Rimonabant, a potent CB1 cannabinoid receptor antagonist, is a G?(i/o) protein inhibitor

Rimonabant is a potent and selective cannabinoid CB1 receptor antagonist widely used in animal and clinical studies. Besides its antagonistic properties, numerous studies have shown that, at micromolar concentrations rimonabant behaves as an inverse agonist at CB1 receptors. The mechanism underpinning this activity is unclear. Here we show that micromolar concentrations of rimonabant inhibited G?i/o-type G proteins, resulting in a receptor-independent block of G protein signaling. Accordingly, rimonabant decreased basal and agonist stimulated [35S]GTP?S binding to cortical membranes of CB1- and GABAB-receptor KO mice and Chinese Hamster Ovary (CHO) cell membranes stably transfected with GABAB or D2 dopamine receptors. The structural analog of rimonabant, AM251, decreased basal and baclofen-stimulated GTP?S binding to rat cortical and CHO cell membranes expressing GABAB receptors. Rimonabant prevented G protein-mediated GABAB and D2 dopamine receptor signaling to adenylyl cyclase in Human Embryonic Kidney 293 cells and to G protein-coupled inwardly rectifying K+ channels (GIRK) in midbrain dopamine neurons of CB1 KO mice. Rimonabant suppressed GIRK gating induced by GTP?S in CHO cells transfected with GIRK, consistent with a receptor-independent action. Bioluminescent resonance energy transfer (BRET) measurements in living CHO cells showed that, in presence or absence of co-expressed GABAB receptors, rimonabant stabilized the heterotrimeric G?i/o-protein complex and prevented conformational rearrangements induced by GABAB receptor activation. Rimonabant failed to inhibit G?s-mediated signaling, supporting its specificity for G?i/o-type G proteins. The inhibition of G?i/o protein provides a new site of rimonabant action that may help to understand its pharmacological and toxicological effects occurring at high concentrations.