Difference in glutamate release between retina and cerebral cortex following ischemia

The difference in ischemic tolerance between the retina and cerebral cortex may be attributable to a difference in glutamate release during ischemia. Glutamate release in the retina and the cerebral cortex was compared in rats. A dialysis electrode for real-time glutamate measurement was perfused with L-glutamate oxidase, and the current evoked between two voltage-clamped electrodes was detected. Two electrodes were implanted in the retina through the choroid and cerebral cortex in 12 anesthetized rats, each mounted on a stereotaxic frame. Global ischemia was induced by ligation on both carotid arteries and hypotension was induced by blood withdrawal. Under control conditions, the glutamate concentration in the retina was 164 +/- 231 (mean +/- standard deviation) microM, being significantly higher (P < 0.05) than that in the cerebral cortex (83 +/- 105 microM). In 10 of the 12 animals, the glutamate concentration in the retina decreased to a minimum of 134 +/- 149 microM (P < 0.01, compared with the value for the cerebral cortex), but that in the cortex increased to 410 +/- 305 microM (averaged highest value). Immediately after the start of reperfusion, the glutamate concentration in the cortex decreased rapidly to 101 +/- 27 microM, but that in the retina increased gradually to almost the control level (148 +/- 204 microM). In the other two animals, the glutamate concentration remained unchanged. In conclusion, glutamate release in the retina does not proceed as rapidly as that in the cerebral cortex during 20 min of ischemia, and in fact decreases. This opposite trend shown by the two organs may be due to the slow depletion rate of ATP in the retina. This may explain the differing neuronal tolerance to ischemia in these two organs.