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What is Serotonin?

By Olivia Guy-Evans, published June 14, 2021


Key Takeaways
  • Serotonin is a neurotransmitter (chemical messenger) produced within the central nervous system (CNS) that contributes to feelings of happiness.
  • Too little serotonin has shown associations with depressed feelings, sadness, and fatigue. Too much serotonin, however, could result in serotonin syndrome, which could lead to symptoms of restlessness, hallucinations, and confusion.
  • Serotonin is also known as a hormone within the enteric nervous system of the body, primarily found within the gastrointestinal tract (gut).
  • In the enteric nervous system, serotonin plays a role in numerous biological processes such as controlling cardiovascular function, bladder control, and bowel movements.
  • Serotonin in the brain, however, is of interest to psychologists as its role as a neurotransmitter is thought to contribute to many important functions such as playing a role in mood, especially in relation to mood disorders such as depression and anxiety.

Serotonin is a neurotransmitter

The scientific name for serotonin is 5-hydroxytryptamine (5-HT) and is a neurotransmitter of the monoamine group that contain amino acids. The monoamine group of neurotransmitters play a role in many functions such as decision-making, emotions, happiness, rewards, and have associations with mental health conditions as a result.

Within the brain, serotonin mostly originates in the brain stem within a cluster of nuclei called the Raphe nuclei. Serotonergic fibers are then synthesized from the Raphe nuclei and projected to the nucleus accumbens, part of the basal forebrain that is known as the circuit area for rewards. Here it is then projected throughout the brain, including the lobes of the brain, hippocampus, cerebellum, and spinal cord.

During neurotransmission, serotonin is released into the synaptic cleft from the terminals of the presynaptic neuron. When it reaches this gap, the serotonin will either be taken up by serotonin receptors on the postsynaptic neuron and continues down the next neuron via electrical impulses, or the serotonin may get degraded by an enzyme called monoamine oxidase, or it will be taken back up into the presynaptic neuron by the serotonin transporter (SERT).

There are 15 types of serotonin receptors with 6 families that are G-protein coupled receptors which work by mediating cellular responses (5-HT1, 5-HT2, 5-HT4, 5-HT5, 5-HT6, 5-HT7) and 1 family that consists of ligand-gated ion channels (5-HT3) which works to depolarise the plasma membranes. Serotonin is under the classification of an inhibitory neurotransmitter as it has inhibitory effects on the neurons, decreasing the likelihood that the neurons will fire action potential.

This contrasts with excitatory neurotransmitters which have excitatory effects on the neurons. Serotonin does not therefore stimulate the brain; it instead balances out the excessive excitatory effects of other neurotransmitters.


Functions

Behavior

Since serotonin is projected from the brain stem and reaches most regions of the brain, it has a wide array of effects on many aspects of behavior.

The neuropsychological processes modulated by serotonin can include having a role in attention, perception, reward, anger, aggression, memory, motor skills, and appetite. In fact, it is difficult to identify a human behavior that is not regulated by serotonin in some way.

Mood

Since serotonin is projected from the brain stem and reaches most regions of the brain, it has a wide array of effects on many aspects of behavior.

The neuropsychological processes modulated by serotonin can include having a role in attention, perception, reward, anger, aggression, memory, motor skills, and appetite.

In fact, it is difficult to identify a human behavior that is not regulated by serotonin in some way.

Sleep

Elsewhere in the body, serotonin helps with sleep. There are specific regions in the brain that control when we fall asleep, regulate sleep patterns, and controls when we wake up.

The parts of the brain that are responsible for regulating sleep also have serotonin receptors. Serotonin is responsible for stimulating the parts of the brain that control sleep and wakefulness.

Whether someone is sleeping or is awake is dependant on which serotonin receptor is used. Melatonin, which is a hormone vital to the functioning of sleep, requires serotonin so that melatonin can be produced.

Physical health

Outside of the brain, serotonin also has important roles in other parts of the body, with most of the serotonin being found in the gastrointestinal tract rather than the brain.

Serotonin is required in the gut to promote healthy digestion. Similarly, serotonin helps with maintaining bone health, eating, sexual function, and healing wounds by blood clotting.

Low Serotonin

In regard to serotonin within the brain, there are many symptoms that can be associated with low levels of this neurotransmitter:

  • Down in mood or feeling depressed
  • Aggression
  • Feelings of anxiety
  • Irritability and frustration
  • Low self-esteem
  • Problems with memory
  • Poor appetite
  • Issues with sleeping and insomnia in worse cases
  • Impulsivity

Low levels of serotonin have been associated with some mental health conditions, such as mood disorders. As serotonin helps to regulate mood, people with low serotonin may have a low mood or a less stable mood without understand why this is.

If low moods persist because of low serotonin levels, this could result in depression. Depression is categorized as feelings of intense sadness, hopelessness, chronic fatigue, and suicidal thoughts.

Likewise, anxiety disorders can be attributed partly to low serotonin levels. For instance, obsessive-compulsive disorder (OCD) is an anxiety disorder in which an individual uses compulsive behaviors to deal with intrusive anxious thoughts.

Also, people who have schizophrenia, a condition in which individuals may experience unusual thoughts, develop delusions and experiencing hallucinations, has also thought to be related to low serotonin levels.

A cause of low levels of serotonin could be due to not producing enough of this neurotransmitter. An amino acid called tryptophan is essential for the production of serotonin.

This amino acid is only obtained from food, so if there is a deficiency of this, less serotonin will be made as a result. Similarly, vitamins B6 and D deficiencies have been linked to lower levels of serotonin.

