The Elegant Discovery of a Fear Memory Engram

Introduction

The purpose of this post is to make as accessible as possible the sequence of experiments that culminated in the first thorough evidence for the existence of an engram. An engram can be characterized as the rain’s physical representation of a memory in a constellation of neurons that were active while the memory was acquired. The following post is a synthesis of an already very well written review of Sheena A Josselyn’s life work on discovering engrams for fear learning in the lateral amygdalae of mice.

Primer

Specific ensembles of neurons in the lateral amygdala (LA) are necessary for specific fear memories.

  • CREB (cAMP response element-binding protein) is a cellular transcription factor tthought to be involved in the formation of long-term memories, and has has been shown to play an important role in the formation of fear memories
  • Viral vectors over-expressed CREB in a subset of LA mouse neurons and facilitated fear memory formation AND localized the memories to the neurons over-expressing CREB, the engram.
  • Selective ablation of neurons over-expressing CREB (and therefor ‘containing’ the fear memory) selectively erased that particular fear memory, did not impact other fear memories, and did not preclude future fear memory formation.

Experiments

The following is an attempt to create an easily accessible synopsis of decades of work and the innumerable experiments it took to discover the existence of a fear engram.

Experiment 1 - Josselyn

Experiment 2 - Josselyn

Experiment 3 - Josselyn

Experiment 4 - Josselyn.png

About the Author: Daniel Stern is a first year graduate student in the Neuroscience PhD program at UCSD. His life can be characterized as a gradual shift from east to west coast, having recently escaped both Chicago and the prospect of becoming a lawyer in NYC. He is very happy and humbled to be doing something he loves instead.

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