Effect of hyperthermia on glutamate release in ischemic penumbra after middle cerebral artery occlusion in rats.

Using microdialysis, we investigated the effect of hyperthermia on glutamate release in penumbral cortex of rats with 2 h of either normothermic (37 degrees C) or hyperthermic (39 degrees C) middle cerebral artery (MCA) occlusion. Penumbral blood flow (CBF) was measured by laser-Doppler flowmetry. CBF values (expressed as % preischemic values) in normothermic and hyperthermic groups were 24 +/- 11 (SD) and 24 +/- 16%, respectively, during ischemia and 102 +/- 81 and 147 +/- 79% during recirculation. Average extracellular glutamate in the hyperthermic group increased from a baseline of 7 +/- 2 microM to a peak of 217 +/- 184 microM at 10-20 min after onset of ischemia but returned to near baseline after 60 min. Glutamate in the normothermic group increased from 4 +/- 2 microM to a peak of 26 +/- 17 microM at 10-20 min after MCA occlusion but fell to near-baseline before recirculation. Thus reuptake systems appeared to remain functional in ischemic penumbra, even during hyperthermia. Ischemic glutamate release was significantly higher in hyperthermic than in normothermic rats: average values of individual rats' peak levels were 251 +/- 221 microM and 37 +/- 34 microM, respectively. The ischemic CBF threshold value for glutamate release was 33% of control in the normothermic group but 61% in the hyperthermic group.