What Does Serotonin Do?

Serotonin is a potent neurochemical that plays a role in everything from easing anxiety to regulating blood sugar levels. Researchers have been studying serotonin since 1937, and they're still finding exciting new facts about this important chemical.

Studies of its role in the human body are ongoing, but it's best known for modulating mood and behavior. Increasing available serotonin is one of the most common treatments for depression, anxiety, and other behavioral conditions. 

Scientists have also discovered that most of the serotonin in the body is produced in the gut and that serotonin significantly impacts GI tract function. There is a growing understanding that gut disorders and mood disorders may be more closely linked than previously thought. 

Scientists are exploring how balancing serotonin may improve both mental and gastrointestinal  health. In addition to traditional medications that improve serotonin levels, there is new research on the relationship between probiotic supplements and serotonin in the brain and the gut. 

What Is Serotonin?

Serotonin, also called 5-hydroxytryptamine or 5-HT, is an important chemical neurotransmitter that also acts as a hormone. It is derived from the essential amino acid tryptophan. The body does not produce essential amino acids, so the tryptophan your body needs for serotonin production must come from the food you eat.

About 5% of the serotonin in the body is produced in the brain, and it is known for playing a role in regulating mood. The remainder of the body's serotonin is made in the gut, which is lined with serotonin-producing cells. Some of this gut-produced serotonin is released into the bloodstream and circulates to the rest of the body where it acts as a hormone and intracellular signaling molecule. 

The two supplies of serotonin do not mix, and they can't transfer from the gut to the brain or vice versa. The brain is insulated from peripheral serotonin by the blood-brain barrier (BBB), a protective network of blood vessels that prevents harmful substances and certain biochemicals from affecting brain tissue. Even some medications cannot penetrate the BBB.

Despite this barrier, serotonin in the gut can have an effect on the brain via the gut-brain axis. Serotonin in the gut can bind to receptors on the endings of the vagus nerve that connects the gut and brain. When stimulated by the binding of serotonin in the gut, the vagus nerve typically uses glutamate to transmit the signal to the brain. In this way, the vagus nerve, which stretches from the gut to the brain, can transmit signals from one organ to the other without serotonin having to cross the blood-brain barrier.

Researchers have also discovered that peripheral serotonin affects numerous organ system functions, including cardiovascular function, bowel motility, sexual function, and bladder control.

How Does Serotonin Work?

Neurotransmitters like serotonin are chemicals that act as messengers, sending signals from nerve cells to other cells within the body. Neurotransmitter chemicals reside inside nerve cells, where they await nerve activity. When a nerve cell, sometimes called a neuron, receives a signal, neurotransmitters are released from the nerve cell into the synapse between the nerve and another cell. 

Within the synapse, the neurotransmitter binds to a receptor on the outside of a target cell, which may be a muscle, gland, or nerve cell. This binding prompts the target cell to perform a required action, such as muscle movement or the release of additional chemicals in the body.  For example, when you eat, intestinal serotonin signals the pancreas via the vagus nerve to release bile to help with digestion.

Serotonin in the Brain

Serotonin plays a role in mood, perception, anger, aggression, appetite, memory, sexuality, and attention

While only 5% of the body’s serotonin is produced in the brain, it plays a very important role there. Serotonin receptors exist throughout the brain, and it is known to affect nearly every neuropsychological and behavioral function. Serotonin plays a role in mood, perception, anger, aggression, appetite, memory, sexuality, and attention.

Serotonin Deficiency & Mood Disorders

Lack of proper serotonin levels or mutations in serotonin transporters and/or receptors can result in negative mood and behavioral effects. Serotonin deficits have been linked to psychological conditions such as depression, anxiety, obsessive-compulsive behaviors, panic disorders, phobias, post-traumatic stress disorder, and attention deficit hyperactivity disorder (ADHD). 

There are medications to improve how the body processes serotonin, and they are generally prescribed to manage symptoms of depression. The most common type of medication for serotonin deficiency is a selective serotonin reuptake inhibitor (SSRI). These medications increase the amount of available serotonin for signaling within a synapse.

Serotonin in the Gut

The vast majority of serotonin production in the body takes place in the gut. The research into the full role of serotonin on gastrointestinal (GI) and digestive function is ongoing, but research shows that serotonin plays a role in every aspect of digestion. 

When taste buds detect food, serotonin from taste bud cells initiates a cascade of signals to the brain to create the sensory experience of taste. The mechanical elements of digestion, including the action of food moving through the intestines, are initiated by serotonin, as is the release of digestive fluids to break food down. 

The effects of serotonin also extend outside the intestines to trigger pancreatic enzyme secretion and turn food into energy. 

