SLC6A4 and depressive symptom measurement: Previously, it was shown that LPS-induced depression-like behavior (despair and anhedonia, i.e., the inability to experience pleasure probably caused by reduced ability to experience reward), as measured in the tail-suspension test [17] and in the intracranial self-stimulation (ICSS) paradigm, respectively, was abolished in knockout (SERT−/−) animals [10,11,46].