This brain territory also shows abnormalities in the brains of people with schizophrenia. In one study of people with people with bipolar disorder, the left side of the hippocampus was significantly larger than the right, while in patients with schizophrenia the hippocampus volume was decreased. In both schizophrenia and bipolar disorder the pathways of the neurotransmitter dopamine appear to be important. Dopamine has been a target of scientific investigation since researchers first observed that certain drugs that reduce the action of dopamine in the brain also reduce psychotic symptoms. Although not all cases are familial, genetic factors appear to play a major role in bipolar disorder.
Researchers believe that more than one genetic defect is necessary to trigger bipolar disorder.
Investigators suspect that important genetic abnormalities rest on chromosome 18 and 21. Recently, the first genetic defect specifically associated with bipolar disorder was identified. Serotonin is an important neurotransmitter that plays a prominent role in depression and sleep. Environmental factors also play a role, since not all cases of bipolar disorder are associated with a family history of the disease. No single cause may ever been found for this complex disease.