This study investigates the effect of microglial activation on microglial glutamate transporters in vitro. Stimuli known to activate microglia and/or to be associated with pathological conditions with an impaired astroglial glutamate uptake were compared. Morphological changes and marked increases in ED1 antigen expression were found after 8-h incubation of rat microglia in 56 mM KCl, 1 ng/ml lipopolysaccharide (LPS), and 100 microM glutamate, as well as in acidic and basic conditions, showing that the cells were activated. Of the stimuli used, only LPS induced a significant release of the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6), and was the only stimulus that increased microglial GLT-1 expression and glutamate uptake capacity after 12-h incubation. This effect was probably mediated by TNF-alpha, since this cytokine mimicked the effect of LPS. Furthermore, the effect of LPS was blocked by thalidomide, an inhibitor of TNF-alpha synthesis. Additionally, neutralizing antibodies against TNF-alpha also blocked the increase, indicating TNF-alpha as an inducer of GLT-1 expression in microglia. It was also found that preincubation with glutamate dose-dependently inhibited the microglial glutamate uptake. This could suggest different physiological functions for microglial and astroglial glutamate uptake and might indicate a reciprocal control of GLT-1 expression between microglia and astrocytes.

Herpes simplex virus (HSV) can enter the central nervous system and cause encephalitis (HSV-1) or meningitis (HSV-2). Microglia, the immunocompetent cells of the central nervous system, are potentially able to detect viral infections. Microglia have been shown to express the glutamate transporter GLT-1 during pathological events, leading to increased microglial glutamate uptake and glutathione synthesis. This study aims to address the role of GLT-1 and glutathione, a major antioxidant with antiviral properties, during HSV infections. Using neuron-enriched mixed primary cultures from rat, it was found that microglia have higher resistance to HSV infections than neurons or astrocytes after 24 h incubation with HSV. Purified microglia in culture were used to further address this. It was found that microglia were able to detect HSV and responded by releasing tumor necrosis factor-alpha (TNF-alpha) and upregulating GLT-1 after 24 h incubation with 1 PFU/cell HSV-1 or HSV-2. Furthermore, the microglial glutathione levels were not significantly diminished after 24 h. Inhibition of the microglial glutathione synthesis with 200 microM buthionine sulfoximide (BSO) led to significantly more infected cells after 24 h incubation with 1 PFU/cell HSV-1 or HSV-2. These data indicate that the higher resistance in microglia against HSV infections may be due to the expression of GLT-1, which can maintain the glutathione levels and provide a mechanism for microglial self-defense against HSV.