The present study examined temporal changes in activity of large conductance, Ca2+-activated potassium (BKca) channels in postischemic CA1 pyramidal neurons at 2, 6, 24 and 48h after reperfusion. These changes in activity and possible cellular mechanisms were examined using the inside-out configuration of patch clamp. The unitary conductance of postischemic BKca channels increased transiently to 1.9% of the control at 2h after reperfusion, and recovered to the control level thereafter. A persistent increase in [Ca2+]i sensitivity of BKca channels was observed in postischemic CA1 neurons with the maximal sensitivity to [Ca2+]i at 6h after reperfusion while channel voltage- dependence showed no obvious changes. Kinetic analyses showed that the postischemic enhancement of BKca channel activity was due to longer open times and shorter closed times as there was no significant changes in opening frequency after ischemia. Glutathione disulphide markedly increased BKca channel activity in normal CA1 neurons, while reducing glutathione caused a decrease in BKca channel activity by reducing the sensitivity of this channel to [Ca2+]i in postischemic CA1 neurons. Similar modulatory effects on postischemic BKca channels were also observed with another redox couple, DTNB and DTT, suggesting an oxidation modulation of BKca channel function after ischemia. The present results indicate that a persistent enhancement in activity of BKca channels, probably via oxidation of channels, in postischemic CA1 pyramidal neurons may account for the decrease in neuronal excitability and increase in fAHP after ischemia. The ischemia-induced augmentation in BKca channel activity may be also associated with the postischemic neuronal injury.

It has been recently reported that potassium channel increases activities in CA1 pyramidal neurons of rat hippocampus following transient forebrain ischemia. To understand the role of the enhanced potassium current in the pathogenesis of neuronal damage after ischemia, we examined the effects of tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on the neuronal injury of CA1 region induced by 15 min forebrain ischemia using a four-vessel occlusion model. Adult rats received intracerebroventricular administration of either TEA or 4-AP after ischemia or TEA before ischemia and once each day for 7 days. In the postischemic TEA treated-rats, the neuronal injury in hippocampal CA1 region was signifi-cantly less than that of the controls. In contrast, neither preischemic infusion of TEA nor postischemic treatment of 4-AP had any neuroprotective effects. The present study demonstrates that postischemic application of TEA protects hippo-campal CA1 pyramidal neurons against ischemic insult, suggesting that potassium channels may play important roles in the pathogenesis of CA1 neuronal death after transient forebrain ischemia.

The electrophysiological responses of CA3 pyramidal neurons and dentate granule (DG) cells in rat hippocampus were studied after transient forebrain ischemia using intracellular recording and staining techniques in vivo. Approximately 5 min of ischemic depolarization was induced using 4-vessel occlusion method. The spike threshold and rheobase of CA3 neurons remained unchanged up to 12h following reperfusion. No significant change in spike threshold was observed in DG cells but the rheobase transiently increased 6-9h after ischemia. The input resistance and time constant of CA3 neurons increased 0-3h after ischemia and returned to control ranges at later time periods. The spontaneous firing rate in CA3 neurons transiently decreased shortly following reperfusion, while that of DG cells progressively decreased after ischemia. In CA3 neurons, the amplitude and slope of excitatory postsynaptic potentials (EPSPs) transiently decreased 0-3h after reperfusion, and the stimulus intensity threshold for EPSPs transiently increased at the same time. No significant changes in amplitude and slope of EPSPs were observed in DG cells, but the stimulus intensity threshold for EPSPs slightly increased shortly after reperfusion. The present study demonstrates that the excitability of CA3 pyramidal neurons and DG cells after 5min ischemic depolarization is about the same as control levels, whereas the synaptic transmission to these cells was transiently suppressed after the ischemic insult. These results suggest that synaptic transmission is more sensitive to ischemia than membrane properties, and the depression of synaptic transmission may be a protective mechanism against ischemic insults.

