視丘下核神經元上之回返性鈉離子電流之電壓及鈉離子流向依賴性之探討

Abstract

中樞神經的自發性放電已興起全世界電生理實驗與臨床上的癲癇及運動失調廣泛的聯結。視丘下核這個腦區之神經細胞，表現有顯著之回返性鈉離子電流。之前的實驗提出了一種可能性，就是回返性鈉離子電流和電壓及離子流向有關，只是尚缺乏多電壓及多重細胞內外濃度的調控所得到之數據。再者，先前的研究提出了內生性蛋白物質調控回返性鈉離子電流之假設，雖然合理解釋了不活化態鈉離子通道去活化前重新開啟的機制，然而回返性鈉離子電流於不同狀態下的性質仍無法只用這個假設中之回返體來完全了解。在我們的實驗當中，逐步去改變去極化與再極化電壓，觀察在這樣的實驗方法所引發之回返性鈉離子電流。結果顯示去極化電壓越高，所引發之回返性鈉離子電流隨之增加。同時當再極化電壓改變時，回返性鈉離子電流也有電壓依賴性。在-50mV到-10mV之間，越負的電壓所得到之回返性鈉離子電流越大。進一步地，我們在細胞外用氯化銫分子取代部分的氯化鈉分子之後改變靜止膜電位，除了改變暫時性鈉電流之流向外，更改變了回返性鈉電流的流向，於是發現在同樣電位調控下，與向內之暫時性鈉電流相比，向外的暫時性鈉電流，其之後引發之回返性鈉電流，有著較小的電流大小卻又較快的起始至尖端電流之速度，同時也有較快的電流發生速率。而向外的回返性鈉電流相較於向內的回返性鈉電流其發生速率亦增加，如此的現象讓人懷疑內生性蛋白物質與鈉離子通道之互動可能與不同電壓或鈉離子流向所致不同之親和力有關。於是我們作了在不同前置電位下的不活化態恢復比例曲線，也發現了不活化過程與電位間之相關，這項發現暗示著前人假設中的回返體可能與通道本身之不活化機制有著很大的關聯。

Spontaneous firing in central nervous system plays an important role in epilepsy and idiopathic motor disorders. Subthalamic neurons show manifest resurgent Na+ currents. Previous studies suggest the possibility of voltage- and flow-dependence of sodium resurgent currents, but direct evidence is deficient. On the other hand, a hypothesis had been proposed that an endogenous protein may play a role in the generation of recurrent sodium currents. Although this hypothesis well explains well why inactivated channels reopens upon repolarization, some important molecular details remain to be characterized. In this study, step depolarization and steps repolarization to different voltage are designed to evoke transient sodium currents and resurgent sodium currents. We found incremental resurgent sodium currents following steps to more depolarized pre-pulse voltages between -20mV and +40mV. Voltage-dependence of the repolarizing pulse is also present between -50mV and -10mV. We also replaced part of the extracellular sodium chloride by cesium chloride to shift the reversal potential and thus changed the direction of either transient or resurgent sodium ion flow. Interestingly, outward transient sodium currents are followed by less noticeable resurgent sodium currents with the same voltage protocols if compared with inward transient sodium currents in the same neuron. However, the time to peak resurgent sodium currents is shorter when the preceding pulse elicits outward Na+ currents. The development of resurgent currents is also apparently “speeded” by outward transient sodium currents and outward resurgent sodium currents themselves. We conclude that the affinity between the endogenous protein and sodium channels may be modulated by both conformation changes of the channel and the direction of ionic flow through the pore.