Reciprocal inhibition of G-protein signaling is induced by CB1 cannabinoid and GABAB receptor interactions in rat hippocampal membranes

Cannabinoid CB1 and the metabotropic GABAB receptors have been shown to display similar pharmacological effects and co-localization in certain brain regions. Previous studies have reported a functional link between the two systems. As a first step to investigate the underlying molecular mechanism, here we show cross-inhibition of G-protein signaling between GABAB and CB1 receptors in rat hippocampal membranes. The CB1 agonist R-Win55,212-2 displayed high potency and efficacy in stimulating guanosine-5′-O-(3-[35S]thio)triphosphate, [35S]GTPγS binding. Its effect was completely blocked by the specific CB1 antagonist AM251 suggesting that the signaling was via CB1 receptors. The GABAB agonists baclofen and SKF97541 also elevated [35S]GTPγS binding by about 60%, with potency values in the micromolar range. Phaclofen behaved as a low potency antagonist with an ED50 ≈ 1 mM. However, phaclofen at low doses (1 and 10 nM) slightly but significantly attenuated maximal stimulation of [35S]GTPγS binding by the CB1 agonist R-Win55,212-2. The observation that higher concentrations of phaclofen had no such effect rule out the possibility of its direct action on CB1 receptors. The pharmacologically inactive stereoisomer S-Win55,212-3 had no effect either alone or in combination with phaclofen establishing that the interaction is stereospecific in hippocampus. The specific CB1 antagonist AM251 at a low dose (1 nM) also inhibited the efficacy of G-protein signaling of the GABAB receptor agonist SKF97541. Cross-talk of the two receptor systems was not detected in either spinal cord or cerebral cortex membranes. It is speculated that the interaction might occur via an allosteric interaction between a subset of GABAB and CB1 receptors in rat hippocampal membranes. Although the exact molecular mechanism of the reciprocal inhibition between CB1 and GABAB receptors will have to be explored by future studies it is intriguing that the cross-talk might be involved in balance tuning the endocannabinoid and GABAergic signaling in hippocampus.