Adenosine is an endogenous nucleoside generated locally in tissues under conditions of hypoxia, ischemia, or inflammation. It modulates a variety of physiological functions of many tissues including brain and heart. Adenosine exerts its action via four specific adenosine receptors (also named P1 purinergic receptors): A1-Adenosine Receptor (A1AR), A2A-Adenosine Receptor (A2AAR), A2B-Adenosine Receptor (A2BAR), and A3-Adenosine Receptor (A3AR). All are integral membrane proteins and are members of the G protein-coupled receptor superfamily, which share a common structure of seven putative transmembrane domains, an extracellular NH2 terminus, cytoplasmic COOH terminus, and a third intracellular loop important for binding G proteins. The adenosine receptors can be distinguished on the basis of their differential selectivity for adenosine analogs.