New data on neurogenesis, pattern separation, context discrimination and stress

One of the leading hypothesized functions for adult hippocampal neurogenesis in memory is pattern separation. Loosely defined, pattern separation is the process of making similar patterns of neural activity more distinct. This is clearly relevant for learning and memory since we have many experiences that are similar to each other but nonetheless must be remembered as distinct. For example, the girl who sat behind me in 2nd year organic chemistry bore a striking similarity to the woman who later became the mother of my firstborn child (long, dark curly hair, sense of humour etc). But perhaps due to a dysfunctional hippocampus it wasn’t until halfway through the term that I was able to discriminate these 2 individuals.

In its true form, pattern separation is a neurophysiological computation that is very difficult to measure since we know very little about how information is represented, in terms of action potential firing patterns in assemblies of cells (i.e. how can you measure how information has changed if you don’t have a good handle on what the incoming neural activity meant in the first place?). There has been some progress suggesting the dentate gyrus may pick up on minor changes in the environment and perform such a function. And so behaviourists have been keen to test whether the dentate gyrus and immature neurons are important for this function, using tasks such as discriminative context fear conditioning (is this the place where I received a shock?) or object location tests (did these objects move just a tiny bit since I saw them last?). When the dentate gyrus is compromised, or when neurogenesis is reduced, we sometimes see deficits in these behaviours. If you have a look at the pattern separation blog you’ll see an impressive interdisciplinary discussion of what these findings mean (and don’t mean!). In short, they are consistent with a pattern separation role but they don’t prove that the dentate is actually performing pattern separation at a physiological level.

Here I present some new data on adult neurogenesis, context fear discrimination, and stress hormones. It’s been on my hard drive since 2008. Which is ridiculous since it reflects many long days of putting mice into boxes and the findings are pretty intriguing, if inconclusive.

So finally I wrote it up and have published it on FigshareDownload it there and read along.

The basic idea is that I was training neurogenesis-deficient GFAP-TK mice in a discriminative context fear paradigm. The hypothesis was that, if the dentate gyrus and adult neurogenesis is important for pattern separation, then we would expect that the TK mice would be impaired, and show similar levels of freezing in the so-called “safe” and “shock” contexts. This is now obvious given work by McHughTronel, Sahay, Niibori, Kheirbeck.

Figure 1 - circles vs stripes
Fig 1-circles vs stripes

To make the discrimination challenging, I started with a discrimination paradigm where the 2 contexts were quite similar and the only difference was the pattern on the walls of the 2 contexts: circles or stripes. During the training session it appeared to be too challenging – the mice showed no discrimination whatsoever. Interesting finding #1: when tested 1 week later, the WT mice did show a discrimination whereas the TK mice did not. To get the most out of the experiment, I re-tested the mice the following day: mice that were tested in the shock context on test 1 were tested in the safe context on test 2 and vice versa. Interesting finding #2: There was a carry over effect such that the WT mice again discriminated, but on test 2 they now froze more in the safe context! On test 2 corticosterone levels were also greater in the mice tested in the safe context.

This experiment (“Circles vs Stripes”) suggests to me that neurogenesis may indeed be involved in some sort of pattern separation function, since the TK mice never successfully discriminated. But it is interesting that WT mice only discriminated during the test. Usually, context fear memories become more generalized with time (see Wiltgen, Biedenkapp, Wang) but here they are becoming more accurate. I don’t have a solid explanation for this but wonder if the simplicity of the context difference plays a role. If mice were able to form a simple stimulus-shock association (circle-shock or stripe-shock association, rather than complex context-shock association) then these memories might not subject to the same generalization/interference processes that typically occur during consolidation. This result is also a reminder that memory may be intact, even when there isn’t behavioural evidence. Regarding the reversal effect, the paradigm is different but reminiscent of findings by Beracochea showing that stress can alter which of 2 context memories dominates at the time of retrieval. It is also worth noting that blood samples were taken 30min after testing for corticosterone measurements, using a submandibular cheek-lancet method. This is a stressful procedure and may have altered the memory retrieved on test #1, and contributed to the carryover effect on test #2.

Figure 2 - mo diff
Figure 2 - mo diff

To see if we could pull out a context discrimination difference during training, I repeated the experiment but changed many more features between the 2 contexts (shape, odours etc). This variation was code named MO DIFF since the contexts were made “more different” and I have kept that name since this isn’t a journal. If anything, the TK mice now did a better job of discriminating (at least during training). Compared to Circles vs Stripes there was weaker discrimination during Mo Diff testing and also fewer reversal/carryover effects between tests #1 and #2. TK mice had huge elevations in corticosterone compared to WT mice at the time of fear memory retrieval.

