光化學遺傳分析感覺神經在痛覺中所扮演的角色

Abstract

臨床上，疼痛主要來自兩種情況，一種是外在的刺激例如強烈的物理、化學或熱刺激；另外一種則是沒有刺激卻自發產生的疼痛。目前研究指出這些不同的刺激性疼痛是由許多不同的神經負責；但自發性疼痛是否有特定的神經所負責的，目前仍未清楚。在CFA誘發慢性疼痛的模型下，機械性痛覺敏感比起疼痛造成的後肢平衡異常恢復時間較快，因此我們假設在痛覺神經中有特定的神經群集負責產生自發性疼痛。在這裡，我們發展出利用光化學遺傳學的方式透周邊活化特定的神經類型，首先我們繁殖出帶有luminopsin 3且為Cre-dependent的轉殖老鼠，使得特定的神經群集帶有LMO3並且可以被螢光酵素coelenterazine (CTZ)所活化，藉此分析這些神經是屬於反應刺激性或自發性的疼痛。結果顯示，在有瞬態感受器電位陽離子通道V1(TRPV1) 以及電壓依賴型鈉離子通道的神經群集(Nav1.8)表現luminopsin 3的轉殖鼠在打入CTZ後在機械性痛覺敏感以及縮腳反應皆有顯著變化；而在酸敏感性離子通道蛋白1a以及1b (ASIC1a, 1b)的基因轉殖鼠只發生機械性痛覺敏感，此外我們透過應用Sluka 的疼痛模型來對於慢性疼痛的產生進行研究，結果發現帶有ASIC1a 或是TRPV1 的神經可能是造成慢性疼痛的原因之一。總結來說，我們建立了新的小鼠模型透過利用光化學遺傳學的方法去活化特定神經藉此分析其在痛覺中所扮演的角色並分析出特定的幾種神經群集是否參與了刺激性疼痛、自發性疼痛或是慢性疼痛的產生。

Clinically, pain can be divided into evoked pain and ongoing pain. Evoked pain is pain triggered by noxious thermal stimuli (thermal nociception), mechanical stimuli (mechanical nociception) or chemical stimuli (chemical nociception); ongoing pain is the spontaneous pain behaviors without external stimuli such as flinching or guarding behaviors. Accumulated evidence has shown that several nociceptive neuron populations involved in different evoked pain sensation such as thermal nociception, mechanical nociception and chemical nociception. However, little is known about the nociceptive neuron populations contribute to ongoing pain. In CFA test, we found mechanical hyperalgesia resolved earlier than guarding pain behaviors in mice. Therefore, we hypothesized nociceptive neurons contribute to either evoked or ongoing pain are distinguishable. Moreover, there may be a key receptors involved in pain chronicity. Here, we have developed a method to activate specific nociceptive neuron subtypes via chemo-optogenetics and probe their roles in evoked pain or ongoing pain. We first generated a Cre-dependent reporter mouse line that carries a luminopsin 3, a fusion protein of channelorhodosin and luciferase. We then used ctz (coelenterazine), a subtract of LMO3 (luminopsin 3) channel to activate LMO3-positive dorsal root ganglion neurons via peripheral nerve terminals and tested the evoked and ongoing pain behaviors in specific Cre-line::LOM3 mice. Results showed after CTZ injection, transient receptor potential cation channel subfamily V member 1 (TRPV1) and sodium gated ion channel 1.8 (Nav1.8) positive neurons contributed to both evoked and ongoing pain sensation; ASIC1a and ASIC1b positive neurons involved in evoked pain only. Moreover, we found that ASIC1a and TRPV1 positive neurons were induced prolonged primary and secondary pain response by modified acid-saline induced muscle (ASM) pain model. Together, we have established a mouse model that can probe the roles of sensory neuron subtypes in nociception via chemo-optogenetics and found out the key nociceptors contribute to chronic paindevelopment.