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Serotonin Research

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Serotonin Research

Serotonin is one of the most well-known neurotransmitters (made from the amino acid tryptophan) and has multiple effects on the brain. According to research, it is key in mood stabilisation, and it is also known for being the happy hormone. It aids in feelings of happiness you may have throughout your day. It functions as a neurotransmitter in your central nervous system, and a hormone in your peripheral nervous system.

Serotonin Research Chemical form of serotonin StudySmarterThe chemical form of Serotonin.

Serotonin also helps in:

  • Sleeping.

  • Digestion/eating.

  • Healing wounds.

Serotonin levels affect your mood and other bodily functions. Normal levels of serotonin usually mean that you will feel relaxed, relatively happy and calm, and more focused. You will generally feel comfortable.

Depending on how drastically and how long serotonin levels change, problems can begin to arise. Simply having fluctuating levels is normal. Increases and decreases occur naturally, and they are why we can react to certain situations appropriately.

Serotonin levels affect sleep. For instance, serotonin is associated with melatonin synthesis, and it also reduces REM sleep (rapid eye movement sleep), which aids in the natural sleep cycle. In some cases, low levels of serotonin result in insomnia.

Serotonin and aggression

As you can see, research on serotonin indicates it plays an important role in mood regulation.

The orbitofrontal cortex (OFC) is associated with self-control and higher cognitive functions. Normal levels of serotonin in the OFC are a large part of the reason why you can inhibit impulsive behaviours. The OFC sends important messages to other areas of the brain, notably the amygdala, for emotional processing.

The amygdala (involved in the limbic system) is important in aggression, and as you can read in some of our other explanations, if stimulated it can induce aggressive behaviours.

If we connect all of this information, we can see that:

  • Serotonin is a mood regulator: it inhibits or dampens certain moods, in this case, aggression.

  • If serotonin levels are low in the OFC, impulsive behaviours are not regulated or inhibited.

  • In some cases, the amygdala may have been stimulated, and it will send aggressive impulses to the OFC.

  • If serotonin levels are indeed low, these impulses ARE NOT inhibited, and the person will act on these impulses.

This is the serotonin-deficiency hypothesis. According to it, the causes for serotonin deficiency can vary. The OFC is unable to control these aggressive impulses coming from the limbic system.

Serotonin Research Brains and Aggression StudySmarterA happy brain and sad brain, flaticon.com.

When you describe or discuss a theory in an exam, these studies will help you gain marks in your evaluations of these theories. So, try to remember as many as you can and what theories they support!

Consider the following studies:

Coccaro et al. (2007)

In this study, ten people suffering from intermittent explosive disorder (IED, a disorder associated with reactive aggressive behaviour where people are known to perform poorly on facial emotion recognition tasks) were compared with ten healthy individuals (the control group). They were asked to react to emotional faces whilst being measured by an fMRI. The researchers measured and compared amygdala and OFC reactivity to faces.

  • The activation levels were then also measured against previous aggressive behaviours from the individuals.

  • Those with IED had exaggerated amygdala reactivity and diminished OFC activation to angry faces, compared to the controls. The findings show an amygdala-OFC dysfunction in response to processing angry faces, supporting the connection between the OFC and the amygdala.

Mann et al. (1990)

In this study, 35 participants were given a questionnaire that assessed their aggression levels. They were also given the drug dexfenfluramine, which lowers serotonin levels in the brain. As we know, serotonin is important in mood regulation.

  • Aggression levels rose in males but not in females following the administration of dexfenfluramine.

  • This shows there is a difference in how serotonin and aggression work within the different sexes (an issue with beta bias, something which should be acknowledged in other studies!)

Brown et al. (1979)

In this study, military men who were struggling to settle into military life had their cerebrospinal fluid levels measured for certain factors, notably serotonin (5HIAA, where serotonin has been broken down).

