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Abstract:
We have recently reported that inactivation of the midline thalamic nucleus reuniens (RE) dramatically impaired the rat performance of a previously learnt spatial task (Mei et al., 2018). Our findings supported an emerging view that the RE may contribute to spatial memory by coordinating interactions between the hippocampus (HPC) and the prefrontal cortex (PFC). The HPC-PFC interactions occurring at different frequency ranges (e.g. theta, beta, or gamma) are thought to support specific steps of information processing. However, the detailed mechanisms of task-specific cross-regional interactions within the RE-HPC-PFC network remain poorly understood. We recorded extracellular activity from the RE, dorsal HPC and medial PFC in the rats learning a complex spatial task on a cross-word maze over 5 training sessions. We aim at characterizing the learning-induced dynamics of cross regional interactions. During task performance, the strongest theta power was present in the HPC, while it was much weaker in the RE and PFC. In the beginning of each trial, we observed systematic surge of the theta-synchronization between the PFC and RE, which was followed by increases in the RE-HPC and HPC-PFC theta-coherence. This activity dynamics seemed to be associated with a retrieval of the correct trajectory at the beginning of the trial. Thus, a decision making (e.g. right or left turn) on the maze likely required retrieval of cortically-stored information and its integration into active network. A strong theta-coherence within the entire RE-HPC-PFC network accompanied errors (e.g. entries to the wrong maze alleys). Besides, we compared the coherence between the RE, HPC, and PFC across training days. Our preliminary results further support the idea that the RE may indeed gate the bidirectional information flow within the HPC-PFC pathway. The RE may contribute to spatial learning as a network element enabling retrieval and storing task-related information in working memory. It remains, however, unclear if the RE is required for consolidation of spatial memory.
