Analysis and Quantification of GPCR Allosteric Receptor–Receptor Interactions Using Radioligand Binding Assays: The A2AR-D2R Heteroreceptor Complex Example

There is a large body of biochemical and biophysical experimental evidences which establishes the existence of G protein-coupled receptors (GPCRs) as homo- and heteroreceptor complexes. The results indicate that there are allosteric interactions across the receptor–receptor interface of homo- and heteroreceptor complexes that modulate the binding properties of their receptor protomer components in terms of affinity and density, and thereby change their pharmacology. In the adenosine A2A-dopamine D2 heteroreceptor complexes (A2AR-D2R), the activation of the A2AR protomer by its standard receptor agonist CGS21680 causes a conformational change in the A2AR-D2R heteroreceptor complex. The allosteric wave passes over the receptor interface, invades the orthostatic dopamine binding site of the dopamine D2R protomer, and reduces the affinity of the high but not the low affinity D2R agonist binding site. In view of the complex nature of allosteric mechanisms, the detection, analysis, and quantification of the effects of this phenomenon rely on the use of competition radioligand binding assays to ensure proper demonstration of the high and low affinity D2R agonist binding sites. Outlined in this chapter is simple but useful experimental approaches for measuring the allosteric receptor–receptor interactions at GPCR heteroreceptor complexes. The readers will also find tips and discussion on the pitfalls of these assay and instructions for data analysis.