Low Ouabain Concentration Differentially Modulates Spontaneous and Evoked Synaptic Transmission: Role of NCX Subtypes

Low concentrations of ouabain exert neuromodulatory effects beyond classical Na+/K+-ATPase inhibition, but the underlying mechanisms remain incompletely understood. Here, we investigated the involvement of Na+/Ca2+ exchangers (NCXs) in the modulation of spontaneous and evoked excitatory synaptic transmission by 1 nM ouabain. In cultured cortical neurons, ouabain reduced miniature excitatory postsynaptic current (mEPSC) frequency by approximately 4-fold without affecting amplitude. This effect was prevented by the broad-spectrum NCX inhibitor CB-DMB but persisted in the presence of the preferential NCX1 inhibitor SEA0400. In hippocampal CA1, ouabain increased the slope of evoked field excitatory postsynaptic potentials (fEPSPs) by approximately 34%. SEA0400 also potentiated fEPSPs, with additional potentiation observed upon sequential application of ouabain and SEA0400. In contrast, CB-DMB (NCX1–3 inhibitor) increased the fEPSP slope but prevented further potentiation by ouabain. These findings demonstrate differential modulation of spontaneous and evoked excitatory synaptic transmission by nanomolar ouabain and support the involvement of NCX-sensitive mechanisms, potentially including NCX2 and/or NCX3. Our results suggest that functional coupling between Na+/K+-ATPase and NCX-dependent Na+ and Ca2+ homeostasis contributes to the neuromodulatory effects of low ouabain concentrations.