30 November 2009

For new brain cells, go to the wild

ResearchBlogging.orgThis post was chosen as an Editor's Selection for ResearchBlogging.orgFor a while, people thought that vertebrate brains did not get new neurons throughout life. Songbirds provided one of the first known counterexamples, when it was discovered that some birds generated new neurons, associated with song centers, every year. Later, mammals were discovered to generate new neurons in adulthood, most interestingly in the hippocampus.

The hippocampus is deeply involved in all manner of learning and memory, but particularly spatial learning and memory. The natural hypothesis is making new neurons might be related to learning.

Demonstrating what causes new neurons to form is tricky, though. For example, one standard spatial learning test, the Morris water maze, is set up so that animals (rats, actually) that learn the task move around less (swim, actually) than animals that don’t learn the task. Learning and the physical activity are inextricably linked, so you cannot easily say which caused new neural growth.

Researcher Lara LaDage and colleagues tried to address this problem by using a different animal and a different learning task. Chickadees are very good at spatial learning because they store food in little hiding spots, and come back for it later. Many birds are dramatically better at these kinds of spatial memory tasks than humans.

LaGade and company captured a group of young chickadees (Poecile gambeli) in the wild, and split them into three groups.

  1. Banded, let loose to live free in the wild, and recaptured three months later.
  2. Captured and kept in in captivity for three months, where they were allowed to cache food and do associative learning.
  3. Captured and kept in in captivity for three months, where they were not allowed to cache food.

A lot of the difficulty in running an experiment like this is trying to make sure everything is the same except one variable. With the free-living chickadees, you just have to accept that you can’t control everything. The trick is in making everything the same for the two groups of lab animals, a key one being keeping the animals’ general activity levels the same. LaDage and colleagues measured the chickadees’ perching in the lab, and were able to show they moved about the same distance, eliminating the confound between activity and learning.

All of this was done to see the impact on the brain. LaDage and colleagues identified new neurons by labeling a protein called doublecortin. Doublecortin shows up fairly specifically in new neurons, so by using an antibody to it, they could look in various regions of the hippocampus to see how many new neurons had formed in roughly the last month.

All the birds grew new neurons, but the free range birds had more new hippocampal neurons – both in sheer raw numbers and proportionately – than the captive birds allowed to cache food. The captive birds that cached food had more new neurons than the captive birds that did not cache food. A simplistic summary might be that animals’ brains show greater capacity to change and adapt when they get to be animals in the environment that natural selection has shaped them for.

Reference

LaDage, L., Roth, T., Fox, R., & Pravosudov, V. (2010). Ecologically relevant spatial memory use modulates hippocampal neurogenesis Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2009.1769

Picture by user jerryoldenettel on Flickr, used under a Creative Commons license.

2 comments:

MFK said...

This is a great paper, and it raises the question of why it has been so hard to find adult neurogenesis in laboratory mammals. Given that an 'enriched' environment seems to increase neurogenesis, is it possible that such 'little' neurogenesis we see in lab mammals has to do with their poor rearing? Do you know of any studies that have loooked at adult neurogenesis in wild rats or mice?

Zen said...

I think it's entirely plausible. I recall that some people have shown differences in neural structure (e.g., number of dendrites, etc.) in rats raised in "enriched environments" (a.k.a. "rat heaven" -- big enclosures, many animals, tubes, places to run, etc.) rather than cages. I don't know if anyone has done equivalent experiments for neurogenesis. But then, I haven't looked.

A slightly similar effect on neuronal plasticity was seen in barn owls, which showed greater ability to learn if allowed to catch food in the lab rather than being fed things they didn't have to catch (Bergan et al. 2005. Hunting Increases Adaptive Auditory Map Plasticity in Adult Barn Owls. The Journal of Neuroscience 25(42):9816-9820. doi: 10.1523/jneurosci.2533-05.2005

While they were not looking at neurogenesis, it also showed a similar principle: ecologically relevant tasks reveal greater capacity of nervous system to change.