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Serotonin is a chemical that has a wide variety of functions in the human body. It is sometimes called the happy chemical, because it contributes to wellbeing and happiness.
The scientific name for serotonin is 5-hydroxytryptamine, or 5-HT. It is mainly found in the brain, bowels, and blood platelets.
Serotonin is used to transmit messages between nerve cells, it is thought to be active in constricting smooth muscles, and it contributes to wellbeing and happiness, among other things. As the precursor for melatonin, it helps regulate the body’s sleep-wake cycles and the internal clock.
It is thought to play a role in appetite, the emotions, and motor, cognitive, and autonomic functions. However, it is not known exactly if serotonin affects these directly, or if it has an overall role in co-ordinating the nervous system.
It appears to play a key role in maintaining mood balance. Low serotonin levels have been linked to depression.
Serotonin is created by a biochemical conversion process that combines tryptophan, a component of proteins, with tryptophan hydroxylase, a chemical reactor. Together, they form 5-hydroxytryptamine (5-HT), or serotonin.
Serotonin is most commonly believed to be a neurotransmitter, although some consider it to be a hormone. It is produced in the intestines and the brain. It is also present in the blood platelets and the central nervous system (CNS).
As it occurs widely throughout the body, it is believed to influence a variety of body and psychological functions.
Serotonin cannot cross the blood-brain barrier, so any serotonin that is used inside the brain must be produced inside the brain.
As a neurotransmitter, serotonin relays signals between nerve cells, or neurons, regulating their intensity.
It is believed to play a key role in the central nervous system (CNS) and in the general functioning of the body, and especially the gastrointestinal (GI) tract. Studies have found links between serotonin and bone metabolism, breast milk production, liver regeneration, and cell division.
Serotonin influences most brain cells both directly and indirectly.
Bowel function: Most of the body’s serotonin is found in the GI tract, where it regulates bowel function and movements. It also plays a part in reducing the appetite while eating.
Mood: In the brain, serotonin impacts levels of mood, anxiety, and happiness. Illicit mood-altering drugs such as Ecstasy and LSD cause a significant rise in serotonin levels.
Clotting: Serotonin contributes to the formation of blood clots. It is released by platelets when there is a wound. The resulting vasoconstriction, or narrowing of the blood vessels, reduces blood flow and helps blood clots to form.
Nausea: If you eat something that is toxic or irritating, the gut produces more serotonin to increase transit time and expel the irritant in diarrhea. This also stimulates the nausea area in the brain, resulting in nausea.
Sexual function: Serotonin appears to inhibit sexual activity. Selective serotonin reuptake inhibitors (SSRIs) boost serotonin levels in people with depression, but between 20 and 70 percent of people who take them experience a range of symptoms related to sexual dysfunction.
It is not clear precisely what causes depression, but a key theory in the last 50 years is that it may involve an imbalance of neurotransmitters or hormones in the body.
Depression has been linked to low levels of serotonin, but whether this contributes to depression or results from it remains unclear.
SSRIs are approved by the Food and Drug Administration (FDA) to treat depression. They are the most commonly prescribed antidepressants. Examples include fluoxetine (Prozac), citalopram (Celexa) and sertraline (Zoloft).
Normally, once a neurotransmitter has transmitted its neural impulse is reabsorbed into the body. SSRIs prevent the serotonin from being reabsorbed, leading to higher levels of serotonin in the synapses.
However, scientists are now questioning the role of serotonin or any single neurotransmitter in triggering depression.
Low levels of serotonin have been linked with:
- poor memory
- low mood
They may also lead to the following symptoms:
- craving for sweet or starchy foods
- difficulty sleeping
- low self-esteem
These are common symptoms of depression, but there appears to be little evidence linking them directly to low serotonin levels.
It may also be that this is only true for people who are already susceptible to the symptoms of depression.
When people use recreational drugs, such as MDMA and ecstasy, large amounts of serotonin are released. This can lead to serotonin depletion and a low mood, confusion, and other symptoms that last several days.
Studies have also shown that these drugs may also damage the nerves that contain serotonin, with possible long-lasting adverse effects.
SSRIs increase serotonin levels by preventing serotonin neurotransmitters from being reabsorbed. Serotonin levels remain high in the brain. This is believed to elevate mood.
