Ischemia-induced glutamate release in the dentate gyrus. A microdialysis study in the gerbil

The levels of extracellular glutamate were measured in the dentate gyrus by using an in vivo brain microdialysis method to determine whether the ischemia-induced glutamate release might be correlated with the neuronal vulnerability to ischemia. A microdialysis membrane was placed in CA4 (vulnerable to ischemia) and the molecular and granule cell layers of the dentate gyrus (resistant to ischemia) of gerbils. A significant increase in glutamate levels was induced in the normal dentate gyrus during 10-min ischemia. The increase was completely suppressed during the first 5 min of ischemia when CA4 neurons were eliminated. Thus, it was indicated that during the first 5 min of ischemia glutamate was released mostly from CA4 neurons but not from granule cells of the dentate gyrus. During the second half of 10-min ischemia, a significant increase in glutamate release was induced even in the dentate gyrus where CA4 neurons were eliminated; this increase was significantly suppressed by inhibiting proliferation of astrocytes. A large part of glutamate that was released during the second half of 10-min ischemia was considered to be attributable to glutamate release from astrocytes.