Parallel emergence of stable and dynamic memory engrams in the hippocampus

Area of Science:

• Neuroscience
• Cognitive Science
• Memory Research

Background:

• Memories are encoded by neuronal groups called engrams, assembled through synaptic modification.
• Engrams are initially stored in the hippocampus and later transferred to cortical areas.
• The hippocampus, particularly the dentate gyrus, plays a crucial role in memory processing and pattern separation.

Purpose of the Study:

• To investigate the stability and specificity of neuronal representations across hippocampal subfields during a multi-day spatial memory task.
• To reconcile the apparent paradox of transient hippocampal memory storage with long-term memory recall.
• To record neuronal activity in all major hippocampal subfields over an extended period.

Main Methods:

• Chronic two-photon calcium imaging in head-fixed mice.
• A multi-day spatial memory task in a virtual environment.
• Recording neuronal activity in dentate gyrus, CA1, CA2, and CA3 hippocampal subfields.

Main Results:

• Pyramidal neurons in CA1-CA3 exhibited precise, context-specific, but continuously changing spatial representations.
• Granule cells in the dentate gyrus displayed a stable spatial code over many days with low place- or context-specificity.
• Synaptic weights in the hippocampal trisynaptic loop are constantly reassigned, utilizing a stable dentate gyrus code for dynamic downstream representations.

Conclusions:

• The dentate gyrus provides a stable foundation for memory encoding.
• Hippocampal areas CA1-CA3 dynamically update representations based on the stable dentate gyrus code.
• This suggests a mechanism for reconciling short-term hippocampal processing with long-term memory.