SLC6A4 and depressive symptom measurement: Considering that BDNF protein and mRNA levels are decreased in the ventral hippocampus of SERT−/− rats [25,26,27,43], we sought to investigate whether BDNF gene overexpression in the ventral hippocampus was able to restore BDNF levels and anxiety- and depression-like behavior in SERT−/− rats.