前端無顆粒狀腦島皮質之第五層錐狀神經元與丙胺基丁酸神經元細胞對自二三層傳入訊息之突觸傳導比較研究

Abstract

前端無顆粒狀腦島皮質(rostral agranular insular cortex, RAIC)是大腦皮質中 島葉的前端,在人腦中位於顳葉的內側,又稱大腦的第五葉。島葉是腦邊緣系統的 一部份,和很多感覺調控都有關係,包括內臟感覺、情緒等,也參與在疼痛反應的 調控上。過去研究指出當島葉有病變或是受到藥物抑制時,會有鎮痛的反應。這表 示在正常狀態下,活化島葉會造成痛覺過敏。過去腦核磁照影也指出島葉會參與在 調控慢性痛的迴路裡。慢性痛是一個非常麻煩的病症,對於患者會持續產生疼痛感 覺和影響患者的生活品質,過去研究指出慢性痛的生成可能和大腦中疼痛核區的 突觸長期性改變有關。而突觸長期性的改變,或稱可塑性(Plasticity),通常和核 區內興奮性和抑制性細胞的調控失衡有關,而影響一個核區的活化。所以我們假設: 島葉是因為核區內興奮性和抑制性細胞的調控失衡,而活化核區,最後影響慢性痛 的生成。實驗室過去的研究已提出一個有力的證據,在前端無顆粒狀腦島皮質中, 第五層的錐狀神經元與丙胺基丁酸神經元細胞會在慢性痛後有差異性的活化,只 有投射細胞錐狀神經元被活化。 因此,為了更進一步探討慢性痛在突觸上產生的可塑性變化,我進行了全細 胞鑲嵌技術(Whole-cell patch clamp)同時紀錄第五層的錐狀神經元與丙胺基丁酸 神經元細胞,探討這兩種細胞對來自於第二三層同樣的興奮性刺激的突觸生理,並 比較其在疼痛前和疼痛後的差異。我的研究主要在於疼痛處理前做探討,分為兩個 部分,突觸後與突觸前反應。 (1) 突觸後反應:可以發現在錐狀神經元的興奮性突觸後電流較丙胺基丁酸 神經元細胞來得慢也比較長。同時我也測量了N-甲基-D-天門冬胺酸接 iv 受器(NMDA receptor)在突觸後神經元的比例,我發現錐狀神經元細 胞中的N-甲基-D-天門冬胺酸接受器較丙胺基丁酸神經元細胞有較大的 比例。 (2) 突觸前反應:我紀錄了配對脈衝比率(Paired-pulse ratio),發現錐狀細 胞的前突觸皆為配對脈衝促進性,但丙胺基丁酸神經元細胞有些呈現促 進性,而有些呈現抑制性。還發現投射到丙胺基丁酸神經元細胞的前突 觸趨勢性較高的神經濾胞釋放機率。 最後,由於先前研究指出慢性痛會誘發磷酸化細胞外信號調節激酶(pERK), 所以我測試了該激酶對於島葉突觸的影響。我發現當誘發磷酸化細胞外信號調節 激酶(pERK)時,會增加興奮性突觸後電流。而進一步的疼痛研究將是我們未來 的目標。

It has been shown that inhibition or lesion of the rostral agranular insular cortex (RAIC) results in analgesia, it suggests that RAIC tonically produces hyperalgesia signal. RAIC is a cortical area where nociceptive output originates, and it has been reported to activate in chronic pain perception. It’s believed that chronic pain is associated with the long-term change in synaptic plasticity. Moreover, the imbalance of excitatory and inhibitory (E/I) synaptic signaling in neural circuits is responsible to modulate synaptic plasticity in certain behavior disorders. In our lab, previous study had reported that the induction of chronic pain induced differential activation in pyramidal cells and GABAergic neurons in RAIC. We propose here that E/I imbalance in RAIC may contribute to the increased cortical output of nociceptive signal in chronic pain. To test this possibility, we compared synaptic transmission of layers 2/3 (L2/3) inputs onto layer 5 (L5) pyramidal cells (PC), which are the descending projection neurons, and onto local GABAergic interneurons (IntN) in RAIC. We performed dual-patch recording from a paired IntN-PC in layer 5, and elicited EPSC by putting an electrode in layer 2/3. We found functional connectivity in 34.2% of all recorded IntN-PC pairs. There was no significant difference in data sampled from IntN-PC pairs with and without functional connectivity, and all data were pooled. Our data showed no significant difference in paring-pulse ratio between transmission at L2/3-PC synapses and at L2/3-IntN synapses. L2/3-IntN seemed to have higher releasing probability than L2/3-PC synapse in quantum study. The ratio of NMDA and non-NMDA EPSCs component was larger at L2/3-PC synapses than at L2/3-IntN synapses. Furthermore, the rising and decay of EPSCs were much faster at L2/3-IntN synapse than at L2/3-PC synapse. We further examined the modulation of pERK on IntN-PC pairs by applying PKC activator Phorbol 12,13- diacetate (PDA). PDA enhanced the postsynaptic currents at L2/3-PC synapses and L2/3- IntN synapses. The further issue of chronic pain model is under studying.