Rimonabant, a potent CB1 cannabinoid receptor antagonist, is a G alpha(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 underpin- ning this activity is unclear. Here we show that micromolar concentrations of rimonabant inhibited Gai/o- type G proteins, resulting in a receptor-independent block of G protein signaling. Accordingly, rimona- bant decreased basal and agonist stimulated [35S]GTPgS 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 GTPgS 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 Kþ channels (GIRK) in midbrain dopamine neurons of CB1 KO mice. Rimonabant suppressed GIRK gating induced by GTPgS in CHO cells transfected with GIRK, consistent with a receptor-independent action. Biolumines- cent resonance energy transfer (BRET) measurements in living CHO cells showed that, in presence or absence of co-expressed GABAB receptors, rimonabant stabilized the heterotrimeric Gai/o-protein complex and prevented conformational rearrangements induced by GABAB receptor activation. Rimo- nabant failed to inhibit Gas-mediated signaling, supporting its specificity for Gai/o-type G proteins. The inhibition of Gai/o protein provides a new site of rimonabant action that may help to understand its pharmacological and toxicological effects occurring at high concentrations