How does serotonin enter the brain?

As a neurotransmitter (messenger substance), serotonin fulfills important tasks in the brain because it transmits information between nerve cells in the nervous system. This information is needed to regulate body temperature and appetite, control sleep-wake cycles, and influence mood and drive. Which process is controlled by serotonin depends on which serotonin receptor the neurotransmitter docks on, because different receptors are responsible for each process.

The largest proportion of serotonin found in the body is formed in the intestine. The basis for this is the amino acid L-tryptophan, which is converted into serotonin in two steps with the help of special enzymes. In the first step, L-tryptophan is converted into the amino acid 5-hydroxytryptophan (5-HTP for short).

5-HTP is found in various types of bananas and in the African black bean (Griffonia). Its seed extracts are often used in dietary supplements to increase serotonin levels.

As an amino acid, 5-HTP itself has no other function than to be converted into serotonin in the body - in the second step of synthesis. Once this process is complete, serotonin can be released as needed. It performs important functions such as contracting blood vessels and controlling intestinal movements. However, it cannot enter the brain because it cannot cross the so-called blood-brain barrier.

What is the blood-brain barrier?

The blood-brain barrier is a kind of barrier that protects the brain. On the one hand, it ensures that the brain is supplied with all important nutrients by allowing them to pass through. Conversely, breakdown products that are no longer needed are also allowed to pass through for removal. However, it is impenetrable to potentially harmful substances such as toxins or even pathogens. Also, not all substances with positive effects (such as serotonin) can cross the blood-brain barrier. For these cases, precursors of the respective substances are typically administered, which can pass the blood-brain barrier and then transform into the desired substance.

How does serotonin enter the brain?

Since serotonin cannot cross the blood-brain barrier, it cannot enter the brain from the outside. Rather, it must be produced directly there, which it is. About 10% of the serotonin in the body is produced in the brain to fulfill its tasks there. The formation of the neurotransmitter corresponds to the synthesis of the messenger substance in the intestine, i.e. it takes place in two steps via the intermediate product 5-HTP.

Conclusion on serotonin in the brain

Serotonin cannot cross the blood-brain barrier, therefore it cannot enter the brain from the body (especially the gut). For this reason, serotonin is also formed in the brain and can take over important tasks right there, such as the regulation of various body processes (sleep-wake rhythm, appetite, body temperature, etc.).