Cannabinoid drugs differ in their rank order of potency to produce analgesia versus other central nervous system effects. We propose that these differences are due to unique agonist-bound cannabinoid CB1 receptor conformations that exhibit different affinities for individual subsets of intracellular signal transduction pathways. In order to test this hypothesis, we have used plasmon-waveguide resonance (PWR) spectroscopy, a sensitive method that can provide direct information about ligand-protein and protein-protein interactions, and can detect conformational changes in lipid-embedded proteins. A recombinant epitope-tagged human cannabinoid CB1 receptor was expressed in insect Sf9 cells, solubilized and purified using two-step affinity chromatography. The purified receptor was incorporated into a lipid bilayer on the surface of the PWR resonator. PWR spectroscopy demonstrated that cannabinoid agonists exhibit high affinity (KD=0.2+/-0.03 nM and 2+/-0.4 nM for CP 55,940 and WIN 55,212-2, respectively) for the purified epitope tagged hCB(1) receptor. Interestingly however, these structurally different cannabinoid agonists shifted the PWR spectra in opposite directions, indicating that CP 55,940 and WIN 55,212-2 binding leads to different hCB1 receptor conformations. Furthermore, PWR experiments also indicated that these CP 55,940-and WIN 55,212-bound hCB1 receptor conformations exhibit slightly different affinities to an inhibitory G protein heterotrimer, Gi1 (KD=27+/-8 nM and KD=10.7+/-4.7 nM, respectively), whereas they strikingly differ in their ability to activate this G protein type.