How Your Gut Microbiome Can Affect Your Brain
Gut Brain Axis - this bidirectional communication network includes the central nervous system (in the brain), autonomic nervous system (outside the brain), enteric nervous system (in the gut) and HPA axis (hypothalamic pituitary adrenal). This 2 way communication is the link between the emotional and cognitive parts of the brain with intestinal function.The autonomic nervous system sends signals from the intestinal wall through enteric, spinal and vagal pathways to the central nervous system and vice versa.
The Vagus Nerve - an essential part of the gut brain axis. It is one of the connections between the brain and GI tract and acts by sending information from gut to brain and brain to gut through its fibers This nerve is the main component of the parasympathetic nervous system (a branch of the autonomic nervous system). The vagus nerve is responsible for different functions like digestion, heart rate, respiratory rate and certain reflex actions. Preliminary evidence has shown that vagus nerve stimulation seems to be a beneficial add on to treatment for depression, PTSD as well as anxiety and mood disorders.
Enteric microorganisms are distributed among the GI tract. The enteric microbiota interacts not only with the enteric nervous system in the gut but also with the central nervous system in the brain through neuroendocrine and metabolic pathways. Studies have shown that gut bacteria can activate neural pathways and central nervous system signaling systems.
Several different mechanisms have been identified as to how gut bacteria can affect the nervous system:
-altering activity of HPA axis
-vagal nerve stimulation
-secretion of short chain fatty acids
-affect permeability of blood brain barrier
Dysbiosis and inflammation of the gut have been linked to causing several mental illnesses including anxiety and depression and healthy gut function has been linked to normal central nervous system function.
Majority of studies conducted on the relationship between the gut and the brain have been done in animal models but preliminary studies on humans indicate gut bacteria interventions can alter neurotransmitters. In animal models, different pathways of communication have been identified along the gut brain axis which are driven by the immune system, vagus nerve and modulation of neuroactive compounds by gut bacteria.
Studies conducted on germ free animals (without a gut microbiome) demonstrated:
-bacterial colonization of the gut is central to the development of the enteric and central nervous system
-the absence of microbial colonization associated to an altered expression of neurotransmitters
-microbiota influences stress reactivity and anxiety like behavior
-presence of microbiota results in modulation of serotoninergic system
-gut bacteria can produce and consume neurotransmitters like serotonin, GABA, norepinephrine, and dopamine. Influence of these neurotransmitters by gut bacteria have an influence on host physiology.
A study done in 2016 did a fecal microbiota transplant, basically they took gut bacteria from the stool of depressed patients and orally administered it to rats who did not have a gut microbiome. The recipient animals displayed behavioural and physiological features of depression (including alterations in tryptophan metabolism).
