Optogenetic stimulation of a hippocampal engram activates fear memory recall

March 29, 2021

by Liu and Ramirez, ..., Tonegawa (Nature 2012)

memory-engrams contextual-fear-conditioning dentate-gyrus auditory-fear-conditioning

Background

Previously, ablating/inactivating memory engram neurons prevented retrieval of fear response

c-fos is an immediate early gene often used as marker of recent neuronal activity

Experimental (Exp): 5 days habituation in Context A. Fear conditioning in context B (tone+shock).
Optogenetically activate Context B neurons when mouse back in Context A (Fig 3A)
No shock (NS): same as experimental group, but no shock in context B.
Optogenetic activation had no effect on freezing response (Fig 3B)
EYFP: same as experimental group, but no optogenetic activation in Context A (Fig 3C)

3ABCDEF

Freezing by experimental group is relatively small. Authors hypothesized this might be due to imprecision with tagging engram neurons
Authors added two more groups
Exp 1-Day: shortened DOX-off period from 2 days to 1 day (allowing for better targeting of active cells during fear conditioning), showed higher freezing (Fig 3D)
Exp-Bi: Exp used unilateral injection of virus and optogenetic stimulation. Now use bilateral intervention. (Fig 3E)
Takeaway: optogenetically activating engram neurons, not other neurons, induces fear recall

Is the previous result context specific?
First test whether different contexts activate same dentate gyrus (DG) cells
Take mice off DOX, place in context C, then place back on DOX, fear condition in Context B and kill mice 1.5hr later
- ChR2–EYFP used to identify Context C cells and c-fos used to identify Context B cells
- Overlap at chance levels (Fig 4G)

Having established that contexts are assigned to different DG populations, now test whether activating non-fear context invokes fear response
Repeat previous experiment, adding fear conditioning before killing the mice. Then optogenetically activate the (neutral) Context C. Freezing at habituation levels