No Evidence for Endocannabinoid-Induced G Protein Subtype Selectivity at Human and Rodent Cannabinoid CB₁ Receptors

Area of Science:

• Neuroscience
• Pharmacology
• Molecular Biology

Background:

• The endocannabinoid system (ECS) utilizes endogenous ligands like arachidonoyl ethanolamide (AEA) and 2-arachidonoyl-glycerol (2-AG).
• Cannabinoid receptor 1 (CB1) is a key G protein-coupled receptor in the ECS, with identified human variants (hCB1, hCB1a, hCB1b).
• CB1's interaction with specific inhibitory G proteins (G$_{i/o}$) is not fully understood.

Purpose of the Study:

• To compare AEA and 2-AG-induced activation of various G protein subtypes at CB1.
• To compare responses across human CB1 variants (hCB1, hCB1a, hCB1b).
• To explore species differences using rodent CB1 receptors (mCB1, rCB1).

Main Methods:

• Activation of G protein subtypes in HEK293 cells expressing CB1 was measured using TRUPATH biosensors and a G protein dissociation assay.
• Biosensor performance was validated using Z-factor analysis.
• Pathway potencies and efficacies were analyzed to determine G protein subtype selectivity for AEA and 2-AG.

Main Results:

• AEA acted as a partial agonist, while 2-AG acted as a full or high-efficacy agonist across pathways.
• Direct evidence confirmed hCB1 receptor coupling to G$_{12}$ and G$_{13}$ proteins.
• No evidence of G protein subtype selectivity was found for either ligand across human variants or rodent receptors.

Conclusions:

• AEA and 2-AG do not exhibit G protein subtype selectivity at the CB1 receptor.
• This finding is consistent across human CB1 variants (hCB1, hCB1a, hCB1b) and rodent receptors (mCB1, rCB1).
• The study provides a comprehensive comparison of ligand-activated G protein coupling profiles at CB1.