A fundamental property of the hippocampus is its ability to rapidly encode memories while simultaneously keeping them distinct. Recording from hippocampal neurons one can clearly see that different populations of neurons are active as a rat explores two environments. This is thought to be one mechanism by which information is kept distinct in the brain.

For the last 15-20 years it has been thought that the dentate gyrus (DG), a major subfield of the hippocampus, serves to take small changes in incoming sensory information and orthogonalize them (i.e. make them more different). This idea was built in part on the fact that there are many more DG neurons than upstream cortical neurons. Thus, the DG could use completely different populations of neurons to represent different sets of incoming information and then pass on these representations to CA3, which may bind them into coherent events/memories (the interconnectedness of CA3 neurons, via “recurrent collatorals”, is thought to be a mechanism by which the different components of a memory are bound together).

However, a “problem” arose when Leutgeb et al. found that it is always the same population of dentate granule neurons (~1% of the total population) that are active as an animal explores different environments, even very different ones. This was a bit of a surprise. Still consistent with the proposed role of the DG in orthogonalizing information, however, was the fact that the DG neurons fired (i.e. generated action potentials, which transmit information from neuron to neuron) at different rates/frequencies in the different environments. Thus, changes in sensory information were represented by changes in patterns of activity within the same population of cells, not by recruiting different populations of cells. This is but one study – the question of how the DG encodes and extracts information is far from settled (e.g. what are the other 99% of granule neurons doing? Surely there is a situation in which they are active, no?). But the findings were robust and raise many questions, namely: How does the same population of DG neurons activate different populations of downstream CA3 neurons, during different experiences?

Until now I had been in denial, fixated on trying to understand what types of behavioral experiences might activate different populations of dentate gyrus neurons. But maybe now it’s time to face the data.

The consensus, both in vitro (e.g. here and here) and in vivo (here), seems to be that if DG neurons are sufficiently active they can reliably activate CA3 neurons. Can a single population of DG neurons account for the amount of CA3 activity seen in the behaving animal? Well, 1% activation of the total DG population (1 million neurons) is 10 000 DG neurons. Each DG neuron contacts about 10 CA3 neurons. So if all active DG neurons activated all their downstream targets, then you’d expect about 100 000 active CA3 neurons – a third of the population. Indeed, about 20% of CA3 neurons are active when a rat explores a novel environment. So it’s possible. But it’s probably unlikely.

One reason it’s unlikely is that it doesn’t explain how different populations of CA3 neurons are activated by different experiences if it is the same population of DG neurons that are always driving them. In other words, since DG neurons are relatively hard-wired to CA3 neurons, how could a given DG neuron activate a CA3 neuron under some conditions and not others? One answer is that maybe it doesn’t – quite a while ago, McNaughton et al. showed that, even when the DG is lesioned, CA3 neurons are still able to selectively encode spatial locations as a rat traverses the environment, probably due to direct inputs from the cortex. And so perhaps the primary function of the DG is not to selectively activate different CA3 populations. However, the DG could certainly shape activity within CA3 or insert unique information into the CA3 network. How?

One possible mechanism, which may be dead obvious to electrophysiologists, is frequency itself. Leutgeb et al. found that frequency of activity is how DG neurons encode information and so frequency of activity may also be the way DG neurons transmit information to CA3 during different experiences.

It has been known for some time now that the output of DG neurons, the mossy fiber axons, show extraordinary frequency-dependent synaptic facilitation. Basically, as a DG neuron fires more action potentials over shorter periods of time, the amount of neurotransmitter it releases onto CA3 neurons increases (thereby increasing the likelihood a CA3 neuron will in turn fire action potentials and be recruited to participate in memory encoding). This means that at low firing rates, a DG neuron will activate some CA3 neurons and, at higher firing rates, it will recruit different or at least additional CA3 neurons.

Wouldn’t this cause a problem where, as DG firing rates increase, it is not different populations of CA3 neurons that become activated, but more populations? Well, it is known that some DG neurons increase their activity, and others decrease their activity, as an animal has different experiences, so the net activity in CA3 could remain constant, while still activating different CA3 populations. However, the DG-CA3 circuitry is certainly complicated enough to allow for other mechanisms. For example, while the dentate gyrus projects to CA3, and it is connections between these hippocampal regions that are thought to encode memories, DG neurons actually contact more inhibitory interneurons than CA3 neurons. Furthermore, there is a wide variety of synaptic connections between DG neurons and interneurons and these connections can be made weaker or stronger in a state- and frequency-dependent manner. Suffice it to say, by firing at different frequencies, it is plausible that a given DG neuron could activate different populations of interneurons, which in turn could inhibit different populations of downstream CA3 neurons, making them less likely to participate in memory encoding.

