Solubilization of the cannabinoid receptor from rat brain and its functional interaction with guanine nucleotide-binding proteins

The present investigation was undertaken to characterize cannabinoid receptor binding in the absence of the membrane environment, inasmuch as cannabinoid drugs have been noted to influence the behavior of integral membrane proteins. The zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) was able to solubilize the cannabinoid receptor from rat brain membranes, with the greatest yield and specific activity being obtained at a detergent/protein ratio of 0.5:1. [3H]CP-55940 bound to a single class of binding sites in the CHAPS extract, which exhibited a Kd of 0.94 nM as determined by nonlinear regression analysis of equilibrium binding data. The order of potency for cannabinoid agonists in heterologous equilibrium binding studies was CP-55244 > or = desacetyllevonantradol > delta 9-tetrahydrocannabinol > cannabinol > cannabidiol, consistent with the relative affinities for these agonists in brain membrane preparations. CP-55243, the biologically inactive enantiomer of CP-55244, competed for binding of [3H]CP-55940 by < 50% at 1 microM, similar to its poor affinity for the receptor in membranes. The CHAPS-solubilized cannabinoid receptor exhibited functional interactions with guanine nucleotide-binding proteins (G proteins). GTP and nonhydrolyzable analogs decreased [3H]CP-55940 binding by 75%. The concentration-effect curves for guanine nucleotides exhibited a potency order similar to that observed for other G protein-linked receptors. Kinetic analyses indicated that GTP analogs increased the rate of agonist dissociation, decreasing the t1/2 from 60 min at 0-4 degrees to a multiphasic dissociation that exhibited a component having a t1/2 of < 1 min. The cannabinoid agonist desacetyllevonantradol was able to reduce pertussis toxin-catalyzed ADP-ribosylation of G proteins by 50%, demonstrating a receptor effect on G protein functions. These studies demonstrate that the membrane environment is not necessary for agonist binding to the cannabinoid receptor. Furthermore, the cannabinoid receptor maintains its functional interactions with pertussis toxin-sensitive G proteins in detergent solution.