SSRIs that the FDA has approved to treat depression are:
- Citalopram (Celexa)
- Escitalopram (Lexapro)
- Fluoxetine (Prozac)
- Paroxetine (Paxil, Pexeva)
- Sertraline (Zoloft)
- Vilazodone (Viibryd)
Does serotonin really relieve depression?
SSRIs have been used since the 1980s to treat depression by boosting serotonin levels.
Medication such as SSRIs are believed to relieve the symptoms of depression by boosting serotonin levels in the body, but exactly how this works is unclear.
Some scientists have suggested that, while SSRIs seem to work for some people, increasing serotonin levels is unlikely to directly improve symptoms of depression.
One problem is that it is possible to measure serotonin levels in the bloodstream, but not in the brain. Researchers do not know whether serotonin levels in the bloodstream reflect the serotonin levels in the brain, or if SSRIs can really affect the brain.
In 2014, a mouse study suggested that serotonin may not play a role in depression. Researchers created some mice that were not able to produce serotonin in their brains. The mice showed no signs of depression, even when placed under stress.
In 2015, however, other scientists found that mice that lacked serotonin were more susceptible to social stressors than healthy control mice.
While SSRIs appear to help some people with depression, some scientists now argue that “simple biochemical theories that link low levels of serotonin with depressed mood are no longer tenable.”
One editorial published in the BMJ in 2015 refers to the use of SSRIs to treat depression as “the marketing of a myth.”
SSRIs are also used to treat symptoms of anxiety, panic disorder, and obsessive-compulsive disorders.
Adverse effects of SSRIs
SSRIs have some side effects, but these usually decrease after 2 to 4 weeks of use.
- weight loss or gain
- increased sweating
- sleepiness or insomnia
- dry mouth
- suicidal thoughts
- sexual dysfunction
SSRIs and suicide
SSRIs do not begin to work on depression for a couple of weeks. During the first month of treatment it is possible for symptoms of depression to worsen before positive changes are noticed. Report any suicidal thoughts immediately.
There are reports of some younger users of Zoloft experiencing suicidal ideation.
The FDA requires all antidepressants to carry a black-box warning about the danger of suicide during the initial stages of treatment, especially in children.
Other uses of SSRIs
SSRIs have been used for conditions other than depression.
Antiemetics: The antiserotonergic drugs that act on the serotonin receptors, such as ondansetron (Zofran), are important for treating nausea caused by chemical toxins, including drugs used in chemotherapy and general anesthesia. They act centrally on the part of the brain involved with nausea.
Migraine: Serotonergic vasoconstrictive antimigraine drugs, or triptans, such as almotriptan (Axert), can reduce migraine symptoms and are well tolerated.
Appetite suppressants: Serotonergic appetite suppressants, such as fenfluramine (Pondimin) and chlorphentermine, have been used for reducing appetite, but they are no longer licensed for this purpose.
Parkinson’s disease: The serotonergic system has been linked to cognition, emotion, and motor behavior. Changes in this system may therefore have an effect on the motor and non-motor features commonly associated with Parkinson’s disease. Serotonergic drugs have been used to treat Parkinson’s disease. They are no longer licensed, but research continues in this area.
Premenstrual syndrome (PMS): The cause of this condition is unknown, but an oversensitivity to one of the female hormones, progesterone, seems to reduce the level of serotonin in the brain. Serotonin inhibitors are sometimes used to relieve symptoms around the time when they occur.
Other possible uses of SSRI-type drugs include the treatment of obesity and irritable bowel syndrome (IBS).
Serotonin syndrome, or serotonin toxicity, can happen if a person takes two serotonin-boosting drugs at the same time.
There is excessive stimulation of the CNS and peripheral serotonin receptors. It is usually the result of a drug interaction, but at least one medical condition can trigger it.
Drug interactions: The use of prescribed medications, illegal drugs, or dietary supplements can lead to serotonin syndrome, for example, if a person is susceptible or if two drugs that affect serotonin levels are taken at the same time. Symptoms can appear 6 to 8 hours after taking the drug.
The FDA have warned, for sample, against taking SSRIs with triptans, a medication to treat migraine, because of the risk of serotinin syndrome.
Drugs that target serotonin levels cannot be given with MAOI’s. MAOIs are another approach to treating depression and work on different neurotransmitters and their receptors.