This ties in loosely to a peculiarity of the dentate gyrus that, until now, has just been a source of pretty histological images (to me) – the variability of calbindin expression in dentate gyrus neurons. Calbindin is a protein that binds calcium, it acts as a buffer, and gives DG neurons their property of facilitation (briefly: A single action potential in a DG neuron will travel down the axon and trigger the opening of calcium channels in the synaptic terminal at a CA3 neuron. Calcium is necessary for neurotransmitter release and subsequent activation of the CA3 neuron. Calbindin will bind this small amount of calcium, thereby preventing neurotransmitter release and CA3 activation. However, as the number and frequency of action potentials increases, calbindin will fail to effectively “mop up” the extra calcium and neurotransmission will proceed.). If you look at the picture at the top of this post, you can see that the amount of calbindin varies greatly in DG neurons. Immature DG neurons, identified by PSA-NCAM expression, are devoid of calbindin (arrows point to clear examples) and even when they are quite mature (10w of age) 40% will still be devoid of calbindin (see my data in this montage). Lastly, calbindin expression can be modified by experience. So the variable and modifiable expression of calbindin might be yet another mechanism by which DG neurons are capable of shaping activity in CA3 neurons. Indeed, at least one study, from Robert Sapolsky’s lab, has shown that genetically altering calbindin expression in the dentate gyrus dramatically influences DG-CA3 physiology and impairs memory.

Thanks to A.P. for posing the question.

Reference

Leutgeb JK, Leutgeb S, Moser MB, & Moser EI (2007). Pattern separation in the dentate gyrus and CA3 of the hippocampus. Science, 315 (5814), 961-6 PMID: 17303747

One positive thing that came out of looking at these very immature neurons was that I got the chance to see several examples of pyknotic (dying) cells. Older, adult-born neurons also die, particularly after an experience (see here and here), but it’s infrequent and hard to visualize. However, a relatively large proportion of new neurons die within a few days of their birth making them easier to find – the cluster of cells shown below is an example that caught my attention.

You can clearly see two BrdU-labeled cells (in green; marked with arrowheads) that also express doublecortin (DCX; red). The blue stain, Hoechst, stains DNA allowing for the visualization of all cell nuclei. Collectively, these 3 stains tell us that the cells are 1-day-old (because BrdU was injected 1 day before brains were collected), that they’re neurons (because they express the immature marker DCX) and that they’re dying (because BrdU and Hoechst both label DNA and show that the DNA is condensed in a ball, as is typically seen when cells undergo pyknosis). The arrow points to a lucky, neighboring neuron that is not dying.

Why were these two cells born if they’re only going to die 24 hours later?

I can understand the speculation that neural activity influences the survival of more mature neurons in a “use it or lose it” manner – essentially, if a memory is stored in a young neuron there must be a mechanism to ensure that the neuron, and therefore the memory, survives. But is it possible that a similar mechanism also influences the survival of very immature neurons? It’s hard to imagine, since very young neurons do not have synapses and cannot participate in memory processing/storage. Consistent with this idea, Tashiro has shown that NMDA receptors (a synaptic ingredient essential for many forms of memory) regulate the survival of 2-3 week-old neurons, which are just beginning to form synapses, but not younger neurons that have not yet formed synapses. However, the possibility remains that learning could do something to these 1-day-old neurons – e.g. epigenetically imprint them – so that they have some sort of cellular memory that causes them to subsequently participate in certain behaviors but not others. Since information is typically thought to be stored at synapses, I can’t imagine that these memories could be terribly specific but they could bias a young neuron to be more involved in a general class of behavior (e.g. spatial memory vs. stress) that is associated with certain broad differences in activity (e.g. firing patterns, neuromodulators, hormones). It would be really cool if someone shows this.

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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?

In this spreadsheet I’ve tried to provide summary answers to these questions. The data can be sorted by the type of behavior examined (e.g. depressive behaviors, memory etc), how neurogenesis was manipulated (e.g. via irradiation, transgenic tools or exogenous factors like anti-mitotic drugs), and behavioral effect.

It should be noted that I essentially took authors’ claims at face value and nothing here should be blindly accepted as evidence for or against a behavioral function for neurogenesis – read the papers! Task, neuronal age, and other methods should all be considered. Also, at this time, I have only entered data for a fraction of the studies, namely those that have claimed to use a technique specific for reducing neurogenesis. In reality, no such technique exists and I’d like to enter the same data for all studies that correlate neurogenesis with behavior, even those that have manipulated neurogenesis using methods that have widespread effects in the nervous system (e.g. exercise, enriched environment). If you’d like to assist let me know. And if you have any suggestions about how to improve the list let me know – as far as databases go I’m quite a novice.

