Stimulation of glutamine synthetase activity by excitatory amino acids in astrocyte cultures derived from aged mouse cerebral hemispheres may be associated with non-n-methyl-d-aspartate receptor activation

Abstract

We have been using glial cells derived from aged mouse cerebral hemispheres (MACH) at several passages to study the responsiveness of astrocytes to microenvironmental signals in culture. In the present study, we examined the effects of excitatory amino acids on the activity of glutamine synthetase, a marker for astrocytes. MACH glia cell passages 25 to 29 were used. Culture groups were Dulbecco's modified Eagle's medium +10% fetal bovine serum (control); glutamate 100 μM; γ-amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA) 50 μM; kainic acid 10 μM; N-methyl-d-aspartate (NMDA) 10 μM. In all treated groups glutamine synthetase activity was significantly higher than in controls. We speculate that this increase represents an enhanced differentiation of immature astrocytes. In a second series, we examined the effects of glutamate receptor antagonists on glutamine synthetase activity as follows. MACH cultures were treated with glutamate 100 μM in combination with either L(+)-2-amino-3-phosphonopropionic acid (L-AP3; 50 μM); D(−)-2-amino-5-phosphonopentanoic acid (D-AP5; 50 μM) or 6,7-dinitroquinoxaline-2,3-dione (DNQX; 50 μM). The increase in GS activity produced by glutamate was exhibited by the non-selective NMDA receptor antagonist, DNQX, but not by the metabotropic receptor antagonist, L-AP3 or a selective NMDA receptor antagonist, D-AP5. We also found that in cultures treated with glutamate, a number of astrocytes resembled “reactive astrocytes” morphologically. These astrocytes were absent in cultures treated with glutamate + DNQX. The findings provide supportive evidence that astrocytes from aged mouse cerebral hemispheres respond to excitatory amino acids and that this response is mediated by non-NMDA receptor activation.