The midline thalamus bidirectionally connects the medial prefrontal cortex (mPFC) and hippocampus (HC) creating a unique cortico-thalamo-cortical circuit fundamental to memory and executive function. While the anatomical connectivity of midline thalamus has been thoroughly investigated, little is known about its cellular organization within each nucleus. Here we used immunohistological techniques to examine cellular distributions in the midline thalamus based on the calcium binding proteins parvalbumin (PV), calretinin (CR), and calbindin (CB). We also examined these calcium binding proteins in a population of reuniens cells known to project to both mPFC and HC using a dual fluorescence retrograde adenoassociated virus-based tracing approach. These dual reuniens mPFC-HC projecting cells, in particular, are thought to be important for synchronizing mPFC and HC activity. First, we confirmed the absence of PV⁺ neurons in the midline thalamus. Second, we found a common pattern of CR⁺ and CB⁺ cells throughout midline thalamus with CR⁺ cells running along the nearby third ventricle (3V) and penetrating the midline. CB⁺ cells were consistently more lateral and toward the middle of the dorsal-ventral extent of the midline thalamus. Notably, single-labeled CR⁺ and CB⁺ zones were partially overlapping and included dual-labeled CR⁺/CB⁺ cells. Within RE, we also observed a CR and CB subzone specific diversity. Interestingly, dual mPFC-HC projecting neurons in RE expressed none of the calcium binding proteins examined, but were contained in nests of CR⁺ and CB⁺ cells. Overall, the midline thalamus was well organized into CR⁺ and CB⁺ rich zones distributed throughout the region, with dual mPFC-HC projecting cells in reuniens representing a unique cell population. These results provide a cytoarchitectural organization in the midline thalamus based on calcium binding protein expression, and set the stage for future cell-type specific interrogations of the functional role of these different cell populations in mPFC-HC interactions.

中文翻译：

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与前额叶-海马回路相关的中线丘脑的钙调蛋白和钙结合蛋白结构

中线丘脑双向连接内侧前额叶皮层 (mPFC) 和海马体 (HC)，形成独特的皮质-丘脑-皮质回路，对记忆和执行功能至关重要。虽然已经彻底研究了中线丘脑的解剖连通性，但对其每个细胞核内的细胞组织知之甚少。在这里，我们使用免疫组织学技术检查基于钙结合蛋白小白蛋白 (PV)、钙结合蛋白 (CR) 和钙结合蛋白 (CB) 的中线丘脑中的细胞分布。我们还使用基于双荧光逆行腺相关病毒的追踪方法检查了已知投射到 mPFC 和 HC 的 reuniens 细胞群中的这些钙结合蛋白。这些双团聚 mPFC-HC 投射细胞，特别是，被认为对于同步 mPFC 和 HC 活动很重要。首先，我们确认没有 PV⁺中线丘脑中的神经元。其次，我们在整个中线丘脑中发现了一种常见的 CR ⁺和 CB ^{+细胞模式，CR +}细胞沿着附近的第三脑室 (3V) 运行并穿透中线。CB ⁺细胞始终更侧向并朝向中线丘脑的背腹范围的中间。值得注意的是，单标记 CR ⁺和 CB ⁺区域部分重叠，包括双标记 CR ⁺ /CB ⁺细胞。在 RE 中，我们还观察到了 CR 和 CB 子区域的特定多样性。有趣的是，RE 中的双 mPFC-HC 投射神经元不表达所检查的钙结合蛋白，但包含在 CR ⁺和 CB ⁺细胞的巢中。总体而言，中线丘脑被很好地组织成分布在整个区域的 CR ⁺和 CB ⁺丰富区域，双 mPFC-HC 投射细胞代表独特的细胞群。这些结果提供了基于钙结合蛋白表达的中线丘脑中的细胞结构组织，并为未来细胞类型特异性询问这些不同细胞群在 mPFC-HC 相互作用中的功能作用奠定了基础。