Lithium has been the gold standard for bipolar treatment for about 50 years. We still don't know exactly how it works, but it has been proven through many studies that it really does help to control mania, and decrease depression. It has also been shown to be protective against suicide, although it can be lethal in overdose.
Researchers at UCLA have "used a novel method of three-dimensional magnetic resonance imaging (MRI) to map the entire surface of the brain in people diagnosed with bipolar disorder."
Findings: "Bipolar patients who were taking lithium had a striking increase in gray matter in the cingulate and paralimbic regions of the brain," she said. "These regions regulate attention, motivation and emotion, which are profoundly affected in bipolar illness." They have measured up to a 15% increase in gray matter in these brain regions.
I find this reassuring, since lithium has had a bad reputation, and is somewhat mysterious in its workings. I do wonder how lithium would affect 'normal' brains though.