Cannabidiol is a negative allosteric modulator of the cannabinoid CB₁ receptor

Background and Purpose Cannabidiol has been reported to act as an antagonist at cannabinoid CB₁ receptors. We hypothesized that cannabidiol would inhibit cannabinoid agonist activity through negative allosteric modulation of CB₁ receptors. Experimental Approach Internalization of CB₁ receptors, arrestin2 recruitment, and PLCβ3 and ERK1/2 phosphorylation, were quantified in HEK 293A cells heterologously expressing CB₁ receptors and in the STHdh^Q7/Q7 cell model of striatal neurons endogenously expressing CB₁ receptors. Cells were treated with 2-arachidonylglycerol or Δ⁹-tetrahydrocannabinol alone and in combination with different concentrations of cannabidiol. Key Results Cannabidiol reduced the efficacy and potency of 2-arachidonylglycerol and Δ⁹-tetrahydrocannabinol on PLCβ3- and ERK1/2-dependent signalling in cells heterologously (HEK 293A) or endogenously (STHdh^Q7/Q7) expressing CB₁ receptors. By reducing arrestin2 recruitment to CB₁ receptors, cannabidiol treatment prevented internalization of these receptors. The allosteric activity of cannabidiol depended upon polar residues being present at positions 98 and 107 in the extracellular amino terminus of the CB₁ receptor. Conclusions and Implications Cannabidiol behaved as a non-competitive negative allosteric modulator of CB₁ receptors. Allosteric modulation, in conjunction with effects not mediated by CB₁ receptors, may explain the in vivo effects of cannabidiol. Allosteric modulators of CB₁ receptors have the potential to treat CNS and peripheral disorders while avoiding the adverse effects associated with orthosteric agonism or antagonism of these receptors.