| Summary: | 碩士 === 國立陽明大學 === 神經科學研究所 === 106 === The hippocampus plays a key role in memory function as lesion to this structure usually result in learning disabilities. There are three sub-fields within the hippocampus, the CA3 field, CA2 field, and CA1 field. Neurons in different parts (dorsal, ventral, proximal, and distal) of the hippocampal CA1 region not only have distinct features, but also display diverse functions. In addition, different portions of the hippocampal CA1 field communicate with different brain areas, and some topographic connections have also been mapped. In recent years, more and more varieties (e.g., anatomical structure, electrophysiology firing pattern, gene expression, cell type, etc.) have been recognized within the hippocampus, and therefore, the hippocampus has gradually been considered to be a heterogeneous structure. However, it remains unknown whether neurons in each part of the hippocampal CA1 region also connect with other brain regions in a topographic organization or they intermingle with each other. To address these questions, the most effective and straightforward approach is injecting several neuronal tracers into different sites within restricted sub-region simultaneously. Yet, this approach is under the premises that 1) tracers’ locations are well-separated, 2) tracers do not diffuse to surrounding brain regions, 3) and the labelling efficiency between tracers is comparable. Unfortunately, only few studies have ever examined the labelling efficiency between tracers. Therefore, in this study, we aim at assessing the efficiency between retrograde tracers in tracing the connectivity of the CA1 field of the dorsal hippocampus.
We first sub-divide the dorsal hippocampal CA1 region into 4 different parts (the dorsal septal part, dorsal temporal part, dorsal proximal part, and dorsal distal part) and then assess the labelling efficiency by co-injecting 2 tracers into one of the sub-domains simultaneously. We demonstrate that the labelling efficiency between different types of retrograde tracers, retrobeads (beads) and cholera toxin B subunit conjugates (CTB), is not comparable. To our surprise, the labelling efficiency between the same types of retrograde tracers is also unequal. Here, we show that the labelling efficiency of red beads surpassed green beads even with 5-fold less in volume. In addition, the labelling efficiency of CTB488 is obviously higher than CTB594, but is slightly lower than CTB555. Overall, our findings demonstrate that most of the time, the labelling efficiency between tracers is not comparable. Consequently, we should pay more attention to the difference in labelling efficiency if using more than 2 tracers at the same time. Otherwise, we are prone to underestimate the connectivity of at least one region.
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