Psilocybin and neuroplasticity — what is BDNF and why does it matter?

BDNF (Brain-Derived Neurotrophic Factor) is a protein that supports the growth, maintenance, and differentiation of neurons — it's sometimes called 'miracle-gro for the brain' in pop science. It plays a key role in synaptic plasticity (the ability of synapses to strengthen or weaken over time, which is the cellular basis of learning and memory). Low BDNF is associated with depression, PTSD, and chronic stress. Conventional antidepressants (SSRIs) increase BDNF with chronic use — this is thought to be part of their mechanism. Psilocybin has been shown to increase BDNF expression in rodent studies (Ly et al., 2018, Cell Reports) within hours, much faster than SSRIs.

The Ly et al. 2018 paper is important: they showed psychedelics (psilocin, DMT, LSD) promote 'psychoplastogen' activity — increased dendritic complexity, synaptogenesis, and BDNF-TrkB signaling in cortical neurons. This is significant because it suggests psychedelics could produce rapid structural brain changes that support lasting therapeutic effects. The 'critical period reopening' hypothesis (Bhatt et al., 2023) suggests psychedelics may temporarily restore juvenile-like plasticity, creating a window where new learning, habit change, and therapeutic integration are more available.

Caveat: most neuroplasticity data is from rodents. Human neuroimaging can show some proxy markers (functional connectivity changes) but we don't have direct human BDNF-in-brain measurements after psilocybin — blood BDNF measurements are noisy proxies. The mechanism is biologically plausible and well-supported in animals, but the direct causal link to human therapeutic outcomes is still inferred, not proven.