TPH2 and depressive disorder: To evaluate the impact of genetically induced brain 5-HT deficiency on changes in body weight and depression- and anxiety-like behaviors following chronic HFD, the current work examined the tryptophan hydroxylase 2 (Tph2) R439H knock-in (KI) mouse line, which harbors a partial loss-of-function mutation in the brain 5-HT synthesis enzyme, Tph2 (Beaulieu et al., 2008).