N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)4-methyl-1H-pyr
azole-3-carboxamide hydrochloride (SR141716A), a cannabinoid CB1 receptor a
ntagonist, has inverse agonist effects in cannabinoid CB1 receptor-expressi
ng cell lines, brain and peripheral organs. These studies characterized SR1
41716A-inhibited G-protein activity by measuring [S-35]GTP gammaS binding.
Maximal inhibition of basal [S-35]GTP gammaS binding in cerebellar membrane
s was 50%. The EC50 value for inhibition of [S-35]GTP gammaS binding was 4.
4 muM, whereas the K-c for inhibition of R(+)-[2,3-dihydro-5-methyl-3-[(mor
pholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methano
ne mesylate (WIN 55,212-2)-stimulated [S-35]GTP gammaS binding was 0.6 nM.
[S-35]GTP gammaS autoradiography was used to examine the regional specifici
ty of SR141716A inhibition. SR141716A inhibited basal [S-35]GTP gammaS bind
ing in all regions examined, with inhibition ranging from approximately 20%
in caudate-putamen to 40% in hippocampus. These studies demonstrate that S
R141716A is a competitive antagonist at nanomolar concentrations, whereas i
t inhibits basal receptor-mediated G-protein activity at micromolar concent
rations. These data suggest that the apparent inverse agonist effect is eit
her not cannabinoid CB1 receptor-specific or that SR141716A is binding to d
ifferent sites on the cannabinoid CB1 receptor to produce inverse agonist v
ersus competitive antagonist effects. (C) 2001 Elsevier Science B.V. All ri
ghts reserved.