Preconditioning with Cortical Spreading Depression Decreases Intraischemic Cerebral Glutamate Levels and Down‐Regulates Excitatory Amino Acid Transporters EAAT1 and EAAT2 from Rat Cerebal Cortex Plasma Membranes

Abstract

Abstract:We previously reported a 50% reduction in cortical infarct volume following transient focal cerebral ischemia in rats preconditioned 3 days earlier with cortical spreading depression (CSD). The mechanism of the protective effect of prior CSD remains unknown. Recent studies demonstrate reversal of excitatory amino acid transporters (EAATs) to be a principal cause for elevated extracellular glutamate levels during cerebral ischemia. The present study measured the effect of CSD preconditioning on (a) intraischemic glutamate levels and (b) regulation of glutamate transporters within the ischemic cortex of the rat. Three days following either CSD or sham preconditioning, rats were subjected to 200 min of focal cerebral ischemia, and extracellular glutamate concentration was measured by in vivo microdialysis. Cortical glutamate exposure decreased 70% from 1,772.4 Ϯ 1,469.2 M-min in sham-treated (n ϭ 8) to 569.0 Ϯ 707.8 M-min in CSD-treated (n ϭ 13) rats ( p Ͻ 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 days but not at 0 or 21 days after CSD. Semiquantitative lane analysis showed 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-regulation coincides with the time frame reported for induced ischemic tolerance. These data are consistent with reversal of glutamate transporter function contributing to glutamate release during ischemia and suggest that down-regulation of these transporters may contribute to ischemic tolerance induced by CSD.

Normobaric Hypoxia Induces Tolerance to Focal Permanent Cerebral Ischemia in Association with an Increased Expression of Hypoxia-Inducible Factor-1 and its Target Genes, Erythropoietin and VEGF, in the Adult Mouse Brain

“…Because normobaric hypoxia (9% O 2 , 8 hours) has been reported to induce tolerance against kainate-induced seizures (Pohle and Rauca, 1994), one might suppose that the mechanisms of hypoxic preconditioning involve a reduction of the excitotoxic events during focal ischemia. Such a hypothesis has already been suggested for other models of ischemic tolerance (Douen et al, 2000). Although we did not investigate these hypotheses, we cannot exclude a potential implication of these signaling pathways in the development of ischemic tolerance.…”

Nitric Oxide Produced During Sublethal Ischemia Is Crucial for the Preconditioning-Induced Down-Regulation of Glutamate Transporter GLT-1 in Neuron/Astrocyte Co-Cultures

“…Although numerous studies have reported increased tolerance to ischemia following preconditioning of the mammalian brain with OGD, cortical spreading depression (CSD), or middle cerebral artery occlusion (MCAO), the mechanism underlying this neuroprotection is unclear (17)(18)(19)(20). Some studies have reported that neurons themselves acquire resistance against ischemia through preconditioning (19,20).…”

Functional Significance of the Preconditioning-induced Down-regulation of Glutamate Transporter GLT-1 in Neuron/astrocyte Co-cultures

“…Although numerous studies including by us have reported increased tolerance to ischemia following preconditioning of the mammalian brain with OGD, cortical spreading depression (CSD), or middle cerebral artery occlusion (MCAO), the mechanism underlying this neuroprotection is unclear (20)(21)(22)(23). Some studies have reported that neurons themselves acquire tolerance against ischemia through preconditioning (22, 23).…”