Dopamine hypothesis

Dopamine hypothesis

Schizophrenia is caused by elevated levels of dopamine at key synaptic sites within subcortical regions of the brain e.g. the mesolimbic pathway and the limbic system

Synaptic transmission overload

• The excess of DA leads to too many impulses being transmitted, arguably triggering positive symptoms of schiz

• This could be caused by a number of factors, including the presynaptic neurons releasing too much DA

Post-synaptic level

• There may not be enough dopamine being broken down after ST, leaving too much DA in the surrounding cells

• Reuptake processes may be impaired, leaving too much dopamine in the synapse

• Abnormalities in dopamine can manifest in odd behavioural patterns found in some schizophrenia patients e.g. visual hallucinations

D2 receptors

Higher than normal concentrations of dopamine have been found in thepost-mortems of patients suffering from schizophrenia

Evidence from drugs

Drugs that increase dopamine production make symptoms of schizophrenia worse. Amphetamines increase dopamine activity and can bring about positive symptoms of schizophrenia

Davis et al (1991) - revised dopamine hypothesis

• In the prefrontal cortex there are no D2 receptors and actually there appears to be a deficiency of dopamine in this region (i.e. the mesocortical pathway) are more responsible for negative symptoms of schizophrenia like avolition

• This is hypodopaminergia

Research Support (+)

• Randrup & Munkvad created schiz like behaviour in a sample of rats by giving them dopamine agonists (amphetamines)

• The rats were given amphetamines to raise dopamine levels three times a day for six days. The rats showed lasting abnormalities including being unable to filter out irrelevant sounds 

• This was then reversed using antipsychotic medication 

Incomplete explanation (-)

• Most individuals with schiz are prescribed antipsychotics, which work by reducing dopamine, hence correcting dopamine levels

• Antipsychotics have been shown to be effective in 85% of patients with schizophrenia, which poses the question as to why decreasing the dopamine activity of the remaining 15% isn’t decreasing their symptoms

Lacks cause and effect (-)

• E.g. does high dopamine cause schizophrenia or does schizophrenia cause high dopamine?

• Perhaps there is a third unknown factor that actually causes both, such as a variant TPH gene as found by Hong et al (2001) to be associated with schizophrenia in Chinese patients

• As TPH plays a role in serotonin production, faults here could impact dopamine activity as serotonin can have an inhibitory effect on dopamine

Applications (+)

• Understanding that schiz is linked to elevated levels of dopamine in the mesolimbic pathway and limbic system has led clinicians to prescribe antipsychotics to patients

• Antipsychotics work by blocking the activity of dopamine and have shown to be effective in up to 85% of patients with schiz.