Mice were mutated to have a flawed 'Clock' gene, "which helps control the body's circadian rhythms: waking, sleeping and eating times as well as the maintenance of proper body temperature, heart and blood functions, and hormone levels."
This created manic mice:
"...the flawed Clock gene induced a manic state in mice that had a profile similar to that of humans suffering from bipolar disorder or manic depression, a condition during which a person cycles from deep depression to manic behavior.
The mutant mice showed increased response to reward stimuli in several hedonic scenarios. When trained that depressing a pedal would trigger an enjoyable electrical stimulation to the forebrain, the mice pressed it more frequently than their wild (unaltered) counterparts. They acted the same way when the reward was a sugar solution. In the presence of cocaine the mutant mice needed smaller doses than the normal mice to activate their reward circuits, indicating that they were hyperactive. McClung notes that manic people, who are prone to drug abuse, shopping sprees, compulsive gambling and other high-impulse behaviors, "tend to find rewarding things more rewarding" than others do. "
To prove it, they gave the mice lithium, which is still our gold standard for treating mania. After 10 days on lithium, the mice were already starting to revert to more normal behavior.
The hope is that someday we will be able to actually 'cure' bipolar disorder by fixing the flawed brain regions.
Here's how it worked for the mice: "...the scientists used a targeted virus to return a proper copy of Clock to cells in the ventral tegmental area, a midbrain region critical in the reward pathway involved in production of the pleasure system neurotransmitter dopamine. After the virus was introduced, the mutant mice began to act more like the control animals but were still a bit more hyper, McClung notes. "
How cool would that be? My job would be SO much easier.