The midline thalamic nucleus reuniens is essential for spatial memory retrieval

Neural processing in the medial prefrontal cortex (mPFC) and hippocampus (HPC) as well as mPFC-HPC interactions are essential for spatial information acquisition, storage and flexible use. The cross-regional interactions are supported by direct and indirect anatomical pathways. Hippocampal projections to the mPFC critically contribute to spatial encoding. Little is known about the functional role of multi-synaptic communication between HPC and mPFC through the nucleus reuniens of the midline thalamus (nRe), which is reciprocally connected with both HPC and mPFC. To study the role of nRE for memory consolidation we tested the effect of nRe inactivation (muscimol, 0.27µg/µl, 0.27 µl) on acquisition and performance of a HPC- and mPFC-dependent task on a crossword maze. Rats with chronically implanted cannulas received intra-nRe injection immediately after each learning session or prior task performance. To get reward, rats were required to follow specific maze-trajectory, which depending on the start position contained 6 or 7 decision cross-points. There was no effect of post-learning nRe inactivation on the task acquisition, while nRe inactivation before the probe trail (24h after reaching a learning criterion) produced a strong performance deficit in well-trained rats. Our results suggest the direct HPC-mPFC pathway may be sufficient for spatial learning; however indirect HPC-mPFC communication via nRe may be essential for retrieval of newly acquired spatial information. The effects of nRe inactivation on the remote memory will be presented