Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across the synapse from one neuron to another. Different neurotransmitters have different effects on the body and mind. For GCSE Psychology, you need to know about three key neurotransmitters: serotonin, dopamine, and adrenaline (also called epinephrine).

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Serotonin

Serotonin is a neurotransmitter that plays a key role in regulating mood, sleep, appetite, and emotional well-being.

Function	Description
Mood regulation	Adequate serotonin levels are associated with feelings of well-being and happiness
Sleep	Serotonin is involved in regulating the sleep-wake cycle
Appetite	Influences feelings of hunger and satiety
Social behaviour	Affects social interactions and impulse control

Serotonin and Depression

Low levels of serotonin are associated with depression:

• Selective Serotonin Reuptake Inhibitors (SSRIs) — such as fluoxetine (Prozac) — are antidepressant drugs that work by blocking the reuptake of serotonin in the synapse
• This leaves more serotonin available in the synapse, increasing its effect on the post-synaptic neuron
• SSRIs are one of the most commonly prescribed treatments for depression

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Dopamine

Dopamine is a neurotransmitter involved in reward, motivation, pleasure, and movement.

Function	Description
Reward and pleasure	Released when we experience something pleasurable (eating, socialising, achieving goals) — creates feelings of enjoyment
Motivation	Drives us to seek out rewarding experiences
Movement	Helps control voluntary movement — the motor cortex relies on dopamine
Learning	Involved in reinforcement learning — we repeat behaviours that produce dopamine release

Dopamine and Addiction

Dopamine plays a central role in addiction:

• Addictive substances (e.g. nicotine, alcohol, cocaine) cause a surge of dopamine in the brain's reward system
• This creates an intense feeling of pleasure that the person wants to repeat
• Over time, the brain adapts to the high dopamine levels by reducing the number of dopamine receptors (tolerance)
• The person then needs more of the substance to achieve the same effect
• Without the substance, dopamine levels are abnormally low, leading to withdrawal symptoms and cravings

Dopamine and Parkinson's Disease

Parkinson's disease is caused by the death of dopamine-producing neurons in the brain, leading to:

• Tremors (shaking)
• Slow movement
• Stiffness
• Difficulty with balance and coordination

This demonstrates the role of dopamine in motor control.

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Adrenaline (Epinephrine)

Adrenaline functions as both a neurotransmitter and a hormone. It is produced by the adrenal glands (located above the kidneys) and is central to the fight or flight response.

Function	Description
Fight or flight	Prepares the body for rapid action in response to threat
Increased heart rate	Pumps more blood to muscles
Increased breathing rate	Takes in more oxygen
Release of glucose	Provides energy for muscles
Pupil dilation	Improves vision in threatening situations
Suppressed digestion	Diverts blood flow from digestion to muscles

Adrenaline is released in stressful or exciting situations and produces the physical sensations associated with fear, anxiety, and excitement (racing heart, rapid breathing, sweating).

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Summary Table

Neurotransmitter	Key Functions	Too Little	Too Much	Relevant Condition
Serotonin	Mood, sleep, appetite	Depression, anxiety	Serotonin syndrome (rare, from drug interactions)	Depression
Dopamine	Reward, motivation, movement	Parkinson's disease, depression	May contribute to schizophrenia symptoms	Addiction, Parkinson's
Adrenaline	Fight or flight response	Low energy, poor stress response	Chronic anxiety, panic attacks	Anxiety disorders

Mapping neurotransmitters to behaviour

flowchart LR
    A["Three key<br/>neurotransmitters"] --> B[Serotonin]
    A --> C[Dopamine]
    A --> D[Adrenaline]
    B --> B1[Mood, sleep, appetite]
    B1 --> B2[Low → Depression]
    B2 --> B3[SSRIs block reuptake]
    C --> C1[Reward, motivation, movement]
    C1 --> C2["Low → Parkinson’s<br/>treated with L-DOPA"]
    C1 --> C3[Surge → Addiction]
    D --> D1[Fight or flight]
    D1 --> D2["Adrenal glands<br/>raises heart rate"]

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Key Points

• Neurotransmitters are chemical messengers that cross synapses.
• Serotonin regulates mood, sleep, and appetite — low levels linked to depression.
• Dopamine is involved in reward, motivation, and movement — linked to addiction and Parkinson's.
• Adrenaline triggers the fight or flight response — prepares the body for action.
• Drugs (e.g. SSRIs) work by altering neurotransmitter activity at synapses.

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Worked study: the dopamine hypothesis and Parkinson's disease

Aim: To investigate the role of dopamine in motor control by studying patients with Parkinson's disease, and to evaluate whether replacing dopamine could restore movement.

Procedure: From the 1960s onwards, neurologists including Arvid Carlsson (Nobel Prize 2000) and Oleh Hornykiewicz examined post-mortem brains of Parkinson's patients and compared neurotransmitter levels with those of controls. They measured dopamine concentrations in the basal ganglia — a region of the brain involved in the control of voluntary movement. They also trialled the drug L-DOPA (a chemical precursor that the brain converts into dopamine) on Parkinson's patients to see if symptoms could be reduced. Patients' motor symptoms (tremor, bradykinesia, rigidity) were measured before and after treatment.

Findings: Parkinson's patients had severely reduced dopamine levels in the basal ganglia — often less than 20% of typical levels. Administering L-DOPA produced a dramatic reduction in motor symptoms: tremors decreased, stiffness eased, and some patients regained the ability to walk. However, benefits diminished over time and long-term L-DOPA use could cause dyskinesias (involuntary movements) and psychiatric side effects including hallucinations.

Conclusion: Dopamine is essential for normal motor control. Parkinson's disease is caused by the death of dopamine-producing neurons in the substantia nigra, demonstrating a clear causal link between neurotransmitter levels and behaviour. Drug treatments (L-DOPA) that restore dopamine levels improve symptoms, supporting the biological approach to psychology.

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