Intracellular Zn2+ Accumulation Contributes to Synaptic Failure, Mitochondrial Depolarization, and Cell Death in an Acute Slice Oxygen-Glucose Deprivation Model of Ischemia

Despite considerable evidence for contributions of both Zn2+ and Ca2+ in ischemic brain damage, the relative importance of each cation to very early events in injury cascades is not well known. We examined Ca2+ and Zn2+ dynamics in hippocampal slices subjected to oxygen-glucose deprivation (OGD). When single CA1 pyramidal neurons were loaded via a patch pipette with a Ca2+-sensitive indicator (fura-6F) and an ion-insensitive indicator (AlexaFluor-488), small dendritic fura-6F signals were noted after several (similar to 6-8) minutes of OGD, followed shortly by sharp somatic signals, which were attributed to Ca2+ (Ca2+ deregulation). At close to the time of Ca2+ deregulation, neurons underwent a terminal increase in plasma membrane permeability, indicated by loss of AlexaFluor-488 fluorescence. In neurons coloaded with fura-6F and a Zn2+-selective indicator (FluoZin-3), progressive rises in cytosolic Zn2+ levels were detected before Ca2+ deregulation. Addition of the Zn2+ chelator N,N,N',N'-tetrakis(2-pyridylmethyl) ethylenediamine (TPEN) significantly delayed both Ca2+ deregulation and the plasma membrane permeability increases, indicating that Zn2+ contributes to the degenerative signaling. Present observations further indicate that Zn2+ is rapidly taken up into mitochondria, contributing to their early depolarization. Also, TPEN facilitated recovery of the mitochondrial membrane potential and of field EPSPs after transient OGD, and combined removal of Ca2+ and Zn2+ markedly extended the duration of OGD tolerated. These data provide new clues that Zn2+ accumulates rapidly in neurons during slice OGD, is taken up by mitochondria, and contributes to consequent mitochondrial dysfunction, cessation of synaptic transmission, Ca2+ deregulation, and cell death.