Background
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CB~1~and CB~2~cannabinoid receptors are G protein-coupled receptors which have been described to couple mainly to the G~i/o~subfamily of G proteins. However, in some cell types and upon stimulation with certain cannabinoid agonists, activation of other G protein subtypes has also been observed. GPR55 is an orphan G protein-coupled receptor which has been suggested to be a novel member of the cannabinoid receptor family.

Methods
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In this study we wanted to characterize the G protein activation properties of the two known cannabinoid receptors and GPR55 following stimulation with different cannabinoid ligands, using bioluminescence resonance energy transfer (BRET). We monitored the activation of different G protein subtypes (G~o~, G~q~, G~s~or G~12~) using *Renilla*luciferase-tagged wild type or chimeric Gα~o~subunits (i.e. Gα~o~with the C-terminal 5 amino acids replaced with those of Gα~q~, Gα~s~or Gα~12~, respectively) co-expressed with EYFP-tagged α~1~α~11~subunit and the receptor in CHO cells.

Results
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We found that CB~1~was able to activate all four subtypes of G proteins, with different pharmacokinetic properties, following stimulation by non-selective (WIN55 and 2-AG) or CB~1~-selective (ACEA) cannabinoid agonists. Basal activity of CB~1~could also be detected with G~o~and G~12~subtypes, as the CB~1~inverse agonist AM251 caused significant BRET increase (i.e. G protein subunit association) when tested with these G proteins. In contrast, CB~2~showed no G protein activation other than G~o~, upon either WIN55 or 2-AG stimuli. Stimulation of GPR55 with WIN55, 2-AG or AM251 did not alter the activity of the tested G proteins even at considerably high ligand concentrations.