Recreational drugs: The use of recreational drugs, such as ecstasy and MDMA, leads to a high and sudden release of serotonin. If used with prescribed serotonergic drugs, this can increase the risk of serotonin syndrome.
Carcinoid tumors: These cancerous tumors, commonly found in the GI tract, can cause too much serotonin to be released. Most carcinoid tumors have no symptoms. They are often found during tests or procedures for other conditions.
Serotonin syndrome leads to excessive nerve activity.
Signs and symptoms include:
- agitation and restlessness
- increased heart rate and blood pressure
- pupil dilation
- loss of muscle co-ordination
- muscle rigidity
Severe serotonin syndrome is life-threatening in 2 to 12 percent of cases.
Symptoms include a high fever, irregular heartbeat, seizures and unconsciousness.
The use of a single drug does not usually result in serotonin toxicity.
Treatment focuses on managing symptoms and attempting to restore normal serotonin levels.
Discontinuing the medication may be enough to stop the symptoms.
Severe serotonin syndrome may need hospital treatment. Medications can be given to relax or paralyze the muscles, control heart rate and blood pressure, and, in some cases, to block the production of serotonin.
Oxygen and intravenous fluids may be given to maintain normal levels of oxygen in the bloodstream and to treat fever and dehydration.
As well as SSRIs and illegal drugs, there may be other ways of increasing serotonin levels in the body.
Mood induction: Alterations in thought, either through psychotherapy or self-induction, could increase levels of serotonin if the interaction between serotonin synthesis and mood is a two-way relationship.
Light: Already used as a treatment for seasonal affective disorder (SAD), a few studies have suggested that it can be used to treat depression as well.
Exercise: Exercise has an antidepressant effect, and some research has suggested that it can increase brain serotonin function.
Diet: Foods that have higher levels of tryptophans could be linked to improved mood and cognition, possibly due to increased serotonin levels. Also serotonin is available as a supplement, which can be purchased online.
These are all areas that require further study, as current knowledge remains speculative.
Tryptophans are an amino acid that can be found in food. Some research has linked a higher intake of dietary tryptophan with more positive mood scores, possibly because tryptophan boosts serotonin levels.
High-protein foods: Some foods, such as turkey, eggs, and cheese, are believed to contain tryptophan and to increase tryptophan levels in the blood.
Bananas: These contain serotonin, and they have been recommended for lifting mood. But they can only improve a person’s mood if the serotonin they contain will reach the brain.
Tryptophan is a precursor, a main ingredient that the body needs to make serotonin. Eating foods high in this essential chemical doesn’t mean that the body will absorb and use it. But making tryptophan available when needed will improve serotonin synthesis.
In some studies, however, older people who were given tryptophan supplements performed better on cognitive tests than they had before.
There is a growing interest in the idea that gut microbiota might influence the CNS and cognition through a link known as the gut-brain access. In this case, serotonin in the digestive system may be able to influence mood.
The following foods are said to contain tryptophans:
- soya products
- talbina, a dish made from barley
However, while foods may contain tryptophan, it may be in too small an amount to make a difference. Most studies have used supplemental forms of tryptophan.
A protein known as α-lactalbumin, for example, is found in milk. It contains more tryptophan than most proteins. However, scientists suggest that it is probably not practical to consume the amount that would be needed to make a difference.
In the 1980s, corn that was bred with a higher tryptophan content was successful in preventing pellagra. Population studies suggest that people for whom this corn was a major part of the diet showed fewer aggressive tendencies, although this could have been for other reasons. It is possible that the tryptophans in the corn increased people’s serotonin levels.
Studies in which people consumed tryptophan-enriched drinks and other nutrient-based treatments have produced mixed results.
Until now, there is little evidence that a specific diet can affect mood or depressive symptoms.
However, following a healthful and varied diet has been found to produce a range of health benefits and to boost overall wellbeing.
The 2015-2020 Dietary Guidelines for Americans urge people to focus on a varied diet and to aim to obtain nutrients from food sources as far as possible.
Tryptophan supplements are available online, but they should not be used without first speaking to a doctor, because of the risk of serotonin syndrome.
Much remains unknown about serotonin. The difficulties surrounding the study of brain functions means that it will be some time before a full knowledge of serotonin can be acquired.