SR 141716A acts as an inverse agonist to increase neuronal voltage-dependent Ca2+ currents by reversal of tonic CB1 cannabinoid receptor activity

The CB1 cannabinoid receptor antagonist SR 141716A abolished the inhibition of Ca2+ currents by the agonist WIN 55,212-2. However, SR 141716A alone in creased Ca2+ currents, with an EC50 of 32 nM, in neurons that had been micr oinjected with CBI cRNA. For an antagonist to elicit an effect, some recept ors must be tonically active. Evidence for tonically active CBI receptors w as seen as enhanced tonic inhibition of Ca2+ currents. Preincubation with a nandamide failed to enhance the effect of SR 141716A, indicating that anand amide did not cause receptor activity. Under Ca2+-free conditions designed to block the Ca2+-dependent formation of anandamide and sn-2-arachidonylgly cerol, SR 141716A again increased the Ca2+ current. The Ca2+ current was to nically inhibited in neurons expressing the mutant K192A receptor, which ha s no affinity for anandamide, demonstrating that this receptor is also toni cally active. SR 141716A had no effect on the Ca2+ current in these neurons , but SR 141716A could still antagonize the effect of WIN 55,212-2. Thus, t he K192 site is critical for the inverse agonist activity of SR 141716A. SR 141716A appeared to become a neutral antagonist at the K192A mutant recept or. Native cannabinoid receptors were studied in male rat major pelvic gang lion neurons, where it was found that WIN 55,212-2 inhibited and SR 141716A increased Ca2+ currents. Taken together, our results demonstrate that a po pulation of native and cloned CBI cannabinoid receptors can exist in a toni cally active state that can be reversed by SR 141716A, which acts as an inv erse agonist.