Gu Ning, Vervaeke Koen, Hu Hua, Storm Johan F
Department of Physiology at IMB and Centre for Molecular Biology and Neuroscience (CMBN), University of Oslo, PB 1103 Blindern, N-0317 Oslo, Norway.
J Physiol. 2005 Aug 1;566(Pt 3):689-715. doi: 10.1113/jphysiol.2005.086835. Epub 2005 May 12.
In hippocampal pyramidal cells, a single action potential (AP) or a burst of APs is followed by a medium afterhyperpolarization (mAHP, lasting approximately 0.1 s). The currents underlying the mAHP are considered to regulate excitability and cause early spike frequency adaptation, thus dampening the response to sustained excitatory input relative to responses to abrupt excitation. The mAHP was originally suggested to be primarily caused by M-channels (at depolarized potentials) and h-channels (at more negative potentials), but not SK channels. In recent reports, however, the mAHP was suggested to be generated mainly by SK channels or only by h-channels. We have now re-examined the mechanisms underlying the mAHP and early spike frequency adaptation in CA1 pyramidal cells by using sharp electrode and whole-cell recording in rat hippocampal slices. The specific M-channel blocker XE991 (10 microm) suppressed the mAHP following 1-5 APs evoked by current injection at -60 mV. XE991 also enhanced the excitability of the cell, i.e. increased the number of APs evoked by a constant depolarizing current pulse, reduced their rate of adaptation, enhanced the after depolarization and promoted bursting. Conversely, the M-channel opener retigabine reduced excitability. The h-channel blocker ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride; 10 microm) fully suppressed the mAHP at -80 mV, but had little effect at -60 mV, whereas XE991 did not measurably affect the mAHP at -80 mV. Likewise, ZD7288 had little or no effect on excitability or adaptation during current pulses injected from -60 mV, but changed the initial discharge during depolarizing pulses injected from -80 mV. In contrast to previous reports, we found that blockade of Ca2+-activated K+ channels of the SK/KCa type by apamin (100-400 nm) failed to affect the mAHP or adaptation. A computational model of a CA1 pyramidal cell predicted that M- and h-channels will generate mAHPs in a voltage-dependent manner, as indicated by the experiments. We conclude that M- and h-channels generate the somatic mAHP in hippocampal pyramidal cells, with little or no net contribution from SK channels.
在海马锥体细胞中，单个动作电位（AP）或一串动作电位之后会跟随一个中等幅度的超极化后电位（mAHP，持续约0.1秒）。mAHP背后的电流被认为可调节兴奋性并引起早期的放电频率适应性变化，从而相对于对突然兴奋的反应而言，减弱对持续兴奋性输入的反应。最初认为mAHP主要由M通道（在去极化电位时）和h通道（在更负的电位时）引起，而不是由小电导钙激活钾通道（SK通道）引起。然而，在最近的报道中，有人提出mAHP主要由SK通道产生，或者仅由h通道产生。我们现在通过在大鼠海马脑片中使用尖锐电极和全细胞记录技术，重新研究了CA1锥体细胞中mAHP和早期放电频率适应性变化的潜在机制。特异性M通道阻滞剂XE991（10微摩尔）抑制了在-60毫伏时通过电流注入诱发的1 - 5个动作电位后的mAHP。XE991还增强了细胞的兴奋性，即增加了由恒定去极化电流脉冲诱发的动作电位数量，降低了它们的适应速率，增强了去极化后电位并促进了爆发式放电。相反，M通道开放剂瑞替加滨降低了兴奋性。h通道阻滞剂ZD7288（4 - 乙基苯基氨基 - 1,2 - 二甲基 - 6 - 甲基氨基嘧啶氯化物；10微摩尔）在-80毫伏时完全抑制了mAHP，但在-60毫伏时几乎没有作用，而XE991在-80毫伏时对mAHP没有明显影响。同样，ZD7288在从-60毫伏注入电流脉冲期间对兴奋性或适应性几乎没有影响，但改变了从-80毫伏注入去极化脉冲期间的初始放电。与之前的报道相反，我们发现蜂毒明肽（100 - 400纳米）阻断SK / KCa型钙激活钾通道未能影响mAHP或适应性。一个CA1锥体细胞的计算模型预测，正如实验所示，M通道和h通道将以电压依赖的方式产生mAHP。我们得出结论，在海马锥体细胞中，M通道和h通道产生体细胞mAHP，而SK通道的净贡献很小或没有。
