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 rat s. A dialysis electrode for real-time glutamate measurement was perfused wi th L-glutamate oxidase, and the current evoked between two voltage-clamped electrodes was detected. Two electrodes were implanted in the retina throug h the choroid and cerebral cortex in 12 anesthetized rats, each mounted on a stereotaxic frame. Global ischemia was induced by ligation on both caroti d arteries and hypotension was induced by blood withdrawal. Under control c onditions, the glutamate concentration in the retina was 164 +/- 231 (mean +/- standard deviation) mu M, being significantly higher (P < 0.05) than th at in the cerebral cortex (83 +/- 105 mu M). In 10 of the 12 animals, the g lutamate concentration in the retina decreased to a minimum of 134 +/- 149 mu M (P < 0.01, compared with the value for the cerebral cortex), but that in the cortex increased to 410 +/- 305 mu M (averaged highest value). Immed iately after the start of reperfusion, the glutamate concentration in the c ortex decreased rapidly to 101 +/- 27 mu M, but that in the retina increase d gradually to almost the control level (148 +/- 204 mu M). In the other tw o animals, the glutamate concentration remained unchanged. In conclusion, g lutamate release in the retina does not proceed as rapidly as that in the c erebral cortex during 20 min of ischemia. and in fact decreases. This oppos ite 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 isc hemia in these two organs. (C) 2000 Elsevier Science Ltd. All rights reserv ed.