The role of nucleus reuniens thalami for cortical-hippocampal interaction underlying spatial memory in the rat

Abstract

The hippocampus (HPC) and prefrontal cortex (PFC), two critical brain regions which support several basic brain functions, are essential for our daily life. For instance, as one of the most important brain functions, the memory process has been demonstrated to be correlated to both HPC and PFC. By remembering (memory consolidation) and recalling (memory retrieval) the episode we experienced before, the memory process guides our current life. Considering their contribution to memory and the anatomical connections between them, the interaction between PFC and HPC has drawn a lot of attention and there is an increasing number of studies talking about the PFC-HPC interplay. However, the mechanism of the PFC-HPC interaction still remains relatively unclear by now. The cross-regional coordination between spatially-distributed brain structures must rely on the direct/indirect anatomical connections. In rodents, while the direct connection between medial prefrontal cortex (mPFC) and HPC have already been elaborately studied, the indirect mPFC-HPC communication through the nucleus reuniens (RE) has not yet been well understood. In thesis, we aimed to investigate the role of RE in the mPFC-RE-HPC circuit. We first studied the RE contribution to the spatial memory process in a crossword maze task. By temporarily inactivating RE with muscimol injection, we revealed the important role of RE in spatial memory retrieval and/or “online” processing of spatial memory. Next, we performed multi-site recording in mPFC, RE and HPC in freely-moving rats. We observed the synchronization between RE and mPFC, between RE and HPC in high gamma frequency range. Moreover, we found a strong RE-HPC cross-frequency coupling around the high gamma synchrony event, which could be correlated to the memory demand. In the end, it was revealed that all nodes of mPFC-RE-HPC circuit could synchronize in high gamma range, during which the cross-frequency coupling between RE and HPC was significantly stronger. To summarize, the current work sheds light on the RE contribution to mPFC-HPC interaction and reveals the potential mechanism for mPFC and HPC to communicate/coordinate indirectly.