Serotonin, Synapses, and Setting: Gitte Moos Knudsen on the Neurobiology of Psychedelic Action

Psychedelics neuroscience is closely tied to serotonin signaling, with stimulation of the 5-HT2A receptor described as the primary driver of classic psychedelic experiences. Gitte Moos Knudsen connects this receptor mechanism to neuroplasticity ideas including changes in synaptic density, highlighting PET imaging targets like SV2A and the need for better in vivo work on synaptic alterations after treatment. She also discusses circuit-level findings such as altered global integration and reduced integrity of high-level networks like the default mode network. The interview emphasizes that neuroimaging interpretations have important caveats, especially the assumption of stable neurovascular coupling in fMRI because serotonergic psychedelics can affect vasculature. She also argues that dosing and setting interact in measurable ways: PET SV2A studies and observations about “therapy room” versus noisy scan conditions suggest environment can modulate synaptic density changes. Finally, Knudsen discusses biomarkers and response prediction, leaning toward plasma concentration and approximate 65% 5-HT2A receptor occupancy as a practical target, while taking an open view on whether hallucinogenic effects must be separated from therapeutic outcomes and how much post-session integration is truly needed.