Engrams and circuits crucial for systems consolidation of a memory

TLDR

Episodic memories first depend on rapid hippocampal synaptic plasticity and later consolidate into neocortical networks, yet the specific engrams and circuits mediating this neocortical consolidation remain unknown. We identified that prefrontal cortical engram cells, crucial for remote contextual fear memory, are quickly formed during learning through hippocampal‑entorhinal and amygdalar inputs, mature over time with hippocampal support, while hippocampal engrams fade and amygdalar engrams persist, illustrating the functional reorganization underlying systems consolidation.

Abstract

Episodic memories initially require rapid synaptic plasticity within the hippocampus for their formation and are gradually consolidated in neocortical networks for permanent storage. However, the engrams and circuits that support neocortical memory consolidation have thus far been unknown. We found that neocortical prefrontal memory engram cells, which are critical for remote contextual fear memory, were rapidly generated during initial learning through inputs from both the hippocampal-entorhinal cortex network and the basolateral amygdala. After their generation, the prefrontal engram cells, with support from hippocampal memory engram cells, became functionally mature with time. Whereas hippocampal engram cells gradually became silent with time, engram cells in the basolateral amygdala, which were necessary for fear memory, were maintained. Our data provide new insights into the functional reorganization of engrams and circuits underlying systems consolidation of memory.