GPR55 and the vascular receptors for cannabinoids.

CB1 and CB2 receptors mediate most responses to cannabinoids but not some of the cardiovascular actions of endocannabinoids such as anandamide and virodhamine, or those of some synthetic agents, like abnormal cannabidiol (abn-cbd). These agents induce vasorelaxation which is antagonised by rimonabant but only at high concentrations relative to those required to block CB1 receptors. Vasorelaxation to anandamide is sensitive to Pertussis toxin (though that to abn-cbd is not), and so is thought to be mediated by a G protein-coupled receptor through Gi/o. An orphan receptor, GPR55, apparently a cannabinoid receptor, is activated by abn-cbd, but is not the receptor mediating vasorelaxation to this agent, as the response persists in vessels from GPR55 knockout mice. However, the activity of anandamide in GPR55 knockout mice is not yet reported and so the role of GPR55 as a cannabinoid receptor mediating vascular responses has yet to be finalised.