Tsutsui Hidekazu, Oka Yoshitaka
Misaki Marine Biological Station, Graduate School of Science, University of Tokyo, Misaki, Miura, Kanagawa 238-0225, Japan.
J Physiol. 2002 Mar 15;539(Pt 3):743-53. doi: 10.1113/jphysiol.2001.013061.
It has been previously shown that the "large cell" in the corpus glomerulosum (CG) of a teleost brain has a low-pass temporal filtering property. It fires a single spike only in response to temporally sparse synaptic inputs and thus extracts temporal aspects of afferent activities. To explore the ionic mechanisms underlying this property, we quantitatively studied voltage-gated Na(+) channels of the large cell in the CG slice preparation of the marine filefish by means of whole-cell patch clamp recordings in the voltage-clamp mode. Recorded Na(+) current was well described using the Hodgkin-Huxley "m(3)h" model. It was revealed that the Na(+) channels have a novel feature: remarkably slow recovery from inactivation. In other words, the time constant for the "h" gate was extremely large (approximately 100 ms at -80 to -50 mV). In order to test whether the analysed Na(+) current serves as a mechanism for filtering, the behaviour of the membrane model incorporating the Na(+) channel was simulated using a computer program called NEURON. In response to current injections, the membrane model displayed low-pass filtering and firing properties similar to those reported in real cells. The present results suggest that slow removal of Na(+) channel inactivation serves as a crucial mechanism for the low-pass temporal filtering property of the large cell. The simulation study also suggested that velocity and/or amplitude of a spike propagating though an axon expressing Na(+) channels of this type could potentially be modulated depending on the preceding activities of the cells.
先前的研究表明，硬骨鱼脑肾小球（CG）中的“大细胞”具有低通时间滤波特性。它仅在对时间上稀疏的突触输入作出反应时才发放单个动作电位，从而提取传入活动的时间特征。为了探究这种特性背后的离子机制，我们通过电压钳模式下的全细胞膜片钳记录，对海鲀CG切片制备中的大细胞的电压门控Na(+)通道进行了定量研究。使用霍奇金-赫胥黎“m(3)h”模型很好地描述了记录到的Na(+)电流。结果表明，Na(+)通道具有一个新特征：失活后的恢复非常缓慢。换句话说，“h”门的时间常数极大（在-80至-50 mV时约为100 ms）。为了测试所分析的Na(+)电流是否作为一种滤波机制，我们使用名为NEURON的计算机程序模拟了包含Na(+)通道的膜模型的行为。在电流注入时，膜模型表现出与真实细胞中报道的类似的低通滤波和发放特性。目前的结果表明，Na(+)通道失活的缓慢消除是大细胞低通时间滤波特性的关键机制。模拟研究还表明，通过表达这种类型Na(+)通道的轴突传播的动作电位的速度和/或幅度可能会根据细胞的先前活动而受到潜在调节。
