Posted by pdg1234567 on January 15, 2008, at 20:47:31
Wonder if any of this true and if so what it means for those of us that suffer from depression? Could it be an immune response?
We know, or suspect, that 'friendly bacteria' have all sorts of physical health benefits, particularly in aiding digestion and boosting our immune systems. But a chance observation and studies on mice suggest that they may also have a positive effect on our mood by boosting levels of serotonin in the brain.
Dr Mary O'Brien, an oncologist at the Royal Marsden Hospital in London, was trying out an experimental treatment for lung cancer that involved vaccinating patients with Mycobacterium vaccae, bacteria common in soil and dirt. Her aim was to boost the patients' immune systems, helping them to fight the cancer more effectively. But there was an unexpected side-effect: not only did the patients show fewer symptoms of the cancer, but their mood and vitality also improved.
Intrigued by the observation, Dr Christopher Lowry - a Wellcome Trust Research Career Development Fellow at Bristol University - and colleagues turned to mice to find out whether this feeling of wellbeing arises because the immune response to M. vaccae induces the brain to produce serotonin.
When they tested the effects of the bacteria on mice's performance in a swimming test - a common test to predict potential antidepressant effects of drugs - they found that control mice swam for an average of two-and-a-half minutes, but mice injected with M. vaccae paddled enthusiastically for four. This suggested that the bacteria had the same effect on mood as antidepressant drugs.
The team also found that mice injected with M. vaccae had higher levels of cytokines - proteins produced by the immune system that can trigger responses in the brain. Cytokines actually act on sensory nerves that run from organs such as the heart and the lungs to the brain, and stimulate a brain structure called the dorsal raphe nucleus. Dr Lowry's team found a group of cells within this nucleus that release serotonin into the limbic system (the brain's emotion-generating area) in response to sensory-nerve stimulation.
The findings, published in Neuroscience in April 2007, suggest that the bacteria activate immune cells that release cytokines, which then act on receptors on the sensory nerves to boost serotonin production.
Exposure to a wide range of common bacteria in early life appears to promote healthy development of the immune system. Without such exposure, the immune system seems more likely to mistake the body's own cells for invaders and launch attacks on them. This could be one of the mechanisms underlying the recent increase in frequency of asthma and eczema: children brought up in 'over-hygienic' conditions, without exposure to harmless bugs, may have malfunctioning immune systems, making them more prone to these conditions.
Dr Lowry's research takes this 'hygiene hypothesis' a step further by linking exposure to microbes with good mental as well as physical health, and suggests a role for the immune system in mood disorders such as depression.
Further studies will be needed to determine if M. vaccae or other bacteria do have antidepressant properties through activation of this group of serotonin neurons. If they do, it may in the future be possible to treat clinical depression with a vaccination.
poster:pdg1234567
thread:806782
URL: http://www.dr-bob.org/babble/20080114/msgs/806782.html