(Kyodo) _ A team of Japanese scientists said Tuesday that it has confirmed a possible link between mitochondrial dysfunction in brain nerve cells and bipolar disorder. The team, which includes scientists at the RIKEN Brain Science Institute based in Wako, Saitama Prefecture, published their findings in the online edition of the U.S. scientific journal Molecular Psychiatry on Tuesday. There are different hypotheses on the cause of bipolar disorder, with some indicating the involvement of neurotransmitters, but the disease's exact mechanism is unknown. Bipolar disorder, also known as manic-depressive disorder, is characterized by recurrent depressive and manic phases. In their experiment, the team created a mouse whose mitochondria in brain nerve cells were set to dysfunction through genetic engineering. Normal mice became active in the dark and stopped its activity as it turned brighter, but the disabled mice continued to be active in the bright environment for a while, showing insomnia-like symptoms seen among bipolar patients. The team has long pointed out the link between mitochondrial dysfunction and bipolar disorder. "There is a possibility to help resolve the disease's mechanism and to develop new drugs," said the team's leader, Tadafumi Kato