Conscious memories are critically dependent upon bilateral hippocampal formation, and interhemispheric commissural projections made by mossy cells and CA3 pyramidal cells. GABAergic interneurons also make long-range axonal projections, but little is known regarding their commissural, inter-hippocampal connections. We used retrograde and adeno-associated viral tracing, immunofluorescence and electron microscopy, and in vitro optogenetics to assess contralateral projections of neurochemically defined interneuron classes. We found that contralateral-projecting interneurons were 24-fold less common compared to hilar mossy cells, and mostly consisted of somatostatin- and parvalbumin-expressing types. Somatostatin-expressing cells made denser contralateral axonal projections than parvalbumin-expressing cells, although this was typically 10-fold less than the ipsilateral projection density. Somatostatin-expressing cells displayed a topographic-like innervation according to the location of their somata, whereas parvalbumin-expressing cells mostly innervated CA1. In the dentate gyrus molecular layer, commissural interneuron post-synaptic targets were predominantly putative granule cell apical dendrites. In the hilus, varicosities in close vicinity to various interneuron subtypes, as well as mossy cells, were observed, but most contralateral axon varicosities had no adjacent immunolabeled structure. Due to the relative sparsity of the connection and the likely distal dendritic location of their synapses, commissural projections made by interneurons were found to be weak. We postulate that these projections may become functionally active upon intense network activity during tasks requiring increased memory processing.

中文翻译：

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表达生长抑素的中间神经元形成对侧海马的轴突投影。

意识记忆在很大程度上取决于双侧海马的形成以及苔藓细胞和CA3锥体细胞的半球合缝投影。GABA能神经元也能进行远距离轴突投射，但对它们的连合，海马间连接了解甚少。我们使用了逆行和腺相关病毒示踪，免疫荧光和电子显微镜以及体外光遗传学来评估神经化学定义的中间神经元类别的对侧预测。我们发现对侧投射的中间神经元比肺门苔藓细胞少见24倍，并且主要由生长抑素和小白蛋白表达类型组成。生长抑素表达细胞比对白蛋白表达细胞的对侧轴突投射更密集，尽管这通常比同侧投影密度小10倍。表达生长抑素的细胞根据其生长激素的位置表现出地形样的神经支配，而表达小白蛋白的细胞则主要支配CA1。在齿状回分子层中，连合后神经元突触后的目标主要是假定的颗粒细胞顶端树突。在hilus中，观察到与各种中间神经元亚型以及苔藓细胞非常接近的静脉曲张，但是大多数对侧轴突静脉曲张没有相邻的免疫标记结构。由于连接的相对稀疏性和突触可能位于远端树突状位置，因此发现由中间神经元产生的连合投影很弱。