The mechanism question — how do psychedelics actually work in the brain — is more settled than most popular coverage suggests, and less settled than the research establishment sometimes implies.
Dr. Gitte Moos Knudsen, Professor of Neurology at the University of Copenhagen and Director of the Neurobiology Research Unit at Rigshospitalet, [laid out the current state of knowledge in an interview with Psychedelic Alpha](https://psychedelicalpha.com/news/serotonin-synapses-and-setting-gitte-moos-knudsen-on-the-neurobiology-of-psychedelic-action/). The short version is useful to have.
## What we know
The core mechanism of classic psychedelics — psilocybin, LSD, DMT, mescaline — is stimulation of the serotonin 2A receptor (5-HT2A). This is established. When researchers administer 5-HT2A receptor antagonists before a psychedelic, the subjective experience is almost completely blocked. The receptors in question are primarily located in pyramidal neurons in the cortex.
This is why "serotonin" keeps coming up in explanations of how psychedelics work, but it's a different serotonin story than the one attached to SSRIs. SSRIs block the reuptake of serotonin to increase its availability across synapses generally. Psychedelics are direct agonists at a specific serotonin receptor subtype. The pharmacology is related but distinct — which is part of why combining the two is clinically complicated and actively studied.
## What we're still figuring out
What happens downstream of that receptor activation is where things get uncertain. The leading hypothesis is that psychedelics promote neuroplasticity — the brain's capacity to form new connections. Animal studies and some human imaging work suggest this is real. The most interesting frontier right now is synaptic density: measuring actual changes in the number of synaptic connections using a target called SV2A, which has only recently become imageable in living humans via PET scanning.
Knudsen describes this as largely unexploited territory. The question of *when* those synaptic changes occur — immediately during the experience, in the days after, weeks later — is not well characterized. It matters clinically, because if the therapeutic window involves a period of heightened neuroplasticity, knowing its timeline would change how you design treatment protocols.
## The imaging problem
FMRI studies of psychedelic states have produced striking images of increased brain connectivity and decreased default mode network activity. Knudsen flags an underappreciated complication: fMRI measures blood flow as a proxy for neural activity, and psychedelics affect the blood vessels themselves, not just the neurons. Neurovascular coupling — the relationship between neural firing and blood flow — may be altered under psychedelics in ways that confound fMRI interpretation.
This doesn't invalidate the fMRI findings, but it means they need to be interpreted carefully. The brain under psilocybin may look more connected on fMRI partly because the drug is doing something to the vasculature, not only because the neurons are firing differently.
## Why this matters
Understanding mechanism matters for several reasons. It constrains which compounds are likely to work for which conditions. It suggests what biomarkers might predict who responds to treatment — finding those predictive markers is one of Knudsen's research goals. And it bears directly on the question of what the experience itself is doing.
The hard problem of consciousness — why physical processes in the brain give rise to subjective experience at all — doesn't get solved by mapping 5-HT2A receptors. But psychedelics are among the most powerful tools available for studying how changes in brain chemistry produce radical changes in conscious experience. That's why the neuroscience keeps attracting researchers who started out thinking about something else entirely.