Serotonin reuptake inhibitor suppresses the activation of human platelets by a combination of thrombopoietin and collagen through inhibition of Rac and Rho/Rho-kinase

Tramadol and duloxetine, reuptake inhibitors of serotonin and noradrenaline, are widely used analgesics. Cytoplasmic serotonin in human platelets reportedly regulates the activity of low-molecular-weight GTP-binding proteins via serotonylation, leading to the modulation of platelet functions. We recently showed that the combination of thrombopoietin and collagen in the low doses synergistically induces human platelet activation via Rac and Rho/Rho-kinase. In the present study, we investigated the effects of tramadol and duloxetine on the synergistic effect, and the mechanism. Tramadol reduced the platelet aggregation and the release of PDGF-AB by the combination of thrombopoietin and collagen in the low doses. The aggregation and the release were also inhibited by duloxetine. Not reboxetine, a specific inhibitor of noradrenaline transporter, but fluvoxamine and sertraline, specific inhibitors of serotonin transporter suppressed the aggregation and the re-lease. Tramadol, duloxetine, fluvoxamine and sertraline but not reboxetine attenuated the levels of GTP-Rac and GTP-Rho, and phospho-cofilin induced by the combination. Taken together, our results strongly suggest that tramadol and duloxetine, not as noradrenaline reuptake inhibitor but as serotonin reuptake inhibitor, suppress the activation of Rac and Rho/Rho-kinase elicited by the combination of subthreshold thrombopoietin and collagen, leading to the attenuation of human platelet activation.