One of the key questions concerning glutamate toxicity is how a transient NMDA exposure can lead to a delayed death of neurons. To address this issue, we performed whole-cell recording on acutely isolated hippocampal CA1 neurons to monitor the membrane response after NMDA exposure. Transient NMDA exposure (100 microM, 10 min) induced an inward current (postexposure current; Ipe) which was associated with a Ca2+- and Na+-permeable cation conductance. Ipe continuously increased (in the absence of NMDA) until death of the neuron occurred. Application of NMDA in the absence of extracellular calcium failed to trigger Ipe and neuronal death. Postexposure suppression of Ipe protected against NMDA toxicity. These results indicate that a cation current, which is induced by an increase in intracellular calcium concentration ([Ca2+]i) and is itself partly carried by Ca2+, links the initial NMDA exposure to neuronal death.

Whole cell voltage-clamp recordings from acutely isolated hippocampal CA1 pyramidal cells from adult guinea pigs were used to evaluate divalent cations as possible blockers of the postexposure current (Ipe). Ipe is a cation current that is triggered by the rise in intracellular Ca2+ concentration that occurs after the application of a toxic level of N-methyl-D-aspartate (NMDA). Once triggered, Ipe continues to grow until death of the neuron occurs. Ipe may be a critical link between transient NMDA exposure and cell death. Ipe was blocked by micromolar concentrations of Zn2+. The Zn2+ effect had an IC50 of 64 microM and saturated at 500 microM. Prolonged Zn2+ block of Ipe revealed that the maintenance of a steady Ipe is not dependent on Ipe-mediated Ca2+ influx but that the continuous growth in Ipe is dependent on Ipe-mediated Ca2+ influx. The availability of an effective blocker of Ipe should facilitate the investigation of the intracellular activation pathway of Ipe and the role of Ipe in neuronal death.

No abstract