THE VAGUS NERVE

Or It Takes Guts to Alter a Brain PART I

vagus  en.wikipedia.org Henry Gray (1918) Anatomy of the Human Body

The longest and most extensive cranial nerve in our body, the vagus nerve connects the brain to the lungs, heart, and gastrointestinal (GI) tract. Within the lungs, the bronchial branch stimulates the bronchi, while the cardiac branch impacts heart rate. The vagal fibers that reach the GI drive peristalsis, the wavelike, muscular contractions required for food to pass through the digestive system. And amazingly, this neural connection also provides a link in gut microbiome-brain interactions.

First, a little history…

The record of gut-brain interactions has an ancient history. Rufus of Ephesus who lived in the 2nd century, noted that the snipping of the vagus nerve (a vagotomy) stopped peristaltic movements within the stomach. Galen of Pergamum, the great physician and philosopher, identified the vagus nerve in his De Anatomicis Administrationibus (On Anatomical Procedures). Relatively recently, a vagotomy was considered an important therapy to combat peptic ulcer disease. But what about the gut microbiota?

In 2011, Drs. Javier Bravo and Paul Forsythe reported that Lactobacillus rhamnosus*  impacted brain chemistry and behavior via the vagus nerve. Researchers fed mice either a standard chow diet (SCD) or an SCD+ Lactobacillus rhamnosus diet. L. rhamnosus is a probiotic with anti-inflammatory properties. Later, the mice underwent a variety of behavioral tests. Mice fed the probiotic enriched diet exhibited reduced depressive-like and anxiety-like behaviors. Moreover, the SCD+ L. rhamnosus mice also displayed unique differences in GABA receptor levels. GABA, gamma-aminobutyric acid, is a major neurotransmitter. Deficits in GABA signaling are linked with depression. The mice that ingested L. rhamnosus displayed an increased expression of GABA receptors in the hippocampus and a decreased expression of GABA receptors in the amygdala and prefrontal cortex**. This suggests that a microbe within the gut modulates the brain—an organ far, far, away. How?! The exact mechanism is unknown, but when researchers snipped the mouse vagus nerve the L. rhamnosus diet no longer altered behavior or GABA expression. Gut microbes produce many modulatory molecules, including neurotransmitters, these molecules may stimulate the vagus nerve, impacting brain and behavior. Remember: the vagus nerve is only one of many pathways involved in gut-brain communication.***  Could gut bacteria be a useful tool to treat depression? What do you think?

Interesting work Dr. Bravo and Dr. Forsythe!

*The lactic acid bacteria are typically found in our guts and yogurt!

**Hippocampus: brain region associated with learning, memory, and much more // Prefrontal cortex: the ‘smart’ brain region associated with cognitive thinking and much more // Amygdala: brain region associated with fear

***I’ll post about vagus-independent gut-brain communication later!

Sources + Additional Information:

Vagus Nerve: http://www.britannica.com/science/vagus-nerve

Galen: https://archive.org/details/b20457194

Johnson, Leonard R., Ed. Physiology of the Gastrointestinal Tract, Two Volume Set. Elsevier Limited: Oxford, 2012.

The Bravo et al. study: http://www.pnas.org/content/108/38/16050.full

For a Great Review on Gut-Brain Interactions, click: http://icds.uoregon.edu/wp-content/uploads/2011/07/CryanDinan2012_MindAlteringOrganisms.pdf

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