Another cause of low serotonin levels can result from not having enough serotonin receptors in the brain, or the receptors not functioning properly.

When serotonin leaves the presynaptic neuron, it could be broken down in the synaptic cleft too quickly or it could be reabsorbed back into the presynaptic neuron too soon, stopping it from reaching the next neurons during neurotransmission.


Increasing Serotonin Levels

The occipital lobes can be divided into several functional areas, although there are no anatomical markers distinguishing these areas.

Brain imaging has revealed that neurons in the occipital cortex create an ongoing visual map of information taken in by the retinas.

Similarly, it is worth noting that the motor cortex plays a role in the muscles of the eyes, which are heavily relied on by the occipital lobes.

Often, medication is prescribed to individuals who wish to treat some of the symptoms or mental health conditions associated with low levels of serotonin. Below are some of the main types of medications and their functions:

Selective Serotonin Re-uptake Inhibitors (SSRIs)

The most prescribed antidepressant medication are selective serotonin re-uptake inhibitors (SSRIs).

These are used to treat conditions such as depression, anxiety, panic disorders, obsessive-compulsive disorders, and phobias. SSRIs work by blocking the re-uptake of serotonin from the neuron that released it.

Since the SSRIs are preventing serotonin being reabsorbed into the presynaptic neuron, there will more serotonin circulating around the synaptic cleft.

This makes it more likely that serotonin will reach the receptors of the postsynaptic neuron, so it will be able to influence the brain and increase mood as a result. Some types of SSRI include Citalopram (Celexa), Fluoxetine (Prozac), and Sertraline (Zoloft).

Tricyclic antidepressants (TCAs)

An older classification of antidepressant, tricyclic antidepressants (TCAs) work in a similar fashion to SSRIs in the sense that they also block the re-uptake of serotonin from returning to the presynaptic neuron.

However, TCAs also work by blocking the re-uptake of another neurotransmitter called norepinephrine (also known as noradrenaline), which also affects mood.

Despite working in a similar way to SSRIs, TCAs are known to not be as tolerable as SSRIs, having more side effects. Similarly, SSRIs have shown to be more effective than TCAs in terms of treating depression with anxiety (Lane, Baldwin, & Preskorn, 1995).

Monoamine oxidase inhibitors (MAOIs)

Another older classification of antidepressant are monoamine oxidase inhibitors (MAOIs).

Typically, when serotonin enters the synaptic cleft for neurotransmission, some of the neurotransmitter gets removed by an enzyme called monoamine oxidase. MAOIs, however, will work to prevent this from happening.

This ultimately means that there will be more serotonin circulating in the synaptic cleft, making it more likely that it will reach the receptors of the postsynaptic neuron.

MAOIs can also have an effect on other neurotransmitters in the brain which can cause unwarranted side effects. This type of antidepressant is not prescribed as much as SSRIs due to the side effects and because of associated dietary precautions that need to be taken when using the medication.

MAOIs could have adverse reactions when mixed with other drugs and in rare cases, can cause dangerously high levels of serotonin, known as serotonin syndrome.


Too Much Serotonin

Although serotonin is beneficial in maintaining a good mood and providing people with happy feelings, too much serotonin can be detrimental.

Having a surplus of serotonin in the brain can come as a result of the medications that are being taken to increase low serotonin levels.

Having too much serotonin in the brain can result in a condition called serotonin syndrome.

This syndrome can arise after starting to take a new medication, or when increasing the dosage of an existing medication. Some of the milder symptoms associated with serotonin syndrome are as follows:

  • Confusion
  • Dilated pupils
  • Restlessness
  • Rapid heart rate
  • High blood pressure
  • Headaches
  • Shivering and goose bumps

Mild cases of serotonin syndrome may go away within a day of stopping the medications causing the symptoms, although if not treated it could result in worsened symptoms such as seizures, irregular heartbeat, unconsciousness, or even death in the worse cases.

Therefore, if someone is considering taking medication for curing symptoms of low serotonin or associated mental health conditions, they would be advised to start off with a small dosage, before potentially increasing this.

About the Author

Olivia Guy-Evans obtained her undergraduate degree in Educational Psychology at Edge Hill University in 2015. She then received her master’s degree in Psychology of Education from the University of Bristol in 2019. Olivia has been working as a support worker for adults with learning disabilities in Bristol for the last four years.

How to reference this article:

Guy-Evans, O. (2021, June 14). What is serotonin . Simply Psychology. https://www.simplypsychology.org/what-is-serotonin.html

APA Style References

Berger, M., Gray, J. A., & Roth, B. L. (2009). The expanded biology of serotonin. Annual review of medicine, 60, 355-366.

Lane, R., Baldwin, D., & Preskorn, S. (1995). The SSRIs: advantages, disadvantages and differences. Journal of psychopharmacology, 9(2_suppl), 163-178.

Mayo Clinic. (2019, October 8). Tricyclic antidepressants and tetracyclic antidepressants. https://www.mayoclinic.org/diseases-conditions/depression/in-depth/antidepressants/art-20046983

White, M. A. (2020, July 13). Serotonin deficiency: Symptoms and treatment. Medical News Today. https://www.medicalnewstoday.com/articles/serotonin-deficiency#symptoms

Scaccia, A. (2020, August 19). Serotonin: What You Need to Know. Healthline. https://www.healthline.com/health/mental-health/serotonin#takeaway

Salters-Pedneault, K. (2021, May 28). What Is Serotonin? Very well mind. https://www.verywellmind.com/what-is-serotonin-425327

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