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 [(35)S]guanosine-5'-(gamma-O-thio) triphosphate ([(35)S]GTPgammaS) binding by the high efficacy agonist WIN 55,212-2 was reduced by only 20-25% in most brain regions, with the exception of striatum/GP where the decrease in stimulation was as predicted (approximately 50%). Furthermore, although the efficacies of the cannabinoid partial agonists, methanandamide and (9)-tetrahydrocannabinol, were similarly lower in heterozygous mice, their relative efficacies compared with WIN 55,212-2 were generally unchanged. Saturation analysis of net WIN 55,212-2-stimulated [(35)S]GTPgammaS binding showed that decreased stimulation by WIN 55,212-2 in striatum/GP of heterozygous mice was caused by a decrease in the apparent affinity of net-stimulated [(35)S]GTPgammaS binding. The apparent maximal number of binding sites (B(max)) values of net WIN 55,212-2-stimulated [(35)S]GTPgammaS binding were unchanged in cerebellum and striatum/GP of heterozygous mice, but decreased in cingulate cortex, with a similar trend in hippocampus. Moreover, in every region except cingulate cortex, the maximal number of net-stimulated [(35)S]GTPgammaS 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.
Selley
D E
DE
Department of Pharmacology and Toxicology and Institute for Drug and Alcohol Studies, Virginia Commonwealth University, Richmond, Virginia, USA. deselley@hsc.vcu.edu
Rorrer
W K
WK
Breivogel
C S
CS
Zimmer
A M
AM
Zimmer
A
A
Martin
B R
BR
Sim-Selley
L J
LJ
D000818:Animals; D001095:Arachidonic Acids / Q000494:pharmacology; D048588:Benzoxazines; D001665:Binding Sites; D001921:Brain / Q000187:drug effects / Q000378:metabolism*; D002186:Cannabinoids / Q000494:pharmacology*; D002462:Cell Membrane / Q000378:metabolism; D002531:Cerebellum / Q000378:metabolism; D003342:Corpus Striatum / Q000378:metabolism; D003433:Crosses, Genetic; D013759:Dronabinol / Q000494:pharmacology; D005260:Female; D019204:GTP-Binding Proteins / Q000378:metabolism*; D005917:Globus Pallidus / Q000378:metabolism; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006179:Gyrus Cinguli / Q000378:metabolism; D006579:Heterozygote; D006624:Hippocampus / Q000378:metabolism; D007700:Kinetics; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009025:Morpholines / Q000494:pharmacology; D009281:Naphthalenes / Q000494:pharmacology; D009928:Organ Specificity; D043882:Receptors, Cannabinoid; D011955:Receptors, Drug / Q000172:deficiency / Q000235:genetics / Q000502:physiology*
D001095:Arachidonic Acids; D048588:Benzoxazines; D002186:Cannabinoids; D009025:Morpholines; D009281:Naphthalenes; D043882:Receptors, Cannabinoid; D011955:Receptors, Drug; C088155:methanandamide; D016244:Guanosine 5'-O-(3-Thiotriphosphate); C070417:(3R)-((2,3-dihydro-5-methyl-3-((4-morpholinyl)methyl)pyrrolo-(1,2,3-de)-1,4-benzoxazin-6-yl)(1-naphthalenyl))methanone; D013759:Dronabinol; D019204:GTP-Binding Proteins