Eero Castrén has investigated the role of neurotrophic factors and neuronal plasticity in the antidepressant drug action. He has shown that both typical and fast-acting antidepressants act by directly binding to brain-derived neurotrophic factor (BDNF) receptor TrkB and allosterically increasing BDNF signaling. He recently showed that psychedelic compounds LSD and psilocin directly bind to TrkB with high affinity and thereby promote plasticity and antidepressant effects, but not the hallucinogenic effects. These findings demonstrate that all antidepressants act though the same mechanism, binding to TrkB. He has shown that through increased TrkB signaling, antidepressants reactivate a juvenile-like plasticity in the adult rodent brain. Through enhanced plasticity, antidepressants increase the sensitivity of neuronal networks to environmental influences, which provides a new paradigm where active engagement of the patient together with drug-promoted plasticity is beneficial