視丘下核神經元上回返性鈉通道電流之離子依賴性

Abstract

在神經系統的電氣訊號傳遞上，受電壓調控而開啟的鈉離子通道扮演著關鍵性的角色。一旦鈉離子通道功能受損，很容易造成神經傳訊方面的失調。當細胞膜去極化會促使鈉離子通道開啟，而後很快會進入不活化狀態。當細胞膜處於再極化時，會促使鈉離子通道由不活化態回復到休息態。此一回復過程，依據回復之分子機制之不同，可能伴隨或不伴隨有鈉電流之發生。有此等電流之發生者，即稱該電流為回返性鈉電流。有學者提出看法，認為回返性鈉電流的產生，是由某些內生性物質在鈉離子通道開啟時塞住通道的洞口而造成。然而回返性鈉電流的詳細機制仍尚未充分了解。我們選擇在游離的視丘下核神經細胞改變鈉離子電流的流向，並在胞外給予NMDG+、Cs+，或是K+等離子替代正常胞外溶液的Na離子濃度，進行全細胞箝制技術來記錄回返性鈉電流，然而我們卻發現到這些替代Na+的離子會出現於回返性鈉電流之中。另外，我們也嘗試額外給予二價離子Ca2+。發現Ca2+可依存在於胞外的離子不同，而對於回返性電流有不同的作用效果，因此綜合這兩點我們認為當在出現回返性電流時，鈉離子通道的選擇性已大部喪失。為了檢測離子選擇性喪失的結構形成的時間，我們從胞外給予150mM Na+ + 2mM Ca2+和150mM K+ + 2mM Ca2+這兩款溶液進行一系列time course的實驗，發現在這兩種胞外溶液下鈉離子通道選擇性喪失的構造出現以及完全形成的時間並無太大差異。最後我們給予四級基胺類離子並探討胞外給予之四級基胺類離子對於回返性鈉電流的作用。我們發現100μM TMA，TEA，TPrA及TBA對於短暫性與回返性Na+電流有顯著不同的抑制效果，但100μM TPentylA及10μM THexA則對兩者皆有近乎完全之抑制效果，顯示鈉離子通道在出現短暫性與回返性Na+電流時的孔洞外口構型有顯著的不同，也印證著前述改變鈉離子流向的實驗結果—兩種電流發生時鈉離子通道的選擇性構造並不相同。

In nervous system, voltage gated sodium channel (VGSC) has a crucial role in electrical signaling. Dysfunction of sodium channels may therefore result in serve neurological disorders. When the cell membrane is depolarized, sodium channels are activated (driven open) and then inactivated. Upon membrane repolarization, sodium channels will be recovered from the inactivation state to the resting state. There could be Na+ currents or not during the recovery process according to the molecular mechanism of the recovery. The sodium currents during recovery, if exciting, are called resurgent currents (INaR). It has been proposed that the resurgent currents are generated because the sodium channel pore is blocked by an endogenous open-channel blocker which could compete with the inactivating particle. However, the detailed mechanism underlying the genesis of resurgent currents has not been fully understood. We recorded resurgent sodium currents with whole cell patch clamp, from the dissociated subthalamic nucleus (STN) neurons, while substituting NMDG+, Cs+, or K+ for extracellular Na+ ions. We found that these substituting ions could constitute part of the resurgent currents. Addition of extracellular Ca2+ would have different effect on the resurgent currents according to the extracellular ionic species. The results indicate loss of ion selectivity of the sodium channel when resurgent current is generated. To investigate the kinetics of selectivity loss, we studied the time course of development of resurgent currents in 150mM K+ + 2mM Ca2+ and 150mM Na+ + 2mM Ca2+ both solutions, and found no difference in these two conditions. We have also explored the effect of extracellular tetra-alkylammonium ions on the resurgent sodium currents. 100μM TMA, TEA, TPrA and TBA have evidently different effects on the transient and resurgent currents, while 100μM TPentylA and 10μM THexA essentially abolish both currents, demonstrating different conformations of the external pore mouth of the sodium channel when the transient and resurgent currents are generated.