heres a concise page describing low serotonin causing depression

Serotonin levels can dictate if you feel depressed or not. Antidepressant medications like Paxil, Zoloft, St. John's Wort, and Prozac work by preventing serotonin destruction and loss. These antidepressants inhibit serotonin uptake ( or reuptake) by the neurons in the brains. Low serotonin levels cause depression.
http://www.diagnose-me.com/cond/C14113.html