自然遺忘是一種記憶修剪及簡化的方式，以利於長期記憶的儲存。研究記憶如何長期儲存，是一個相當熱門的研究主題 ，其中幼年健忘為一種自然遺忘的模式，為一種研究遠程記憶機制之重要平台。目前文獻上有數個假說用來解釋幼年情境式記憶在成年時無法被提取的原因，但仍未達共識。大腦中海馬迴在情境式記憶之學習中，扮演著關鍵角色。而後壓皮質則與遠程情境式記憶提取有關，與海馬迴有密不可分的相互連結。然而，海馬迴與後壓皮質網絡在幼年健忘及遠程記憶提取中的關聯性則尚未釐清。
為了研究幼年的海馬迴在記憶學習時參與的分子機轉，我們首先確認幼年鼠 (出生後20日)相較於成鼠 (出生後60日)表現長期記憶缺陷於物件位置記憶與情境式恐懼記憶之作業。同時，在低強度學習條件下，幼年鼠海馬迴CA1的神經興奮度低於成年鼠。藉由電生理，幼年鼠的Schaffer Collateral-CA1 突觸的基礎興奮性突觸傳導與早期長期增益現象皆顯著低於成年鼠。反之，幼年鼠則較容易去增益現象。在神經突觸蛋白質的表現量上，幼年鼠海馬迴CA1的NMDA受體的次體GluN2B、PKMζ和PP2B都顯著高於成年鼠。進一步，我們觀察到CaMKII之Thr286自體磷酸化位點、GluA1之Ser831磷酸化位點，以及PKMζ生合成都會在維持早期長期增益現象出現。在單一次高頻電刺激的條件下，幼年鼠都顯著低於成年鼠。再者，我們發現在幼年鼠給予藥物阻斷NMDA受體的次體GluN2B或PP2B均能有效地改善早期長期增益現象及長期記憶的表現。在神經迴路的研究上，我們也發現海馬迴會投射至後壓皮質，並於遠程情境式記憶提取時提高神經活性在顆粒狀後壓皮質、非顆粒狀後壓皮質、基底外側杏仁核、齒狀迴、外側內嗅皮質與後鼻皮質。再者，透過活化幼年記憶的後壓皮質印痕細胞，可以維持記憶至兩週之久。這代表後壓皮質在遠程記憶有重要角色。
為了研究神經迴路於後壓皮質分區在遠程記憶提取之角色，我們使用順向與逆向的病毒標定法。我們確認小鼠的顆粒狀後壓皮質的興奮性神經從第五層投射至海馬迴背側CA1與非顆粒狀後壓皮質的表層。我們發現化學與光遺傳學抑制顆粒狀後壓皮質至CA1路徑，而非透過顆粒狀後壓皮質至非顆粒狀後壓皮質路徑，則有選擇性參與遠程記憶提取。藉由這一系列的研究，我們提供了早期長期增益效應維持發育不成熟與後壓皮質印痕細胞是與幼年健忘之發源有關性。我們也發現了一個顆粒狀後壓皮質參與遠程恐懼記憶透過至CA1之路徑。本研究，我們提出了一個幼年健忘模式可能是合適的動物模式去研究遺忘跟遠程記憶之機轉。

Natural forgetting is a process which shapes and simplifies our memory for long-term storage. To study the mechnism for permanent memory saving, we examined childhood amensia (CA) with the property of natural forgetting to clarify the mechanism of remote memory. Several hypotheses have been raised to explain mechanisms underlying the phenomenon that episodic memories acquired during infancy or early childhood cannot be recalled in adult. According to previous study, the episodic memory formation initially requires hippocampus for acquisition and gradaully transfer to neocortex for remote memory storage. Retrosplenial cortex (RSC) is a neocortex area densely interconnected with hippocampus and contributes to the remote contextual fear memory (CFM) retrieval.
Thus, we aim to study molecular bases contributes to childhood memory acquisition in hippocampus and the circuit mechanisms through the subdivisions of RSC [granular (RSG) and agranular retrosplenial area (RSA)] involved in remote memory retrieval for the better understanding of permanent memory saving.
To investigate molecular mechanisms for childhood memory acquisition in hippocampus, we first confirmed that juvenile [postnatal day 20 (P20)] mice showed deficits in long-term memory retention compared to adult (P60) mice by using object-location memory (OLM) and contextual fear conditioning (CFC) tasks. Meanwhile, the neuronal excitability of hippocampal CA1 was significant decreased in P20 than P60 mice after weak CFC training paradigm. By electrophysiology approach, P20 mice was significantly lower in basal transmission and early-phase long-term potentiation (E-LTP) at Schaffer collateral-CA1 synapses, compared to P60 mice. Conversely, P20 mice have a greater susceptibility to induce time-dependent reversal of LTP than P60 mice by low-frequency afferent stimulation. In protein expression levels, the GluN2B subunit of N-methyl-D-aspartate receptors (NMDARs), protein kinase M zeta(PKMzeta) and protein phosphatase 2B (PP2B) were significantly increased in P20 than P60 mice in hippocampal CA1 region. Furthermore, we observed that the phosphorylation of calmodulin-dependent protein kinase II alpha (CaMKIIalpha) at Thr286, GluA1 phosphorylation at Ser831 and the protein levels of PKMzeta biosynthesis occurred during the ensuing maintenance of E-LTP were significantly lower after single train of high-frequency stimulation (HFS) in P20 than P60 mice. Moreover, we found that pharmacological blockade of GluN2B-containing NMDARs or PP2B significantly rescued impairment of E-LTP and long-term memory in P20 mice.
To study whether the RSC contribute to childhood memory retrieval, we found that hippocampus formation sends the projection to RSC and increased neuronal activity in RSG, RSA, basal lateral amygdala (BLA), dentate gyrus (DG), lateral enterhinal cortex (LEnt) and postrihinal cortex (POR) after remote CFM. Moreover, reactivation of the childhood memory engram in the RSG can remind and maintain the memory after 2 weeks by HFS protocol of optogenetic stimulation. It may exhibit the important role of RSG in remote memory. To explore the neuronal circuitry of RSC subdivisions for remote memory retrieval, our results in the use of anterograde and retrograde viral tracing approaches identified the layer V of RSG excitatory projections to the dorsal CA1 and the RSA superficial layers in mice. Chemogenetic or optogenetic silencing exhibits the RSG to CA pathway contributes to the remote CFM retrieval, but not the RSG to RSA. By series of experiments, we provide evidence to know the developmental immaturity of the E-LTP maintenance and the engram of RSG are related to the occurrence of CA. We also indentified remote CFM retrieval is required RSG, and provide circuit evidence that direct RSG to CA1 projection contribute to the remote CFM retrieval. In this study, we raise the possibility that the CA is an appropriated animal model to further investigate the mechanisms of forgetting and remote memory.

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