A quantitative pharmacology model for cannabinoid CB₁ receptor mediated by Gi/Gs protein competition

Background and Purpose: Orthosteric agonism of the CB1 receptor normally associates with Gi signalling resulting in a net inhibition of cAMP production. Empirical evidence shows CB1 causes a net cAMP stimulation through Gs coupling under two conditions: co-stimulation with the D-2 receptor and high-level CB1 expression. Two hypotheses have been proposed to account for these paradoxical effects, (1) Gi is consumed by coupling to D-2 or extra CB1 and excess CB1 binds to Gs and (2), the formation of dimers CB1-CB1 or CB1-D-2 switches Gi/Gs preference. This study explored the mechanisms of Gi/Gs preference based on a mathematical model of the CB1 receptor.Experimental Approach: The model was based on Hypothesis 1 and known mechanisms. The model was calibrated to align with multiple types of data (cAMP, Gi dissociation and internalisation). The key step of Hypothesis 1 was examined by simulation from the model. An experiment was proposed to distinguish Hypothesis 1 and 2.Key Results: The model successfully descripted multiple types of data under Hypothesis 1. Simulations from the model indicated that precoupling of G protein with receptors is necessary for this hypothesis. The model designed experiments to distinguish Hypothesis 1 and 2 by increasing Gi & Gs in parallel with CB1 overexpression. The two hypotheses result in distinct cAMP responses.Conclusion and Implications: A mathematical model of CB1-regulated Gi/Gs pathways was developed. It indicated Hypothesis 1 is feasible and G protein precoupling is a key step causing cAMP signalling switch. The model-designed experiments provided guides for future experimentation.