Cannabinoid Receptors are seven-pass transmembrane proteins that belong to the rhodopsin family of G protein-coupled receptors. There are at least two types of Cannabinoid receptors, CB1R and CB2R, which are activated by a family of endogenous arachidonic acid derivatives, such as the endocannabinoids and D9-THC. CB1R is expressed in the central nervous system and upper GI tract, while CB2R is expressed by selected neuronal cell types and hematopoietic cells.