The probiotic bacteria in the gut indirectly influence the central nervous system, but scientists have not identified specific mechanisms. In this study, Bravo and coworkers introduced the gut bacteria Lactobacillus rhamnosus into normal, healthy mice and looked for changes in behavior and brain chemistry. Mice fed bacteria-laden broth showed fewer behaviors related to stress, anxiety, and depression compared with mice fed broth without bacteria. Bacteria-fed mice also had significantly lower levels of the stress hormone corticosterone. The authors also found relatively high expression of a GABAB receptor subunit in the brain’s frontal cortices, where it is normally reduced during depression; and relatively low expression of a GABAA receptor subunit in parts of the brain where it is normally increased in stressed or anxious animals. Surgical severance of the vagus nerve, which communicates changes in the gastrointestinal tract to the brain, prevented the bacteria-induced effects on behavior and GABA receptors. The authors suggest that these findings highlight the important role of bacteria in the bidirectional communication between gut and brain, and they suggest that certain organisms may be useful therapeutic adjuncts in stress-related disorders such as anxiety and depression.

Bravo JA et al.,  Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve.  Published online before print August 29, 2011, doi: 10.1073/pnas.1102999108 PNAS August 29, 2011.

http://www.pnas.org/content/early/2011/08/26/1102999108