Biological explanation Schizophrenia

• The initial link between schizophrenia and dopamine was first discovered in the 1950’s when researcher was being carried out to support Parkinsons disease.
• Initially it was quite a basic theory. It assumed that too much dopamine results in schizophrenia, and the demonstration of schizophrenic symptoms. 

  1968 – J.J. Griffiths – induced psychosis in non-schizophrenic volunteers, with the administration of dextro-amphetamine (which increases dopamine in the brain), it was found that the participants demonstrated the abrupt onset of paranoid delusions and showed a cold and detached emotional response. 
• Dopamine receptor sites 

  This original research was also complicated by the discovery of several subtypes of dopamine receptor sites, D1-D5, which are widely distributed in the cerebral cortex. And also subcortically (under the grey matter) in the limbic system. 
• Sleeman and Lee (1975) – demonstrated the impact of antipsychotic drugs on the D2 receptor, as D2 receptors are found mainly in the limbic system, as a result they allowed the research on the dopamine hypothesis to progress, and the D2 receptors became the main focus of the hypothesis. 

  This ultimately led to the revised dopamine hypothesis
• The Limbic System 

  Most recent research on the dopamine hypothesis has focused on the limbic system. The limbic system is made up of a variety of ‘subcortical structures’ that are engaged in many functions, but most notably emotions, memory formation and arousal. Nerve pathways leave from the limbic system to many other subcortical structures and also to the cerebral cortex; two of the main pathways associated with schizophrenia include the mesolimbic pathway and mesocortical pathway. 
• Mesolimbic pathway – This pathway carries signals from the ventral tegmental area to the nucleus accumbens. Too much dopamine either from neurons that fire too often or too quickly, cause overstimulation and ultimately positive symptoms of schizophrenia such as hallucinations and delusions.  
• Mesocortical pathway – This pathway carries signals from the ventral tegmental area to the frontal lobe. This nerve pathway is vital in emotional responses, motivation and cognition. Kenneth Davies et al (1991) note that too little dopamine (hypofunction) is evident in D1 receptors of the frontal lobe of many individuals with the cognitive impairments and negative symptoms of schizophrenia.