Abnormal Ca2+ homeostasis before cell death revealed by whole cell recording of ischemic CA1 hippocampal neurons

Slices were made from the hippocampus of gerbils following transient ischemia achieved by clamping the carotid arteries for 5 min, and changes in the electrophysiology of CA1 pyramidal neurons were studied by whole cell patch-clamp recording as well as conventional intracellular recording. The great majority of CA1 neurons in slices made 2.5-3 days after ischemia showed reduced resting potentials and were easily depolarized by prolonged low-frequency stimulation or by tetanic stimulation of the Schaffer collateral/commissural input. This stimulus-induced depolarization was accelerated by intracellular injection of D-myo-inositol 1,4,5-triphosphate, which depolarized membrane potentials towards 0 mV without synaptic input stimulation. Intracellular application of BAPTA, a Ca2+ chelator, effectively blocked the stimulus-induced depolarization. When recording from ischemic neurons with patch pipettes containing both D-myo-inositol 1,4,5-triphosphate and BAPTA, excitatory postsynaptic currents were transiently potentiated by stimulation, but the membrane potential did not show stimulus-induced depolarization and remained steady for long periods. These results lend support to the view that the intracellular Ca2+ regulation system is severely disturbed following ischemia, and that input fiber stimulation leads to abnormal Ca2+ accumulation in ischemic neurons resulted in neuronal death. The reduction of free Ca2+ inside the ischemic neuron by BAPTA apparently saves neurons which are otherwise destined to delayed neuronal death.