A team of scientists has shown that brain levels of serotonin, the happy hormone, are regulated by the amount of bacteria in the gut during early life.

The research results, detailed in the journal Molecular Psychiatry, show that normal adult brain function depends on the presence of gut microbes during development. Serotonin, the major chemical involved in the regulation of mood and emotion, is altered in times of stress, anxiety and depression and most clinically effective antidepressant drugs work by targeting this neurochemical.

The team used a germ-free mouse model to show that the absence of bacteria during early life significantly affected serotonin concentrations in the brain in adulthood. The research also highlighted that the influence is sex dependent, with more marked effects in male compared with female animals.

Finally, when the scientists colonized the animals with bacteria prior to adulthood, they found that many of the central nervous system changes, especially those related to serotonin, could not be reversed indicating a permanent imprinting of the effects of absence of gut flora on brain function.

This builds on earlier work, showing that a microbiome-gut-brain axis exists that is essential for maintaining normal health which can affect brain and behavior.

“As a neuroscientist these findings are fascinating as they highlight the important role that gut bacteria play in the bidirectional communication between the gut and the brain, and opens up the intriguing opportunity of developing unique microbial-based strategies for treatment for brain disorders”, said senior author Prof John F Cryan, Head of the Department of Anatomy & Neuroscience at University College Cork.

This research has multiple health implications as it shows that manipulations of the microbiota (e.g. by antibiotics, diet, or infection) can have profound knock-on effects on brain function.

“We’re really excited by these findings,” said lead author Dr Gerard Clarke. “Although we always believed that the microbiota was essential for our general health, our results also highlight how important our tiny friends are for our mental wellbeing.”

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Bibliographic information: Clarke G., Grenham S., Scully P., Fitzgerald P., Moloney R.D., Shanahan F., Dinan T.G. and Cryan J.F. 2012. The microbiome-gut-brain axis during early life regulates the hippocampal serotonergic system in a sex-dependent manner. Mol Psychiatry, advance online publication; doi:10.1038/mp.2012.77