Serotonin is a neurotransmitter primarily found in the brain, intestines, and blood. In psychology, serotonin plays a significant role in mood regulation, emotional well-being, sleep, appetite, and cognitive functions. Abnormalities in serotonin levels or its neural pathways are often associated with mood disorders like depression and anxiety.
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.
The scientific name for serotonin is 5-hydroxytryptamine (5-HT), a monoamine group neurotransmitter that contains amino acids.
The monoamine group of neurotransmitters plays a role in many functions, such as decision-making, emotions, happiness, rewards, and has 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 continue 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).
Serotonin can act as both excitatory and inhibitory, depending on the type of receptors it binds to and the location within the brain. Its effect is context-dependent.
In some brain regions or circuits, serotonin can increase neuronal activity (excitatory); in others, it can decrease activity (inhibitory). The diverse roles of serotonin in mood, appetite, sleep, and other functions arise from its complex modulatory effects across different brain regions.
There are 15 types of serotonin receptors with 6 families that are G-protein coupled receptors that 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 an 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
One of the key interests in serotonin, and its most well-known function, is its effect on modulating mood. Serotonin is considered to be a natural mood stabilizer, and when functioning normally, it is believed to help people with feeling happy, calm, focused, and emotionally stable.
Serotonin is also thought to regulate anxiety and reduce depressed feelings.
It is important to note that serotonin does not work in isolation and often uses other neurotransmitters, such as dopamine, to help alleviate mood.
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 control 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 awake is dependent 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 understanding 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 experience 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, vitamin 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 reabsorbed into the presynaptic neuron too soon, stopping it from reaching the next neurons during neurotransmission.
Increasing Serotonin Levels
Increasing serotonin levels can enhance mood, alleviate depression, and regulate sleep and appetite.
Proper serotonin balance is essential for emotional and mental well-being.
Often, medication is prescribed to individuals who wish to treat some of the symptoms or mental health conditions associated with low serotonin levels. Below are some of the main types of medications and their functions:
Selective Serotonin Re-uptake Inhibitors (SSRIs)
The most prescribed antidepressant medication is 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 from being reabsorbed into the presynaptic neuron, there will be 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 antidepressants, 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 not to be as tolerable as SSRIs, having more side effects. Similarly, SSRIs have been 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 antidepressants is 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.
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 with a small dosage before potentially increasing this.
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
