Download Optical Stimulation of Engram-bearing Cells

Optogenetic Stimulation of a Hippocampal Engram
Activates Fear Memory Recall
Xu Liu1, Steve Ramirez1, Petti T. Pang1, Corey B. Puryear1, Arvind Govindarajan1, Karl Deisseroth2, & Susumu Tonegawa1
1. RIKEN-MIT Center for Neural Circuit Genetics and The Picower Institute for Learning and Memory, MIT, Cambridge, MA
2. Department of Bioengineering and Department of Psychiatry and Behavioral Sciences, Stanford University, Stanford, CA
Abstract
The memory engram theory proposes that
a specific memory is encoded and stored in a
defined population of neurons. In order to test
this hypothesis directly, one needs to show that
the activation of such a population of neurons
can cause the recall of that specific memory.
We constructed an activity-dependent,
doxycycline-regulatable labeling system by
combing a transgenic mouse expressing
tetracycline transactivator (tTA) driven by the cfos promoter and an adeno-associated virus
(AAV) encoding channelrhodopsin-2 (ChR2)
under the control of the tetracycline response
element (TRE).
In the experimental group, we labeled a
population of cells in the dentate gyrus (DG) that
were active during fear conditioning with ChR2.
Control groups consisted of animals with either
ChR2 labeling a DG cell population active during
a non-fear memory learning task, or EYFP
labeling a DG cell population active during fear
conditioning, followed by light stimulation.
Our study tests the hypothesis that a DG cell
population active during fear memory learning is
sufficient for fear memory recall upon activation,
and thus may provide direct evidence for the
cellular basis of a memory engram.
Labeling and Activating a Memory Engram
Optical Stimulation of Engram-bearing Cells
Induces Post-training Freezing
Verified Implant Placements
Expected
behavior
Labeling and Activating Engram-bearing
Cells with ChR2
ChR2
EYFP
ChR2
Crosses mark the tip of optic fiber as verified by histology.
EYFP
Conclusions
Optical activation of DG cells active during fear learning
is sufficient for fear memory recall.
Experimental Mice Show Reduced Freezing
during Context, but not Tone, Probe Trials
Light preferentially activated cells expressing ChR2,
but not EYFP, as shown by c-Fos signal.
Verification by Electrophysiology
Combining Transgenics and Optogenetics
a
Five sessions of light-induced fear memory recall elicit a
decrease in freezing to the original training context.
Here we have shown that optical activation
of hippocampal cells that were active during fear
conditioning is sufficient to elicit freezing
behavior. Our results argue that defined cell
populations can form a cellular basis for fear
memory engrams.
The memory engram that we selectively
labeled and manipulated is likely contextual in
nature, as previous studies have demonstrated
that
hippocampal
interventions
affect
conditioned freezing responses to a context, but
not a tone, which is consistent with our finding
that only experimental groups showed
decreased freezing during context, but not tone,
probe trials.
The approach and methods described here
will be a powerful tool for mapping multiple
component engrams, each contributing to an
overall memory.
Extinction by Light or Context
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Freezing
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Acknowledgements
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Context
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When an animal is kept off Dox diet, only active c-Fos
expressing cells will become labeled by ChR2.
in vivo head-fixed recordings from anesthetized animals
show light response in ChR2-expressing animals.
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The Tonegawa Lab @ MIT
Grace Lin
Xiaoning Zhou
Shu Huang Mike Ragion
Tomas Ryan Roger Redondo
Alex Rivest Jennie Young
This work is supported by NIH grants R01MH078821, P50-MH58880 to S.T. and RIKEN
Brain Science Institute.