Glutamate transporter blockade affects Ca(2+) responses in astrocytes

Brief pretreatment of astrocytes in culture with glutamate (500 microM for 20 min), was earlier shown to significantly enhance the Ca(2+) responses to a depolarizing pulse. It is known that malfunction of glutamate transporters increases extracellular glutamate concentration. We hypothesized that pretreatment of astrocytes with glutamate in conditions where the glutamate transporter activity is blocked should cause further elevation of the Ca(2+) responses to a depolarizing pulse. To test the hypothesis we pretreated astrocytes in culture (primary rat astrocyte cultures) with glutamate (500 microM) and glutamate transport inhibitor, threo-beta-hydroxy-aspartate (200 microM, TBHA) or glutamate (500 microM) in Na(+) free extracellular solution for 20 min. The Ca(2+) responses were elicited by depolarization of the astrocyte to evoke voltage-gated Ca(2+) currents. Paradoxical attenuation of the Ca(2+) transients was observed when the glutamate pretreatment was done in conditions that blocked glutamate transport, accompanied by faster rise and decay times. When the experiments were done on astrocyte pairs that were pretreated with glutamate and TBHA, we observed attenuated Ca(2+) responses in the adjoining cell when compared with the depolarized cell. The results were contrary to our earlier observation of heightened responses in the adjoining cell of the astrocyte pair, in cells pretreated with glutamate alone. The attenuated Ca(2+) responses in astrocytes would imply decrease in the vesicular release of glutamate and ATP. Extracellular glutamate concentration dependent regulation of the Ca(2+) signaling mechanism thus seems to operate in astrocytes, which may be important in regulating the neurotoxic accumulation of glutamate in the extracellular space and the synapse.