The actions of benzophenanthridine alkaloids, piperonyl butoxide and (S)-methoprene at the G-protein coupled cannabinoid CB₁ receptor in vitro

This investigation focused primarily on the interaction of two benzophenanthridine Alkaloids (chelerythrine and sanguinarine), piperonyl butoxide and (S)-methoprene with G-protein-coupled cannabinoid CB(1) receptors of mouse brain in vitro. Chelerythrine and sanguinarine inhibited the binding of the CB(1) receptor agonist [(3)H]CP-55940 to mouse whole brain membranes at low micromolar concentrations (IC(50)s: chelerythrine 2.20 μM; sanguinarine 1.10 μM). The structurally related Isoquinoline Alkaloids (berberine and papaverine) and the phthalide isoquinoline ((-)-β-hydrastine) were either inactive or considerably below IC(50) at 30 μM. Chelerythrine and sanguinarine antagonized CP-55940-stimulated binding of [(35)S] GTPγS to the G-protein (IC(50)s: chelerythrine 2.09 μM; sanguinarine 1.22 μM). In contrast to AM251, both compounds strongly inhibited basal binding of [(35)S]GTPγS (IC(50)s: chelerythrine 10.06 μM; sanguinarine 5.19μM). Piperonyl butoxide and S-methoprene inhibited the binding of [(3)H]CP-55940 (IC(50)s: piperonyl butoxide 8.2 μM; methoprene 16.4 μM), and also inhibited agonist-stimulated (but not basal) binding of [(35)S]GTPγS to brain membranes (IC(50)s: piperonyl butoxide 22.5 μM; (S)-methoprene 19.31 μM). PMSF did not modify the inhibitory effect of (S)-methoprene on [(3)H]CP-55940 binding. Our data suggest that chelerythrine and sanguinarine are efficacious antagonists of G-protein-coupled CB(1) receptors. They exhibit lower potencies compared to many conventional CB(1) receptor blockers but act differently to AM251. Reverse modulation of CB(1) receptor agonist binding resulting from benzophenanthridines engaging with the G-protein component may explain this difference. Piperonyl butoxide and (S)-methoprene are efficacious, low potency, neutral antagonists of CB(1) receptors. Certain of the study compounds may represent useful starting structures for development of novel/more potent G-protein-coupled CB(1) receptor blocking drugs.