Age-induced activation of hippocampal astrocytes. Immunohistochemical visualization of GFAP revealed low-to-moderate expression in astrocytes distributed throughout DG, CA1 and CA3 of adults. GFAP immunoreactivity was markedly increased, and astrocyte morphology indicated activation, throughout the hippocampal formation of aged rats. No qualitative differences between aged intact and aged impaired rats were evident, and no subregion-specific alterations in astrocyte activation were observed. Increased GFAP immunoreactivity was associated with astrocyte hypertrophy, but not astrocyte proliferation, as total populations of GFAP+ glia were not changed between groups in CA1, CA3 or DG. Blue: Hoechst; Red: GFAP.