Molecular probes for the cannabinoid receptors

Cannabinoids produce most of their biochemical and pharmacological effects by interacting with CB1 and CB2 cannabinoid receptors, both of which are G-protein coupled membrane-bound functional proteins. CB1 is found in the central nervous system and in a variety of other organs including heart, vascular endothelium, uterus, vas deferens, testis and small intestine. Conversely, the CB2 receptor appears to be associated exclusively with the immune system and is found in the periphery of the spleen and other cells associated with immunochemical functions. Although both CB1 and CB2 have been cloned and the primary sequences are known, their three dimensional structures and the amino acid residues at the active site, critical for ligand recognition, binding and activation have not been characterized. In the absence of any X-ray crystallographic and NMR data, information on the structural requirements for ligand-receptor interactions is obtained with the help of suitably designed molecular probes. These ligands either interact with the receptor in a reversible fashion (reversible probes) or, alternatively, attach at or near the receptor active site with the formation of a covalent bond (irreversible probes). Subsequently, information related to ligand binding and receptor activation is further amplified with the help of receptor mutants and computer modeling. This review focuses on molecular probes related to the classical and non-classical cannabinoids that have been reported since the discovery of the first cannabinoid receptor over a decade ago.