Abstract
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 α(2A)-adrenoceptors (α(2A)Rs), as measured in an in vitro GTPγS G protein assay. Mitragynine's in vitro α(2A)R antagonist pharmacology contrasts with rat behavioral pharmacology studies that suggest mitragynine behaves as an in vivo agonist at rat α(2)Rs. 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 Gα(i/o) proteins coupled to α(2A), α(2B), and α(2C)Rs, as well as Gα(q) and Gα(11) coupled to α(1A)R. Additionally, we examined rat and human α(2A)R-mediated cAMP inhibition, α(2A)R-mediated β-arrestin2 recruitment, and tested α(2)R or α(1)R-mediated ERK phosphorylation in wild-type, β-arrestin 1/2 knockout, and Gα(q/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 α(2)Rs, but, aligned with our previous GTPγS results, demonstrate that mitragynine is a low-potency, competitive α(2A)R antagonist at Gα(i1), cAMP, and β-arrestin2 transducers. However, we show that mitragynine is a low-potency (EC(50) ∼3 μM), partial agonist at α(1A)R-Gα(11) and stimulates ERK phosphorylation via Gα(q/11)-coupled α(1)Rs, supporting in vivo studies that suggest mitragynine is an α(1)R agonist. Nevertheless, the agonist effects of mitragynine at α(1A)R-Gα(11) were modest compared to clonidine, a partial agonist control that also activated all α(2)R transducers. Mitragynine's dual α(2A)R antagonist/α(1)R partial agonist pharmacology might contribute to mitragynine's psychostimulant-like properties.