What causes schizophrenia?

From Nature:

News.2009 Researchers in Sweden have revealed a surprising change in brain biochemistry that occurs during the training of working memory, a buffer that stores information for the few second required to solve problems or even to understand what we are reading. The discovery may have implications for understanding disorders in which working memory is deficient — such as schizophrenia and attention deficit hyperactivity disorder (ADHD).

Working memory depends on the transmission of signals in certain parts of the brain by the chemical dopamine and one of its receptors, the D1 receptor, particularly in the parietal and frontal regions of the cortex. The efficiency of working memory drops off as people age. But the 'use-it-or-lose-it' adage holds true — working memory can be improved through training. Torkel Klingberg, a neurologist at the Karolinska Institute in Stockholm, and his colleagues studied what happened to D1 receptors in the brains of healthy young men during such training.

More here.