Reboxetine blocks NMDA receptors in a voltage-, use-, and activity-dependent manner

Abstract Reboxetine is a selective norepinephrine reuptake inhibitor which has initially been approved as an antidepressant. In addition, it has been used off label for panic disorder and attention deficit hyperactivity disorder, particularly when methylphenidate is ineffective. As the action of other known monoamine reuptake inhibitors, including fluoxetine and atomoxetine, turned out to be effective blockers of NMDA receptors, we analyzed whether reboxetine similarly inhibits NMDA receptors. The whole-cell configuration of the patch-clamp technique was used to analyze the effect of reboxetine on recombinant NMDA receptors transiently expressed in tsA201 cells. Reboxetine reversibly blocked NMDA receptors in a concentration-dependent manner with a half-maximal inhibition of 20.4 ± 3.6 µM, when NMDA (50 µM) and glycine (10 µM) were used to activate GluN2A containing receptors. For GluN2B a corresponding value of 12.6 ± 3.1 µM was estimated. Inhibition increased at higher agonist concentrations, consistent with an uncompetitive, activity-dependent mechanism. Furthermore, the block occurred in a voltage- and use-dependent manner, indicating an open-channel blocking mechanism with an interaction site located deep within the channel pore (zδ = 0.94). In addition, a modest subunit dependence was observed, with greater inhibition of GluN2B- than GluN2A-containing receptors. Finally, Reboxetine belongs to a rare group of NMDA receptor antagonists whose efficacy increases with the concentration of the agonist. This activity-dependent profile is therapeutically attractive because it predicts preferential suppression of pathological NMDA-receptor over-activation while leaving low-frequency physiological signaling comparatively unaffected.