The activity of serotonin systems have been to mood.  Tryptophan depletion leads to symptom relapse in people in remission from depression.

University of Vermont

Cellular: 
-Simultaneous inhibition and excitation
-Complicated cell responses
Behavioral: 
-Anxiety
-Depression 
-Mood

Serotonin Roles and Functions

Serotonin Reference Library

Tryptophan depletion

Studies have shown that low amounts of serotonin in the brain have been implacated in anxiety. One way this phenomena was shown was via a force swim test, as well as through learned helplessness.
           

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