Sodium influx pathways during and after anoxia in rat hippocampal neurons.
A prominent feature of cerebral ischemia is the excessive intracellular accumulation of both Na(+) and Ca(2+), which results in subsequent cell death. A large number of studies have focused on pathways involved in the increase of the intracellular Ca(2+) concentration [Ca(2+)](i), whereas the elevation of intracellular Na(+) has received less attention. In the present study we investigated the effects of inhibitors of different Na(+) channels and of the Na(+)/Ca(2+) exchanger, which couples the Na(+) to the Ca(2+) gradient, on ischemic damage in organotypic hippocampal slice cultures. The synaptically evoked population spike in the CA1 region was taken as a functional measure of neuronal integrity. Neuronal cell death was assessed by propidium iodide staining. The Na(+) channel blocker tetrodotoxin, and the NMDA receptor blocker MK 801, but not the AMPA/kainate receptor blocker NBQX prevented ischemic cell death. The novel Na(+)/Ca(2+) exchange inhibitor 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943), which preferentially acts on the reverse mode of the exchanger, leading to Ca(2+) accumulation, also reduced neuronal damage. At higher concentrations, KB-R7943 also inhibits Ca(2+) extrusion by the forward mode of the exchanger and exaggerates neuronal cell death. Neuroprotection by KB-R7943 may be due to reducing the [Ca(2+)](i) increase caused by the exchanger.