In the present study, we found that amyloid-beta peptide enhanced glutamate release from primary cultured rat microglia via the Na+-dependent glutamate transporter, which was activated by extracellular K+. Glutamate transport current was measured by a conventional whole-cell patch recording mode under voltage-clamp conditions. With the pipette solution containing 10 mM glutamate and 100 mM Na+, an increase of the external K+ concentration from 0 to 10 mM evoked an outward current, resulting from co-extrusion of glutamate and Na+. The inward current, reflecting forward glutamate transport, was also activated by external glutamate. Both these reverse and forward glutamate transport currents were three-fold greater in microglia incubated with a relatively low concentration of amyloid-beta peptide (25-35) (5 microM) for four days. The glutamate-activated inward current was blocked by D,L-threo-beta-hydroxyaspartate in a dose-dependent manner (ranging from 0.001 to 1 mM), but not by a high concentration of kainate (1 mM). The glutamate concentration released from microglia upon high-K+ stimulation was also significantly increased (up to 170 microM) after treatment with amyloid-beta peptide (25-35). These results suggest that, at the pathological sites where extracellular K+ concentration may increase, the activation of microglia by amyloid-beta peptide causes an increase in extracellular glutamate concentration via reverse glutamate transporter, and therefore this mechanism may contribute to the pathogenesis of neuronal dysfunction and death in Alzheimer's disease.