Allosteric mechanisms within the adenosine A2A -dopamine D2 receptor heterotetramer

The structure constituted by a G protein coupled receptor (GPCR) homodimer and a G protein provides a main functional unit and oligomeric entities can be viewed as multiples of dimers. For GPCR heteromers, experimental evidence supports a tetrameric structure, comprised of two different homodimers, each able to signal with its preferred G protein. GPCR homomers and heteromers can act as the conduit of allosteric interactions between orthosteric ligands. The well-known agonist/agonist allosteric interaction in the adenosine A(2A) receptor (A(2A)R)-dopamine D-2 receptor (D2R) heteromer, by which A(2A)R agonists decrease the affinity of D2R agonists, gave the first rationale for the use of A(2A)R antagonists in Parkinson's disease. We review new pharmacological findings that can be explained in the frame of a tetrameric structure of the A(2A)R-D2R heteromer: first, ligand-independent allosteric modulations by the D2R that result in changes of the binding properties of A(2A)R ligands; second, differential modulation of the intrinsic efficacy of D2R ligands for G protein-dependent and independent signaling; third, the canonical antagonistic Gs-Gi interaction within the frame of the heteromer; and fourth, the ability of A(2A)R antagonists, including caffeine, to also exert the same allosteric modulations of D2R ligands than A(2A)R agonists, while A(2A)R agonists and antagonists counteract each other's effects. These findings can have important clinical implications when evaluating the use of A(2A)R antagonists. They also call for the need of monitoring caffeine intake when evaluating the effect of D2R ligands, when used as therapeutic agents in neuropsychiatric disorders or as probes in imaging studies. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'. Published by Elsevier Ltd.