Evaluation of the cannabinoid CB2 receptor-selective antagonist, SR144528:: further evidence for cannabinoid CB2 receptor absence in the rat central nervous system

The aim of this study was to characterize the activity of the cannabinoid CB2 receptor selective antagonist, N-{(1S)-endo-1,3,3-trimethyl bicyclo[2.2.1] heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide} (SR144528) in a number of biochemical assays and to look for evidence of cannabinoid CB2 receptors in the rat central nervous system. SR144528 displaced [H-3]CP 55,940 ((-)-3-[2-hydroxyl-4-(1,1-dimethylheptyl)-phenyl]-4-[3-hydroxypropyl]cyclohexan-1-ol) from binding sites in CB2- and CB1-transfected cells (K-i = 0.67 +/- 0.30 and 33.0 +/- 5.09 nM) and from rat cerebellum and whole brain membrane homogenates (K-i = 54.7 +/- 9.70 and 54.8 +/- 7.86 nM). In the GTP gamma S binding assay, SR144528 antagonized a number of cannabinoid receptor agonists (K-B values ranging from 26.3 to 76.6 nM) in rat cerebellar membranes and in rat whole brain membranes (K-B = 50.8 nM). SR144528 also antagonized CP 55,940-stimulated GTP gamma S binding in a CB2-expressing cell line (K-B = 6.34 nM). In Xenopus oocytes co-expressing the CB1 receptor and G-protein coupled inwardly rectifying K+ channels (GIRK 1/4), SR144528 antagonized WIN 55212-2 ((R)-(+)-[2,3-dihydro-5 -methyl-3-[(4-morpholinyl)methyl]pyrolo[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthalenyl)methanone)-stimulated currents (K-B = 558 nM). In summary, this report characterizes the cannabinoid CB2 receptor-selective cannabinoid antagonist, SR144528, and additionally suggests an absence of cannabinoid CB2 receptors in the rat central nervous system, an observation confirmed by Northern blot. (C) 1999 Elsevier Science B.V. All rights reserved.