Scientists are exploring evidence that major depression may be in part an intestinal sensation, orchestrated by the microbiome, trillions of microorganisms living in and around our body, that influence our health and well-being.
In a series of studies, researchers found that the microbial menagerie that lives in our digestive tract can help regulate brain function, including mental health. Recent findings from scientists in the US, Europe and China relate our feelings of stress, anxiety and severe depression to alterations among hundreds of species of microbes that live in the gut that some researchers have begun to call psychobiome.
In contrast, other gut bacteria appear to produce some of the same substances that doctors use to treat depression and may play a natural role in maintaining our emotional balance.
“The feeling of discomfort, if you will, is often associated with gastrointestinal disorders,” said microbiologist Jack Gilbert of the University of California, San Diego and the Scripps Institution of Oceanography, who helped pioneer the study of the human intestinal microbiome. It is “chemically altering the nerve signals that enter the brain, which alter brain chemistry and therefore behavior, mood and, we believe, depression and anxiety.”
As evidence, some scientists have been able to infect mice and rats with mental disorders, including depression and anxiety, by transplanting stool samples, which contain intestinal microbes, from human patients to laboratory animals, according to several recent studies. . “When you give these mice the germs of depression, they start behaving in a depressive way,” said psychiatrist Julio Licinio of New York State University Upstate Medical University in Syracuse. These behavioral changes in mice affect things like appetite, weight gain, and activities like swimming. Dr. Licinius studied the biology of depression and helped design some of the experiments. “It’s actually transmissible,” he said.
Bacteroids, seen here in a color scanning electron micrograph, are the most common bacteria found in the human intestinal tract.
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So far, however, no one has been able to distinguish specific species of microbes related to a mental illness. This month, an international research team first identified dozens of species of intestinal microbes involved in depression by comparing patients diagnosed with the disorder with healthy people. These 47 species are a small fraction of the microbial diversity of the gut, which includes other unicellular organisms, thousands of species of viruses, and fungi.
New research by neuroscientist Peng Xie at China’s first affiliated hospital at China’s Chongqing Medical University reveals a potential mechanism for a mental illness that affects approximately 350 million people worldwide, several experts said. The research was published in the journal Science Advances.
Scientists are quick to discover how these microbes interact with the human central nervous system, what signals they send to the brain, and how this alters a person’s behavior or risk of mental illness, in hopes of new treatments and diets for to diseases of the mind.
“The big race is to understand what role all of these play in various brain diseases,” said Emeran Mayer, a medical psychologist at the University of California, Los Angeles who studies the brain and gut microbiome and has written “The Mind-Gut Connection “If you already have genetic risk factors for Parkinson’s disease or Alzheimer’s or major depression, this is a factor that could push you over the edge and turn it into a disease.” .
Not so many years ago, the only microbes that attracted medical attention were germs that caused infections and disease.
But the indiscriminate use of antibiotics and other sanitation measures eliminated the damage caused by bacteria at the expense of the protection they can provide. Unintended health consequences ranged from increased liver disease, type 2 diabetes and asthma to premature birth and antibiotic-associated diarrhea, according to a 2019 review published in the Journal of Experimental Medicine and many other microbiology studies. .
Over the past decade, advances in low-cost, high-speed gene sequencing machines have allowed researchers to study millions of microorganisms that cannot normally be grown in a laboratory. In these studies, researchers can determine if the genetic material belongs to bacteria, although a biomarker called the ribosomal RNA gene from the 16s, which only appears in microbes.
As a result, the study of the microbiome is one of the hottest new fields in medicine, with more than 15,000 scientific papers published last year alone. “There’s a lot of excitement now in the field of psychiatry about this,” said John Cryan at University College Cork in Ireland, which studies the microbiome and neurobiology of stress.
Microbiologists estimate that the human gut contains more than 100 trillion microorganisms. Together they weigh about 5 pounds, about a large mango and a little more than the human brain, according to the European Society of Neurogastroenterology and Motility.
In addition, where the human genome carries about 22,000 genes encoding proteins, researchers estimate that the human microbiome provides about eight million genes encoding unique proteins, or 360 times more bacterial genes than human genes, according to the Human Institutes’ Microbiome Project. National Health. .
These microbes seem especially adaptable to changes in the environment, diet, and the biochemistry of emotions. Although no one still knows exactly why, patients with various psychiatric disorders, including depression, bipolar disorder, schizophrenia, and autism spectrum disorder, have significant alterations in the composition of their gut microbiome.
Microbes appear to be in almost constant communication with the brain directly affecting nerve signals and indirectly through chemicals absorbed into the bloodstream, Dr. Gilbert, who is also a scientific advisor to a small microbiome company called Holobiome in Cambridge, Mass. seeks new ways to treat depression, insomnia and other illnesses.
Some common intestinal bacteria, for example, help generate neurotransmitters such as serotonin, which affects mood-related neuronal activity and memory. It is commonly used to treat depression. Others make an amino acid called gamma-aminobutyric acid that naturally blocks some brain signals. It is used in medications to relieve anxiety and improve mood.
“Bacteria hijack parts of the body’s systems that we know affect emotional regulation,” Dr. Cryan said. “This has led us to the idea that by targeting gut microbes, we can have behavioral effects that will impact overall well-being.”
Write to Robert Lee Hotz at [email protected]
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