Neuronal stimulation leading to upregulation of glutamate transporter-1 (GLT-1) in rat microglia in vitro

As previously reported, activated microglia facilitate the expression of a glial cell-type glutamate transporter, glutamate transporter-1 (GLT-1; EAAT2), around injured motoneurons in axotomized rat facial nucleus. This phenomenon suggests that the motoneurons stimulate microglia to enhance the levels of GLT-1. In the present study, we investigated the effects of neuronal stimulus on the uptake of glutamate (Glu) by microglia and on the expression of GLT-1 protein in microglia in vitro. A 14C-Glu uptake experiment revealed that microglia enhance uptake of Glu by stimulation with neuronal conditioned medium (NCM). The NCM-stimulated uptake was significantly suppressed in the presence of dihydrokinate (a specific GLT-1 inhibitor), suggesting that GLT-1 is a major glutamate transporter for the uptake. Furthermore, immunoblotting analysis revealed that the amounts of GLT-1, but not another glial cell-type glutamate transporter glutamate-aspartate transporter (GLAST: EAAT1), increased significantly in microglia by treatment with NCM. Altogether, neuronal stimulus was found to promote the uptake of Glu in microglia, probably due to the increased levels of GLT-1.