Figure 3 - stress+mo diff
Figure 3 - stress+mo diff

For the last experiment I had some mice that had been subjected to chronic stress so I figured why not then test them on Mo Diff? The mice in Mo Diff didn’t remember super well and chronic stress enhances fear conditioning so…we found that these mice indeed discriminated very well during training and testing. No difference between WT and TK mice during training but TK mice discriminated identically on tests #1 and #2. In contrast, WT mice again showed a carryover effect such that there was no discrimination on test #2.

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Final thoughts: This dataset may raise more questions than it answers and for this reason my work with GFAP-TK mice then took a more straightforward route, eliminating memory from the equation and investigating whether new neurons are important for innate responses to psychological stress. In any case:

  1. The data support a role for neurogenesis in context discrimination, and potentially pattern separation, but it suggests that new neurons may bias towards both separation or generalization depending on the conditions.
  2. New neurons may be important for accurate consolidation of memory
  3. Neurogenesis regulates stress hormone levels during memory retrieval
  4. Testing order strongly influences whether mice express fear in the appropriate context

Reference: Snyder, Jason; Cameron, Heather A. (2013): Reduced adult neurogenesis alters behavioural and endocrine discriminative fear conditioning. figshare.
http://dx.doi.org/10.6084/m9.figshare.884597

Forming and recalling memories. Artificially.

ResearchBlogging.org

Memory manipulation has become one of the most hotly pursued topics in neuroscience. After all, much or of who are is based on what we’ve learned, including memories that we can consciously recall as well as acquired desires and habits that can lead to problems like addiction. In rodents, we’ve known for decades that damage to the hippocampus can erase recently-formed memories. Studies of reconsolidation have shown us that when a memory is retrieved it becomes labile and allows for new information to be added, thereby creating an updated version. More recently we (humans) have been able to identify the neurons involved in memory formation and show that killing them, and only them, results in memory erasure. Bringing us even closer to the stuff of movies, studies by Garner et al. in Science and Liu et al. in Nature have now artificially controlled memory formation and recall. We’re essentially talking about reactivating memory by pushing a button. Yes – you can say “dude, whoah” now. Continue reading

Someone finally dissects the role new neurons play in fear conditioning

Based on a true story – how progress is made in the field of adult neurogenesis*

  1. A group of scientists reduce neurogenesis and report a memory deficit.
  2. A second group repeats the experiment, with only a few minor differences in protocol, and fails to find a memory deficit.
  3. A third group, using the same species as the first group but a protocol more similar to the second group, replicates the original finding but only when the experiment is performed on Wednesdays.
  4. Faith is restored.
  5. Five groups report no such neurogenesis-dependent memory deficit.
  6. It is reported that developmental exposure to strontium reduces adult neurogenesis by 40% AND produces the much sought after memory deficit. In a technical tour de force follow-up experiment, artisanal cheeses restore neurogenesis and reverse the memory deficits. Causation is established.
  7. BDNF.
  8. Everyone proclaims the role of neurogenesis in memory and is totally confused at the same time.
  9. Someone systematically examines all of the variables in the memory test to determine whether or not the whole thing is a hoax and they should just change careers**.
  10. We have never gotten this far.

Even at level 8, the neurogenesis-fear conditioning story was one of the more convincing arguments of new neuron functionality. With this study by Drew et al. we may soon be jumping for joy as we appear to be graduating to level 9.

The contribution of adult neurogenesis to contextual fear conditioning was greatest when mice were only given a brief training experience – mice lacking adult neurogenesis showed reduced fear of a context where they previously received a single footshock during a brief (3 min) exploration session. With longer exposures to the context, or additional footshocks, neurogenesis-deficient mice showed normal memory. This finding could be explained by the fact that young neurons have a lower threshold for synaptic plasticity, allowing them to encode fleeting experiences that would be forgotten if left to mature neurons.

So, brief training protocols may now likely be my first choice, at least when using mice. In fact, the only times I have observed contextual fear memory deficits in mice has been after brief training protocols almost identical to those used by Drew et al. So we just might have taken a big step forward. If not, check back in 5 years for my revised “How progress is made” list.

