Differential receptor-G-protein coupling evoked by dissimilar cannabinoid receptor agonists.

Multiple affinity states are revealed by agonist competition for radioantagonist [3H]SR141716A binding to rat brain CB1 cannabinoid receptors. Desacetyllevonantradol (DALN), a tricyclic cannabinoid, and WIN55212-2, an aminoalkylindole, both bound in two discrete affinity states (30% high affinity), but the ratios of the IC50 revealed distinct differences. Other affinity-state differences between the agonists were: Na+ reduced the affinity for the membrane-bound receptor by 10-fold for DALN but minimally for WIN55212-2; a nonhydrolysable GTP analogue decreased the fraction of high-affinity WIN55212-2 binding but not that of DALN unless Na+ was also present. Detergent solubilisation increased the fraction of high-affinity binding for both agonists but eliminated any effect of Na+ on the agonist affinities. In detergent solution, the GTP analogue reduced the WIN55212-2 high-affinity fraction but not that of DALN, even though the IC50 values increased for both DALN and WIN55212-2. The differential modulation of CB1 receptor-G-protein coupling by Na+ and guanine nucleotides is dependent upon the cannabimimetic agonist bound.