The effects of cannabinoid drugs on cAMP production were examined in mammalian brain. The cannabinoid receptor agonist (R)-(+)-[2,3-dihydro-5-methyl-3-[(4-morpholinyl)methyl]pyrrolo[1,2,3,-d,e-1,4-benzoxazin-6-yl]-(1-naphthalenyl) methanone (WIN55,212-2) decreased forskolin-induced cAMP accumulation in a concentration-dependent manner (10(-8)-10(-5) M) in membranes from several rat and human brain regions, this effect being antagonized by 10(-5) M N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A). Furthermore, high micromolar concentrations of SR141716A evoked a dose-dependent increase in basal cAMP in rat cerebellum and cortex, as well as in human frontal cortex. This effect was antagonized by WIN55,212-2 and abolished by N-ethylmaleimide, consistent with the involvement of cannabinoid CB(1) receptors through the activation of G(i/o) proteins. These results suggest a ligand-independent activity for cannabinoid CB(1) receptor signaling cascade in mammalian brain.