Corticosterone inhibits expression of the microglial glutamate transporter GLT-1 in vitro

The present study investigates the effect of the glucocorticoid corticosterone on microglial glutamate transporters in vitro. Microglial cultures obtained from rat cerebral cortex were found to express the excitatory amino acid transporter GLT-1, but not GLAST, and this expression was increased by 1ng/ml lipopolysaccharide after 12 h of stimulation. This increase has previously been shown to be mediated by tumor necrosis factor-α, a cytokine released by microglia during pathological conditions. Furthermore, lipopolysaccharide increased the microglial release of tumor necrosis factor-α and 1μM corticosterone inhibited this effect. Corticosterone also inhibited the lipopolysaccharide-induced increase of the GLT-1 expression as well as the expression in non-activated cells. The effect of corticosterone on the GLT-1 expression was dose dependent and accompanied by similar effects on the microglial glutamate uptake capacity. Additionally, exogenous tumor necrosis factor-α was found to counteract the effect of corticosterone on microglial GLT-1 expression. The effect of corticosterone appeared to be glucocorticoid receptor specific since 10μM of the glucocorticoid receptor antagonist mifepristone inhibited the effect. Thus, corticosterone decreased the microglial uptake of glutamate by decreasing the expression of glutamate transporters, probably due to the inhibited microglial tumor necrosis factor-α release. These results provide insights into the mechanisms behind microglial glutamate transporter expression during pathological conditions, and contribute to the debate about the beneficial or harmful effects of glucocorticoids.