BDNF and depressive disorder: Considering the individual enantiomers, in a chronic social defeat stressand learned helplessness models of depression, a TRkB antagonist wasable to block the antidepressant effects of both (S)-ketamine (Yang et al.,2018a) and (R)-ketamine (Yang et al., 2015).Interestingly, (R)-ketamine induced greater effects on reduceddendritic spine density, BDNF-TrkB signalling and synaptogenesis inthe prefrontal cortex and hippocampus compared with (S)-ketamine and(R)-ketamine showed a greater potency and longer-lastingantidepressant effect than (S)-ketamine in this model (Yang et al.,2015).