Vol. 288, Issue 2, 478-483, February 1999

Large Receptor Reserve for Cannabinoid Actions in the Central Nervous System¹

Andrew N. Gifford, Magalie Bruneus, S. John Gatley, Ruoxi Lan, Alexandros Makriyannis and Nora D. Volkow

Medical Department, Brookhaven National Laboratory, Upton, New York (A.N.G., M.B., S.J.G., N.D.V.); and Department of Pharmaceutical Sciences and Molecular and Cell Biology, University of Connecticut, Storrs, Connecticut (R.L., A.M.)

The receptor occupancy required to produce cannabinoid effects in the central nervous system was determined in both a neurochemical and a behavioral assay for cannabinoid actions. In the neurochemical experiments, performed on superfused rat hippocampal slices, electrically evoked [³H]acetylcholine release was inhibited by the cannabinoid agonist, WIN 55212 to 2 with an EC₅₀ of 0.005 µM and maximum effect of 79%. In parallel experiments examining binding of the radiolabeled CB1 antagonist [¹³¹I]AM 281 {N-(morpholin-4-yl)-5-(4-[¹³¹I]iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide} to living hippocampal slices, WIN 55212 to 2 inhibited [¹³¹I]AM 281 binding with an EC₅₀ of 1.3 µM. From these two sets of data it was determined that 50% of maximal inhibition of [³H]acetylcholine release in hippocampal slices occurs at a receptor occupancy of only 0.13% and 95% of maximal inhibition at a receptor occupancy of 7.5%, suggesting the presence of a receptor reserve that is large compared with other G protein-coupled receptor systems in the central nervous system. In behavioral experiments, WIN 55212 to 2 inhibited spontaneous locomotor activity in mice with an ED₅₀ of 0.3 mg/kg, i.v.. In in vivo binding experiments using [¹³¹I]AM 281, WIN 55212 to 2 failed to produce significant inhibition of radiotracer binding in the mouse brains, except at very high doses (10 mg/kg or greater, i.v.). By contrast, the CB1 antagonist SR 141716A (10 mg/kg, i.p.), completely abolished specific [¹³¹I]AM 281 binding. These experiments suggest that behavioral effects of cannabinoids, like neurochemical effects, are produced at very low receptor occupancy.