一氧化氮在中樞神經元軸突傷害反應中扮演之角色

Abstract

為瞭解中樞內投射到周邊的神經元在軸突受傷後，一氧化氮和自由基環境對其存活的影響，我們以截斷顏面神經元周邊軸突的近端及遠端為實驗設計，探討四種調控胞內自由基的酶、受傷細胞存活的情形、其細胞體附近星形神經膠細胞和小膠細胞的反應、以及其在中樞內殘存的軸突退化的情形。所謂近端是指截斷處在腦幹表面，遠端則是在莖乳孔處。顏面運動神經元在遠端軸突受傷後，神經性一氧化氮合成酶的表現提高，調控神經性一氧化氮合成酶活性的酶calcineurin的表現降低，同時錳―超氧化物岐化酶的表現會在一週時上升，之後維持正常。然而在近端軸突受傷後，神經性一氧化氮合成酶的表現同樣會提高，calcineurin表現也會降低，但錳―超氧化物岐化酶的表現則反而降低，特別是在手術後四週，且細胞死亡情形較遠端受傷嚴重；此外，神經細胞裡也有粒線體腫脹的情況。然而以TUNEL染色，並沒有觀察到細胞凋亡的現象，也未觀察到有受傷細胞胞核裂成小葉之情形，因此推測細胞不是經由凋亡的方式死亡，而造成細胞死亡的主要原因可能來自錳―超氧化物岐化酶表現的降低，進而失去對粒線體內superoxide代謝的控制，而非對NO生成的調控。此外，軸突受傷後，星形神經膠細胞的突起轉為肥大，小膠細胞則從具有重複分枝突起轉變成變形蟲狀而少有突起的型態，這些現象的發生在近端軸突受傷後較遠端受傷者早。最後，我們發現顏面神經在周邊軸突受傷後，位於中樞內未受傷的近端軸突快速的以逆行性的方向退化，這和中樞內投射的紅核脊髓徑神經元的軸突受傷後，其近端軸突以前向式且緩慢的情形退化截然不同。因此，雖然中樞內投射到中樞內、外的神經元的胞體、樹突、及部分的軸突同樣都在中樞神經系統中，但這兩類細胞可能具先天性差異因而對軸突傷害反應不同。

To understand the role of NO and free radical environment on the survival of periphery-projection central neurons following axonal injury, we studied the temporal expressions of four free radical enzymes, the severity of cell loss, the perineuronal astrocytic and microglial reactions, and the degeneration of the proximal central axons of facial motoneurons following close and distant axotomies at the brainstem surface and stylomastoid foramen, respectively. Distant lesion upregulated NOS, persistently downregulated the NOS-activating enzyme calcineurin, transiently upregulated the Mn-SOD expressions of these neurons, and resulted in mild neuron loss. On the other hand, close lesion upregulated NOS but downregulated calcineurin transiently and Mn-SOD expression specifically at 4-week-post-injury. This led to severe cell loss and neurons were found to contain swollen mitochondria 2–4 weeks postinjury. Neither nuclear fragmentation nor TUNEL reactivity was identified. This suggests that downregulation of Mn-SOD expression and the subsequent mitochondrial free radical toxicity are likely to be responsible for the death of axotomized periphery-projection central neurons. Close axotomy caused an earlier transformation of reactive glial morphologies, including the hypertrophy of astrocytic processes and metamorphosis of ramified microglia to amoeboid cells, than distant injury. Unlike rubrospinal neurons whose central axons degenerated slowly and anterogradely only following the severe loss of cell bodies elicited by close axotomy, the central axons of facial motoneurons degenerated quickly and retrogradely independent of the severity of their cell body loss. This shows that although central neurons projecting within and outside the CNS have cell bodies, dendrites and part of their axons in the same CNS environment, they are inheritably different and react differently to axonal injuries.