Hypoxia is a major cause of neonatal encephalopathy and seizures, and an increased neuronal excitability may be an underlying mechanism. To determine the role of Ca2+-activated K+ channels in hyperexcitability, we measured large unitary conductance (>200 pS, BKca) currents in symmetrical 140/140 mM K+ using inside-out configuration in CA1 pyramidal cells acutely dissociated from the hippocampus of rats exposed to normoxia or hypoxia (at 10% inspired 02) for 4 weeks after birth. About 53% of the patches contained BKca channels in the normoxic group, but only 20% in the hypoxic one. There were no differences in channel conductance or reversal potential between the groups. Yet, the open probability of BKca channels was much less in hypoxic neurons than that in the control, because of a decrease in channel open time and a prolongation of the closed time. These were partially recovered by an oxidizing but not by reducing agent, suggesting an involvement of redox mechanism. Results indicate that the Ca2+-activated K+ channel activities in hippocampal CA1 neurons are modulated by hypoxia during maturation. The reduction in BKca activity may contribute to hypoxia-induced neuronal hyperexcitability.

It has been shown that intracellular Ca2+ in hippocampal CA1 neurons is elevated during ischemia and at early period following reperfusion. This Ca2+ overload has been suggested to be involved in ischemic brain damage. In normal CAI neurons, the major mechanism allowing Ca2+ entry from the extracellular compartment is the opening of voltage-gated Ca2+ channels. The aim of the present study was to explore whether L-type calcium channel in hippocampal CA1 neurons changed at early period of reperfusion after ischemia. Transient forebrain ischemia in a duration of 15 min was induced by the use of the 4-vessel occlusion method in rats. Single L-type calcium currents were recorded in cell-attached patches of actually dissociated hippocampal CAI neurons. After ischemia, average total patch current of L-type Ca2+ channels significantly increased in CAI neurons when compared with that of control. This ischemia-induced enhancement in channel function was due to a higher channel open probability. Further analysis of single channel kinetics showed a prolonged open time and an increased opening frequency in postischemic channels. It is suggested that the functional enhancement in L-type calcium channels may partially account for the postischemic increase in intracellular Ca2+ concentration of CA1 neurons following ischemia.

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.

Using inside-out configuration of patch clamp techniques, ATP modulation of BKca channels was studied in hippocampal CAI pyramidal neurons of adult rat. Intracellular ATP application markedly increased BKca channel activity, and this ATP-produced increase in BK, channel activity was characterized by a higher opening frequency with no changes in channel open times. In the presence of specific inhibitor against protein kinase A, H-89, ATP did not induce any increase in the channel activity. Furthermore, adding H-89 after addition of ATP reversed the modulation produced by ATE In contrast, protein kinase C inhibitor chelerythrine exerted no apparent effects on ATP-induced channel activation. The present study suggests that BKca channels from hippocampal CA1 pyramidal neurons could be modulated by ATP via a functionally associated orotein kinase A-like orotein.