Cannabinoid agonists stimulate both receptor- and non-receptor-mediated signal transduction pathways in cells transfected with and expressing cannabinoid receptor clones.

The physiologic activity of (-)-delta 9-tetrahydrocannabinol, the most active component of marijuana, and of many synthetic cannabimimetics may be mediated either through receptor binding and functional coupling to specific signal transduction pathways or through nonspecific interaction with cell membrane components. The cloning of the human and rat cannabinoid receptors has provided the opportunity to investigate the binding properties and signal transduction pathways directly associated with these receptors. Cannabinoid receptor cDNA was transfected into and stably expressed in fibroblast cell lines that do not contain native cannabinoid receptors, thus allowing comparison with untransfected cells. Binding constants measured using [3H]CP55,940 indicated that the rat and human cloned cannabinoid receptors were similar to native cannabinoid receptors measured in brain and neural cell lines. The cloned receptors coupled to the inhibition of cAMP accumulation, as previously demonstrated. CP55,940 binding and inhibition of cAMP accumulation were absent in untransfected cells. Cannabinoid agonist-stimulated release of arachidonic acid and increase in intracellular calcium were observed in both transfected and untransfected cells. Stereoselectivity of cannabinoid agonists was demonstrated for binding and functional inhibition of cAMP accumulation, but not for the release of arachidonic acid and intracellular calcium. Therefore, cannabinoid agonists can stimulate signaling pathways through both receptor- and non-receptor-mediated pathways in the same cell.