In vivo cellular uptake of glutamate is impaired in the rat hippocampus during and after transient cerebral ischemia: A microdialysis extraction study

Using microdialysis in CA1 of the rat hippocampus, we studied the effect of transient cerebral ischemia on in vivo uptake and on extracellular levels of glutamate during, and at different time points after ischemia. H-3-D-asp artate (test substance), and C-14-mannitol (reference substance), were adde d to the dialysis perfusate, and the cellular extraction of H-3-D-aspartate was calculated from scintillation analysis of fractionated dialysate sampl es. The extraction of H-3-D-aspartate was studied both in a tracer like con dition with a perfusate concentration of 0.2 muM, and in a condition of hig h saturation level, with 1.0 mM D-aspartate added to the perfusate. In betw een radioisotope perfusions, dialysate was sampled for analysis of amino ac id content by HPLC. During ischemia, extraction of H-3-D-aspartate (0.2 muM ) declined to a maximum reduction of 68%. In the hours after ischemia, extr action of H-3-D-aspartate (0.2 muM) was decreased by 32%. In the days after ischemia, there was a progressive decline in extraction of H-3-D-aspartate (1.0 mM), reaching a reduction of 89% on Day 4 after ischemia. Extracellul ar glutamate remained at control levels at all time points after ischemia. The present study is the first to investigate uptake of glutamate in the in tact rat brain in relation to cerebral ischemia. Evidence is provided that uptake of Glu is restrained during ischemia, and that in the hours after is chemia, the extracellular turnover of glutamate is decreased. In the course of the days after ischemia, degeneration of CA1 pyramidal cells occurs con comitantly with a progressive decline in glutamate transport ability, possi bly of pathogenetic importance to CA1 pyramidal cell loss. (C) 2001 Wiley-L iss, Inc.