Interazione intestino-cervello

Brain and gut: how the microbiota influences our mood

The intestine is increasingly called “the second brain“, but is that the reason for this name? We know that the intestine hosts an invisible ecosystem, the microbiota intestinalcomposed of an endless community of microorganisms which, in addition to supporting digestion, send continuous signals to the brain through theaxis intestine-brainwhich follows neuroendocrine, immune and other pathways. Recent studies show us that this communication is bidirectional, from the intestine to the brain and vice versa, and can influence ours humoron emotions and even on the mental health. It’s one symbiotic relationship which increasingly reveals to us the complex network of interactions that defines not only our body, but also our own behavior.

How do the intestine and brain communicate?

The fastest and most direct route between the intestine and the brain is the vagus nervewhich is also the longest nervous pathway in the human bodyand transmits signals in both directions (i.e. both from the gut to the brain and vice versa). It starts from the medulla oblongata, a deep area at the base of our head that regulates basic functions such as heartbeat, breathing and circadian rhythms, and extends through the neck, trunk and abdomen, branching out to the heart, lungs, stomach and, indeed, the intestine. Being a nerve, the main mode of transmission of communication is through electrical impulseswhich are influenced by the intestine through the ability of intestinal bacteria to synthesize neurotransmitters such as gamma aminobutyric acid (for friends, GABA), serotonin, dopamine and norepinephrine.

Intestine-brain vagus nerve
The vagus nerve is the longest nerve pathway in the body and acts on several organs responsible for autonomous functions

The vagus nerve and the autonomic nervous system are connected to the enteric nervous system which is, as the name suggests, a network of neurons located in the gastrointestinal tract. Then there are neuroendocrine pathwaysof which thehypothalamic-pituitary-adrenal axis. Chronic stress can activate this axis and promote the secretion of cortisol called, precisely, the stress hormone. The immune pathways constitute another way in which the brain and intestine communicate: the intestinal microbiota influences the level of cytokines inflammatory and anti-inflammatory, which in turn influence brain function.

More generally, i intestinal bacteria they produce a surprising variety of molecules that affect the central nervous system: some important neurotransmitters, as we have seen, but also neurotoxins such as D-lactic acid and ammonia which negatively impact cognitive functions. Also very important short chain fatty acids produced by intestinal microorganisms that can cross the blood-brain barrier of the brain and impact on inflammatory processes and neural plasticityor the brain’s property of creating new connections, therefore new memories and knowledge.

Gut-brain axis

The microbiota and mood disorders

When the balance between the various bacterial populations is altered we enter a condition called dysbiosis. An imbalance in the microbiota can lead to reduced production of neurotransmitters such as serotonin, while increasing inflammation and, over time, a vicious cycle that more and more research links to a greater risk of anxiety And depression. Some studies have shown that patients with a diagnosis of depression have a lower microbial diversity in the intestine compared to healthy individuals, with alterations in the composition of the intestinal flora. Experiments conducted transplanting parts of the microbiota from depressed patients to laboratory rats he has observed in the latter behaviors and physiological characteristics linked to depression. The “depressed” microbiota affected the mice’s mood.

Microbiota dysbiosis

Imbalances in the regulation of the hypothalamic-pituitary-adrenal axis is also associated with mood disordersdepression and an increase in stress. The process works like a vicious circle: chronic stress can alter the composition of the microbiota, which in turn alters the response of the vagus nerve and axis; these, in turn, becoming dysregulated they retroact on stress and the intestine by altering the microbiota and producing large quantities of cortisolwhich influences intestinal permeability by promoting inflammation.

Just as dysbiosis worsens mood, the healthy microbiota promotes good psychological resistance to life’s difficulties. The short chain fatty acids produced by intestinal bacteria have in fact a decisive role in maintaining organic and cerebral homeostasis: an example is the butyratewhich has been shown to have anti-inflammatory effects and help improve mood.

Targeted studies have found an increase in Actinobacteria And Enterobateriaceae within the gut microbiota of people diagnosed with bipolar disorder And major depressionand a significant decrease in such bacteria Faecalibacterium.

In summary, alterations in the intestinal microbiota impact the nervous systemon the endocrine system and on immune system. Both directly and indirectly, the state of health and “coexistence” of the various components of the intestinal microbiota (bacteria, viruses and fungi), impacts our brain, therefore our perceptions, our sensitivity and our psychic state. One last unsolicited curiosity: one of the ways in which medicine addresses the problem of dysbiosis is fecal transplant. Yes, that’s exactly what you think.