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.
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 |
| Authors: | *M. RIZZI, K. POWELL, J. HEFENDEHL, A. FERNANDES, M. HAUSSER; 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 |
Brain Windows 
This looks like a very interesting abstract. Hasn’t Svoboda’s lab published work that is essentially the same? Not fear conditioning, but learning?
There is a very useful review from Hippocampus (2002) 12:609-36 where Martin and Morris supply the following criteria for the search for the engram:
Four criteria relevant to the assessment of the synaptic plasticity and memory (SPM) hypothesis:
DETECTABILITY: If an animal displays memory of some
previous experience, a change in synaptic efficacy should
be detectable somewhere in its nervous system.
MIMICRY: If it were possible to induce the appropriate
pattern of synaptic weight changes artificially, the animal
should display ‘apparent’ memory for some past
experience which did not in practice occur.
ANTEROGRADE ALTERATION: Interventions that prevent
the induction of synaptic weight changes during a learning
experience should impair the animal’s memory of that
experience.
RETROGRADE ALTERATION: Interventions that alter the
spatial distribution of synaptic weight changes induced by
a prior learning experience (see detectability) should alter
the animal’s memory of that experience.
Keep up the nice work on your weblog!
Hi Graham,
I think you are referring to Daniel Huber’s seminal paper on ChR2 control of behavioral choice. That paper simply demonstrated that a mouse could be trained to detect the brain activity associated with ChR2 stimulation of a few hundred neurons in the brain and go left or right for its reward depending on the presence or absence of the stimulation. His presentation at CSHL in ’07 was covered here (before blogging about talks was banned), and was tremendously exciting at the time. However, it hardly scratches the surface of how real sensory stimuli are encoded. It was a necessary characterization step for future experiements that will be aimed at decoding, modifying and recreating the patterns of activity associated with real sensory stimuli.
There will be a Nature Insights piece from Daniel, Dan O’Conner and Karel that discusses these issues coming out sometime this month.
Thanks for the heads up on that review, I need to check it out, sounds good!
Nature Reviews Neuro has a more recent review (2008) covering the SPM hypothesis by Neves, Cooke and Bliss: http://www.ncbi.nlm.nih.gov/pubmed/18094707
Also similar work (although not quite as direct as Chr2 stim) was published last year by Reijmers and Mayford using their TetTag mouse, where two different IEG promoters were used to drive different fluorescent proteins. These different fluorophores were used to separately labels cells involved in learning and recall and then overlap was assessed between the populations. After labeling the memory-involved neurons, the next step is of course to control them as in this SfN poster.
It’s definitely nice to see this work progressing! Undeniable proof that science fiction is slowly becoming reality.
[…] help but be a little cautious of the validity of these findings. This was how I felt upon seeing similar findings presented at SfN way back in 2009 – cool, but really? The specificity of the induced recall […]
[…] help but be a little cautious of the validity of these findings. This was how I felt upon seeing similar findings presented at SfN way back in 2009 – cool, but really? The specificity of the induced recall […]