Glutamate uptake inhibitor, L-trans-pyrrolidine-2,4-dicarboxylate (PDC, 20 muM) elevated basal and N-methyl-D-aspartate (NMDA, 100 muM)-induced extracellular glutamate accumulation, while it did not augment kainate (100 muM)-induced glutamate accumulation in cultured cerebellar granule neurons. However, pretreatment with PDC for 1 h significantly reduced NMDA-induced glutamate accumulation, but did not affect kainate-induced response. Pretreatment with glutamate (5 muM) for 1 h also reduced NMDA-induced glutamate accumulation, but did not kainate-induced response. Upon a brief application (3-10 min), PDC did neither induce elevation of intracellular calcium concentration ([Ca(2+)](i)) nor modulate NMDA-induced [Ca(2+)](i) elevation. Pretreatment with PDC for 1 h reduced NMDA-induced [Ca(2+)](i) elevation, but it did not reduce kainate-induced [Ca(2+)](i) elevation. These results suggest that glutamate concentration in synaptic clefts of neuronal cells is increased by prolonged exposure (1 h) of the cells to PDC, and the accumulated glutamate subsequently induces selective desensitization of NMDA receptor.
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