Tag Archives: frankland

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.


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.

Virus: a new tool for generating pretty pictures

Now that I have something to show for it, let this be a formal announcement that I’ve returned to Toronto to join Paul Frankland’s lab (and therefore the larger Josselyn-Frankland group). I’ve always liked their work and one of the techniques I’m excited to learn is the use of viruses to alter gene expression in neurons. BECAUSE THIS WILL ALLOW ME TO TAKE PRETTY PICTURES!!! I will also say that this will be a short (but hopefully sweet) stay as I’ll be leaving at the end of the year to start my own lab in the Psychology Department at the University of British Columbia (!).

Now, on with the pictures! As always, I recommend high-res viewing (click on the image to view it, bigger, on Flickr).

Using a retrovirus, which infects dividing cells, I made the amazing discovery of four adult-born cells which all had the exact same shape and were located right next to each other!
Continue reading Virus: a new tool for generating pretty pictures

Saving the best for last: neurogenesis, plasticity and memory. #SFN11

blue dcx

Previously, I wrote about new SFN data on the role for newborn neurons in regulating emotion. The second half of the SFN meeting rounded out the story because the bulk of the functional presentations focussed on the role of new neurons in that other, classic function of the hippocampus: memory. Spanning synaptic plasticity, circuit function, and then linking it all to behavior, we have quite a complete story here.


Every time I get worked up about all various neurogenesis findings I think about one acronym that returns me to a state of inner peace: ACSF-LTP. Yes, I plagiarized that last line from my previous post. We all know about LTP right? The ability of synapses to strengthen their connections in response to activity? It has been used for decades as a physiological model of memory formation. It’s pretty well accepted that newborn neuron ACSF-LTP is a unique form of LTP – one that is insensitive to GABAergic inhibition (hence “Artificial Cerebro Spinal Fluid” LTP, in contrast to LTP that also requires inhibition of GABA neurotransmission), one that requires a the NR2B subunit of the NMDA receptor, and one that is induced more easily than that of mature neurons. ACSF-LTP has quite a history: Continue reading Saving the best for last: neurogenesis, plasticity and memory. #SFN11

#SFN10 Itinerary Pt. 2

Continuing on…

1) 31.20/C37 – Dentate network activity modulates integration of newborn granule cells
1Inst. of Cell Biology, Swiss Federal Inst. of Technol. (ETH), Zürich, Switzerland; 2Brain Res. Inst., Univ. of Zürich, Zürich, Switzerland; 3Univ. of Lausanne, Lausanne, Switzerland; 4Brain Res. Inst., Univ. of Zurich, Zürich, Switzerland; 5Swiss Federal Inst. of Technol. (ETH), Zürich, Switzerland

This looks interesting because there is so little known about how neuronal activity regulates neurogenesis. In 2007 Toni et al. suggested that dendritic filopodia on new neurons form synapses with presynaptic boutons that have already formed a synapse onto a different (presumably mature) neuron. The question addressed here is whether altering excitability/plasticity at those pre-existing synapses affects the subsequent formation of new neuron synapses. In other words, if you make old neurons more plastic, will they outcompete new neurons for synaptic space? Seems maybe they do.

2) 203.9/KKK52 – Coding of temporal context in the hippocampus: Do rate codes offer insight into a time-of-day signature?
1Dept of Neurosci., Univ. of Lethbridge, Lethbridge, AB, Canada; 2Ctr. for Neural Circuits and Behavior, Neurobiol Section, Div. of Biol Sci., UCSD, LA JOLLA, CA

We all know the hippocampus is important for episodic (-like) memory yet activity in hippocampal neurons is usually only measured in relation to spatial information. Memories also often contain temporal information and Rob Sutherland (one of the authors) has shown that rats indeed integrate time-of-day information into their memories. Here, measuring electrophysiological activity in hippocampal neurons, it is reported that 1) hippocampal neurons fire when a rat is in a specific spatial location, as expected; 2) when nearby contextual features are altered (square vs circular exploration chamber) the same population of neurons are active in the same places but they fire at different rates (rate coding); 3) what is unique here: hippocampal neurons also used a rate code to differentiate between a given context explored in the morning vs the afternoon. Thus, time-of-day is a contextual feature that is encoded in the hippocampus. Interestingly, it is reported that the rate codes for spatial and temporal information are carried by different populations of neurons.

3) 330.6/A6 – Experience specific information encoding by newborn neurons of the adult dentate gyrus
Salk Inst., La Jolla, CA

This presentation builds on Kee 2007, who showed that 10-week-old neurons are only activated in the water maze if they were functional at the time of the original water maze training, and Tashiro 2006 who claimed that, when a mouse re-experiences something, it is new neurons that were in their critical period during the original experience that are activated. We still have a ways to go before we understand how faithful new neurons are in their responding to different experiences – their enhanced plasticity and unique physiology has caused some speculation that they could be promiscuous, participating in many different experiences. This study seems like it may have the best evidence that young neurons are in fact quite selective in what they encode.

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