Role of dopamine in schizophrenia and Parkinson's disease

The neurotransmitter dopamine (DA) and the dopaminergic neurones play an important role in schizophrenia and Parkinson's disease (PD). A decrease in DA in the substantia nigra of the brain has been implicated as the cause of PD. By contrast, it is argued that a functional excess of DA or oversensitivity of certain DA receptors is one of the causal factors in schizophrenia. These factors are reflected in the treatment of both conditions; drugs aimed at increasing DA are prescribed to patients with Parkinson's disease, while most antipsychotic drugs block and reduce the effects of DA. In schizophrenia the antipsychotic effects of traditional 'neuroleptic' drugs such as chlorpromazine are highly correlated with their ability to block DA receptors and reduce the effects of DA. This article examines the role of DA in these two conditions.