Optogenetic stimulation of a hippocampal engram activates fear memory recall

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

Research Questions

• Does activating memory engram neurons invoke a retrieval of fear response?

Experiments

Exp 1 (Cellular)

Exp 2 (Neural, Behavioral)

• 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)

• 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

Exp 3 (Neural)

• 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)

Exp 4 (Neural, Behavioral)

• 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

• Takeaway: activating other engram cells does not invoke freezing response