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標題: | 中腦三叉神經感覺核區第三型酸敏感離子通道之特性 Acid-sensing ion channel 3 in the trigeminal mesencephalic neurons |
作者: | Yi-Hsien Su 蘇怡嫻 |
指導教授: | 閔明源(Min, Ming-Yuan) |
關鍵字: | 酸敏感離子通道,中樞神經系統,三叉神經感覺核區,痛覺, ASIC,CNS,Me5,nociception, |
出版年 : | 2011 |
學位: | 碩士 |
摘要: | 酸敏感離子通道(ASICs)為一種在與胞外氫離子結合後便會開啟的陽離子通道,可偵測出細胞外氫離子濃度的增加,其中,又以ASIC3對氫離子的敏感度最高,可感知組織中pH值產生的微小變化,前人研究得知ASICs在發炎反應、缺氧等組織酸化的情況下會被活化,且ASIC3被發現與酸引起的痛覺最有關連。ASICs大多表現於周邊感覺神經元上,在中樞神經系統(CNS)中,我們首次發現中腦三叉感覺核區(Me5)存在ASIC3,而Me5是CNS唯一具有感覺訊息傳入神經元細胞本體之處,因此,Me5中之ASIC3便顯得非常獨特。藉由細胞膜箝制技術,我們發現若於記錄時給予低pH值的酸溶液,會引發一流向胞內之電流,其電流大小會隨pH值降低而增加,其次,ASIC抑制劑的存在會顯著減小酸溶液所引發的電流,而且,此一酸引發的電流具有ASIC3特有的快速desensitization之特性。在Asic3基因剔除小鼠的實驗中,酸溶液所引發的電流較小,或甚至無法引發電流,且其電流desensitization的速度顯著較慢。另外,單一細胞反轉錄PCR實驗結果顯示,多數Me5的神經元都含有Asic3,反之,Asic3基因剔除小鼠的Me5神經元皆不表現Asic3。在酸引發痛覺模式小鼠的實驗中,於咀嚼肌注射兩劑酸溶液而引發的慢性痛會使Me5神經元的動作電位閾值下降,也就是在慢性痛的情形下,Me5神經元的較容易被興奮;在Asic3基因剔除小鼠中,同樣注射兩劑酸溶液的小鼠,其Me5神經元的動作電位閾值較基因正常的小鼠顯著高出許多,顯示ASIC3在酸引發的慢性痛中占有一定的重要性,但若單獨比較Asic3基因剔除小鼠在正常生理與酸引發慢性痛的動作電位閾值,卻又發現在正常生理情況下,Asic3基因剔除小鼠Me5神經元的動作電位閾值比酸引發慢性痛時要低。綜合以上實驗結果,我們認為Me5的神經元的確存在具有生理功能之ASIC3,且其功能與感知酸引發的痛覺有關,然而,含有ASIC3的Me5神經元如何參與或調控酸引發之痛覺,仍需仰賴後續研究提供解答。 Acid-sensing ion channels (ASICs) have been identified to be proton-gated cationic channels that can sense the extracellular acidification. Among them, ASIC3 is the most sensitive, detecting a slight pH change. ASIC3 is predominately expressed in the peripheral sensory neurons, where the channels respond to noxious and mechanical stimulation. In addition, it is the major subtype participating in pain caused by tissue acidosis such as inflammation and ischemia. Once activated, ASIC3 depolarizes the cell and increases its excitability. However, ASIC3 has not been reported in the central nervous system. Here, we provide evidence for the existence of functional ASIC3 in the trigeminal mesencephalic neurons (Me5), which innervate to masseter muscle spindles and are the only known primary afferent neurons in the brain. Local application (duration: 20 ms) of buffer of various pH induced fast inward currents in Me5 neurons in a dose-dependent manner (EC50 = pH 6.0). The acid-induced currents were reversely blocked by the pan-ASIC blocker, amiloride indicating the involvement of ASICs mediated currents (IASIC). In wild-type (WT) mice, IASIC was induced in all Me5 neurons tested for pH 5.0 (n=46) and pH 6.0 (n=7) with mean amplitudes 931±102pA and 334±90pA, respectively. In contrast, IASIC was only induced in 11 out of 14 cells tested for pH 5.0 and 5 out of 10 cells tested for pH 6.0 in Asic3 knock-out (Asic3-/-) with the respective mean amplitude being 465±103pA and 115±45pA, both of which were significantly smaller than the corresponding values of the WT mice. Furthermore, the desensitization time constant of IASIC was significantly faster in WT (0.89±0.05 s; n=12) than in Asic3-/- (1.21±0.10 s; n=9). These results support that IASIC of Me5 neurons involved ASIC3. Single-cell RT-PCR was used to investigate the composition of ASIC subtypes (1a, 1b, 2a, 2b and 3) in Me5 neurons. In WT mice, ASIC3 was detected in 17 out of 29 cells, with some of them (8 out of 17) co-expressing with other ASIC subtypes. Acid-induced muscle pain model established by injecting two separate doses of acidic saline into the masseter muscles was performed to study the function of ASIC3. In WT mice, the action potential (AP) threshold was significantly lower in the group injected pH 4.0 saline than that injected pH 7.4 saline which indicated that the mice subjected to acid-induced pain have neurons with higher excitability. In respect to the groups with acid-induced pain, AP threshold was significantly lower in WT than in Asic3-/- which revealed the role of ASIC3 in the acid-related masseter muscle pain in Me5 neurons. Moreover, in Asic3-/- mice, AP threshold was higher in the pH 4.0 saline-injected group. Together, our results show that the majority of acid-evoked currents in Me5 neurons are ASIC3-like. Furthermore, the physiological function of ASIC3 in Me5 neurons is probably related to the sensation of acid-induced pain. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/28663 |
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