Treatment of mice with a ‘friendly’ bacteria, normally found in the soil, altered their behavior in a way similar to that produced by antidepressant drugs, reports research published in the latest issue of Neuroscience.
These findings, identified by researchers at the University of Bristol and colleagues at University College London, aid the understanding of why an imbalance in the immune system leaves some individuals vulnerable to mood disorders like depression.
Dr Chris Lowry, lead author on the paper from Bristol University, said: "These studies help us understand how the body communicates with the brain and why a healthy immune system is important for maintaining mental health. They also leave us wondering if we shouldn’t all be spending more time playing in the dirt."
This discovery was an accident. Dr. Lowry was experimenting with the use of M. vaccae to treat lung cancer and found that the mood and cognitive function of lung cancer patients were also boosted by the M. vaccae vaccination.
Interest in the project arose after human cancer patients being treated with the bacteria Mycobacterium vaccae unexpectedly reported increases in their quality of life. Lowry and his colleagues reasoned that this effect could be caused by activation of neurons in the brain that contained serotonin.
When the team looked closely at the brains of mice, they found that treatment with M. vaccae activated a group of neurons that produce the brain chemical serotonin. The lack of serotonin in the brain is thought to cause depression in people, thus M. vaccae’s effects on the behavior of mice may be due to increasing the release of serotonin in parts of the brain that regulate mood.
As expected, cytokine levels rose. They then looked directly in their animals' brains for the effect of those cytokines.
Cytokines actually act on sensory nerves that run to the brain from organs such as the heart and the lungs. That action stimulates a brain structure called the dorsal raphe nucleus. It was this nucleus that Dr Lowry focused on. He found a group of cells within it that connect directly to the limbic system, the brain's emotion-generating area. These cells release serotonin into the limbic system in response to sensory-nerve stimulation.
The consequence of that release is stress-free mice. Dr Lowry was able to measure their stress by dropping them into a tiny swimming pool. Previous research has shown that unstressed mice enjoy swimming, while stressed ones do not. His mice swam around enthusiastically.
It is worth noting that this work fits in a larger context: the argument (known as the hygiene hypothesis) that humans are suffering more auto-immune diseases such as allergies and asthma due to a lack of exposure to bacteria, digestive tract worms (which might be key to prevention and treatment of inflammatory bowel disease- also see here), and other pathogens. According to this theory people living in modern clean industrialized societies with purified water, refrigerators, automated farms, flush toilets, warm showers, and hand soap the immune system doesn't get exposed to pathogens it is designed to handle. The immune system is designed to work properly only in the presence of those pathogens. So it goes awry and starts attacking things it ought not attack. Considerable amounts of evidence (see here and here) supports the idea that getting dirty might be good for you.
This latest result suggests that other functions of the immune system (e.g. interactions with the nervous system) aren't getting sufficiently stimulated in modern society. So maybe we are suffering from an epidemic of depression (and other mental illnesses while we are at it?) due to excessive purity of our environments. Well, I'm sure glad as a kid that I liked to go out in the yard and build dirt castles and mud walls. City kids didn't have that advantage.
What I'd like to know: Do kids with dogs have a lower risk of getting depressed when they grow up? Do kids who grow up on pig and cow farms similarly have lower risks of adult depression? Also, does depression vary by country due to different vaccination regimes used in different countries?
The larger lesson: We are not in our ancestral environments which we are genetically adapted to. So all bets are off. We need to develop technologies that adapt us to our new environments. Speaking of which: In addition to vaccinations that give our immune systems needed exercise we also need to reshape our work environments to give ourselves more exercise in cubicle land. My suggestion: move exercise bicycles and steppers into meeting rooms and training rooms so that we can get exercise while getting training and giving project status reports.
|Share |||Randall Parker, 2007 April 07 09:09 AM Brain Depression|