Depression And The Puzzle Of Long-Lasting Treatment Effects
Depression is commonly treated with daily medications that can take weeks to work and often fail to help everyone. Psilocybin, the psychedelic compound found in certain mushrooms, has drawn attention because clinical studies show that one or two supervised doses can reduce depressive symptoms for months or even years. Until recently, scientists were unsure how such a brief treatment could lead to such long-lasting benefits.
A new preclinical study published in Neuropsychopharmacology offers an important clue. The findings suggest that psilocybin’s long-term antidepressant effects may come from lasting changes in how brain cells function electrically rather than from permanent structural changes in the brain.
Why Scientists Looked Beyond Brain Growth
Earlier research focused on neuroplasticity, especially structural plasticity. Structural plasticity refers to physical changes in the brain, such as the growth of dendritic spines, which are small protrusions that help neurons communicate. Short-term studies often show an increase in these spines shortly after psychedelic exposure.
The unanswered question was whether these physical changes last long enough to explain mood improvements that persist for months. If the structures disappear, something else must be maintaining the antidepressant effects.
How The Psilocybin Study Was Designed
Researchers from LSU Health Sciences Center studied male Wistar Kyoto rats, a strain commonly used to model stress-related and depression-like behaviors. The animals received a single dose of psilocybin, a selective serotonin receptor compound called 25CN-NBOH, or a placebo.
Behavioral testing was conducted five weeks and twelve weeks later using the forced swim test. This test measures passive versus active coping behavior and is widely used to evaluate antidepressant effects in animal studies.
Long-Lasting Behavioral Changes After One Dose
Rats that received psilocybin or 25CN-NBOH showed reduced immobility compared to controls. This antidepressant-like effect was present at both five and twelve weeks, long after the drugs had left the body. The durability of the behavioral change suggested that a stable shift in brain function had occurred.
What The Brain Analysis Revealed
After twelve weeks, researchers examined the medial prefrontal cortex, a region involved in mood regulation and decision-making. Surprisingly, dendritic spine density had returned to normal levels. Gene expression related to synaptic structure was also unchanged. These findings ruled out long-term structural plasticity as the main explanation.
The team then turned to functional plasticity, which refers to how neurons process electrical signals. Using electrophysiology, they measured how individual neurons fired. Psilocybin-treated neurons were more excitable and closer to their firing threshold, meaning they could activate more easily. These electrical changes persisted months after treatment.
Why Functional Plasticity Matters For Depression
These results suggest that neurons may “remember” the psychedelic experience through altered electrical properties rather than lasting physical growth. Even though structural changes fade, the brain circuits continue to function differently. This may help explain why people report lasting mood improvements after just one or two psilocybin sessions.
The fact that a selective serotonin 5-HT2A receptor compound produced similar effects also points to this receptor as a key driver of long-term antidepressant changes.
Limitations And What Comes Next
The study used only male rats, so future research must include females to better reflect human biology. Animal models also cannot fully capture the complexity of human depression. Still, these findings offer a promising framework for understanding how psychedelic-assisted therapy may work.
Future studies will likely focus on the transition period between early structural changes and long-term functional shifts. Mapping this timeline could help refine dosing strategies and improve therapeutic outcomes.
Citations
Kramer HM, Hibicke M, Middleton J, Jaster AM, Kristensen JL, Nichols CD. Psychedelics produce enduring behavioral effects and functional plasticity through mechanisms independent of structural plasticity. Neuropsychopharmacology. 2026;51(3):641–649. doi:10.1038/s41386-025-02272-3. https://pubmed.ncbi.nlm.nih.gov/41224969/
Carhart-Harris RL, Goodwin GM. The therapeutic potential of psychedelic drugs: past, present, and future. Neuropsychopharmacology. 2017. https://pmc.ncbi.nlm.nih.gov/articles/PMC5603818/