Dopamine Hypothesis

• Initially dopamine hypothesis suggested that an excess (too much) of the neurotransmitter dopamine in certain regions of the brain is associated with positive symptoms of SZ
• SZ are thought to have abnormally high numbers of dopamine receptors on receiving neurons, resulting in more dopamine binding and more neurons firing
• Dopamine hypothesis - evidence

  • Drugs that increase dopamine levels - amphetamines and L dopa both increase dopamine levels and are linked to SZ symptoms
  • Drugs that decrease dopamine levels - all antipsychotic drugs block dopamine levels and decrease hallucinations
• The revised dopamine hypothesis - positive

  Proposed that positive symptoms of SZ are caused by an excess of dopamine in subcortical areas of the brain, especially the mesolimbic pathway
• The revised dopamine hypothesis - negative

  The negative and cognitive symptoms arise from a deficit of dopamine in areas of the prefrontal cortex (mesocortical pathway)
• Evidence for revised hypothesis

  • Neural imaging - PET scans show lower levels of dopamine in prefrontal cortex of SZ
  • Animal studies - Study reduced dopamine levels in prefrontal cortex in rats, resulting in cognitive impairments. This was then reversed using an atypical antipsychotic