Preconditioning with cortical spreading depression decreases intraischemiccerebral glutamate levels and down-regulates excitatory amino acid transporters EAAT1 and EAAT2 from rat cerebral cortex plasma membranes

We previously reported a 50% reduction in cortical infarct volume following transient focal cerebral ischemia in rats preconditioned 3 days earlier wi th cortical spreading depression (CSD). The mechanism of the protective eff ect of prior CSD remains unknown. Recent studies demonstrate reversal of ex citatory amino acid transporters (EAATs) to be a principal cause for elevat ed extracellular glutamate levels during cerebral ischemia. The present stu dy measured the effect of CSD preconditioning on (a) intraischemic glutamat e levels and (b) regulation of glutamate transporters within the ischemic c ortex of the rat. Three days following either CSD or sham preconditioning, rats were subjected to 200 min of focal cerebral ischemia, and extracellula r glutamate concentration was measured by in vivo microdialysis. Cortical g lutamate exposure decreased 70% from 1,772.4 +/- 1,469.2 mu M-min in sham-t reated (n = 8) to 569.0 +/- 707.8 mu M-min in CSD-treated (n = 13) rats (p < 0.05). The effect of CSD preconditioning on glutamate transporter levels in plasma membranes (PMs) prepared from rat cerebral cortex was assessed by western blot analysis. Down-regulation of the glial glutamate transporter isoforms EAAT2 and EAAT1 from the PM fraction was observed at 1, 3, and 7 d ays but not at 0 or 21 days after CSD. Semiquantitative lane analysis showe d a maximal decrease of 90% for EAAT2 and 50% for EAAT1 at 3 days post-CSD. The neuronal isoform EAAT3 was unaffected by CSD. This period of down-regu lation coincides with the time frame reported for induced ischemic toleranc e. These data are consistent with reversal of glutamate transporter functio n contributing to glutamate release during ischemia and suggest that down-r egulation of these transporters may contribute to ischemic tolerance induce d by CSD.