In Vitro Pharmacology of Mitragynine at α-Adrenoceptors.

Mitragynine is a psychoactive alkaloid in Mitragyna speciosa with unique polypharmacology at G protein-coupled receptors. In addition to its well-known partial agonist activity at opioid receptors, mitragynine is an antagonist at human α2A-adrenoceptors (α2ARs), as measured in an in vitro GTPγS G protein assay. Mitragynine's in vitro α2AR antagonist pharmacology contrasts with rat behavioral pharmacology studies that suggest mitragynine behaves as an in vivo agonist at rat α2Rs. This study investigates this apparent discrepancy using recombinant α-adrenoceptors and a range of orthogonal signal transducers. We evaluated whether mitragynine activates any of seven Gαi/o proteins coupled to α2A, α2B, and α2CRs, as well as Gαq and Gα11 coupled to α1AR. Additionally, we examined rat and human α2AR-mediated cAMP inhibition, α2AR-mediated β-arrestin2 recruitment, and tested α2R or α1R-mediated ERK phosphorylation in wild-type, β-arrestin 1/2 knockout, and Gαq/11 knockout cells. Finally, we report binding and enzyme-inhibition profiling results for mitragynine and its major metabolites, 7-Hydroxymitragynine and 9-Hydroxycorynantheidine, at 99 targets. The results did not support the hypothesis that mitragynine (or its primary metabolites) activates α2Rs, but, aligned with our previous GTPγS results, demonstrate that mitragynine is a low-potency, competitive α2AR antagonist at Gαi1, cAMP, and β-arrestin2 transducers. However, we show that mitragynine is a low-potency (EC50 ∼3 μM), partial agonist at α1AR-Gα11 and stimulates ERK phosphorylation via Gαq/11-coupled α1Rs, supporting in vivo studies that suggest mitragynine is an α1R agonist. Nevertheless, the agonist effects of mitragynine at α1AR-Gα11 were modest compared to clonidine, a partial agonist control that also activated all α2R transducers. Mitragynine's dual α2AR antagonist/α1R partial agonist pharmacology might contribute to mitragynine's psychostimulant-like properties.