Evoked postsynaptic potentials of CA1 pyramidal neurons in rat hippocampus were studied during 48h after severe ischemic insult using in vivo intracellular recording and staining techniques. Postischemic CAI neurons displayed one of three distinct response patterns following contralateral commissural stimulation. At early recirculation times (0-12h) approximately 50% of neurons exhibited, in addition to the initial excitatory postsynaptic potential, alate depolarizing postsynaptic potential lasting for more than 100ms. Application of dizocilpine maleate reduced the amplitude of late depolarizing postsynaptic potential by 60% Other CAI neurons recorded in this interval failed to develop late depolarizing postsynaptic potentials but showed a modest blunting of initial excitatory postsynaptic potentials (non-late depolarizing postsynaptic potential neuron). The proportion of recorded neurons with late depolarizing postsynaptic potential characteristics increased to more than 70% during 13-24h after reperfusion. Beyond 24h reperfusion, 20% of CA1 neurons exhibited very small excitatory postsynaptic potentials even with maximal stimulus intensity. The slope of the initial excitatory postsynaptic potentials in late depolarizing postsynaptic potential neurons increased to 150% of control values up to 12h after reperfusion indicating a prolonged enhancement of synaptic transmission. In contrast, the slope of the initial excitatory postsynaptic potentials in non-late depolarizing postsynaptic potential neurons decreased to less than 50% of preischemic values up to 24h after reperfusion indicating a prolonged depression of synaptic transmission. More late depolarizing postsynaptic potential neurons were located in the medial portion of CA1 zone where neurons are more vulnerable to ischemia whereas more non-late depolarizing postsynaptic potential neurons were located in the lateral portion of CA1 zone where neurons are more resistant to ischemia. The result from the present study suggests that late depolarizing postsynaptic potential and small excitatory postsynaptic potential neurons may be irreversibly injured while non-late depolarizing postsynaptic potential neurons may be those that survive the ischemic insult. Alterations of synaptic transmission may be associated with the pathogenesis of postischemic neuronal injury.

We have previously identified three distinct populations of CA1 pyramidal neurons after reperfusion based on differences in synaptic response, and named these late depolarizing postsynaptic potential neurons (enhanced synaptic transmission), non-late depolarizing postsynaptic potential and small excitatory postsynaptic neurons (depressed synaptic transmission). In the present study, spontaneous activity and membrane properties of CA I neurons were examined up to 48 h following-14 min ischemic depolarization using intracellular recording and staining techniques in vivo. In comparison with preis-chemic properties, the spontaneous firing rate and the spontaneous synaptic activity of CA1 neurons decreased significantly during reperfusion; spontaneous synaptic activity ceased completely 36-48h alter reperfusion, except for a low level of activity which persisted in non-late depolarizing postsynaptic potential neurons. Neuronal hyperactivity as indicated by increasing firing rate was never observed in the present study. The membrane input resistance and time constant decreased significantly in late depolariz-ing postsynaptic potential neurons at 24-48h reperfusion. In contrast, similar changes were not observed in non-late depolarizing postsynaptic potential neurons. The rheobase, spike threshold and spike frequency adaptation in late depolarizing postsynaptic potential neurons increased progressively following reperfu-sion. Only a transient increase in rheobase and spike threshold was detected in non-late depolarizing postsynaptic potential neurons and spike frequency adaptation remained unchanged in these neurons. The amplitude of fast afterhy perpolarization increased in all neurons after reperfusion, with the smallest increment in non-late depolarizing postsynaptic potential neurons. Small excitatory postsynaptic potential neurons shared similar changes to those of late depolarizing postsynaptic potential neurons. These results suggest that the enhancement and depression of synaptic transmission following ischemia are probably due to changes in synaptic efficacy rather than changes in intrinsic membrane properties. The neurons with enhanced synaptic transmission following ischemia are probably the degenerating neurons, while the neurons with depressed synaptic transmission may survive the ischemic insult.