SEROTONIN: FUNCTION, PRODUCTION, STRUCTURE - NEUROPSYCHOLOGY - 2025

The Serotonin is a neurotransmitter known as the hormone of happiness, well - being hormone or hormone of love. It occurs mainly in the brain regions and in some parts of the body.

Serotonin is one of the neuronal substances that has motivated more scientific research, since it is one of the most important neurotransmitters in human beings; It has an especially relevant role in regulating the mood and mood of people.

Serotonin neuron. Above is how serotonin is released into the synaptic space from the axon and reuptake. Below is the dendrite with the receptors.

Serotonin is a chemical that is synthesized in the brain, so it is a neurotransmitter, that is, an element that performs a series of brain activities. More specifically, it is a neurotransmitter monoamine.

Although serotonin can also transcend neuronal regions and circulate through other regions of the body, this substance is scientifically interpreted as a neurotransmitter and, in some cases, as a hormonal neurotransmitter.

Where is serotonin produced?

Serotonin molecule

Serotonin (5-HT) is produced mainly in brain regions and in some parts of the body. Specifically, this monoamine is synthesized in the serotonergic neurons of the central nervous system and in the enterchromaffin cells of the gastrointestinal tract.

At the brain level, neurons in the raphe nucleus, a cellular aggregate that makes up the medial column of the brainstem, constitute the epicenter of 5-HT production.

Serotonin is synthesized through L-Tryptophan, an amino acid included in the genetic code that involves the action of important enzymes. The main enzymes are tryptophan hydroxylase (TPH) and amino acid decarboxylase.

Chemical synthesis of serotonin. Source: Evelin Kinari Medina via Wikimedia

With regard to tryptophan hydroxylase, we can find two different types, the TPH1 that is found in various tissues of the body, and the TPH2 that is found exclusively in the brain.

The action of these two enzymes allows the production of serotonin, so when they stop acting, the synthesis of the neurotransmitter stops completely. Once 5-HT is produced, it must be transported to the relevant brain regions, that is, to the nerves of neurons.

This action is carried out thanks to another brain substance, the SERT or 5HTT transporter, a protein that is capable of transporting serotonin to its target nerve.

This transporter is also an important regulator of brain serotonin, since no matter how much it is produced, if it is not transported to the relevant regions, it will not be able to carry out any activity.

Thus, in general, for serotonin to be generated and act in brain regions, the action of two amino acids and a neuronal protein is required.

Serotonin pathways

Source: BruceBlaus via Wikimedia Commons

Within the central nervous system, serotonin acts as a nerve impulse neurotransmitter, with the neurons in the raphe nuclei being the main source of release.

The nucleus of the raphe is a set of neurons located in the brain stem, a place from which the cranial walls start.

The axons of neurons raphe nuclei, that is, the parts of neurons that allow information to be transmitted, establish important connections with critical areas of the nervous system.

Regions such as the deep cerebellar nuclei, the cerebellar cortex, the spinal cord, the thalamus, the striated nucleus, the hypothalamus, the hippocampus or the amygdala are connected thanks to the activity of 5-HT.

As we can see, serotonin starts from a specific region of the brain but quickly spreads through multiple structures and parts of this organ. This fact explains the large number of functions that this substance performs and the importance it contains to establish optimal brain function.

These multiple indirect effects on different areas of the brain also explain a large part of its therapeutic actions.

Neurotransmission of serotonin

Neurotransmission process. Source: Adert ​​via Wikimedia Commons)

Serotonin is released at the presynaptic terminal of neurons, from where it enters the intersynaptic space (the space in the brain between neurons) and acts by binding to specific postsynaptic receptors.

Specifically, in order to communicate from one neuron to another, serotonin must bind to three 5-HT receptors when it is in the intersynaptic space.

In summary: a neuron releases serotonin, this remains in the space between neurons and when it binds to the 5-HT receptor it manages to reach the next neuron. Thus, one of the key elements for the proper functioning of serotonin are these specific receptors.

In fact, many drugs and psychotropic drugs act on this type of receptor, a fact that explains the ability of these elements to produce psychological changes and provide therapeutic effects.

Serotonin functions

Serotonin is probably the most important neurotransmitter in humans. It carries out a large number of activities and performs functions of vital importance for the well-being and emotional stability.

Although it is often known as the substance of love and happiness, the functions of serotonin are not limited to the regulation of mood. In fact, they perform many more actions which are also vitally important for the optimal functioning of both the brain and the body.

This substance that begins in the nuclei of the raphe, transcends to many and very diverse regions of the brain. Thus, it acts both in higher regions such as the hippocampus, the amygdala or the neo-cortex, as well as in more internal regions such as the thalamus, the hypothalamus or the nucleus accumbens, and even participates in more primary regions such as the spinal cord or the cerebellum.

Raphe nuclei (in green). Source: Patrick J. Lynch, medical illustrator via Wikimedia

As is well known, the functions performed by the higher regions of the brain are very different from those performed by the more internal structures, so serotonin can be expected to perform very different functions. The main ones are:

Mood

It is probably the best known function of serotonin, which is why it is known as the hormone of happiness. The increase in this substance almost automatically produces a feeling of well-being, increased self-esteem, relaxation and concentration.

