Background and purpose: Cannabis is the source of at least seventy phytocannabinoids. The pharmacology of most of these has been little investigated, three notable exceptions being Δ9-tetrahydrocannabinol, cannabidiol and Δ9-tetrahydrocannabivarin. This investigation addressed the question of whether the little-studied phytocannabinoid, cannabigerol, can activate or block any G protein-coupled receptor.
Experimental approach: The [35S]GTPγS binding assay, performed with mouse brain membranes, was used to test the ability of cannabigerol to produce G protein-coupled receptor activation or blockade. Its ability to displace [3H]CP55940 from mouse CB1 and human CB2 cannabinoid receptors and to inhibit electrically evoked contractions of the mouse isolated vas deferens was also investigated.
Key results: In the brain membrane experiments, cannabigerol behaved as a potent α2-adrenoceptor agonist (EC50= 0.2 nM) and antagonized the 5-HT1A receptor agonist, R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin (apparent KB= 51.9 nM). At 10 µM, it also behaved as a CB1 receptor competitive antagonist. Additionally, cannabigerol inhibited evoked contractions of the vas deferens in a manner that appeared to be α2-adrenoceptor-mediated (EC50= 72.8 nM) and displayed significant affinity for mouse CB1 and human CB2 receptors.
Conclusions and implications: This investigation has provided the first evidence that cannabigerol can activate α2-adrenoceptors, bind to cannabinoid CB1 and CB2 receptors and block CB1 and 5-HT1A receptors. It will now be important to investigate why cannabigerol produced signs of agonism more potently in the [35S]GTPγS binding assay than in the vas deferens and also whether it can inhibit noradrenaline uptake in this isolated tissue and in the brain.