小鼠海马5-羟色胺2A受体在记忆和空间认知中作用研究

Episodic memory - memory for personal history events occurring in discrete locations and at distinct times, is dependent upon the hippocampus and its network of associated brain structures. Disturbed hippocampal activity is associated with age- and Alzheimer's disease-related memory impairments. Developing effective treatments of cognitive impairments associated with these conditions requires that we understand the neurobiological and neuropharmacological mechanisms of memory and cognition. Serotonin is a neurotransmitter that influences mood states, emotional responding and cognitive processes. Activation of the G-protein coupled 5-HT2AR enhances phospholipase C activity and elevates intracellular Ca2+ and contributes to the behavioral responses of atypical antipsychotics. 5-HT2ARs are abundantly expressed in the hippocampal memory network. Genetic variation of the human 5-HT2AR gene is associated with impaired consolidation of episodic memory, and a decrease in novelty-induced activation of human hippocampus. Further, a significant decline in neocortical 5-HT2AR binding has been found in patients with cognitive impairment. Until recently, it has been difficult to define the 5-HT2AR-selective effects on memory and cognition, due in part to the lack of highly-selective 5-HT2AR agonists, and to studies showing that non-selective 5-HT2 receptor agonists enhanced associative learning, but impaired acquisition of operant responding, and induced hallucination . We recently found that at a low dose, a new 5-HT2AR agonist TCB-2 significantly enhanced consolidation of hippocampal-dependent object memory and contextual fear memory, while at a higher dose TCB-2 impaired spatial navigation in mice. These findings suggest our central hypothesis that hippocampal 5-HT2ARs influence memory and spatial cognition in mice. To test our hypothesis, we will employ the immunoelectron microscopy technique to visualize the subcellular distribution of 5-HT2ARs in the dorsal hippocampus CA1 region (dCA1) and the morphological relation between 5-HT2ARs and postsynaptic glutamate NMDARs. Then the influence of systemic or hippocampal local administration of 5-HT2AR agonist or antagonist on the encoding, consolidation, retrieval, and extinction of hippocampal-dependent spatial memory, non-spatial memory and fear memory will be examined with behavioral paradigms. Finally, we will explore the influence of pharmacological manipulation of 5-HT2ARs on the dynamics of the hippocampal pyramidal neurons and local field potentials, and the correlation between the neuronal activities and cognitive behaviors. The results will shed light on the subcellular distribution of dCA1 5-HT2AR, the role of 5-HT2AR on memory and spatial cognition and underlying mechanisms. Moreover, the results will enrich hippocampal memory study and facilitate the development of novel therapy for memory and cognition impairments.

5-羟色胺（5-HT）调节记忆和空间认知等脑高级功能，但其作用的中枢靶点及受体未见报道。海马接收5-HT能纤维投射并表达5-HT2A受体（5-HT2AR），我们预实验发现5-HT2AR激动剂低剂量时巩固小鼠海马依赖性记忆，而高剂量时削弱空间认知，故我们假设：小鼠海马5-HT2AR调节记忆和空间认知。据此我们首先采用免疫电镜检测5-HT2AR在背侧海马CA1区（dCA1）超微分布及其与谷氨酸NMDA受体的空间关系；其次采用行为学方法观察全身或海马局部给药激动或抑制5-HT2AR对小鼠空间认知，空间、非空间和恐惧记忆的编码、巩固、提取和消退的作用；最后采用多通道在体记录技术记录5-HT2AR对dCA1单个神经元放电和场电位的影响，以及电活动与认知行为的相关性。本课题研究将明确海马5-HT2AR超微分布、对记忆和空间认知的影响及机制，实验结果将丰富海马认知理论，并为改善认知障碍提供新的思路。

海马结构调控学习记忆及认知等脑功能活动，形态学等研究发现海马接受5-羟色胺（5-HT）能神经纤维投射并表达5-HT 2A受体（5-HT2AR）蛋白及mRNA，本课题系统研究5-HT2AR对小鼠记忆和空间认知的调控作用及其海马机制。免疫电镜实验结果提示5-HT2AR免疫阳性颗粒在背侧海马CA1区（dCA1）主要分布于突触后区域，包括突触后膜及突触后细胞内结构，同时5-HT2AR免疫阳性颗粒也存在于突触前膜。5-HT2AR和与突触后膜谷氨酸NMDA受体存在空间毗邻关系；行为学研究发现系统或dCA1区域给予5-HT2AR激动剂TCB-2促进对小鼠对非空间物体记忆和恐惧记忆的巩固作用及恐惧记忆的消退，抑制非空间记忆的记忆信息提取；5-HT2AR激动剂可增加dCA1单个神经元放电频率及细胞外谷氨酸浓度。依托本基金支持，我们还发现药理学阻断5-HT2AR抑制吗啡依赖相关记忆行为及躯体和内脏疼痛。我们还发现药理学激动5-HT2AR异构体5-HT2CR抑制吗啡依赖相关记忆行为。本课题研究结果提示5-HT2AR在海马突触后膜和前膜分布及与NMDA受体存在空间上毗邻关系；5-HT2AR激活促进非空间记忆巩固和恐惧记忆消退，5-HT2AR阻断可抑制吗啡依赖的记忆行为；其机制可能与调控神经元电活动和谷氨酸活动相关。本研究丰富学习记忆的5-HT调控学说，从转化医学角度审视，中枢5-HT2AR可能是调控学习记忆和相关障碍的一个重要的靶点。