The system in the brain is extremely complex. First, scientists have only isolated a handful of neurotransmitters (chemicals that carry electric signals from one brain cell to another) and there are many more neurotransmitters that exist that have not been isolated. Scientists have discovered that there's a correlation between neurotransmitters and mood. And there's also a genetic component (chromosome mutations) that makes some people more predisposed to depression or manic-depression than other people. These chromosome mutations result in certain proteins and/or enzymes not being metabolized correctly. (Region of chromosome 22 linked to bipolar disorder, again)

The only way Scientists can postulate that a drug does something specific in the brain (relieve mania, anxiety, depression, etc.) is to test it in controlled studies in animals or humans. (That's how the effect of lithium was first discovered, ie. by accident on a guinea pig). And these Studies are how the FDA makes its decision on whether or not a certain drug has been proven to be effective for a specific condition.

Serotonin is a molecule that is made up of chemicals just like dopamine, norepinephrine (noradrenaline) and epinephrine are. Proteins, enzymes are molecules that are also made up of chemicals. Every molecule in your body is made up of chemicals like water, carbon, salt, nitrogen, oxygen, iron, potassium, zinc, etc.

Just a story I heard on NPR this week: There is a group of neurons that decipher sound and music so that our brains can understand it. When Stravinsky debuted the "Rite of Spring" in 1913 in Paris the audience had never heard anything like it before. He had placed dissonant music overtop of melodies. The NPR guests said that when this specific group of neurons have trouble decoding sound/music they emit excess dopamine. (And excess dopamine has been found to be associated with schizophrenia along with other factors.) They said the audience that night went completely mad and experienced schizophrenia. In 1914, the "Rite of Spring" was performed again and people had heard it before and had learned to like it (and love it) and that night Stravinsky was hailed as a genius and was carried out of the theater on peoples' shoulders. (The Rite of Spring - Wikipedia, the free encyclopedia, Milestones of the Millennium: The Rite of Spring)

"Monteux believed that the trouble began when the two factions in the audience began attacking each other, but their mutual anger was soon diverted towards the orchestra: "Everything available was tossed in our direction, but we continued to play on". Around forty of the worst offenders were ejected—possibly with the intervention of the police, although this is uncorroborated. Through all the disturbances the performance continued without interruption."

This shows you how complex the brain can behave.