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.

Redox regulation of BKca channels was studied in CA1 pyramidal neurons of adult rat hippocampus by using inside-out configuration of patch clamp. Intracellular application of oxidizing agent 5, 5-dithio-bis (2-nitrobenzoic acid)(DTNB) markedly increased activity of BKca channels and this stimulating action persisted even after washout. In contrast, the reducing agent dithiothreitol (DTT) had no apparent effects on channel activity but could reverse the pre-exposure of DTNB-induced enhancement. The increase in channel activity produced by DTNB was due to shortened closed time as well as prolonged open time. The effects exerted by another redox couple glutathione disulphide and its reducing form were similar as DTNB and DTT. The present results indicate that BKca channels in CA1 pyramidal neurons can be modulated by intracellular redox potential, and that augmentation of BKca channels by oxidative stress might contribute to the postischemic electrophysiological alterations of CA1 pyramidal neurons.

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.

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.

Buyang Huanwu Decoction (BYHWD), a traditional Chinese medicine, has been developed as a drug to be used for treatment of stroke for hundreds of years. However, the underlying mechanisms remain unknown. In the present study, the effects of BYHWD on delayed neuronal death of hippocampus after transient forebrain ischemia were examined in rats. Transient forebrain ischemia in a duration of 15 rain was induced with the four-vessel occlusion method. BYHWD (per 6.65g/kg) was given orally to rats twice each day for 7 days before ischemia. In BYHWD-pretreated rats, the neuronal injury in the hippocampal CA1 region was significantly less than that of controls. Oral administration of BYHWD also markedly attenuated the number of TUNEL-positive neurons and suppressed the expression of caspase-3p20, a product of catalytically active caspase-3, in the CA1 region. Our results suggest that an inhibition of caspase-3 and apoptosis by BYHWD may oartially account for its neuroorotection against ischemic iniury in the hippocampal CA1 region.