  • 5HIAA levels had a significantly negative correlation with a history of aggressive behaviours in these men. To put it simply, they had lower levels of this byproduct of serotonin being broken down.

  • These low levels indicate that there’s less serotonin in the brain, which, as previously established, could explain why they had a history of aggression.

Deneris et al. (2003)

In this study, the PET-1 gene was found to be associated with the neurone development of serotonin.

  • Mice lacking in the PET-1 gene had serotonin neurones that failed to differentiate, and any remaining neurones had issues with serotonin synthesis, uptake, and storage.

  • This resulted in the mice having anxious and aggressive behaviours, suggesting insufficient systems for serotonin production and function are key aspects of aggressive behaviours, at least in mice.

Delville et al. (1997)

In this study, they tested the effects of fluoxetine (a drug that inhibits serotonin reuptake) on mice, assessing aggressive behaviours such as biting.

  • They found that after taking fluoxetine, the mice took a longer time to bite or act aggressively.

  • This suggests that when serotonin remains in the synaptic cleft for longer, it allows for a heightened mood regulation. Specifically, it reduced aggressive behaviours.

However, this is not to say serotonin plays a purely inhibitory role in aggression. This can be seen in the MAOA gene.

You may have come across the links between research on serotonin and depression. This is because depression, when you look at its basic components, is a mood disorder. Those with depression often show low levels of activity in serotonin pathways, which is why they have issues with feelings of sadness, hopelessness, and lack of motivation. Left untreated, this can result in depression.It’s why a lot of treatments for depression involve SSRIs: selective serotonin reuptake inhibitors. They keep serotonin in the synaptic clefts for longer (this is what we mean when we say reuptake inhibitors. They inhibit the ‘reuptake’ of serotonin), to increase the effects of serotonin overall.

When a person takes drugs involving serotonin issues such as serotonin syndrome can occur. The person may feel confused, agitated, have twitching muscles, amongst other symptoms.

Evaluation of Serotonin research on aggression

It is important to evaluate the research on the relationship between serotonin and aggression to establish how reliable and valid it is to make assumptions.

Crockett et al. (2012)

In this study, the diets of 30 random, healthy individuals were manipulated. This altered their serotonin levels.

  • This was done by giving them amino acid mixtures with low levels of tryptophan, the building blocks of serotonin, on the serotonin depletion day. They were given the same mixture but with normal levels of tryptophan on the control day.

  • They were then asked to view emotional faces (angry, neutral, sad) whilst being scanned using an fMRI. This allowed them to see how different regions of the brain communicate with each other in reaction to the stimulus.

  • Researchers found that on serotonin depletion days, the low levels of serotonin resulted in weakened communication between the amygdala and frontal cortex.

  • Those who were naturally more aggressive (established through a personality questionnaire) had an even weaker communication problem between these two areas.

  • Researchers suggest that these low serotonin levels are the reason why the frontal cortex struggles to inhibit and control the amygdala’s aggressive impulses. Without serotonin inhibiting the impulsive responses being sent, they reacted more aggressively to stimuli.

  • This study has high validity, as it used randomised participants and was a double-blind study.

Krakowski (2003)

Krakowski argued that it is difficult to establish a causal relationship between serotonin and aggression.

  • Overall, he suggested that the influence of serotonin in aggression depends on multiple factors, namely the individual's own self-control on their impulses, their emotional regulation, and the social situation at hand.

  • When dysfunction in serotonin occurs in aggressive individuals, whether this results in aggressive behaviours depends on individual differences and the social context the individual is in.


