NTRK2 and depressive disorder: TrkB T1 over-expressing transgenic mice, which show reduced TrkB activation in the brain, are resistant to the effects of antidepressants (Saarelainen et al., 2003), whereas over-expression of TrkB led to resistance to depression-like behavior, with SSRI administration unable to further increase this resistance (Koponen et al., 2005) indicating that TrkB signaling is required for the behavioral benefits of antidepressants.