NEURONAL MECHANISMS OF LATE-PHASE CONSOLIDATION UNDERLYING MEMORY PERSISTENCE

Memory traces persist over time despite the rapid turnover of their molecular substrates. One hypothesis suggests that multiple late-consolidation cycles help stabilize the engram. Previous results demonstrated that inhibiting molecular processes in the hippocampus and prefrontal cortex 12 h after training affects 30-day remote memory in the contextual fear conditioning (CFC) paradigm. Here, we investigate whether neuronal activity 12 hours after memory acquisition defines a critical window for engram stability and memory persistence.
Male and female Fos2A-iCreER underwent CFC with 5 unconditioned stimuli (US) to assess behavioral performance. Next, Fos2A-iCreER:tdTomato mice received either 0US or 5US and were administered 4-hydroxytamoxifen (50mg/kg, i.p.) after acquisition to label active neurons. Memory expression was tested behaviorally, and ensembles were visualized by tdTomato and c-Fos immunofluorescence.
5US conditioning produced a robust fear memory that persisted for 30 days, with no sex differences observed. In addition, 5US mice exhibited higher freezing levels than 0US controls. Immunofluorescence analysis confirmed labeling of tdTomato+ and c-Fos+ neurons, enabling visualization of memory-related neuronal populations recruited during acquisition and retrieval. Quantification of engram size and overlap is ongoing, and this approach is being extended to characterize 30d remote memory engrams. We will further assess the impact of modulations during late-phase consolidation, 12 h after acquisition, on engram stability and memory retrieval.
Together, these preliminary results demonstrate that our protocol induces a persistent contextual fear memory and enables access to underlying neuronal ensembles. These findings provide a framework for future experiments dissecting how late-phase hippocampal and prefrontal manipulations shape memory representations.