Abstract
To understand how structurally distinct ligands regulate CB1 receptor interactions with Gi1, Gi2, and Gi3, we quantified the Gαi and βγ proteins that coimmunoprecipitate with the CB1 receptor from a detergent extract of N18TG2 membranes in the presence of ligands. A mixture of A, R, GGDP (or G_), and ARGGDP (or ARG_) complexes was observed in the presence of aminoalkylindole (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN 55,212-2) for all three RGαi complexes, cannabinoid desacetyllevonantradol for Gαi1 and Gαi2, and eicosanoid (R)-methanandamide for Gαi3. Desacetyllevonantradol maintained RGαi3 complexes and (R)-methanandamide maintained RGαi1 and RGαi2 complexes even in the presence of a nonhydrolyzable GTP analog. The biaryl pyrazole antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboximide hydrochloride (SR141716) maintained all three RGαi complexes. Gβ proteins, and to a certain extent Gγ2, exhibited the same association/dissociation pattern as the Gα proteins. A GDP analog had no influence on any of these association/dissociation reactions and failed to promote sequestration of G proteins. These results can be explained by invoking the existence of an inverse agonist-supported inactive state in the ternary complex equilibrium model. WIN 55,212-2 behaves as an agonist for all three Gi subtypes; SR141716 behaves as an inverse agonist for all three Gi subtypes; desacetyllevonantradol behaves as an agonist for Gi1 and Gi2, and an inverse agonist at Gi3; and (R)-methanandamide behaves as an inverse agonist at Gi1 and Gi2, and an agonist at Gi3. These ligand-selective G protein responses imply that multiple conformations of the receptor could be evoked by ligands to regulate individual G proteins.
- Received August 4, 2004.
- Accepted March 4, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
MolPharm articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|