As a major hippocampal output structure, the subiculum projects to diverse cortical and subcortical areas, and its projection to the medial mammillary nucleus (MM) has been implicated in memory. Major efferent targets of the MM are the anteroventral and anteromedial thalamic nuclei and Gudden’s ventral tegmental nucleus. These projections may play a key role in distributing subicular information. However, it remains unknown whether neurons in the MM that receive monosynaptic input from the subiculum project to these target regions. We addressed this issue with anterograde transsynaptic tracing mediated using adeno-associated virus serotype 1 (AAV1). Injection of AAV1-Cre and a Cre-dependent AAV encoding enhanced yellow fluorescent protein (EYFP) into the rat dorsal subiculum and MM, respectively, labeled the soma of the MM and axons in the anteroventral / anteromedial thalamic nuclei and Gudden’s ventral tegmental nucleus with EYFP. The EYFP-positive neurons in the MM were immunoreactive for glutamate and leu-enkephalin and received perisomatic GABAergic inputs. These results revealed monosynaptic projections from the subiculum to MM neurons projecting to the anteroventral / anteromedial thalamic nuclei and Gudden’s ventral tegmental nucleus. This monosynaptic connection may support a fast and robust signal flow through the hippocampal–mammillothalamic and hippocampal–mammillotegmental pathways.

作为主要的海马输出结构，下突投射到不同的皮层和皮层下区域，其投射到内侧乳头核 (MM) 与记忆有关。MM 的主要传出目标是前腹侧和前内侧丘脑核以及 Gudden 腹侧被盖核。这些投影可能在分发特定信息方面发挥关键作用。然而，MM 中从下丘接收单突触输入的神经元是否投射到这些目标区域仍然未知。我们通过使用腺相关病毒血清型 1 (AAV1) 介导的顺行跨突触追踪解决了这个问题。将 AAV1-Cre 和编码增强型黄色荧光蛋白 (EYFP) 的 Cre 依赖性 AAV 分别注射到大鼠背下托和 MM 中，用 EYFP 标记了前腹侧/前内侧丘脑核和 Gudden 腹侧被盖核中的 MM 和轴突的胞体。MM 中的 EYFP 阳性神经元对谷氨酸和亮氨酸脑啡肽具有免疫反应性，并接受周围的 GABA 能输入。这些结果揭示了从下托到 MM 神经元的单突触投射投射到前腹侧/前内侧丘脑核和 Gudden 腹侧被盖核。这种单突触连接可能支持通过海马-乳突丘脑和海马-乳头盖膜通路的快速而强大的信号流。这些结果揭示了从下托到 MM 神经元的单突触投射投射到前腹侧/前内侧丘脑核和 Gudden 腹侧被盖核。这种单突触连接可能支持通过海马-乳突丘脑和海马-乳头盖膜通路的快速而强大的信号流。这些结果揭示了从下托到 MM 神经元的单突触投射投射到前腹侧/前内侧丘脑核和 Gudden 腹侧被盖核。这种单突触连接可能支持通过海马-乳突丘脑和海马-乳头盖膜通路的快速而强大的信号流。