抗癲癇藥物對於Nav1.7通道回返性鈉電流之作用

Abstract

電壓閘控型鈉離子通道Nav1.7的突變已被發現與遺傳性的疼痛疾病有所關聯，如肢端紅痛症(inherited erythromelalgia, IEM)以及陣發性劇痛症(paroxysmal extreme pain disorder, PEPD)，近年來，一些研究也顯示，這些突變會使Nav1.7的回返性電流(resurgent current, INaR)增強，進而影響神經興奮性。此外，一些抗癲癇藥物也被發現能與電壓依賴型鈉離子通道的幾種不活化態結合，抑制其興奮能力。然而，文獻顯示一些傳統的抗癲癇藥物例如phenytoin，對於 IEM 等疾病並無明顯效果，同時，對於抗癲癇藥物是否能作用於Nav1.7而影響 INaR，也仍然不清楚。本篇論文中，我們使用轉染了Nav1.7的中國倉鼠卵巢細胞(Chinese hamster ovary cell, CHO cell)進行實驗，發現rufinamide此種被認為會結合在電位閘控型鈉離子通道中速不活化態的抗癲癇藥物，對於INaR有約15%的抑制效果，並可以針對反覆性活化的通道加強抑制約30%的INaR，同時也會使INaR的活化曲線右移約13.9mV。藉由分析INaR的各項動力學數據以及模擬軟體的輔助，我們提出包含各種不活化態的Nav1.7模式，用以解釋INaR的產生機制以及rufinamide的作用機制，我們發現通道在快速以及慢速不活化態之間可能存在數種中速不活化態，這些中速不活化態很可能是產生INaR的主要結構，同時也可能是rufinamide傾向結合的狀態。這些結果可能提供了抑制INaR的有效方案，也可能作為緩解IEM以及PEPD症狀之潛在希望。

Mutations in voltage-gated sodium channel (VGSC) Nav1.7 are shown to be linked to inherited pain syndromes such as Inherited erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD). Recent studies indicate that resurgent currents (INaR) could be enhanced in those mutant channels. Antiepileptic drugs (AEDs) are shown to inhibit the excitability of the channels by binding to the inactivated states of the VGSC. However, classic AEDs such as phenytoin have shown no definite effect on pain relief for IEM patients. Also, the effects of AEDs on INaR remain unclear. Using Chinese hamster ovary (CHO) cell transfected with Nav1.7, we found that rufinamide, a new AED showing specific affinity to the intermediate inactivated state of VGSC, significantly inhibits ~15% of INaR, and inhibit ~30% of the INaR generated from the repetitive firing channels. Rufinamide also causes ~13.9mV of the hyperpolarized shift in the activation curve of INaR. By analyzing the kinetic properties of INaR and the data from the simulation program, we propose a new Nav1.7 scheme composed of different types of inactivated states, to describe the mechanism of the generation of INaR more completely and to understand the states which rufinamide effect on. We suggest that some intermediated inactivated states(Iim) may be located between the slow inactivated state(Islow) and the fast inactivated state(Ifast). Nav1.7 most likely generates INaR from some of those Iim, which may also be the primary targets for the binding of rufinamide. The results may implicate an effective way to inhibit resurgent Nav1.7 currents, and thus a potential symptomatic relief of IEM and PEPD.