Mechanisms of ischemic tolerance in the rat brain preconditioned with cortical spreading depression.

Abstract

Cortical spreading depression (CSD) imparts delayed neuroprotection against cerebral ischemia. We hypothesized that prior CSD preconditioning attenuates ischemic excitotoxicity by reducing glutamate synaptic responses. We assessed this hypothesis by measuring cerebral blood flow (CBF), tissue depolarization and N-methyl-d-aspartate (NMDA) receptor activation during subsequent ischemic insult. We also measured cerebral glucose utilization and quantified excitatory amino acid transporters (EAAT) at various time intervals after CSD. The methodology included autoradiography and immunoblotting. CSD preconditioning reduced neocortical infarct volume from 43.3 +/- 21.9 nine (n = 18) to 22.2 +/- 25.3 mm3 (n = 12) (p < 0.01). We found [3H]dizocilpine (MK-801) in vivo distribution increased significantly in the outer neocortex during recurrent CSD, suggesting increases in extracellular glutamate. During focal insult on day 3 after CSD, regional CBF outside the ischemic core was higher at 147 +/- 27 mL 100g-1 min -1 (n = 6) in CSD preconditioned cortex compared with 73 +/- 41 mL 100g-1 min-1 (n = 6) in sham studies (p < 0.005). The ratio of [3H]nimodipine ipsilateral/contralateral volume of distribution (uptake ratio), indicating membrane depolarization, was attenuated in the ischemic core from 1.91 +/- 0.26 (n = 7) in sham studies to 1.30 +/- 0.27 (n = 9) in CSD preconditioned studies (p < 0.02). [3H]Dizocilpine in vivo distribution, used to map NMDA receptor activation, showed restricted distribution of [3H]dizocilpine in the ischemic core. The ipsilateral/contralateral ratio of regional cerebral glucose utilization, measured with [14C]deoxyglucose in the neocortex, was reduced 10% in CSD preconditioned studies on day 3 (p < 0.05) and day 7 (p < 0.001) after CSD. Astrocytic, but not neuronal EAAT, were downregulated in plasma membranes. Semi-quantitative immunoprotein analysis indicated that the ratio of ipsilateral/contralateral band intensities was reduced up to 80% for EAAT2 (p < 0.0001) and 45% for EAAT1 (p < 0.001) on day 3 after CSD. We draw three conclusions from our data: (1) CSD preconditioning improves cerebral blood flow in the periinfarct region and attenuates ischemic tissue depolarization in the ischemic core; (2) CSD preconditioning induces a reduction in regional cerebral glucose utilization; (3) CSD preconditioning downregulates astrocytic glutamate transporters, leading to attenuated glutamate exposure during subsequent ischemic insult. CSD preconditioning may therefore include improvement of blood flow and attenuation of glutamate excitotoxicity.