*or any other field for that matter
**this is not entirely a joke because, in this case, it both 1) appears to not be a hoax, and 2) marks the launch of the next phase of Michael Drew’s career (congrats)

Reference
Drew MR, Denny CA, & Hen R (2010). Arrest of adult hippocampal neurogenesis in mice impairs single- but not multiple-trial contextual fear conditioning. Behavioral neuroscience, 124 (4), 446-54 PMID: 20695644

Everything you always wanted to know about neurogenesis timecourses (but were afraid to ask)

Most studies of adult neurogenesis are concerned with neuronal age. Or at least they should be. This is because new neurons develop from a stage where they have no excitatory synapses to one where they have many. If we assume the traditional view that information is stored at excitatory synaptic connections, then young neurons are initially useless and only become physiologically and behaviorally meaningful when they have matured to a point where they can relay and process information. It is therefore critical that the developmental timecourse of new neurons be mapped out, so we know when new neurons become functionally relevant, or whether they might even have different functions at different ages.

Below are what I hope to be comprehensive visual collages of all published timecourse experiments, where a certain property of new neurons is examined at multiple (≥ 3) different ages. They are grouped by studies of: 1) cell survival, 2) marker expression, 3) functionality, and 4) miscellaneous studies that do not quite fit into the first 3 categories. I’ve ordered the data roughly chronologically and have included the first author’s name and publication year so you can read deeper, if needed. Indeed, if you know these studies already, a brief look at the graphs will bring back the take home message. However, since the data is stripped of text, if the studies are unfamiliar, you’ll have to go to the original source to figure out what the heck they mean (use Pubmed to at least obtain abstracts for the original studies if I didn’t provide a direct link).

Personally, I like timecourse studies for the same reason I like to have all my music albums or books visible at the same time: at a single glance they provide a lot of information – each individual stage of maturation can be interpreted within a bigger picture. The result of these many hours of work will either be a) that the purpose of adult neurogenesis will become immediately clear, or b) that we’ll all have some fancy collages to pin on our bulletin boards and look intelligent.

The survival timecourse

addition of new neurons

New neurons are born and then many die. The survival timecourse answers the questions: How many new neurons are born? Where are they born and where do they end up, anatomically? How many of them survive and can their survival be altered? Survival timecourses are typically performed by injecting animals with a mitotic marker that will label new neurons as they’re being born, e.g. ³H-thymidine (old school), BrdU (tried and true – example), or a GFP-expressing retrovirus (new school). At a later date one can then detect these birthdated new neurons and count them, see where they’re located etc.

Continue reading

A list of experiments that relate adult hippocampal neurogenesis to behavior


The list as a Google spreadsheet (also excel | HTML | RSS feed of updates)
List last updated 3/9/2011.

I’ve always enjoyed making lists. As a kid I can remember writing lists of rhyming words, lists of all the Ocean Pacific clothes I owned, lists of all the people I knew. Many years later, I hope I’ve now made a list that is actually useful.

Adult neurogenesis is now undisputed. Pretty much on a weekly basis there is a new paper that examines both levels of adult hippocampal neurogenesis and behavior, attempting to draw a functional connection. The good news is that the argument for a behavioral function for adult neurogenesis continues to get stronger. The bad news is that there’s a massive pileup of data, and it’s becoming hard to filter through the relevant studies – first you have to find them amongst the 1000+ studies of adult neurogenesis. Then you have to read them. What behaviors are examined? Is there an effect of reducing or enhancing neurogenesis? What method is used to manipulate neurogenesis? What do other studies find that performed a similar analysis? Continue reading

Cell Nov. 13, 2009: Adult Neurogenesis Modulates the Hippocampus-Dependent Period of Associative Fear Memory

Adult Neurogenesis Modulates the Hippocampus-Dependent Period of Associative Fear Memory

Kitamura et al. (2009) Adult Neurogenesis Modulates the Hippocampus-Dependent Period of Associative Fear Memory. Cell. 139:814-827.

It’s great to see this paper finally in print. At SFN 2008 the authors had a poster that generated a lot of excitement, at least in our circles.  And the poster was quite a sight: there was such a profusion of data that the poster poured off the easel, nearly reaching the floor. With 27 (!) supplemental figures in the final article, one has to wonder if this is the final straw that led to this article.

The authors use an ingenious approach to address an idea that has been floating around for a while: that adult neurogenesis regulates memory turnover in the hippocampus. Continue reading