Cannabinoid receptors are currently classified into three groups: central (CB1), peripheral (CB2) and GPR55, all of which are G-protein-coupled. CB1 receptors are primarily located at central and peripheral nerve terminals. CB2 receptors are predominantly expressed in non-neuronal tissues, particularly immune cells, where they modulate cytokine release and cell migration. Recent reports have suggested that CB2 receptors may also be expressed in the CNS. GPR55 receptors are non-CB1/CB2 receptors that exhibit affinity for endogenous, plant and synthetic cannabinoids. Endogenous ligands for cannabinoid receptors have been discovered, including anandamide and 2-arachidonylglycerol.