Soleng A F, Baginskas A, Andersen P, Raastad M
Institute for Basic Medical Sciences, University of Oslo, Oslo, Norway.
J Physiol. 2004 Oct 15;560(Pt 2):491-503. doi: 10.1113/jphysiol.2004.071225. Epub 2004 Aug 19.
The membrane potential changes following action potentials in thin unmyelinated cortical axons with en passant boutons may be important for synaptic release and conduction abilities of such axons. In the lack of intra-axonal recording techniques we have used extracellular excitability testing as an indirect measure of the after-potentials. We recorded from individual CA3 soma in hippocampal slices and activated the axon with a range of stimulus intensities. When conditioning and test stimuli were given to the same site the excitability changes were partly masked by local effects of the stimulating electrode at intervals < 5 ms. Therefore, we elicited the conditioning action potential from one axonal branch and tested the excitability of another branch. We found that a single action potential reduced the axonal excitability for 15 ms followed by an increased excitability for approximately 200 ms at 24 degrees C. Using field recordings of axonal action potentials we show that raising the temperature to 34 degrees C reduced the magnitude and duration of the initial depression. However, the duration of the increased excitability was very similar (time constant 135 +/- 20 ms) at 24 and 34 degrees C, and with 2.0 and 0.5 mM Ca2+ in the bath. At stimulus rates > 1 Hz, a condition that activates a hyperpolarization-activated current (Ih) in these axons, the decay was faster than at lower stimulation rates. This effect was reduced by the Ih blocker ZD7288. These data suggest that the decay time course of the action potential-induced hyperexcitability is determined by the membrane time constant.
具有旁侧终扣的细无髓鞘皮质轴突在动作电位后发生的膜电位变化，可能对这类轴突的突触释放和传导能力很重要。由于缺乏轴内记录技术，我们采用细胞外兴奋性测试作为后电位的间接测量方法。我们在海马切片中记录单个CA3神经元胞体，并以一系列刺激强度激活轴突。当在同一部位给予条件刺激和测试刺激时，在间隔小于5毫秒时，兴奋性变化部分被刺激电极的局部效应所掩盖。因此，我们从一个轴突分支引发条件动作电位，并测试另一个分支的兴奋性。我们发现，在24摄氏度时，单个动作电位使轴突兴奋性降低15毫秒，随后兴奋性增加约200毫秒。通过轴突动作电位的场记录，我们表明将温度升至34摄氏度可降低初始抑制的幅度和持续时间。然而，在24摄氏度和34摄氏度以及浴液中含有2.0毫摩尔和0.5毫摩尔钙离子的情况下，兴奋性增加的持续时间非常相似（时间常数为135±20毫秒）。在刺激频率大于1赫兹（这种情况会激活这些轴突中的超极化激活电流（Ih））时，衰减比在较低刺激频率时更快。Ih阻滞剂ZD7288可减轻这种效应。这些数据表明动作电位诱导的兴奋性增强的衰减时间进程由膜时间常数决定。
