UK scientists suggest that a type of friendly bacteria found in soil may affect the brain in a similar way to antidepressants.

Their findings are published in the early online edition of the journal Neuroscience.

Researchers from Bristol University and University College London discovered using laboratory mice, that a "friendly" bacteria commonly found in soil activated brain cells to produce the brain chemical serotonin and altered the mice's behaviour in a similar way to antidepressants.

They are suggesting this could explain why immune system imbalance could make some people vulnerable to mood disorders like depression.

Lead author, Dr Chris Lowry 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," he added.

Dr Lowry and colleagues became interested in the project when they heard that cancer patients treated with the bacterium Mycobacterium vaccae reported increases in their quality of life. They speculated this could be because the bacteria were activating brain cells to release more serotonin.

When they treated mice with Mycobacterium vaccae they found that it did indeed activate a particular group of brain neurons that produce serotonin - in the interfascicular part of the dorsal raphe nucleus (DRI) of the mice, to be precise. They established this by measuring the amount of c-Fos in the area, a biochemical marker whose presence indicates that serotonin releasing neurons have fired.

Serotonin, also known as 5-HT (short for 5-hydroxytryptamine), is found in the gut, brain, nerves and blood of humans and other animals. There are 14 different receptors that bind to serotonin each working a different property of this highly multi-functional chemical messenger.

Apart from having a range of pharmacological actions, serotonin constricts blood vessels, sends messages between cells in the brain and within the central nervous system, regulates secretion of digestive juices, and helps to control the passage of food through the gut.

Different parts of the brain and the body need different levels of serotonin. In the brain for example, the hypothalamus (involved in mood regulation) needs a lot of serotonin while the cortex (involved in many complex processes like thinking, memory, attention, awareness and consciousness) only needs a little.

The brain keeps serotonin levels in balance using at least three mechanisms. One way is by releasing it, a second way is by inactivating it once it is released into the synaptic space between the nerve endings, and the third way is by absorbing it, a process known as "reuptake".

Low levels of serotonin are linked with a number of disorders including aggression, anxiety, depression, obsessive compulsive disorder (OCD), bipolar disorder, irritable bowel and fibromyalgia.

Antidepressants work by increasing serotonin levels in particular areas of the brain. One type, known as monoamine oxidase (MAO) inhibitors reduce the brain's ability to inactivate the free serotonin. Another type, called selective serotonin reuptake inhibitors (SSRIs) work by slowing down the reuptake process.

The friendly bacteria in this study appear to be having an antidepressant effect in a third way, by increasing the release of serotonin.

"Identification of an immune-responsive mesolimbocortical serotonergic system: Potential role in regulation of emotional behavior."
C.A. Lowry, J.H. Hollisa, A. de Vriesa, B. Pana, L.R. Brunetb, J.R.F. Huntb, J.F.R. Patonc, E. van Kampena, D.M. Knighta, A.K. Evansa, G.A.W. Rookb and S.L. Lightmana.
Neuroscience Available online 28 March 2007
doi:10.1016/j.neuroscience.2007.01.067

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Click here for links to different types of antidepressant (from Wikipedia).

Written by: Catharine Paddock
Writer: Medical News Today