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.

采用膜片钳内面向外式技术，在急性分离成年大鼠海马CAl区锥体细胞上记录到了外向整流氯离子通道（outwadly rectifving chloride channel, ORCC），长时间去极化（I>60mv）刺激后，在30%的游离膜片上记录到有外向整流特性的单通道氯电流，膜电位在-60mV到0mV之间的单通道电导为（16.58±1.54）Ps(n=10），而在0mv到+60mv之间电导为（40.92±3.17）pS，通道开放概率有明显的电压依赖性（膜电位-60mV时，P=0.44±0.12；膜电位为+60mv时，Pa=0.86±0.06，n=10），在对称C1-浓度（150 retool/L）时，通道翻转电位为（-4.17±1.84）mv。当溶液中部分NaCl被葡萄糖酸钠替代后，翻转电位为：（-34.23±4.86）mV（C1-1/[C1一]。-（30mmol/L）/（150 mmol/L）），接近氯离子通道的理论值，这表明通道具有氯离子选择性，浴槽液中分别加入氯通道阻断剂DIDS和SITS可以使+40mV的通道开放概率从（0.83±0.06）和（0.86±0.06）分别降低到（0.12±0.05）和（0.13±0.04）n=5)，冲洗后可使开放概率基本恢复，上述研究结果显示，在成年大鼠海马CAl神经元上存在外向整流氯离子通道。

KATP通道在细胞的新陈代谢与膜兴奋性的耦联中起重要作用，采用膜片钳的内面向外式记录方法，在成年大鼠海马cAl区锥体细胞上记录到一种被胞浆侧ATP和甲糖宁（tolbutarnide，一种KATP通道阻断剂）抑制的Ca2+依赖性钾离子通道，在细胞膜内外的K+浓度均为140mmol/L时，通道的电导为（204±21)f）pS，翻转电位为（3.57±1.13）mv，通道无整流性，通道开放概率及ATP对通道的抑制作用均呈现电压依赖性，该KATP通道与以往报道的“经典”KATP通道有显著不同，其活动受膜电位、胞内Ca2+和ATP三重调节，表明这是一种新型的KATP通道，上述结果表明在海马神经元上至少有两种性质不同的KAlP通道，提示神经元可能通过不同性质的KATP通道感受细胞内的代谢状态，进而调节细胞膜的兴奋性。

短暂脑缺血可对随后的损伤性脑缺血表现出明显的耐受，有研究表明大电导ca2+依赖K+（BKCa）通道活动增强参与了缺血性脑损伤，采用膜片钳的内面向外式，观察了3min短暂脑缺血后6h、24h以及48h大鼠海马cAl区锥体细胞上BKca通道活动的动态变化，短暂脑缺血后BKca通道的单通道电导和翻转电位均未见明显变化，但通道的开放概率则在缺血预处理后的前24h内显著降低，通道动力学分析显示通道关闭时间变长是短暂脑缺血后通道活动降低的主要原因，因为通道的开放时间未发生明显变化，结果提示短暂脑缺血所致的BKca通道活动降低可能与缺血耐受的产生有关.

为探讨钾通道参与神经元凋亡的可能机制，在星形孢菌素（STS）诱导的培养海马神经元凋亡模型上，研究了凋亡时神经细胞容积的动态变化及钾通道在其中的作用。实验结果显示，钾通道阻断剂四乙铵或升高细胞外K+均能够明显抑制STS诱导的神经元凋亡，并且大电导钙激活钾通道（BK）选择性阻断剂iberiotoxin和paxilline具有同样程度的抗细胞凋亡作用，表明钾通道（可能主要是BK通道）参与了STS诱导的培养海马神经元凋亡，在STS诱导神经元凋亡的早期就出现了细胞容积的显著减少，而钾通道阻断剂或升高细胞外K+均可阻断该细胞容积减少，研究结果提示细胞内钾离子的外流可能参与了凋亡性细胞容积减少，这也可能是钾通道介导细胞凋亡的重要机制之一。

Neurophysiological changes of hippocampal neurons were compared before and after transient forebrain ischemia using intracellu-lar recording and staining techniques in vivo. Ischemic depolarization (ID) was used as an indication of severe ischemia. Under halo-thane anesthesia, approximately 13 rain of ID consistently produced severe neuronal dam-age in the CA1 region of rat hippocampus, while CA3 pyramidal neurons and dentate granule cells remained intact. After such se-vere ischemia, approximately 60% of the CA1 neurons exhibited a synaptic potentia-tion. The excitability of these neurons pro-gressively decreased following reperfusion. Approximately 30% of the CA1 neurons showed a synaptic depression following ischemia. The excitability of these neurons transiently decreased following reperfusion. After ischemia of the same severity, both synaptic transmission and excitability of CA3 and granule cells transiently depressed. These data suggest that ischemia-induced synaptic potentiation may be associated with the pathogenesis of neuronal damage fol-lowing ischemia, and that the synaptic de-pression may have protective effects on hip-pocampal neurons after ischemic insult.

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.

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.