Serotonin is most commonly believed to be a neurotransmitter, although some consider it to be a hormone. As it occurs widely throughout the body, it is believed to influence a variety of body and psychological functions.

Serotonin in the Brain v. in the Body

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.

The majority of your Serotonin is in your body- an estimated 90 percent of the body's serotonin is made in the digestive tract. However, serotonin cannot cross the blood-brain barrier, so any serotonin that is used inside the brain must be produced inside the brain.

Serotonin and Depression

Depression has been linked to low levels of serotonin, but whether low serotonin contributes to depression or results from it remains unclear.

Selective serotonin re-uptake inhibitors (SSRIs) 

SSRIs have been used since the 1980s to treat depression by boosting serotonin levels, which suggests that serotonin levels affect mood.

Normally, once a neurotransmitter has transmitted its neural impulse, the serotonin 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.

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.

Serotonin deficiency symptoms

Low levels of serotonin have been linked with:

• poor memory
• low mood

And may also lead to the following symptoms:

• craving for sweet or starchy foods
• difficulty sleeping
• low self-esteem
• anxiety
• aggression

Recreational drugs

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.

Further notes

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.

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