Selective blockade of astrocytic glutamate transporter GLT-1 with dihydrokainate prevents neuronal death during ouabain treatment of astrocyte/neuron cocultures.

Glutamate (Glu) is a major excitatory neurotransmitter of the mammalian central nervous system and under normal conditions plays an important role in information processing in the brain. Therefore, extracellular Glu is subject to strong homeostasis. Astrocytes in the brain have been considered to be mainly responsible for the clearance of extracellular Glu. In this study, using mixed neuron/astrocyte cultures, we investigated whether astrocytic Glu transporter GLT-1 is crucial to the survival of neurons under various conditions. Treatment of the mixed cultures with a low concentration of Glu did not produce significant death of neurons. However, cotreatment with dihydrokainate (DHK), a specific blocker of GLT-1, resulted in significant neuronal death that was suppressed by an antagonist of N-methyl-D-aspartate (NMDA) receptors. These results suggested that astrocytic GLT-1 participated in the clearance of extracellular Glu and protected neurons from NMDA receptor-mediated toxicity. When the cultures were treated with ouabain, an inhibitor of Na(+)/K(+)-ATPase, a low concentration of Glu resulted in massive neuronal death that was also suppressed by cotreatment with an antagonist of NMDA receptors. In this case, however, cotreatment with DHK significantly protected neurons from death, suggesting that GLT-1 was responsible for the death of neurons. The present study provides evidence suggesting that astrocytes use their Glu transporter GLT-1 to protect neurons from Glu toxicity, but, ironically, also use GLT-1 to kill neurons through Glu toxicity depending on their status.