Gao, T. M., E. M. Howard, and Z. C. Xu. Transient neurophysio-logical changes in CA3 neurons and dentate granule ceils after severe forebrain ischemia in vivo. J. Neurophysiol. 80: 2860-2869, 1998. The spontaneous activities, evoked synaptic responses, and membrane properties of CA3 pyramidal neurons and dentate granule cells in rat hippocampus were compared before ischemia and s7 days after reperfusion with intracellular recording and staining techniques in vivo. A four-vessel occlusion method was used to induce ～14 min of ischemic depolarization. No significant change in spontaneous firing rate was observed in both cell types after reperfusion. The amplitude and slope of excitatory postsynap-tic potentials (EPSPs) in CA3 neurons decreased to 50% of control values during the first 12h reperfusion and returned to preischemic levels 24h after reperfusion. The amplitude and slope of EPSPs in granule cells slightly decreased 24-36 h after reperfusion. The amplitude of inhibitory postsynaptic potentials in CA3 neurons transiently increased 24h after reperfusion, whereas that in granule cells showed a transient decrease 24-36 h after reperfusion. The duration of spike width of CA3 and granule cells became longer than that of control values during the first 12h reperfusion. The spike threshold of both cell types significantly increased 24-36h after reperfusion, whereas the frequency of repetitive firing evoked by depolarizing current pulse was decreased during this period. No significant change in rheobase and input resistance was observed in CA3 neurons. A transient increase in rheobase and a transient decrease in input resistance were detected in granule cells 24 36h after reperfusion. The amplitude of fast afterhyperpolarization in both cell types increased for 2 days after ischemia and returned to normal values 7 days after reperfusion. The results from this study indicate that the neuronal excitability and synaptic transmission in CA3 and granule cells are transiently suppressed after severe forebrain ischemia. The depression of synaptic transmission and neuronal excitability may provide protection for neurons after isch-emic insult.

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.

It has been reported previously that the neuronal excitability persistently suppresses and the amplitude of fast afterhyperpolarization (fAHP) increases in CAI pyramidal cells of rat hippocampus following transient forebrain ischemia. To understand the conductance mechanisms underlying these post-ischemic electrophysiological alterations, we compared differences in activities of large conductance Ca2+-activated potassium (BKca) channels in CA1 pyramidal cells acutely dissociated from hippocampus before and after ischemia by using inside-out configuration of patch clamp techniques. (1) The unitary conductance of BKc. channels in post-iscfiemic neurons (295pS) was higher than that in control neurons (245pS) in symmetrical 140/140mM K+ in inside-out patch; (2) the membrane depolarization for an e-fold increase in open probability (P) showed no significant differences between two groups while the membrane potential required to produce one-half of the maximum Po was more negative after ischemia, indicating no obvious changes in channel voltage dependence; (3) the [Ca2+] required to half activate BKca, channels was only 1uM in post-ischemic whereas 2 IxM in control neurons, indicating an increase in [Ca2+], sensitivity after ischemia; and (4) BKca channels had a longer open time and a shorter closed time after ischemia without significant differences in open frequency as compared to control. The present results indicate that enhanced activity of BKc. channels in CAI pyramidal neurons after ischemia may partially contribute to the post-ischemic decrease in neuronal excitability and increase in fAHR.

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.

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.