Serotonin Research - Key takeaways

  • Serotonin is one of the most well-known neurotransmitters and has multiple effects on the brain. It is key in mood stabilisation, and it is also known for being the happy hormone.
  • The orbitofrontal cortex (OFC) is associated with self-control and higher cognitive functions. Normal levels of serotonin in the OFC are a large part of the reason why you can inhibit impulsive behaviours.
  • The amygdala (involved in the Limbic System) is important in aggression, and if stimulated it can induce aggressive behaviours.
  • If serotonin levels are low in the OFC, impulsive behaviours are not regulated or inhibited when aggressive impulses are sent from the amygdala (serotonin deficiency hypothesis).
  • Crockett et al. (2012) suggested that low serotonin levels are the reason why the frontal cortex struggles to inhibit and control the amygdala's aggressive impulses.
  • Similarly, Brown et al. (1979) showed that low levels of serotonin (through low levels of 5HIAA in spinal fluid) had a significantly negative correlation with a history of aggressive behaviours in the men they studied.

Frequently Asked Questions about Serotonin Research

Serotonin is a neurotransmitter that acts as a mood regulator and has widespread inhibitory effects on the brain.

Serotonin can be triggered or released in several ways: through experiencing happy thoughts, memories, or events, through being in sunlight, and through exercise, to name a few. It is made up of tryptophan, which is found in nuts and red meat.

Yes, although it is gradual.

Depending on where in the body it is, serotonin can act as a mood regulator, and is important in feeling relaxed and happy. It also acts as a widespread inhibitor in the brain (as a neurotransmitter). As a hormone, it aids in eating, digestion, and feeling nausea, amongst other functions.

Through regulation of your moods (feeling relaxed, calm, and happy), this can result in you being able to focus more. This will then help you study.


StudySmarter is not a licensed medical practitioner. This information is for educational purposes only

Final Serotonin Research Quiz

Question

What is serotonin?

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Answer

It is a neurotransmitter in the central nervous system and a hormone in the peripheral nervous system.

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Question

What is serotonin key in?

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Answer

Mood regulation and feeling relaxed, calm, and happy.

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Question

What widespread effect does serotonin have on the brain?

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Answer

An inhibitory effect.

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What else can serotonin aid in?


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Answer

Sleeping, digestion, eating, healing wounds.

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Question

What amino acid is the building block of serotonin?


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Answer

Tryptophan.

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Question

How is the orbitofrontal cortex involved in aggression?


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Answer

When serotonin levels are normal, it helps inhibit impulsive, aggressive behaviours received from the amygdala.

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What happens if serotonin levels are low in the OFC?


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Answer

It cannot control or regulate impulsive, aggressive behaviours properly.

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Question

What is the serotonin-deficiency hypothesis?


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Answer

Decreased levels of serotonin cause a reduced inhibitory effect, affecting the OFC and causing more impulsive and aggressive behaviours.

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Question

What did Crockett et al. (2012) find in their study?

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Answer

Lowering serotonin levels resulted in weakened communication between the amygdala and frontal cortex. Their studies suggest that low serotonin levels are the reason why the frontal cortex struggles inhibiting and controlling the amygdala's aggressive impulses.

Show question

Question

What did Coccaro et al. (2007) find?


Show answer

Answer

Those with IED had exaggerated amygdala reactivity and diminished OFC activation to angry faces, compared to the controls. The findings show an amygdala-OFC dysfunction in response to processing angry faces, supporting the connection between the OFC and the amygdala.

Show question

Question

What did Brown et al. (1979) find in their study?


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Answer

Low levels of 5HIAA were found in the spinal fluid of military men with aggressive behaviour tendencies. 5HIAA levels had a significantly negative correlation with a history of aggressive behaviours in these men. To put it simply, they had lower levels of this byproduct of serotonin being broken down.


Show question

Question

How is serotonin associated with melatonin?


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Answer

Serotonin is associated with melatonin synthesis.

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Question

What can low levels of serotonin do to your sleeping patterns? 


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Answer

It can cause insomnia.

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Question

What happens if you have normal levels of serotonin?


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Answer

You have little or no aggressive behaviours and can feel relaxed and in control of your emotions.

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Question

Do serotonin and aggression have a causal relationship?


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Answer

No, context matters. Both the individual and their social environment affect the aggressive behaviours, even with abnormal serotonin levels.

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