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The CA1, an important subregion of the hippocampus, is anatomically and functionally heterogeneous in the dorsal and ventral hippocampus. Here, to dissect the distinctions between the dorsal and ventral CA1 (dCA1 and vCA1) in anatomical connections, we systematically analyzed the direct inputs to dCA1 and vCA1 projection neurons (PNs) with the rabies virus-mediated retrograde trans-monosynaptic tracing system in Thy1-Cre mice. Our mapping results revealed that the input proportions and distributions of dCA1 and vCA1 varied significantly. Inside hippocampal formation, dCA1 and vCA1 PNs shared the same upstream brain regions, but with distinctive distribution patterns along the rostrocaudal axis. The intrahippocampal inputs to the dCA1 and vCA1 exhibited opposite trends, decreasing and increasing gradually along the dorsoventral axis, respectively. For extrahippocampal inputs, dCA1 and vCA1 shared some monosynaptic projections from certain regions such as pallidum, striatum, hypothalamus, and thalamus. However, vCA1, not dCA1, received innervations from the subregions of olfactory areas and amygdala nuclei. Characterization of the direct input networks of dCA1 and vCA1 PNs may provide a structural basis to understand the differential functions of dCA1 and vCA1.

Figure 1. Experimental procedures for the cell-type-specific retrograde monosynaptic tracing of dorsal CA1 or ventral CA1 PNs.

This paper focuses on systematic quantification and detailed analysis of the direct inputs of projection neurons (PNs) in dCA1 and vCA1. By employing the genetically modified rabies virus (RV) tracing system and Thy1-Cre transgenic mice, they represented the complex and varied circuity of CA1 along the hippocampal dorsoventral axis. The whole-brain mapping revealed that inputs to the dCA1 and vCA1 PNs were different along the rostrocaudal axis (RC axis): vCA1 PNs directly integrated information from both intrinsic and extrinsic hippocampal subregions, while the dCA1 PNs preferentially received information from intrinsic hippocampal subregions.

 
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