探索痠和痛的神經迴路

Abstract

痠(Sng)產生的原因是由於組織酸化, 常見於鍛煉後肌肉疲勞，慢性疼痛疾病例如纖維肌痛症(fibromyalgia)。儘管痠過去被歸類爲痛覺，最近一些證據表明痠的感覺可能通過藉由本體感受器(proprioceptor)上第3型酸敏通道(Acid-sensing ion channel 3, ASIC3 )而不是痛覺感受器(nociceptor)來傳遞。爲了驗證痠和痛神經迴路差異，我們建立了在三叉神經系統上痠和發炎痛的模型。理論上在咀嚼肌上注射兩針酸性食鹽水(pH = 4.0)會通過本體感受器以及痛覺感受器上ASIC3來引發痠痛，注射芥末油則是透過痛覺感受器上瞬態電壓感受器陽離子通道蛋白類第1型 (Transient receptor potential ankyrin 1, TRPA1)引發發炎痛。在對於小鼠的機械敏感性測試(von-Frey test)中，與之前的研究相似，注射兩針酸性食鹽水會通過ASIC3來引起長期痛覺敏感化，而注射芥末油引起長期痛覺敏感化與ASIC3無關。我們將轉錄因子c-Fos作爲神經活化標誌，注射第二針7天後，比較痠和發炎痛模型所活化的三叉神經系統中次級感覺神經元所在的核區，位於三叉神經主核（principal trigeminal nucleus，Pr5），脊髓三叉神經核顱側亞核（spinal trigeminal nucleus, Sp5O）脊髓三叉神經核尾側亞核（spinal trigeminal nucleus caudalis, Sp5C）的神經細胞通過ASIC3被酸活化，Sp5C核區細胞通過TRAP1被芥末油活化。最後，我們將帶有不同熒光蛋白的Cre重組酶的類腺病毒（Cre-dependent Adeno-Associated Virus）分別注射到TRAP2 小鼠的Pr5與Sp5C中來標記一周后被注射兩針酸性食鹽水所活化的細胞。Pr5中活化的神經元投射到視丘腹側後內側核（ventral posteromedial nucleus, VPM），這與Sp5C投射到視丘核區是不同。因爲位於Pr5是本體感覺的次級感覺神經元，Sp5C中的是痛覺的次級感覺神經元，這些結果中可以看到在三叉神經系統上痠和發炎症痛活化路徑的差異，證明了痠和痛是不同的神經迴路。

Sng (soreness) is the unpleasant feeling caused by muscle acidosis. Although traditionally classified as pain, recent studies show that Sng is mediated by acid-sensing ion channel 3 (ASIC3) on proprioceptors instead of nociceptors. In this study, we compare neuropathways of the trigeminal sensory system in the Sng and inflammatory pain model. Consistent with previous studies, we observed ASIC3-dependent chronic behavioral hypersensitivity upon repeated acidic saline injection into the masseter muscle. Nevertheless, the behavioral hypersensitivity induced by mustard oil is ASIC3-independent. By comparing the c-Fos activation of secondary sensory neuron activation 7 days after the secondary injection between sng and pain models, neurons in the principal trigeminal nucleus (Pr5), spinal trigeminal nucleus caudalis (Sp5C), and spinal trigeminal nucleus oralis (Sp5O) were activated by acid injection through ASIC3, while only those in the Sp5C were activated by mustard oil. Finally, Cre-dependent Adeno-Associated Virus (AAV) with different fluorescent proteins was injected into Pr5 and Sp5C of the TRAP2 mice, in order to label the active neurons 7 days after repeated acidic saline intramuscular injection. Pr5 projects to the ventral posteromedial nucleus (VPM) which is different from Sp5C projection in the thalamus. Since the Pr5 and SP5C contain the secondary sensory neurons of proprioception and nociception respectively, our results demonstrate that Sng and inflammatory pain display very different neurocircuits in the trigeminal system.