Imaging of Adenosine Receptors

Adenosine is a fundamental molecule of life. It is a part of the DNA and the main degradation product of the central currency of energy metabolism in humans and animals – adenosine triphosphate (ATP). Under pathological conditions like hypoxia, the adenosine concentration can rise severalfold – up to micromolar concentrations. The net effect of adenosine on excitable tissue is inhibitory affecting the release of classical neurotransmitters like glutamate, GABA (gamma-aminobutyric acid), and dopamine. The widely used neurostimulant caffeine exerts its effects as an antagonist at adenosine receptors. Four different types of adenosine receptors have been described in mammals: A1, A2A, A2B, and A3 which are all G-protein-coupled receptors. Over the last 25 years, adenosine receptor ligands, agonists as well as antagonists, have emerged as a class of useful therapeutics. For the A1 and A2A subtypes several antagonist radioligands have been used successfully for PET imaging in humans and animals especially for the brain