PHARMACOLOGICAL MANIPULATIONS AND ENGRAM TAGGING IN THE CONTEXT OF SCHEMA MEMORY IN THE MOUSE HEXMAZE

Previous knowledge networks facilitate memory consolidation, yet the underlying neural mechanisms remain unclear. To investigate the contribution of distinct brain regions, we used the mouse HexMaze task, a complex 2 × 2 m spatial environment comprising multiple learning stages. This paradigm allows rapid integration of new information once a schema has been established. In total, 164 mice were trained in 20-minute sessions two to three times per week over 6–9 months. Pharmacological inhibition of the dorsal hippocampus, prelimbic cortex, and retrosplenial cortex at different learning stages was used to assess the involvement of these brain areas. Overall navigation performance showed limited dependence on hippocampal or cortical activity. However, during the schema phase, the hippocampus was critical for encoding new schema-congruent memories, followed by rapid systems consolidation resulting in hippocampal independence after 48 hours. Immediate early gene analyses revealed a gradual increase in cFos expression in the prelimbic cortex over a three-month period. To examine cellular mechanisms, engram tagging using the TRAP2 transgenic line was performed during the first week of learning. Immunohistochemical analyses three months later demonstrated substantial overlap between neurons active during initial learning and those recruited during subsequent schema-congruent learning. These findings indicate that the hippocampus is essential for long-term memory encoding in the context of prior knowledge, despite not being strictly required for spatial navigation. At the cellular level, the prelimbic cortex shows increasing engagement over time, and early-formed neuronal ensembles remain involved in learning new related information over extended timescales.