<P> Amphetamine, cocaine and similar drugs increase levels of dopamine in the brain and can cause symptoms which resemble those present in psychosis, particularly after large doses or prolonged use . This is often referred to as "amphetamine psychosis" or "cocaine psychosis," but may produce experiences virtually indistinguishable from the positive symptoms associated with schizophrenia . Similarly, those treated with dopamine enhancing levodopa for Parkinson's disease can experience psychotic side effects mimicking the symptoms of schizophrenia . Up to 75% of patients with schizophrenia have increased signs and symptoms of their psychosis upon challenge with moderate doses of methylphenidate or amphetamine or other dopamine - like compounds, all given at doses at which control normal volunteers do not have any psychologically disturbing effects . </P> <P> Some functional neuroimaging studies have also shown that, after taking amphetamine, patients diagnosed with schizophrenia show greater levels of dopamine release (particularly in the striatum) than non-psychotic individuals . However, the acute effects of dopamine stimulants include euphoria, alertness and over-confidence; these symptoms are more reminiscent of mania than schizophrenia . Since the 2000s, several PET studies have confirmed an altered synthesis capacity of dopamine in the nigrostriatal system demonstrating a dopaminergic dysregulation . </P> <P> A group of drugs called the phenothiazines, including antipsychotics such as chlorpromazine, has been found to antagonize dopamine binding (particularly at receptors known as D dopamine receptors) and reduce positive psychotic symptoms . This observation was subsequently extended to other antipsychotic drug classes, such as butyrophenones including haloperidol . The link was strengthened by experiments in the 1970s which suggested that the binding affinity of antipsychotic drugs for D dopamine receptors seemed to be inversely proportional to their therapeutic dose . This correlation, suggesting that receptor binding is causally related to therapeutic potency, was reported by two laboratories in 1976 . </P> <P> Genetic evidence has suggested that there may be genes, or specific variants of genes, that code for mechanisms involved in dopamine function, which may be more prevalent in people experiencing psychosis or diagnosed with schizophrenia . Dopamine related genes linked to psychosis in this way include COMT, DRD4, and AKT1 . </P>

What schizophrenic symptoms are decreased by drugs that selectively block the d2 dopamine receptor