Agonist efficacy and receptor efficiency in heterozygous CB1 knockout mice: relationship of reduced CB1 receptor density to G-protein activation

Heterozygous CB1 receptor knockout mice were used to examine the effect of reduced CB1 receptor density on G-protein activation in membranes prepared from four brain regions: cerebellum, hippocampus, striatum/globus pallidus (striatum/GP) and cingulate cortex. Results showed that CB1 receptor levels were approximately 50% lower in heterozygous mice in all regions examined. However, maximal stimulation of [S-35]guanosine-5'-(gamma -O-thio) triphos phate ([S-35]GTP gammaS) binding by the high efficacy agonist WIN 55,212-2 was reduced by only 20-25% in most brain regions, with the exception of str iatum/GP where the decrease in stimulation was as predicted (approximately 50%). Furthermore, although the efficacies of the cannabinoid partial agoni sts, methanandamide and Delta (9)-tetrahydrocannabinol, were similarly lowe r in heterozygous mice, their relative efficacies compared with WIN 55,212- 2 were generally unchanged. Saturation analysis of net WIN 55,212-2-stimula ted [S-35]GTP gammaS binding showed that decreased stimulation by WIN 55,21 2-2 in striatum/GP of heterozygous mice was caused by a decrease in the app arent affinity of net-stimulated [S-35]GTP gammaS binding. The apparent max imal number of binding sites (B-max) values of net WIN 55,212-2-stimulated [S-35]GTP gammaS binding were unchanged in cerebellum and striatum/GP of he terozygous mice, but decreased in cingulate cortex, with a similar trend in hippocampus. Moreover, in every region except cingulate cortex, the maxima l number of net-stimulated [S-35]GTP gammaS binding sites per receptor was significantly increased in heterozygous mice. These results indicate region -dependent increases in the apparent efficiency of CB1 receptor-mediated G- protein activation in heterozygous CB1 knockout mice.