Metabotropic glutamate type 5, dopamine D2 and adenosine A2a receptors form higher-order oligomers in living cells

G protein-coupled receptors are known to form homo- and heteromers at the plasma membrane, but the stoichiometry of these receptor oligomers are relatively unknown. Here, by using bimolecular fluorescence complementation, we visualized for the first time the occurrence of heterodimers of metabotropic glutamate mGlu(5) receptors (mGlu(5)R) and dopamine D-2 receptors (D2R) in living cells. Furthermore, the combination of bimolecular fluorescence complementation and bioluminescence resonance energy transfer techniques, as well as the sequential resonance energy transfer technique, allowed us to detect the occurrence receptor oligomers containing more than two protomers, mGlu(5)R, D2R and adenosine A(2A) receptor (A(2A)R). Interestingly, by using high-resolution immunoelectron microscopy we could confirm that the three receptors co-distribute within the extrasynaptic plasma membrane of the same dendritic spines of asymmetrical, putative glutamatergic, striatal synapses. Also, co-immunoprecipitation experiments in native tissue demonstrated the existence of an association of mGlu(5)R, D2R and A(2A)R in rat striatum homogenates. Overall, these results provide new insights into the molecular composition of G protein-coupled receptor oligomers in general and the mGlu(5)R/D2R/A(2A)R oligomer in particular, a receptor oligomer that might constitute an important target for the treatment of some neuropsychiatric disorders.