Serotonin deficiencies are associated with depression, suicidal thoughts, obsessive compulsive disorder, insomnia, and aggressive states.

In fact, the majority of drugs to treat these diseases, the SSRI antidepressants, act specifically on serotonin receptors to increase the amounts of this substance in the brain and reduce symptoms.

Bowel function

Despite being considered a neurotransmitter, this substance also carries out activities on a physical level, which is why many consider it a hormone.

Leaving aside the nomenclature with which we refer to serotonin, whether it is a hormone or a neurotransmitter, it has been shown that in the body, the highest amounts of this substance are found in the gastrointestinal tract.

In fact, the large amount of serotonin located in the intestines has made it possible to characterize the gastrointestinal serotonergic system. In this region of the body, 5-HT is responsible for regulating bowel function and movements.

This substance is postulated to play a major role in nutrient absorption, motor activity, and water and electrolyte secretion.

Likewise, serotonin has been described as an important transducer of intestinal luminal information, in such a way that stimuli from the intestinal lumen cause its release, which generates motor responses, secretory, and vascular vasodilator reflexes.

Coagulation

Another of the most important physical functions of serotonin lies in the formation of blood clots. When we suffer a wound, platelets release serotonin automatically to initiate the relevant endogenous regeneration processes.

Thus, when serotonin is released, a vasoconstriction occurs, that is, the arterioles (small arteries) narrow more than normal.

This narrowing allows blood flow to be reduced, contributes to clot formation and, therefore, mitigates bleeding and loses less blood.

If we did not have serotonin in our body, we would not experience vasoconstriction when we wound ourselves and you could lose blood in a dangerous way.

Body temperature

Serotonin also performs basic maintenance functions of the integrity of our body. In this way, it plays an important role in body homeostasis through thermal regulation.

This function is a very delicate balance since a difference of a few degrees in body temperature can lead to the massive death of large groups of cell tissues.

Thus, serotonin allows body temperature to be modulated in such a way that, despite internal or external factors to which the body is exposed, it can maintain a thermal regulation that allows the survival of the body's cells.

Nausea

When we eat something toxic, irritating or that our body does not tolerate properly, the intestine increases the production of serotonin to increase intestinal transit.

This fact allows the body to expel the irritant in the form of diarrhea, as well as to stimulate the vomiting center of the brain to ensure that the substance is evacuated from the body.

Bone density

Studies conclude that persistently high spindle serotonin levels can cause an increase in osteoporosis.

The mechanism of action of the substance that could cause this effect has not yet been precisely described, but correlational studies have been carried out that allow the excess of serotonin in the bones to be associated with the appearance of this disease.

Pleasure

You could say that apart from being the hormone of humor or happiness, serotonin is also the hormone of pleasure. In fact, along with dopamine, it is the main hormone that allows us to experience gratifying sensations.

Thus, for example, after orgasm (both female and male), people release a greater amount of serotonin in different brain regions and, as a consequence, we experience high sensations of pleasure.

Likewise, drugs such as ecstasy, methamphetamine or LSD act on the serotonergic systems, providing feelings of pleasure and increasing the addictive potential of the substances.

Sexuality

A correlation between serotonin levels and sexual libido has been proven.

High serotonin levels reduce anxiety and impulsivity, but also sexual desire, which explains why many antidepressant medications can lower libido.

Likewise, the pleasure provided by the release of 5-HT has also been associated with the generation of feelings and emotions of love.

Dream

Serotonin promotes the release of melatonin, a substance that promotes sleep. During the day, we have high amounts of serotonin in the brain, a fact that allows us to gradually release greater amounts of melatonin.

When melatonin is very abundant, sleep occurs, and when we go to sleep, serotonin levels decrease to interrupt the production of melatonin.

Satiety

Studies carried out in humans indicate that the activation of serotinergic receptors induces a reduction in food intake and appetite.

In this way, serotonin regulates eating behavior through satiety, so high levels of this substance can reduce hunger, while low levels of serotonin can increase it.

References

1. Acuña-Castroviejo D, Escames G, Venegas C, Díaz-Casado ME, Lima-Cabello E, López LC, Rosales-Corral S, Tan DX, Reiter RJ. Extrapineal melatonin: sources, regulation, and potential functions. Cell Mol Life Sci 2014 [Epub ahead of print.
2. Bonasera SJ. and Tecott LH. Mouse models of serotonin receptor function: toward a genetic dissection of serotonin systems. Pharmacol Ther 2000; 88 (2): 133-42.
3. Lam DD. and Heisler LK. Serotonin and energy balance: molecular mechanisms and implications for type 2 diabetes. Expert Rev Mol Med 2007; 9 (5): 1-24.
4. Kim H., Toyofuku Y., Lynn FC., Chak E., Uchida T., Mizukami H., et al. Serotonin regulates pancreatic beta cell mass during pregnancy. Nat Med 2010; 16 (7): 804-8.
5. Walther DJ., Peter JU., Bashammakh S., Hortnagl H., Voits M., Fink H., et al. Synthesis of serotonin by a second tryptophan hydroxylase isoform. Science 2003; 299 (5603): 76