Examining the Three-Dimensional Spatial Architecture of Mouse Amygdala Engram Ensembles

Area of Science:

• Neuroscience
• Memory Research
• Systems Neuroscience

Background:

• Memories are encoded by specific neuron groups (engram ensembles) activated during an event.
• The amygdala is crucial for fear memory, but the spatial organization of its engram neurons is unclear.
• Understanding this organization could reveal principles of memory coding.

Purpose of the Study:

• To investigate the 3D spatial architecture of amygdala engram ensembles.
• To determine if engram geometry reflects memory function (aversive vs. neutral).
• To uncover novel organizational principles of memory coding in the amygdala.

Main Methods:

• Used tissue clearing and light-sheet imaging to map Arc-expressing neurons in the entire mouse amygdala.
• Analyzed spatial distribution of engram neurons during encoding and recall of aversive and neutral memories.
• Applied unsupervised clustering and network graph analyses to assess micro-scale organization.

Main Results:

• Identified meso-scale "hotspots" of engram neuron density in amygdala nuclei, varying with fear conditioning and retrieval.
• Observed increased micro-scale clustering of active neurons in lateral (LA), basal (BA), and central capsular nucleus (CeC) during aversive learning and recall.
• Found increased assortativity and hub nodes in specific amygdala nuclei, indicating structured information flow.

Conclusions:

• Meso- and micro-scale spatial patterns of neuronal activity in the amygdala differ across subregions and vary with memory valence and stage.
• The geometric architecture of engram ensembles may shape information flow, enhancing memory encoding and retrieval fidelity.
• This study provides insights into the structural basis of fear memory processing within the amygdala.