Morphologic changes in cultured astrocytes after exposure to glutamate.

Cultured astrocytes, exposed to glutamate at a dose that is generally neurotoxic in vitro (1 mM), exhibit transient swelling in the absence of cell death. In the present study, we further characterize this response by examining the distribution of intermediate filaments and evaluating cellular ultrastructure in primary cultures of astrocytes after exposure to 1 mM glutamate. In addition, cellular swelling was determined using the nonmetabolizable hexose 3-O-methyl [14C]-glucose (3-MG). Glial fibrillary acidic protein (GFAP) was immunolocalized at the light microscopic level to study the distribution of intermediate filaments. GFAP was immunolocalized to a fine cytoskeletal network in control cultures. Four to 24 h after exposure to glutamate, this detailed localization was replaced by a diffuse, uneven pattern of immunoreactivity. The most prominent ultrastructural changes were identified at 4 and 8 h after glutamate exposure. Nucleoli underwent transformation from a normal compact appearance to a markedly dispersed state. The cell body typically exhibited cytoplasmic lucency, swollen mitochondria, and dilated cisterns. Intermediate filaments within cellular processes appeared widely spaced in comparison to the controls. These ultrastructural changes coincided with findings of increased intracellular water space as determined with 3-MG. These findings demonstrate that astrocytes exposed to 1 mM glutamate exhibit transient morphologic changes that not only suggest cellular swelling but also define a more diverse response that is reflected in the altered immunolocalization of GFAP and in unique changes in the nucleolus.