Cannabinoids as Context-Dependent Modulators of the Adenosine A₃ Receptors: Potential Orthosteric and Allosteric...

Adenosine regulates neurotransmission, immunity, inflammation, and tissue protection via four G protein-coupled receptors, among which the adenosine A₃ receptor (A₃R) remains comparatively understudied despite its therapeutic promise. This study examined whether cannabidiol (CBD) and cannabigerol (CBG) modulate human A₃R signalling, alone or with added adenosine, using cAMP assays, molecular docking, binding-energy calculations, and molecular dynamics simulations. Both cannabinoids inhibited forskolin-stimulated cAMP accumulation in A₃R-expressing cells, an effect absent in non-transfected cells and blocked by the selective A₃R antagonist PSB-10 but not by the selective A_2A receptor antagonist SCH 58261, supporting A₃R-mediated activity under these conditions. CBG was more potent than CBD (IC₅₀ = 0.130 vs. 1.05µM), whereas their maximal inhibitory responses were comparable. In the presence of 100nM CBD, the apparent IC₅₀ of adenosine decreased from 138 to 42nM, whereas 100nM CBG reduced it to 1.0nM, corresponding to an approximately 138-fold increase in apparent adenosine potency; CBG also increased the maximal inhibitory response. These results identify CBG as an effective enhancer of adenosine potency at human A₃R, suggesting it stabilizes an occupancy-dependent secondary pose that fine-tunes the orthosteric agonist-receptor interaction network. Further work, encompassing direct binding studies, proximal signalling assays, receptor-subtype profiling, and structural analyses, is needed to validate this mechanism.