Abundance of microglia in different rat brain regions and responsiveness to LPS. LPS or vehicle was injected into the adult rat hippocampus, cortex, or SN as described in the legend to Figure 1. After 7 d, the brains were removed, and tissue sections through the areas of interest were immunostained for CR3 complement receptors using the monoclonal antibody OX-42 as a marker for microglia. At least six animals were used for each injection, and similar results were obtained in all animals examined. Scale bar, 250 μm.