding state of the D₃ receptor for [³H]dopamine. The A_2A and D₃ receptors appear to interact also at the level of G protein coupling; since 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 where A_2A receptors antagonistically modulate both the affinity and the signaling of the D₃ receptors. D₃ receptor is one of the therapeutical targets for treatment of schizophrenia and, therefore, the A_2A/D₃ receptor interactions could provide an alternative antischizophrenic treatment.

Key words: Adenosine, Dopamine, Gi family, Gs family, cAMP, G protein regulation, Desensitization/uncoupling, Fluorescence techniques, Receptor binding studies, Anti-psychotics