I had no idea the link between serotonin and depression was in doubt. Very interesting:

Via Dormivigilia, I came across a fascinating paper about a man who suffered from a severe lack of monoamine neurotransmitters (dopamine, serotonin etc.) as a result of a genetic mutation. […]

Overall, though, the biggest finding here was a non-finding: this patient wasn’t depressed, despite having much reduced serotonin levels. This is further evidence that serotonin isn’t the “happy chemical” in any simple sense.

On the other hand, the similarities between his symptoms and some of the symptoms of depression suggest that serotonin is doing something in that disorder. This fits with existing evidence from tryptophan depletion studies showing that low serotonin doesn’t cause depression in most people, but does re-activate symptoms in people with a history of the disease. As I said, it’s complicated…

Neuroskeptic: Life Without Serotonin

See also:

Serotonin and Depression: A Disconnect between the Advertisements and the Scientific Literature