AM630 behaves as a protean ligand at the human cannabinoid CB2 receptor

BACKGROUND AND PURPOSE We have investigated how pre-incubating hCB(2) CHO cells with the CB2 receptor antagonists/inverse agonists, AM630 and SR144528, affects how these and other ligands target hCB(2) receptors in these cells or their membranes. EXPERIMENTAL APPROACH We tested the ability of AM630, SR144528 and of the CB1/CB2 receptor agonists, CP55940 and R-(+)-WIN55212, to modulate forskolin-stimulated cAMP production in hCB(2) CHO cells or [S-35]-GTP gamma S binding to membranes prepared from these cells, or to displace [H-3]-CP55940 from whole cells and membranes. Assays were also performed with the CB2 receptor partial agonist, Delta(9)-tetrahydrocannabivarin. Some cells were pre-incubated with AM630 or SR144528 and then washed extensively. KEY RESULTS AM630 behaved as a low-potency neutral competitive antagonist in AM630-pre-incubated cells, a low-potency agonist in SR144528-pre-incubated cells, and a much higher-potency inverse agonist/antagonist in vehicle-pre-incubated cells. AM630 pre-incubation (i) reduced the inverse efficacy of SR144528 without abolishing it; (ii) increased the efficacy of D9-tetrahydrocannabivarin; and (iii) did not affect the potency with which AM630 displaced [H-3]-CP55940 from whole cells or its inverse agonist potency and efficacy in the [S-35]-GTPgS membrane assay. CONCLUSIONS AND IMPLICATIONS These results suggest that AM630 is a protean ligand that can target a constitutively active form of the hCB(2) receptor (R*) with low affinity to produce agonism or neutral antagonism and a constitutively inactive form of this receptor (R) with much higher affinity to produce inverse agonism, and that the constitutive activity of whole cells is decreased less by pre-incubation with AM630 than with the higher-efficacy inverse agonist, SR144528.