GPR18 and Arthritis: GPR55 is activated by a number of exogenous and endogenous cannabinoids.14,15,45 GPR55 is expressed in both normal and OA human chondrocytes at the protein level,19 and abnormal cannabinoid O-1602 was shown to reduce inflammatory pain in a rat model of arthritis, which was thought to be mediated by GPR55.46 McHugh et al.16 reported that phytocannabinoid Δ9-tetrahydrocannabinol and endogenous cannabinoid AEA are ligands of GPR18, suggesting that GPR18 may also act as a cannabinoid receptor; however, GPR18 is yet to be identified in articular cartilage.16