RECOMMENDED ASSAY CONDITIONS: Cells are plated at ~50,000 cells per well in a 96-well clear bottom, black-walled assay plate in growth media lacking Genetecin/G418, and incubated overnight at 37°C, 5% CO2. On the day of the assay, loading media is prepared by mixing equal volumes Fluo-4 (1 mM stock in DMSO) and Pluronic F-127 (20% in DMSO), then diluting 1:500 in growth media (lacking Genetecin/G418, containing 2.5 mM probenecid), to achieve a final Fluo-4 concentration of 1 mM. Cells are incubated for 1 h in loading media at 37°C, 5% CO2, and washed 3 times, 200 mL/well each time, with Assay Buffer (HBSS with Ca++ and Mg++/20 mM HEPES/2.5 mM probenecid, added on the day of the assay). Assay Buffer is added (100 mL/well) and cells are returned to 37°C, 5% CO2. CB1 ligands were prepared as 5 mM stocks in 50% ethanol, and the 3x working stocks of the dilution series were prepared in prewarmed Assay Buffer containing 1% ethanol. Additions of 50 mL 3x working ligand stock to cells in 100 mL Assay Buffer were made with a Flex StationTM (Molecular Devices), and fluorescence was measured at ex. 485 nm, em. 525 nm.
Note: Cannabinoid ligands are poorly soluble in aqueous solution, and typically need to be dissolved in an organic solvent (DMSO or ethanol) before addition to assay buffer. However, such solvents can interfere with detection of intracellular calcium, typically at concentrations exceeding 1% DMSO or ethanol. In order to determine the optimal concentration of solvent to achieve effective ligand solubility with minimal solvent effect on calcium measurements, controls for solvent effects, containing solvent at concentrations equal to that in ligand solutions, should always be run in parallel with ligands.