Association of cognitive deficit with glutamate and insulin signaling in a rat model of Parkinson's disease

Cognitive deficit is a frequent non-motor symptom in Parkinson 's disease (PD) with an unclear pathogenesis. Recent research indicates possible involvement of insulin resistance and glutamate excitotoxicity in PD development. We investigated cognitive performance and the brain glutamate and insulin signaling in a rat model of PD induced by bilateral intrastriatal injection of 6-hydroxydopamine (6-OHDA). Cognitive functions were assessed with Passive avoidance (PA) and Morris Water Maze (MWM) tests. The expression of tyrosine hydroxylase (TH) and proteins involved in insulin (IR, pI3K, ERK) and glutamate receptor (AMPAR, NMDAR) signaling was assessed in the hippocampus (HPC), hypothalamus (HPT) and striatum (S) by immunofluorescence, Western blot and ELISA. Three months after 6-OHDA treatment, cognitive deficit was accompanied by decreased AMPAR activity and TH levels (HPC, S), while levels of the proteins involved in insulin signaling remained largely unchanged. Spearman’s rank correlation revealed a strong positive correlation for pAMPAR-PA (S), pNMDAR-pI3K (HPC) and pNMDAR-IR (all regions). Additionally, a positive correlation was found for TH-ERK and TH-pI3K, and a negative one for TH-MWM/errors and pI3K-MWM/time (S). These results suggest a possible association between brain glutamate (but not insulin) signaling dysfunction and cognitive deficit in a rat PD model, detected three months after 6-OHDA treatement.认知功能障碍是帕金森病（Parkinson's Disease, PD）常见的非运动症状，其发病机制尚未阐明。现有研究表明，胰岛素抵抗与谷氨酸兴奋性毒性可能参与帕金森病的发生发展过程。本研究通过双侧纹状体内注射6-羟基多巴胺（6-hydroxydopamine, 6-OHDA）构建帕金森病大鼠模型，探究其认知功能以及脑内谷氨酸与胰岛素信号通路的变化。研究采用避暗实验（Passive Avoidance, PA）与莫里斯水迷宫（Morris Water Maze, MWM）对大鼠的认知功能进行评估；通过免疫荧光、蛋白质印迹（Western Blot）与酶联免疫吸附测定（Enzyme-Linked Immunosorbent Assay, ELISA），分别在海马体（Hippocampus, HPC）、下丘脑（Hypothalamus, HPT）及纹状体（Striatum, S）中，检测酪氨酸羟化酶（Tyrosine Hydroxylase, TH）以及参与胰岛素信号通路（IR、pI3K、ERK）与谷氨酸受体信号通路（AMPAR、NMDAR）的蛋白表达水平。6-羟基多巴胺造模3个月后，模型大鼠出现认知功能障碍，同时伴随海马体与纹状体中AMPAR活性及TH水平下调，而胰岛素信号通路相关蛋白的表达水平基本未发生改变。斯皮尔曼秩相关分析（Spearman's rank correlation）结果显示，纹状体中pAMPAR与PA评分、海马体中pNMDAR与pI3K水平、以及所有脑区中pNMDAR与IR水平均呈显著正相关；此外，TH与ERK、TH与pI3K水平呈正相关，而纹状体中TH与MWM错误次数、pI3K与MWM逃逸时间均呈负相关。上述结果表明，在6-羟基多巴胺造模3个月后的帕金森病大鼠模型中，脑内谷氨酸信号通路功能异常（而非胰岛素信号通路）与认知功能障碍存在潜在关联。