Evaluating CA3-CA3 long-range connection systems in-vivo
The hippocampal CA3 region receives polysensory information from the entorhinal cortex both directly and via the dentate gyrus. The distinctive connectivity of the CA3 region has been well investigated anatomically. A key distinguishing feature of CA3 connectivity are their extensive local interconnections via recurrent axon collaterals. This anatomical connection topology as well as functional and computational data have led to the concept that the CA3 network is capable of pattern completion. However, most CA3 neurons, in addition to local recurrent connections within a given hippocampal lamina, have long-range connections. Firstly, CA3 neurons project to the contralateral CA3 and CA1 regions, via the commissural connections. Additionally, CA3 pyramidal cells project extensively along the septotemporal axis of the hippocampus, frequently for up to 75% of the total axial extent of the hippocampal formation.
While the bilateral dorsal CA3 recurrent networks share similar cortical inputs and outputs, the cortical and sub-cortical input-output pattern varies greatly along the ventral-dorsal hippocampal axis. This differential connectivity has suggested distinct functionality of the dorsal and ventral hippocampi. The dorsal hippocampus is thought to be implicated preferentially in processing spatial information, while the ventral hippocampal is strongly recruited during processing of emotionally salient memories. This suggests the hypothesis that different information is conveyed in commissural vs. ventral-to-dorsal CA3-CA3 connections. To address this question, it would be required to record the activity of both projections specifically in awake mice. In addition, it would be desirable to achieve imaging of the CA3 and CA1 hippocampal populations. We have achieved these goals by expressing GCaMP6s in either commissural or ventral-to-dorsal CA3-CA3 projections using viral gene transfer and a mouse line expressing Cre recombinase only in CA3 pyramidal neurons. We have established in-vivo multiphoton imaging techniques within the CA3 region that allow us to detect activity in individual CA3 axons derived from ventral or contralateral CA3 neurons. These approaches now allow us to address the recruitment of two different CA3-CA3 systems in awake animals, as well as their role in controlling the activity of CA3 and CA1 neurons.