Serotonin Deficiency & GI Disorders

Doctors and researchers have noted an overlap between people with symptoms of depression or anxiety and those who have chronic digestive problems such as constipation, diarrhea, bloating, pain, and stomach upset. 

Mood regulation and digestion are two of the primary functions affected by serotonin levels and efficacy, and experts have found that those functions are more closely linked than previously thought. Interestingly, people with serotonin deficits can experience bowel disorders, including irritable bowel syndrome, and some doctors have found that prescribing selective serotonin reuptake inhibitors (SSRIs) can help alleviate the GI symptoms.

Excessive Serotonin

While medications, supplements, and medical foods that modulate the activity of serotonin in the body are well tolerated by many people, some are very sensitive. If you have experienced symptoms related to excessive serotonin in the past, be sure to consult with your health care provider before adding a supplement or medical food that may increase serotonin. 

Symptoms of excess serotonin may include:

  • Agitation or restlessness
  • Confusion
  • Diarrhea 
  • Dilated pupils
  • Headache
  • High blood pressure
  • Insomnia
  • Rapid heart rate
  • Shivering or goosebumps
  • Sweating

Is There a Probiotic that Affects Gut & Brain Serotonin?

Researchers have learned that the microbiome in the gut may have substantial effects on certain health conditions. One particular probiotic, Lactobacillus plantarum strain PS128 has been linked to higher brain and gut serotonin levels in preclinical studies. The mechanism behind this is still being studied, but researchers are intrigued by what they have learned.

Early research also shows that certain probiotic strains may affect serotonin-related mood and behavior issues. In one animal study, researchers found that Lactobacillus plantarum strain PS128 had measurable behavior effects in mice. One group of mice was subjected to early life stress (ELS) in the form of maternal separation to trigger enhanced stress responses. Mice in the ELS group showed reduced signs of depression when given PS128, and those without ELS showed reduced anxiety-like behaviors when exposed to PS128. Researchers observed changes to brain serotonin levels and/or metabolism in both sets of mice.

second animal study revealed mice given oral PS128 had increased levels of serotonin and serotonin metabolites in one brain region, but not others. Because the animals in the study were “germ-free” (raised without any microorganisms in their guts), the effects could be attributed to PS128. 

2020 animal study of rats showed that serotonin levels in the hypothalamus were lower when irritable bowel syndrome was present, but ingestion of PS128 correlated with increased serotonin levels. Researchers additionally found that serotonin metabolite levels were higher in the prefrontal cortex in rats with irritable bowel syndrome, but ingesting PS128 lowered the levels.

These preclinical studies indicate that PS128 may modulate serotonin levels and the availability of serotonin for metabolic use in the brain, suggesting that probiotic supplementation may offer solutions to mood disorders and/or gut issues.* Improving both levels and availability of serotonin may have implications for mental health as well as gut health.

How to Boost Serotonin Levels

Salmon and other foods are rich in tryptophan, a precursor to serotonin

For many years, finding ways to increase serotonin levels has been a staple of treating mood disorders. Medications like selective serotonin reuptake inhibitors (SSRIs) have a high degree of efficacy in improving mood disorders such as depression and anxiety. Research shows these drugs increase the availability of serotonin in the body, and recent findings show they have a specific effect on the vagus nerve. Some neurologically active probiotic strains such as PS128 that have shown promising effects on anxiety and depression may also work by affecting serotonin levels or metabolism.  

Serotonin levels can also potentially be boosted by making lifestyle changes. Serotonin is made from dietary tryptophan, and some studies suggest that ingesting food high in tryptophan can increase serotonin levels in the brain, with antidepressant effects. 

Foods that contain high natural levels of tryptophan include:

  • Cheese
  • Eggs
  • Nuts and seeds
  • Oats
  • Pineapple
  • Salmon
  • Tofu
  • Turkey

In addition to dietary changes, exposure to bright light and regular exercise may contribute to higher serotonin levels. 

Serotonin, Mental Health & Gut Health

The role of serotonin is both vital to health and highly complex. The gut-brain axis and serotonin's role in facilitating healthy neurological communication between the two systems may be key to addressing chronic serotonin deficits.

If you’re concerned about your serotonin levels based on mood or sleep issues, talk to your health care provider. There are several options you can explore, including dietary changes, medication, and supplementation. If you’re interested in lifestyle adjustments, you might ask about PS128 probiotics, such as Neuralli Medical Probiotic, to see if the probiotic helps improve your overall wellbeing. 

Recommended reading:

Probiotics for Anxiety and Depression

Do Probiotics Help with Anxiety?

Probiotics for Stress


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