Ca2+ and filopodial responses to glutamate in cultured astrocytes and neurons

Abstract

Neurons and glia exhibit complex homeostatic interactions via shared extracellular space which can involve metabolites, inorganic ions, and neurotransmitters. Focal application of glutamate to both human and rat central nervous system astrocytes in primary culture produced a rapid, transient increase in both cytoplasmic and nuclear Ca²⁺. These Ca²⁺ waves can propagate at up to 15–20 μm/s for long distances (millimetres) through the astrocyte syncitium. Oscillatory Ca²⁺ signals were frequently observed under control conditions and were enhanced by glutamate application. These Ca²⁺ signals were paralleled by rapid extensions of filopodia from the astrocyte cell margin and apical surface near the point of glutamate application. Focal application of glutamate to rat hippocampal neurons also elicited rapid, transient increases in intracellular Ca²⁺. Levels of Ca²⁺ signals were consistently two- to three-fold greater in pyramidal neurons cultured from CA1 than in those from CA3. Filopodial extension was extensive in CA1 neurons, but rare in CA3 neurons, and in either case observable only during the first few days of primary culture. Diversity of glial and neuronal responses to binding the glutamate receptors may reflect their roles in homeostatic interactions.Key words: glutamate, astrocytes, hippocampal neurons, Ca²⁺ signals, filopodia.