RELEASE OF ENDOGENOUS GLUTAMATE, ASPARTATE, GABA, AND TAURINE FROM HIPPOCAMPAL SLICES FROM ADULT AND DEVELOPING MICE UNDER CELL-DAMAGING CONDITIONS

The releases of endogenous glutamate, aspartate, GABA and taurine from hippocampal slices from 7-day-, 3-, 12-, and 18-month-old mice were i nvestigated under cell-damaging conditions using a superfusion system. The slices were superfused under hypoxic conditions in the presence a nd absence of glucose and exposed to hydrogen peroxide. In the adult h ippocampus under normal conditions the basal release of taurine was hi ghest, with a response only about 2-fold to potassium stimulation (50 mM). The low basal releases of glutamate, aspartate, and GABA were mar kedly potentiated by K+ ions. In general, the release of the four amin o acids was enhanced under all above cell-damaging conditions. In hypo xia and ischemia (i.e., hypoxia in the absence of glucose) the release of glutamate, aspartate and GABA increased relatively more than that of taurine, and membrane depolarization by K+ markedly potentiated the release processes. Taurine release was doubled in hypoxia and tripled in ischemia but K+ stimulation was abolished. In both the mature and immature hippocampus the release of glutamate and aspartate was greatl y enhanced in the presence of H2O2, that of aspartate particularly in developing mice. In the immature hippocampus the increase in taurine r elease was 10-fold in hypoxia and 30-fold in ischemia, and potassium s timulation was partly preserved. The release processes of the four ami no acids in ischemia were all partially Ca2+-dependent. High concentra tions of excitatory amino acids released under cell-damaging condition s are neurotoxic and contribute to neuronal death during ischemia. The substantial amounts of the inhibitory amino acids GABA and taurine re leased simultaneously may constitute an important protective mechanism against excitatory amino acids in excess, counteracting their harmful effects. In the immature hippocampus in particular, the massive relea se of taurine under cell-damaging conditions may have a significant fu nction in protecting neural cells and aiding in preserving their viabi lity.