Activity-dependent plasticity in an associative hippocampal circuit: mechanisms, synaptic learning rules and involvement in disease

海马的齿状回（DG）在记忆的形成中起着关键作用，它通过转换记忆的模式来实现。 皮层输入转化为新的模式输出到CA 3区。尽管这种现象的细胞和突触基础 重要的转化仍然知之甚少，DG中的两种兴奋性细胞类型，颗粒细胞（GC）和 肺门苔藓细胞（MC）起主要作用。MCs介导一种内在的异源结合（GC-MC-GC）兴奋性 环路，从相对少量的GC接收强大的输入，并提供高度分布的兴奋性 输出到大量的GC。MC将其联合和连合轴突投射到ipsi-和 DG的对侧内分子层，在那里它们与GC的近端树突突触。此外，委员会认为， MC还将它们的轴突沿着海马的隔颞轴投射，从而在功能上连接 这个结构的不同领域。通过沿着隔颞轴投射到DG的大部分区域，MCs可以 为皮层产生的信息提供重要的上下文内容。为了理解如何 信息在DG中进行处理，以及该回路的失调如何导致疾病，更好的方法是 需要异缔合GC-MC-GC电路及其动态特性的知识。我们有 最近报道，MC-GC突触经历了一种新的突触前，NMDA受体独立的形式， 长时程增强（LTP），需要突触后脑源性神经营养因子（BDNF）/TrkB， 突触前环腺苷酸（cAMP）/PKA信号传导。我们假设这种新形式的可塑性增强了GC 输出在联合MC-GC循环电路，并可能有助于DG依赖形式的学习和 脑疾病，如癫痫。关于这一电路的大量问题仍然没有答案。 初步数据表明MC-GC LTP在体内由经验和癫痫活动诱导， 受内源性系统（例如内源性大麻素和腺苷信号传导）调节，并且它可以是 并伴有抑制性传递的LTP。在这里，使用实验方法的组合， 在体外和体内，我们的目的是（1）表征MC可塑性的突触学习规则，（2）识别 （3）确定MC-GC LTP的性质和作用机制 抑制性LTP，和（4）确定MC可塑性在体内的功能相关性。通过识别主要的 性质和机制的活动依赖性可塑性在一个关键的经常性电路在DG，我们提出的 研究不仅可以提高我们对该回路在DG信息处理中的确切作用的理解， 和记忆编码，但也评估如何失调，这一电路可能有助于大脑疾病， 包括癫痫焦虑精神分裂症和抑郁症

The dentate gyrus (DG) of the hippocampus plays a key role in memory formation by transforming patterns of cortical inputs into new patterns of output to the CA3 area. Although the cellular and synaptic basis of this important transformation remain poorly understood, two excitatory cell types in the DG, granule cells (GC) and hilar mossy cells (MC), play a major role. MCs mediate an intrinsic, hetero-associative (GC-MC-GC) excitatory loop, receiving powerful input from a relatively small number of GCs, and providing highly distributed excitatory output to a large number of GCs. MCs project their associational and commissural axons to the ipsi- and contralateral inner molecular layer of the DG, where they synapse onto proximal dendrites of GCs. Moreover, MCs also project their axons along the septotemporal axis of the hippocampus, thereby connecting functionally diverse areas of this structure. By projecting to most areas of the DG along the septotemporal axis, MCs could provide important contextual content to the information arising from the cortex. In order to understand how information is processed in the DG and how dysregulation of this circuit may contribute to disease, a better knowledge of the hetero-associative GC-MC-GC circuit and its dynamic properties is required. We have recently reported that MC-GC synapses undergo a novel presynaptic, NMDA-receptor independent form of long-term potentiation (LTP) that requires postsynaptic brain-derived neurotrophic factor (BDNF)/TrkB and presynaptic cyclic AMP(cAMP)/PKA signaling. We hypothesize that this novel form of plasticity enhances GC output at the associative MC-GC recurrent circuit, and may contribute to DG-dependent forms of learning and brain disease, such as epilepsy. A large number of questions regarding this circuit remain unanswered. Preliminary data indicates that MC-GC LTP is induced in vivo by experience and epileptic activity, is critically regulated by endogenous systems (e.g. endocannabinoid and adenosine signaling), and it can be accompanied by LTP of inhibitory transmission. Here, using a combination of experimental approaches both in vitro and in vivo, we aim to (1) characterize the synaptic learning rules of MC plasticity, (2) identify the molecular mechanism underlying MC-GC LTP, (3) determine the properties and mechanism underlying inhibitory LTP, and (4) determine the functional relevance of MC plasticity in vivo. By identifying the main properties and mechanisms of activity-dependent plasticity in a crucial recurrent circuit in the DG, our proposed studies may not only improve our understanding of the precise role of this circuit in DG information processing and memory encoding, but also assess how dysregulation of this circuit may contribute to brain disease, including epilepsy, anxiety, schizophrenia and depression.