It is unclear what causes bipolar disorder. It is a chemical imbalance in the brain, but they don't know which chemicals yet. Mood stabilizers work, but docs don't really know why. Many of the newer mood stabilizers are anti-seizure medicines, and it's not clear if there's a link between seizures and bipolar or not. It's also clearly genetic, as it runs in families, but they haven't been able to isolate the gene yet. There also seems to be some argument that bipolar is affected by the environment -- set off perhaps by some "trigger event."
There's a pretty good overview of it here: www.webmd.com/bipolar-disorder/guide/bipolar-disorder-causes
Here's the Depression and Bipolar Alliance's page on what research is being done: www.dbsalliance.org/site/PageServer?pagename=research_supportedresearch
Hope this helps,
SerafenaCo-Moderator, Bipolar ForumBipolar II
"Bipolar disorder can be a great teacher. It's a challenge, but it can set you up to be able to do almost anything else in your life." - CARRIE FISHER