Tetracycline-Regulated Inducible CB 2 Expression in AtT20 Cells: A Functional Assay for Quantifying Ligand Efficacy

Introduction Cannabinoid receptor-2 (CB 2 ) is an emerging therapeutic target for chronic and inflammatory pain, cancer, and neurological disorders. Understanding the efficacy of CB 2 ligands is crucial for future drug design and development. Aims We aimed to establish a simple and robust system to control CB 2 expression using a tetracycline-regulated mammalian expression system (T-REx), to enable application of the Black and Leff operational model to measure the operational efficacy (τ) of CB 2 ligands. Methods Ligand-induced hyperpolarisation of AtT20 cells transfected with T-REx and human CB 2 was measured by FLIPR membrane potential assay. Maximal and submaximal responses of the CB 2 ligands were produced by regulating CB 2 expression using tetracycline. Data were fitted to the operational model of receptor depletion to quantify the efficacy of seven ligands. Additionally, the maximal initial rate of signalling (IR max ), another putative measure of ligand efficacy, was determined. Results AK-F-064, CP55940 and 2-AG exhibited similar efficacy with a τ values of 11.4, 11 and 10.4 respectively, while anandamide (AEA) had the lowest efficacy (τ=1.07) among the tested agonists. The rank order of operational efficacy and IR max was similar and was estimated as: AK-F-064 = CP55940 = 2-AG > 5F-AB-PICA = WIN55212-2 > HU-308 = AEA. Conclusion This inducible expression system provides a reliable platform for quantifying and comparing CB 2 ligand efficacy using the operational model. This approach may facilitate more precise CB 2 -targeted drug development and can be readily extended to other GPCR targets.