HTR5A and major depressive disorder: The third finding is the collection of psychopharmacological data implicating 5-HT in animal models of depression and anxiety [15–17], together with the fact that depleting 5-HT (by dietary depletion of its precursor, tryptophan) in human subjects who have recovered from depression, can reinstate an acute, at times fulminant, re-experience of subjective symptoms of the disease, as assessed by various rating scales [18–21].