嗅周皮质神经元放电模式在tEBC追踪记忆形成中的作用机制探究

Neurodegenerative diseases such as Alzheimer’s disease is characterised by devastating impairments in declarative memory and exerts huge suffering on primary carers. Perirhinal cortex (PER) plays a central role in declarative memory and pathophysiologies associated with Alzheimer’s disease in humans have been shown to develop in PER. Recently, a novel mechanism cantered on muscarinic receptor-dependent endogenous persistent-firing (EPF) neurons in PER has been posited in maintaining a transient memory of the conditioned stimulus (CS) during the stimulus free trace interval in trace Pavlovian conditioning. This trace conditioning is a classic experimental protocol for declarative memory. However, little is known about the neural mechanism underlying PER to support trace Pavlovian conditioning in vivo. ..In this proposal, we will use a combination of in vivo electrophysiology, optogenetics and pharmacological intervention to directly examine the contribution of PER in the acquisition of trace eyeblink conditioning (tEBC) in freely moving rat. The necessity of PER in tEBC will be addressed by evaluating the effect on learning outcome with optical and pharmacological inactivation of PER during training. Levering the millisecond temporal resolution afforded by optogenetics, we will further pin point the most critical time window of PER firing required to support tEBC. To directly examine the neural mechanism, we will perform extracellular electrophysiological recordings in rat PER during tEBC training. We will characterize the firing pattern of PER EPF neurons as correspondence to learning progress and learning outcomes. ..Together, our proposal will provide causal evidence that muscarinic receptor-dependent EPF neurons in PER during the trace interval are necessary to bridge the CS-free interval for acquisition of tEBC. Our results will provide new insights at system level to the neural mechanism underlying declarative memory in the hippocampus-parahippocampal circuit. We also believe that these new understanding on the neuroscience will also help understand the mechanism in neurodegenerative disease and advance therapies for memory disorders.

陈述性记忆是人类长期记忆的一种主要形式。在阿尔兹海默病等多种神经退行性疾病早期常表现出陈述性记忆损伤。在以追踪眨眼条件反射（tEBC）为模型的陈述性记忆研究中发现，内侧前额叶皮质（mPFC）中的内源性持续放电（EPF）神经元在tEBC建立中起到关键作用。损伤实验表明，嗅周皮质（PER）也参与了动物tEBC条件反射的建立。在脑片实验中记录到PER中存在大量EPF神经元。但目前仍缺少PER中EPF参与tEBC的直接证据。本课题将结合光遗传学、药理学及电生理学技术控制并记录大鼠tEBC训练时PER中神经元的放电，系统地探究PER参与tEBC的直接证据。主要研究内容：(1)明确PER EPF是否参与tEBC的建立；(2)分析EPF在tEBC建立时的放电特征。本研究结果为进一步明确陈述性记忆的产生机制及与之相关的精神、神经类疾病的研究提供理论基础

我们通过本课题验证了假设：嗅周皮质（PER）在与任务相关的感觉刺激缺失的情况下，依然能维持在该刺激存在时的表现来引导行为。为了探究PER在记忆和辨别任务中的功能，我们使用“强制二选一（2-alternative forced choice task, 2-AFC)”任务来训练大鼠。在训练中，大鼠需要学会将两种不同频率的声音刺激分别和左边/右边的鼻触装置建立联系来完成空间辨别任务以获取奖励。实验中使用的两种声音刺激是250 Hz和2 kHz的正弦波，二者均有5 ms的上升时间。通过实验发现，动物能够成功完成该任务且表现出令人满意的学习成果（试验正确率≥ 70 % 且至少连续两天内有≥5组实验有5次连续正确的选择试验）。我们在实验的同时采集了PER和PL脑区的LFP数据并进行时频分析。结果表明，在所有试验中，PER的theta波能量从无声延迟的开始到声音刺激时间窗口结束几乎增大了1倍。这种能量上的增大很可能是一种计时机制：当大鼠在中间的鼻触装置等待声音刺激时，这种机制被用于预测声音刺激出现的时间。此外，PER和PL中的beta波峰值均出现在相似的时间点，这可能意味着这两个脑区在相互协调工作。由于beta波活动被认为与有意识的由上至下信息处理有关，实验观察到的beta波增强可能是从PL到PER由上至下的信息处理过程的反映。综上所述，PER的beta波很可能是PER在与任务相关的感觉刺激缺失的情况下依然能维持这种刺激来引导行为的重要神经机制。我们的研究结果不单揭示了PER在感知复杂刺激中的作用，同时为揭示如听觉处理障碍等复杂刺激缺陷的病理基础提供了线索