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Dysfunction of the Na+,K+-ATPase (Na+,K+-pump), due to reduced energy supply or increased endogenous ouabain-like inhibitors, likely occurs under pathological conditions in the central nervous system. In cultured mouse cortical neurons, we examined the hypothesis that a mild non-toxic inhibition of the Na+,K+-ATPase could synergistically sensitize the vulnerability of neurons to normally non-lethal apoptotic signals. Ouabain at a low concentration of 0.1 μM slightly lessened the Na+,K+-pump activity measured as an ouabain-sensitive current, yet did not affect K+ homeostasis and viability of cortical neurons. Co-exposure to 0.1 μM ouabain plus non-lethal C2-ceramide (5 μM) or β-amyloid 1–42 (5 μM), however, induced marked intracellular K+ loss, caspase-3 cleavage, DNA laddering, and synergistically triggered neuronal death. The caspase inhibitor Z-Val-Ala-Asp(OMe)-fluoromethyl ketone (Z-VAD-FMK) predominantly blocked the caspase activation and neuronal death. These results suggest that slight impairment of Na+,K+-pump activity may amplify the disruption of K+ homeostasis in the presence of a non-lethal apoptotic insult, leading to activation of apoptotic cascade and substantial neuronal injury. [Copyright &y& Elsevier]