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. (3)H-D-aspartate (test substance), and (14)C-mannitol (reference substance), were added to the dialysis perfusate, and the cellular extraction of (3)H-D-aspartate was calculated from scintillation analysis of fractionated
dialysate samples. The extraction of (3)H-D-aspartate was studied both in a tracer like condition with a perfusate concentration of 0.2 microM, and in a condition of high saturation level, with 1.0 mM
D-aspartate added to the perfusate. In between
radioisotope perfusions,
dialysate was sampled for analysis of
amino acid content by HPLC. During
ischemia, extraction of (3)H-D-aspartate (0.2 microM) declined to a maximum reduction of 68%. In the hours after
ischemia, extraction of (3)H-D-aspartate (0.2 microM) was decreased by 32%. In the days after
ischemia, there was a progressive decline in extraction of (3)H-D-aspartate (1.0 mM), reaching a reduction of 89% on Day 4 after
ischemia. Extracellular
glutamate remained at control levels at all time points after
ischemia. The present study is the first to investigate uptake of
glutamate in the intact rat brain in relation to
cerebral ischemia. Evidence is provided that uptake of Glu is restrained during
ischemia, and that in the hours after
ischemia, the extracellular turnover of
glutamate is decreased. In the course of the days after
ischemia, degeneration of CA1 pyramidal cells occurs concomitantly with a progressive decline in
glutamate transport ability, possibly of pathogenetic importance to CA1 pyramidal cell loss.