Presynaptic Control of Striatal Glutamatergic Neurotransmission by Adenosine A₁–A_2AReceptor Heteromers

TLDR

The functional role of GPCR heteromers remains debated. The study demonstrates that A1R–A2AR heteromerization enables adenosine to finely tune glutamatergic neurotransmission. Coimmunoprecipitation, bioluminescence, TR‑FRET, and immunogold labeling showed that A1R–A2AR heteromers exist on the cell surface and colocalize in striatal glutamatergic terminals, acting as a switch that modulates glutamate release depending on adenosine concentration. Radioligand binding revealed that A2AR activation lowers A1R agonist affinity, and chronic caffeine alters heteromer function, potentially explaining caffeine tolerance.

Abstract

The functional role of heteromers of G-protein-coupled receptors is a matter of debate. In the present study, we demonstrate that heteromerization of adenosine A 1 receptors (A 1 Rs) and A 2A receptors (A 2A Rs) allows adenosine to exert a fine-tuning modulation of glutamatergic neurotransmission. By means of coimmunoprecipitation, bioluminescence and time-resolved fluorescence resonance energy transfer techniques, we showed the existence of A 1 R–A 2A R heteromers in the cell surface of cotransfected cells. Immunogold detection and coimmunoprecipitation experiments indicated that A 1 R and A 2A R are colocalized in the same striatal glutamatergic nerve terminals. Radioligand-binding experiments in cotransfected cells and rat striatum showed that a main biochemical characteristic of the A 1 R–A 2A R heteromer is the ability of A 2A R activation to reduce the affinity of the A 1 R for agonists. This provides a switch mechanism by which low and high concentrations of adenosine inhibit and stimulate, respectively, glutamate release. Furthermore, it is also shown that A 1 R–A 2A R heteromers constitute a unique target for caffeine and that chronic caffeine treatment leads to modifications in the function of the A 1 R–A 2A R heteromer that could underlie the strong tolerance to the psychomotor effects of caffeine.