Depression-serotonin link may be wrong

The general view is that low levels of the neurotransmitter serotonin are linked with depression. However a number of studies have found, not surprisingly, that the relationship is more complex than that according to a recent review by Geddes in the New Scientist (July 2010). Increased levels of serotonin in some regions of the brain may lead to elevated mood but in other regions there may be negative effects. This may explain why antidepressants that increase serotonin levels (e.g. SSRIs) aren’t always effective. For example, in the short-term people taking SSRIs report feelings of anxiety which may be due to the fact that increasing serotonin levels has negative effects on some serotonin neurons. Boosting serotonin is also associated with increased impulsivity which might explain why SSRIs are linked to suicide.

You can read more here