Dual intracellular signaling pathways mediated by the human cannabinoid CB1 receptor

It has long been established that the cannabinoid CB1 receptor transduces signals through a pertussis toxin-sensitive G(i)/G(o) inhibitory pathway. Although there have been reports that the cannabinoid CB, receptor can also mediate an increase in cyclic AMP levels, in most cases the presence of an adenylyl cyclase costimulant or the use of very high amounts of agonist was necessary. Here, we present evidence for dual coupling of the cannabinoid CB1 receptor to the classical pathway and to a pertussis toxin-insensitive adenylyl cyclase stimulatory pathway initiated with low quantities of agonist in the absence of any costimulant. Treatment of Chinese hamster ovary (CHO) cells expressing the cannabinoid CB, receptor with the cannabinoid CP 55,940, {(-)-cis-3-[2-hydroxy-4-(1,1-dimethyl-heptyl)phenyl]-trans-4-(3-hydroxypropyl) cydohexan-1-ol} resulted in cyclic AMP accumulation in a dose-response manner, an accumulation blocked by the cannabinoid CB1 receptor-specific antagonist SR 141716A, {N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl- 1H-pyrazole-3-carboxamide hydrochloride}. In CHO cells coexpressing the cannabinoid CB1 receptor and a cyclic AMP response element (CRE)-luciferase reporter gene system, CP 55,940 induced luciferase expression by a pathway blocked by the protein kinase A inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamid hydrochloride (H-89). Under the same conditions the peripheral cannabinoid CB, receptor proved to be incapable of inducing cAMP accumulation or luciferase activity. This incapacity allowed us to study the luciferase activation mediated by CB1/CB2 chimeric constructs, from which we determined that the first and second internal loop regions of the cannabinoid CB, receptor were involved in transducing the pathway leading to luciferase gene expression. (C) 1999 Elsevier Science B.V. All rights reserved.