The mGlu 5 Receptor Protomer-Mediated Dopamine D 2 Receptor Trans-Inhibition Is Dependent on the Adenosine A 2A Receptor Protomer: Implications for Parkinson’s Disease

The adenosine A 2A receptor (A 2A R), dopamine D 2 receptor (D 2 R) and metabotropic glutamate receptor type 5 (mGluR 5 ) form A 2A R-D 2 R-mGluR 5 heteroreceptor complexes in living cells and in rat striatal neurons. In the current study, we present experimental data supporting the view that the A 2A R protomer plays a major role in the inhibitory modulation of the density and the allosteric receptor-receptor interaction within the D 2 R-mGluR 5 heteromeric component of the A 2A R-D 2 R-mGluR 5 complex in vitro and in vivo. The A 2A R and mGluR 5 protomers interact and modulate D 2 R protomer recognition and signalling upon forming a trimeric complex from these receptors. Expression of A 2A R in HEK293T cells co-expressing D 2 R and mGluR 5 resulted in a significant and marked increase in the formation of the D 2 R-mGluR 5 heteromeric component in both bioluminescence resonance energy transfer and proximity ligation assays. A highly significant increase of the the high-affinity component of D 2 R (D2R Ki High) values was found upon cotreatment with the mGluR 5 and A 2A R agonists in the cells expressing A 2A R, D 2 R and mGluR 5 with a significant effect observed also with the mGluR 5 agonist alone compared to cells expressing only D 2 R and mGluR 5 . In cells co-expressing A 2A R, D 2 R and mGluR 5 , stimulation of the cells with an mGluR 5 agonist like or D 2 R antagonist fully counteracted the D 2 R agonist-induced inhibition of the cAMP levels which was not true in cells only expressing mGluR 5 and D 2 R. In agreement, the mGluR 5 -negative allosteric modulator raseglurant significantly reduced the haloperidol-induced catalepsy in mice, and in A 2A R knockout mice, the haloperidol action had almost disappeared, supporting a functional role for mGluR 5 and A 2A R in enhancing D 2 R blockade resulting in catalepsy. The results represent a relevant example of integrative activity within higher-order heteroreceptor complexes.