In vitro determination of the efficacy of illicit synthetic cannabinoids at CB₁ receptors

Background and Purpose: The morbidity and mortality associated with recreational use of synthetic cannabinoid receptor agonists (SCRAs) may reflect strong activation of CB₁ receptors and is a major health concern. The properties of SCRA at CB₁ receptors are not well defined. Here we have developed an assay to determine acute CB₁ receptor efficacy using receptor depletion with the irreversible CB₁ receptor antagonist AM6544, with application of the Black and Leff operational model to calculate efficacy. Experimental Approach: Receptor depletion in mouse AtT-20 pituitary adenoma cells stably expressing human CB ₁ receptors was achieved by pretreatment of cells with AM6544 (10 μM, 60 min). The CB ₁ receptor-mediated hyperpolarisation of AtT-20 cells was measured using fluorescence-based membrane potential dye. From data fit to the operational model, the efficacy (τ) and affinity (K_A) parameters were obtained for each drug. Key Results: AM6544 did not affect the potency or maximal effect of native somatostatin receptor-induced hyperpolarization. The τ value of ∆⁹-THC was 80-fold less than the reference CB receptor agonist CP55940 and 260-fold less than the highest efficacy SCRA, 5F-MDMB-PICA. The operational efficacy of SCRAs ranged from 233 (5F-MDMB-PICA) to 28 (AB-PINACA), with CP55940 in the middle of the efficacy rank order. There was no correlation between the τ and K_A values. Conclusions and Implications: All SCRAs tested showed substantially higher efficacy at CB₁ receptors than ∆⁹-THC, which may contribute to the adverse effects seen with these drugs but not ∆⁹-THC.