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. The hippocampus appears to have a temporary role in memory storage — for instance, lesions to the hippocampus often impair recall of recent memories, but have little or no effect on remote memories. One interpretation of this result is that memories are stored in the hippocampus for only a short time and then transferred to other brain structures (likely neocortex) in a process called systems consolidation. This may then free up space in the hippocampus for new memories. Neurogenesis could be a mechanism for this process: for instance, the addition of new neurons to the dentate gyrus of the hippocampus might destabilize old memories and simultaneously provide a fresh substrate for new memories.

LTP lasts longer in irradiated rats (red circles) that lack adult neurogenesis.

Kitamura et al performed a truly heroic number of experiments to provide evidence that adult neurogenesis is involved in memory turnover in the hippocampus. First, they examined whether neurogenesis is important for the persistence of hippocampal long-term potentiation (LTP) in rats. LTP is an activity-dependent increase in the strength of synaptic connections between neurons, and is studied as a cellular model of memory.

They showed that blocking adult neurogenesis via irradiation extends the duration of hippocampal LTP: LTP lasts less than 2 weeks in control rats but lasts more than 3 weeks in irradiated rats (left). Thus, at a cellular/synaptic level, it would seem that neurogenesis causes memories to be cleared from the hippocampus.

Inactivating the hippocampus with TTX impairs memory in irradiated mice (red circles).

Then, in a series of fear conditioning experiments, the authors demonstrated that adult neurogenesis is in fact involved in the clearance of hippocampal memories in mice. In these experiments neurogenesis was blocked with irradiation or a genetic method prior to conditioning. On the surface, both control and neurogenesis-arrested mice acquired fear conditioning normally, and both groups remembered for at least a month. However, a critical difference emerged when the hippocampus was inactivated during remote recall testing: one month after training, hippocampus inactivation failed to impair recall in control mice, replicating a classic effect and suggesting the memory had been transferred to other brain regions. In contrast, in neurogenesis-arrested mice, hippocampal inactivation did impair recall of the remote memory, indicating that the memory still resided in the hippocampus. Importantly, hippocampal inactivation did impair recall in controls 1 day after conditioning, when memories had not yet been consolidated into the neocortex. Collectively, these findings suggest that adult neurogenesis contributes to the consolidation of memory to extra-hippocampal structures.

The authors then go ahead and answer some of the exciting new questions raised by these findings: For example, what happens if, instead of having reduced neurogenesis, you have increased levels of neurogenesis? Well, consistent with their hypothesis, mice that were given access to running wheels had increased neurogenesis (as expected) but as soon as 7 days after conditioning their memory was no longer hippocampal-dependent, unlike controls. Thus, increasing neurogenesis accelerated memory consolidation.

Another interesting finding was that the hippocampal-dependence of memory was prolonged when mice were irradiated only 11 days before conditioning. Since 11-day-old neurons do not even have functional (excitatory) synapses this suggests that it may not be functional young neurons present at the time of learning, but instead the synaptic integration of new neurons after learning that facilitates clearance of memory from the hippocampus.

Why else is this paper cool? Well, to date, adult neurogenesis has been implicated in many different types of behavior but it has been hard to identify common threads – this paper makes a new prediction that neurogenesis is specifically involved in consolidation. It also reconciles a discrepancy in the field – whether or not young neurons are involved in long-term memory. Some studies have found that reducing neurogenesis impairs long-term memory (Snyder 2005, Jessberger, 2009), but others have not observed this effect (Saxe, 2006). Kitamura’s data shows that even if long-term memory appears normal at a behavioral level, reducing adult neurogenesis will have altered the neural substrate of the memory. And so could these memories really be equivalent? This will be an exciting avenue for future studies to explore: how is hippocampus-dependent long-term memory (in neurogenesis-arrested animals) different from hippocampus-independent long-term memory?

It will also be interesting to further explore how arresting neurogenesis affects memory acquisition. If clearing memories from the hippocampus makes room for new memories, then shouldn’t the acquisition of new memories be hindered when clearance stops? Kitamura’s failure to observe significant impairments in memory acquisition may mean that neurogenesis is involved less in memory clearance than in reorganizing memory networks to include extra-hippocampal structures. That is, instead of simply expunging hippocampal memories, new neurons may facilitate the generation of additional memory representations in extra-hippocampal structures.

Another interesting issue is how the apparent give-and-take relationship between hippocampus and neocortex (presumably) arises. The data seem to suggest that the presence of a hippocampal memory representation prevents a cortical memory representation from arising. It’s as if the memory is a discrete entity that can be passed from hippocampus to cortex: as long as the memory remains in the hippocampus, the cortex is deprived. It would be great to know how –and why– this happens.

Kitamura T, Saitoh Y, Takashima N, Murayama A, Niibori Y, Ageta H, Sekiguchi M, Sugiyama H, & Inokuchi K (2009). Adult neurogenesis modulates the hippocampus-dependent period of associative fear memory. Cell, 139 (4), 814-27 PMID: 19914173