Department of Anatomy, Division of Neurobiology, Faculty of Basic Medical Sciences, College of Health Sciences, University of Ilorin, Ilorin, Nigeria.
Center for Studies in Behavioral Neurobiology, Department of Psychology, Concordia University, Montreal, Canada.
J Mol Neurosci. 2021 Jun;71(6):1205-1220. doi: 10.1007/s12031-020-01747-w. Epub 2021 Jan 8.
The hallmarks of Alzheimer's disease (AD) pathology include senile plaques accumulation and neurofibrillary tangles, which is thought to underlie synaptic failure. Recent evidence however supports that synaptic failure in AD may instead be instigated by enhanced N-methyl-D-aspartate (NMDA) activity, via a reciprocal relationship between soluble amyloid-β (Aβ) accumulation and increased glutamate agonist. While previous studies have shown Aβ-mediated alterations to the glutamatergic system during AD, the underlying etiology of excitotoxic glutamate-induced changes has not been explored. Here, we investigated the acute effects of stereotaxic dentate gyrus (DG) glutamate injection on behavior and molecular expression of specific proteins and neurochemicals modulating hippocampal functions. Dependence of glutamate-mediated effects on NMDA receptor (NMDAR) hyperactivation was tested using NMDARs antagonist memantine. DG of Wistar rats (12-weeks-old) were bilaterally microinjected with glutamate (500 mM) with or without daily intraperitoneal (i.p.) memantine injection (20 mg/kg) for 14 days, while controls received either intrahippocampal/i.p. PBS or i.p. memantine. Behavioral characterization in open field and Y-maze revealed that glutamate evoked anxiogenic responses and perturbed spatial memory were inhibited by memantine. In glutamate-treated rats, increased NO expression was accompanied by marked reduction in profiles of glutathione-s-transferase and glutathione peroxidase. Similarly, glutamate-mediated increase in acetylcholinesterase expression corroborated downregulation of synaptophysin and PSD-95, coupled with initiation of reactive astrogliosis (GFAP). While neurofilament immunolocalization/immunoexpression was unperturbed, we found glutamate-mediated reduction in neurogenic markers Ki67 and PCNA immunoexpression, with a decrease in NR2B protein expression, whereas mGluR1 remains unchanged. In addition, increased expression of apoptotic regulatory proteins p53 and Bax was seen in glutamate infused rats, corroborating chromatolytic degeneration of granule neurons in the DG. Interestingly, memantine abrogated most of the degenerative changes associated with glutamate excitotoxicity in this study. Taken together, our findings causally link acute glutamate dyshomeostasis in the DG with development of AD-related behavioral impairment and molecular neurodegeneration.
阿尔茨海默病（AD）病理学的标志包括老年斑的积累和神经纤维缠结，这被认为是突触功能衰竭的基础。然而，最近的证据支持 AD 中的突触功能衰竭可能是由可溶性淀粉样蛋白-β（Aβ）积累和谷氨酸激动剂增加引起的 N-甲基-D-天冬氨酸（NMDA）活性增强引发的。虽然以前的研究表明 AD 期间 Aβ 介导的谷氨酸能系统改变，但兴奋性谷氨酸诱导的变化的潜在病因尚未探索。在这里，我们研究了立体定向齿状回（DG）谷氨酸注射对行为和调节海马功能的特定蛋白质和神经化学物质的分子表达的急性影响。使用 NMDA 受体（NMDAR）拮抗剂美金刚测试了谷氨酸介导的影响对 NMDAR 超激活的依赖性。用谷氨酸（500mM）双侧立体定向微注射 Wistar 大鼠（12 周龄），或每天腹膜内（i.p.）给予美金刚（20mg/kg）14 天，而对照组接受海马内/ i.p. PBS 或 i.p. 美金刚。旷场和 Y 迷宫中的行为特征表明，谷氨酸引起焦虑反应，空间记忆受到干扰，而美金刚可抑制这种反应。在谷氨酸处理的大鼠中，NO 表达增加伴随着谷胱甘肽-S-转移酶和谷胱甘肽过氧化物酶的明显减少。同样，谷氨酸介导的乙酰胆碱酯酶表达增加与突触小体蛋白和 PSD-95 的下调相吻合，同时引发反应性星形胶质细胞增生（GFAP）。神经丝免疫定位/免疫表达不受影响，但我们发现谷氨酸介导的神经发生标志物 Ki67 和 PCNA 免疫表达减少，NR2B 蛋白表达减少，而 mGluR1 不变。此外，在谷氨酸输注大鼠中还观察到凋亡调节蛋白 p53 和 Bax 的表达增加，这与 DG 中颗粒神经元的空泡变性退化相吻合。有趣的是，美金刚阻断了本研究中与谷氨酸兴奋毒性相关的大多数退行性变化。综上所述，我们的研究结果因果关系地将 DG 中急性谷氨酸稳态紊乱与 AD 相关的行为障碍和分子神经退行性变联系起来。
