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 focused 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.
SYNAPTIC PLASTICITY IN YOUNG NEURONS
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:
- It was first shown by Sabrina Wang in Martin Wojtowicz’s lab back in 2000. The first demonstration of a unique functional role for new neurons. That work was done in single neurons from rat, by patch clamping.
- I followed up on that work in the Wojtowicz lab and found that this LTP could be observed in field recordings, i.e. this LTP stood out amongst the background of activity from all granule neurons when you stimulated their synaptic inputs. In fact, it was the only type of LTP observed in the absence of GABA blockers (making it pretty easy to identify and measure, even by novice electrophysiologists such as myself).
- Ge et al (2007) confirmed these findings in single neurons in mice and characterized the “critical period”, showing that only immature neurons displayed this plasticity, and not older adult-born neurons.
- Wang et. al (2008) did a nice trick and increased neurogenesis with fluoxetine and in turn increased ACSF-LTP. Irradiation abolished both.
- Garthe et al (2009) used a chemical method for stopping neurogenesis. ACSF-LTP was gone.
- Sahay et al (2011) also increased neurogenesis and therefore ACSF-LTP, via a novel transgenic method.
AND NOW, I saw a poster by Kheirback et. al, who now used a completely novel method to confirm the existence of this special form of plasticity in new neurons. They used Nestin CreER and a floxed NR2B to delete the NR2B subunit specifically from young neurons. This didn’t kill neurons, it didn’t affect plasticity in mature neurons, but it wiped out ACSF-LTP. This is not entirely surprising because the NR2B subunit is known to endow new neuron with their enhanced plasticity. It just was a novel, elegant and specific way to demonstrate it.
Whereas the behavioral data on new neurons is less advanced and more variable, the fact that ACSF-LTP has been demonstrated in different species (mouse and rat), is absent after using different methods to reduce neurogenesis (irradiation, chemical, transgenic), is enhanced after using different methods for increasing neurogenesis (fluoxetine, transgenic) arguably makes it the strongest support for a real, significant function for hippocampal neurogenesis.
Going back to these NR2B-deficient new neurons, these mice also had behavioral effects: they had impaired ability to behaviorally distinguish contexts during fear conditioning, reduced object exploration, and no preference for a novel object over a familiar object. There was also a tendency, albeit less robust, for these mice to be more innately anxious and fearful.
ACSF-LTP is so well documented because the anatomy and physiology of the inputs to granule neurons is not super complex. In contrast, if there’s one brain region that could really benefit from optogenetics, it’s the dentate gyrus output onto CA3. The connectivity is such that it is practically impossible to stimulate only new neurons with conventional techniques and record from their postsynaptic targets. At least the ones that are further away, like pyramidal neurons. This can be overcome by shining light on the dentate gyrus and ensuring that only new neurons are activated, through selective expression of channelrhodopsin or its variants. This is exactly what Gu et al did. They characterized, for the first time, mossy fiber LTP in new neurons and found that 4-week-old neurons have greater LTP than either 3 or 8-week-old cells. Using archaerhodopsin to inhibit new neurons they also found that these 4-week-old cells are especially important for memory retrieval in fear conditioning and water maze paradigms. The deficits were not huge, but they were consistent. Considering that they used viral methods to express archaerhodopsin, which probably only infects a small proportion of the total new neuron population, the effect these new neurons are having on behavior is quite remarkable.
SO WE HAVE SYNAPSES AND BEHAVIOR, BUT WHAT’S IN THE MIDDLE?
The most commonly-proposed, but least understood, cognitive function for new neurons is pattern separation. Originally a computational term, it refers to the process by which neurons take similar inputs (spatiotemporal patterns of incoming synaptic activity) and maximize their differences. When brains do this well we are able to distinguish very similar experiences in our memory. When it fails you spend 30 minutes looking for your car before you remember that you parked in a different lot. The discrimination of safe from dangerous contexts during fear conditioning is used more and more as a behavioral test of pattern separation. While it may depend on pattern separation by new neurons, this has never been shown. To get at this, Niibori et al. used the catFISH method to see what is happening in CA3 neuronal ensembles when mice don’t have neurogenesis. catFISH uses the spatiotemporal patterns of activity-dependent gene expression to identify neuronal populations that were activated by different experiences. It’s been shown that when rodents are put into 2 different contexts, distinct ensembles of CA3 neurons are activated by the 2 experiences. The memory traces are kept distinct in order to not confuse the experiences. If new neurons are indeed performing a pattern separation function then you’d predict that, in their absence, the same population of CA3 neurons is activated by the 2 experiences. And this is exactly what Niibori et al found. Furthermore, confirming that new neurons are not just performing this separation function, but are actually required to behaviorally distinguish the 2 environments, mice lacking neurogenesis showed similar levels of fear behavior in both a context that was paired with shock as well as a context that had never been paired with shock. And so the hypothesized role for new neurons in pattern separation just got a whole lot stronger.
WHERE ARE WE AT NOW?
There’s only so much functional work going on out there. Some behavior, less electrophysiology, a new method for measuring circuit properties. Your study of a phenomenon feels complete when you’ve seen presentations that cover function at these different levels of analysis. When they replicate and then build upon previous findings it’s even more of a confirmation that progress is being made. I have to say I think it was a pretty good meeting for adult neurogenesis.
Postdoctoral Fellow, NIH/NIMH
Jason studied the synaptic plasticity of adult-born hippocampal neurons during his PhD at the University of Toronto. He is currently investigating the behavioral function of adult neurogenesis at the National Institutes of Health.