Neurotransmission & Major Depressive Disorder (DP IB Psychology): Revision Note

The role of serotonin in MDD

• Neurotransmitters are chemical messengers that pass signals between the presynaptic neuron and postsynaptic neuron across the synaptic cleft

• Serotonin (5-HT) is a monoamine neurotransmitter that helps regulate functions such as sleep, mood, body temperature

• Low or irregular levels of serotonin have been linked to symptoms of MDD

Synaptic transmission

• Synaptic transmission involves a series of events

  • A signal begins as an electrical impulse (an action potential) in the presynaptic neuron

  • When it reaches the end of the axon on the pre-synaptic knob, neurotransmitters are released from structures called vesicles at the presynaptic membrane

  • Neurotransmitters are released into the synaptic cleft, diffuse across the synaptic cleft and temporarily bind with receptor sites on the postsynaptic membrane

  • This stimulates a new electrical impulse (action potential) in the postsynaptic neuron

  • The neurotransmitter molecules left in the synapse are then either broken down by enzymes or reabsorbed (recycled) to prevent continued stimulation

The neurochemical explanation of MDD

• Biological explanations of MDD take the ‘nature’ side of the nature/nurture debate, suggesting behaviour is influenced by innate, inherited traits

• The neurochemical explanation of MDD looks at how neurotransmitters affect behaviour

• Polymorphism in the 5-HTT gene has been linked to the monoamine hypothesis, which suggests that mood disorders are related to levels of monoamines (e.g., serotonin)

• Monoamines are chemicals that transport neurotransmitters like serotonin around the brain 

• Serotonin is strongly implicated in MDD; low or irregular levels are linked to depressive symptoms

• Evidence to support the monoamine hypothesis can be found in the action of selective serotonin reuptake inhibitors (SSRIs), which block the reabsorption of serotonin, leaving more available in the synapse

• Since SSRIs are effective treatments for depression, this supports the idea that serotonin plays a role in the onset and experience of MDD

Research support for the neurochemical explanation of MDD

Caspi et al. (2003)

Aim:

• To investigate the link between variations of the 5-HTT serotonin transporter gene and MDD

Participants:  

• An opportunity sample of 847 participants aged 26 years

• The participants were split into three groups based on their 5-HTT gene alleles

  • Group 1 – two short alleles

  • Group 2 – one short and one long allele

  • Group 3 – two long alleles

Procedure: 

• The participants were asked to report stressful life events that had occurred between ages 21-26

• The Diagnostic Interview Schedule was used to assess incidences of depression over the past year 

• The researchers carried out correlational analyses between the following co-variables:

  • Stressful life events and depression

  • Allele length and depression

  • Perceived stress and allele length

Results: 

• The participants with two short 5-HTT alleles reported greater depressive episodes in response to stressful life events compared to the other two groups

• The participants with two long alleles reported fewer depressive symptoms overall

Conclusion: 

• There may be a relationship between short 5-HTT alleles and vulnerability to stress-induced MDD

• Long 5-HTT alleles may provide protection against stress-induced depression, possibly due to steadier serotonin supply

Evaluation of the neurochemical explanation of MDD

Strengths

• There is clear research support for the monoamine hypothesis based largely on the role of SSRIs in treating depression

• SSRIs are cheap, widely available and effective for many

Limitations

• The evidence supporting a neurochemical explanation is not 100% conclusive

• Antidepressants such as SSRIs do not work for everyone; research has found that only one third of patients responded positively to SSRI treatment

Link to biological reductionism

• Attempting to explain a complex condition such as depression using the neurotransmitter argument is biologically reductionist

• This is because depression cannot be easily defined as a simple neurochemical imbalance, as there are other factors involved, plus depression does not follow a ‘one size fits all’ model: it is different for each depressed person

Link to concepts

Perspectives

• Depression is unlikely to stem from purely biological factors

• MDD may develop due to other reasons:

  • Cognitive – faulty schemas, negative thinking

  • Sociocultural – poverty, abuse, problematic relationships

• Depression should be viewed holistically rather than as a set of symptoms which only a biological treatment (e.g., drugs) can successfully treat

Causality

• The neurochemical explanation supports psychology as a science:

  • Objective methods, clinical data, replicable findings

  • However, cause–effect between serotonin and MDD is not fully established