Preconditioning Protects Lethal Ischemia-induced Injury And Suppresses Large Conductance Ca～(2+)-activated K～+ Channel Activitiy In CA1 Pyramidal Neurons From Rat Hippocampus

Preconditioning of the rat brain with a sublethal cerebral ischemia induces resistance to a subsequent lethal ischemia (ischemic tolerance). A 3-min period of forebrain ischemia in rats produces no appreciable neuronal damage in the CA1 subfield of the hippocampus but can protect against neuronal damage following subsequent longer periods of ischemia, which normally kills CA1 neurons in the hippocampus. Preconditioning-mediated neuroprotection has been shown to initiate several defense mechanisms such as up-regulation of heat shock proteins, bcl-2 protein, reactive oxygen species, as well as an activation of NF-KB while the precise mechanism remains to be elucidated.Pyramidal neurons in the CA1 field of the hippocampus occur delayed neuronal death after transient forebrain ischemia. Glutamate excitotoxicity haas been suggested as a potential mediator of postischemic cell injury. However, neuronal hyperactivity as predicted by excitotoxic hypothesis is not evident in CA1 region after ischemia. Both in vitro and in vivo intracellular recording studies have demonstrated that the spontaneous firing rate and neuronal excitability of CA1 neurons progressively decrease following reperfusion. It has been speculated that increased potassium currents may be responsible for the reduced neuronal excitability after ischemia. Indeed, recent studies reported an enhancement in activities of potassium channels in CA1 pyramidal neurons of rat hippocampus after transient forebrain ischemia. Moveover, it has been showed recently that the enhancement of outward potassium current mediates apoptotic cell death in cultured cortical neurons,and that the apoptotic cell death induced by serum deprivation could be inhibited by potassium channel blocker in both in vitro and in vivo .It has been suggested that the enhancement in large conductance Ca2+-activated potassium (BK) channel activity is involved in the pathogenesis of ischemic neuronal injury. Therefore,we assumed that the suppression of BK channel activity might be involved in ischemic tolerance induced by a ischemic preconditioning. Rats were subjected to forebrain ischemia induced by 4-vessel occlusion for 3-min and only operation without ischemia as preconditioning group and sham-operation group,respectively.,Rats were subjected to forebrain ischemia induced by 4-vessel occlusion for 8-min with preconditioning and without preconditioning as ischemic tolerance group and lethal ischemic group,respectively. Compared to sham-operation group,no histological neuronal changes in rat hippocampus CA1 subfield 7 days after reperfusion were observed in preconditioning group. More than 90% of the CA1 pyramidal cells were destroyed in lethal ischemic group,but more than 89% of the neurons survived in ischemic tolerance group. To address the hypothesis,we used inside-out configuration of patch clamp techniques to investigate the temporal changes in BK channel activity in hippocampus CA1 pyramidal neurons acutely dissociated from preconditioning group at 6h,24h,and 48h following sublethal ischemia and from lethal ischemia group and ischemic tolerance group at 24h following lethal ischemia. The present study found a decrease in BK channel open probability during the first 24h in preconditioning group,rather than an increase observed in lethal ischemia group. Kinetic analyses showed that the suppression of the BK channel activity in preconditioning group was due to a prolongation of the closed time while there was no significant change in open time and the increase in BK channel activity in lethal ischemia group was due to a prolongation of the open time and shortening of the closed time. There were no differences in channel unitary conductance and reversal potential before and after ischemia in groups. In contrast,no apparent changes in BK channel open probability were observed in ischemic tolerance group. It is suggested that the preconditioning-induced suppression of BK channel activity may be associated with ischemic tolerance.