Optogenetic induction of memory recall

18 09 2009

Speaking of reactivating specific memories, at the 2009 Society for Neuroscience meeting, Matteo Rizzi of Michael Häusser’s lab is presenting the realization of an idea that has been floating around in some research proposals I’ve read over the last year.  Express channelrhodopsin-2 under control of the immediate early gene c-fos, induce a strong memory formation via fear conditioning, and then drive the recall of that memory by stimulating the neurons that are expressing ChR2. Immediate early genes are activated shorty after high levels activity in neurons, though the precise patterns are different depending on which promoter (c-fos, Zif268, etc) you use, making precisely HOW they reflect recent neuronal activity patterns unclear.  Nevertheless, the activation of the c-fos based pattern seems close enough to trigger an identical behavioral response as the conditioned stimulus.

Get your ass to Mars!

Not yet, but getting closer...

Electrically-induced fear conditioning is probably the most blunt instrument possible, encoding a very powerful, general ‘fear’ memory, and many things can make a mouse freeze. Thus, this is definitely the low-hanging fruit on the ‘reverse-engineering’ memories tree. Understanding how the information in a memory is distributed across participating neurons is going to take a more sophisticated approach and a lot more work. This result is still incredibly cool, and I’m somewhat surprised it worked by driving ChR2 with c-fos in a hundred cells in the dentate gyrus. That has pretty powerful implications for avenues by which memories can be recalled.  Surely the entire memory is not encoded by only the 100 neurons that were activated! How many other neurons participate, and how does the optical stimulation activate the entire ensemble? Is it even necessary to activate the entire ensemble to drive behavior? The poster will be MOBBED.  I look forward to reading the details.

Program#/Poster#: 388.8/GG103
Title: Memory recall driven by optical stimulation of functionally identified sub-populations of neurons
Location: South Hall A
Presentation Time: Monday, Oct 19, 2009, 10:00 AM -11:00 AM
Wolfson Inst. for Biomed. Res., UCL, London, United Kingdom
Abstract: The mammalian brain is capable of storing information in sparse populations of neurons encompassing several brain areas. Immediate recall of this information is possible upon presentation of a cue or context. Most aspects of this process remain unresolved: are the cells involved in information storage also responsible for its recall? What portion of this distributed circuit needs to be reactivated, in order to achieve successful recall? To answer these questions we selectively expressed a genetically encoded optogenetic probe (Boyden et al., 2005) in neurons engaged during the learning of a specific association. A plasmid encoding channelrhodopsin-2 and EGFP under an immediate early gene promoter (c-fos-ChR2-IRES-EGFP) was electroporated in vivo into granule cells (GCs) of the dorsal dentate gyrus of anaesthetized C57BL/6 mice. Mice were allowed to recover, and then underwent classical delay fear conditioning (consisting of 10-20 pairings of a 5 second auditory tone and a 2 second footshock). An optic fiber was implanted intra-cranially to allow optical stimulation of transfected neurons. Light stimulation (λ = 530 nm; 5 Hz) successfully induced recall of the fear memory, measured as freezing behaviour (n = 27 animals). Post-hoc analysis of the transfected tissue revealed that a remarkably small subpopulation of GCs (<~100 cells) was sufficient to cause this effect. We then tested whether any, comparatively sized, subset of GCs could be equally effective. We transfected neurons with a plasmid encoding ChR2 expression under a general promoter (pCAG-ChR2) to obtain ChR2 expression in a random population of cells. Interestingly, optical stimulation of this population was insufficient to induce memory recall (population data: n=30). Our results therefore suggest that recall of a learned association, sparsely stored in neuronal circuits distributed over several brain areas, can be achieved by the simple reactivation of a very small subset of neurons involved in learning this association. Furthermore, our strategy may also be useful for dissecting the complexities associated with memory storage and recall.
Support: Gatsby Charitable Foundation; Wellcome Trust