Research has found several anti-seizure meds can benefit people with Bipolar disorder and even migraine suffers.
Seems it has multi uses.
Such meds. affect certain areas of the brain, and it's chemistry. CamW or one of the other psychopharmachologists may be able to explain this, making the answer more thorough and understandable, I'm loss for words at the present.
Some medication websites may help too, try googling meanwhile till a reply comes through to your question.
Lots of luck with this.