Young or old it doesn't matter: we need them both

To honor Brain Awareness Week I thought I'd try and discuss a neuroscience paper this week. It's not my field, so you'll have to be patient with me (and you experts out there are more than welcome to pitch in). I found a really fascinating story in the latest issue of Science [1] on the differences in information processing between "young" and "old" neurons. In order to understand the story, I had to take a couple of steps back and review a few things about the brain.

The hippocampus is the part of the brain that's responsible for learning, storing memories and associating them with feelings and emotions. Within the hippocampus lies the dentate gyrus, which is where adult neurogenesis takes place -- the formation of new neurons throughout adulthood. The middle layer of the dentate gyrus contains a type of neurons called granule cells. These are constantly generated and take a few weeks to develop and integrate in the dentate gyrus network. In [1], Marin-Burgin et al. asked the following question:

"Is it solely the continuous addition of new neurons to the network that is important, or are there specific functional properties only attributable to new granule cells (GCs) that are relevant to information processing?"

In order to answer the question, the researchers compared immature granule cells to mature ones in mouse hippocampus. The part that fascinates me the most about these experiments is that in order to "see" the different cells, these neurons are "retrovirally labeled to express red fluorescent protein." What this means is that a genetically engineered retrovirus that preferentially infects this type of cells is used to "infect" them and deliver the fluorescent proteins so that the neural activity can be visualized. Pretty cool, right?

Marin-Burgin et al. found that the dentate gyrus is made of a heterogeneous population of granule cells of different ages and that the different subpopulations have distinct activation thresholds. When given both excitatory and inhibitory input, the ratio of excitation to inhibition favors inhibition in mature granule cells, whereas immature cells have fewer inhibitory inputs (hehe, sounds familiar don't you think?). In other words, younger cells respond more easily and broadly, whereas older cells tend to be more specific. The fact that both are present at all times suggests that this range in different responses is needed for the correct functionality of the dentate gyrus, in particular for the correct storing and integration of novel information.

[1] Marin-Burgin, A., Mongiat, L., Pardi, M., & Schinder, A. (2012). Unique Processing During a Period of High Excitation/Inhibition Balance in Adult-Born Neurons Science, 335 (6073), 1238-1242 DOI: 10.1126/science.1214956