Brains that can't grow new neurons don't respond well to stress… (Getty Images )
What makes people depressed, biologically speaking?
One theory blames brains that can’t generate new neurons. The idea goes back to a 2000 study in the Journal of Neuroscience that found that rats who took antidepressants grew new neurons in a part of the brain called the hippocampus, which is particularly vulnerable to stress. The authors of that study said the new brain cells may well be responsible for easing depressive symptoms in people who took antidepressant medications.
It looks like they were right. A study published online Wednesday by the journal Nature used mice to show that when the hippocampus was unable to grow new neurons, the animals couldn’t tolerate stressful situations and became depressed. The link between neuron growth and depression is not just coincidental but actually causal, according to study leader Jason Snyder of the National Institute of Mental Health.
Synder and his colleagues used a drug to shut down production of new neurons in a group of mice. Under normal conditions, those mice behaved just like any other mice. But when they were subjected to stress, the level of stress hormones remained high much longer than in normal mice.
When these super-stressed mice were given the opportunity to find a food pellet in an open field they had never explored before, only 53% of them did so, even though they were hungry. For the sake of comparison, 92% of normal mice found the pellet despite having endured a stressful experience beforehand.
In another test, both groups of mice were forced to swim in “an inescapable cylinder of water” as researchers measured how long it took for them to give up and stop swimming, which is presumed to be a marker for “despair,” according to the study. The super-stressed mice that couldn’t grow new neurons gave up swimming faster than the normal mice.
In a final test, researchers wanted to see if the super-stressed mice would give up their taste for sugar water. Both groups of mice were allowed to drink plain water and sugar water for three days, and all of the mice preferred the sugar water. Then the researchers withheld both liquids and reintroduced them in different places in their habitats. The normal mice found the sugar water and preferred it as before, but the super-stressed mice settled for the plain water – “a hallmark symptom of depression,” the researchers wrote.
“Elucidating the strong but poorly understood association between stress and depression is critical for development of more effective treatments,” they wrote.
You can read a summary of the study here. For a more detailed discussion of the paper, check out this post from Ed Yong on his blog, Not Exactly Rocket Science.