Glutamate release from microglia via glutamate transporter is enhanced by amyloid-beta peptide

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 mu M) for four days. The glutamate- activated inward current was blocked by D,L-threo-beta-hydroxyaspartare 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 mu M) 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-a 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. (C) 1999 IBRO. Published by Elsevier Science Ltd.