3 讨 论
3.1 嘌呤研究的进展
嘌呤和嘌呤受体是重要的调节递质影响着中枢和周围神经系统功能[4], 近年来成为神经科学领域研究的热点之一。
嘌呤家族分为P1、P2两大类。P1代表着以腺苷(Adenosine)及其A1、A2A、A2B和A3受体的一类核苷;P2家族包括ATP及其核苷酸受体P2X1-7和P2Y1-6,其中,P2X7受体是近年来发现的、有独特特点的受体,在钙离子(Ca2+)内流、细胞凋亡等方面起重要作用[5-10]。研究表明哺乳类及鼠视网膜神经元含有P2X1-7的受体表达,尤其RGC层有P2X7的存在[11]。近年来研究发现P2X7受体的激活参与了大量视网膜疾病的发生、发展[12-14]。例如在玻璃体视网膜病变中,P2X7受体的表达增加[12];受体的激活可引起视网膜周细胞收缩[14]。
我们已有的研究阐明了ATP受体P2X7激活是导致RGC凋亡的重要环节[5-8],提出青光眼视神经损伤嘌呤调节的可能机制:青光眼高眼压引起嘌呤信号ATP释放[2-3],作用于视网膜细胞上P2X7受体,引起Ca2+内流导致RGC凋亡[5]。将嘌呤调节引入青光眼中研究,为探讨青光眼发病机制提供了新的思路与途径。
3.2 NMDA与青光眼发病机制
青光眼是由病理性高眼压引起,以RGC死亡、视功能逐渐丧失为主要特征的一种进行性视神经病变。RGC以凋亡的形式死亡,但目前RGC凋亡的机制尚未完全清楚。
有学说认为升高的眼内压可能降低了视神经乳头的血流灌注,但在没有眼内压升高的视网膜动脉阻塞的病例中,却没有见到在青光眼中应见到的同样改变,表明在正常的血管弹性下,单纯血管因素是不足以引起RGC死亡的;另有人认为,升高的眼内压也可能因为扩张了视神经乳头筛板从而抑制了神经营养因子诸如脑源性BNDF等的轴浆运输,但研究发现RGC的死亡可以先于神经营养因子的缺乏,这表明RGC的死亡在早期可能胞体早有变化,后因神经营养因子的缺乏而加速了死亡进程。另一理论涉及的是兴奋性神经毒损伤:兴奋性神经毒谷氨酸(Glutamate)升高、NMDA受体激活引起RGC细胞内Ca2+升高导致细胞死亡[1]。尽管目前对此学说仍未完全肯定[15,16],但可重复性的研究证实谷氨酸受体激动剂确实对RGC表现出毒性作用[1,17,18]。临床和基础研究表明谷氨酸受体NMDA拮抗剂Memantine能预防压力诱导的RGC死亡,提示谷氨酸在青光眼发生发展中起一定作用[19]。然而,升高的眼内压是如何产生过量的谷氨酸去过激NMDA受体还不完全清楚,最初有研究报道,降低谷氨酸转运子水平并不能逆转这一作用[19,20]。
3.3 P2X7受体/NMDA受体在青光眼发病机制中的作用
研究已表明,嘌呤信号转导在高眼压致谷氨酸增高的过程中起重要中介调节作用。首先高眼压可引起ATP释放。实验发现,压力、损伤、应激等均可刺激嘌呤信号ATP的释放[2,3, 22,23],如轻度的压力可导致内层视网膜星状细胞释放出ATP[22],眼内压升高可检测到视网膜持续的释放出ATP[2,3]。我们将剪去眼前段的新鲜牛眼放入自制压力容器中,牛眼杯内ATP浓度随压力升高而增高[2]。另外,在激光诱导的实验性猴青光眼模型中检测到玻璃体腔ATP水平的增高[2]。最近在临床高眼压青光眼病人房水中也证实有ATP释放(数据待发表)。
ATP受体P2X7激活可导致RGC凋亡 [1-3,5-8]。关键的研究显示P2X7受体激活可诱发出视网膜星状细胞[25]和RGC[8]释放出谷氨酸(Glutamate)。因此,眼内压力、P2X7受体、谷氨酸释放及NMDA受体之间可能存在一定的相互作用,而明确其相互作用机制可能是理解RGC死亡和保护的关键部分。
本研究结果显示,三种NMDA受体拮抗剂MK-801、APV和Memantine均可阻断P2X7受体激动剂BzATP介导的RGC胞内Ca2+升高作用和减少体外培养的BzATP介导的RGC死亡数量。有力地说明,P2X7与NMDA受体之间可能共同介导着RGC兴奋性神经毒损伤机制,且P2X7在NMDA受体的上一环节先起作用。
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