The nucleus reuniens (NR) is an important anatomic and functional relay between the medial prefrontal cortex (mPFC) and the hippocampus (HPC). Whether the NR controls neuronal assemblies, a hallmark of information exchange between the HPC and mPFC for memory transfer/consolidation, is not known. Using simultaneous local field potential and unit recordings in NR, HPC, and mPFC in male rats during slow oscillations under anesthesia, we identified a reliable sequential activation of NR neurons at the beginning of UP states, which preceded mPFC ones. NR sequences were spatially organized, from dorsal to ventral NR. Chemical inactivation of the NR disrupted mPFC sequences at the onset of UP states as well as HPC sequences present during sharp-wave ripples. We conclude that the NR contributes to the coordination and stabilization of mPFC and HPC neuronal sequences during slow oscillations, possibly via the early activation of its own sequences.

SIGNIFICANCE STATEMENT Neuronal assemblies are believed to be instrumental to code/encode/store information. They can be recorded in different brain regions, suggesting that widely distributed networks of networks are involved in such information processing. The medial prefrontal cortex, the hippocampus, and the thalamic nucleus reuniens constitute a typical example of a complex network involved in memory consolidation. In this study, we show that spatially organized cells assemblies are recruited in the nucleus reuniens at the UP state onset during slow oscillations. Nucleus reuniens activity appears to be necessary to the stability of medial prefrontal cortex and hippocampal cell assembly formation during slow oscillations. This result further highlights the role of the nucleus reuniens as a functional hub for exchanging and processing memories.

里诺核（NR）是内侧前额叶皮层（mPFC）和海马（HPC）之间重要的解剖和功能中继。NR是否控制神经元组装，HPC和mPFC之间用于内存转移/合并的信息交换的标志尚不清楚。使用在麻醉下缓慢振荡期间雄性大鼠的NR，HPC和mPFC的同时局部场电势和单位记录，我们确定了在UP状态开始时可靠的顺序激活NR神经元，该状态先于mPFC状态。NR序列是空间排列的，从背侧到腹侧NR。在UP状态开始时NR破坏的mPFC序列的化学灭活以及在尖波波动期间存在的HPC序列。

重要性声明神经元组装被认为有助于编码/编码/存储信息。它们可以记录在不同的大脑区域，表明这种信息处理涉及网络的广泛分布。内侧前额叶皮层，海马和丘脑核单位构成了涉及记忆整合的复杂网络的典型示例。在这项研究中，我们显示空间组织化的细胞集合体在缓慢振荡期间在UP状态发作时被募集到核内。在缓慢振荡过程中，核仁神经元活性对于稳定内侧前额叶皮层和海马细胞装配体形成似乎是必需的。该结果进一步强调了核仁核作为交换和处理记忆的功能枢纽的作用。