Astrocytes respond differently to varying concentrations of LPS. Stimulation of primary astrocytes with 2.5 ng/ml of LPS leads to increased expression of pro-inflammatory genes, while the pro-inflammatory response of primary astrocytes treated with 0.25 ng/ml was insignificant. Gene expression was assessed in primary astrocytes treated 24 hours with LPS and PBS (controls) by quantitative RT-PCR. For each gene, results were expressed as a percentage of the corresponding value in the astrocytes treated with PBS. LPS, lipopolysaccharides; PBS, phosphate buffered saline.