Brain scans taken of people before and after an acute dose of a commonly prescribed SSRI (serotonin reuptake inhibitor) showed changes in connectivity within three hours, researchers said.
"We were not expecting the SSRI to have such a prominent effect on such a short timescale or for the resulting signal to encompass the entire brain," said Julia Sacher of the Max Planck Institute for Human Cognitive and Brain Sciences in Germany.
While SSRIs are among the most widely studied and prescribed form of antidepressants worldwide, it's still not entirely clear how they work.
The new findings show that changes begin to take place right away.
Sacher said what they are seeing in medication-free individuals who had never taken antidepressants before may be an early marker of brain reorganisation.
Study participants let their minds wander for about 15 minutes in a brain scanner that measures the oxygenation of blood flow in the brain.
The researchers characterised three-dimensional images of each individual's brain by measuring the number of connections between small blocks known as voxels (comparable to the pixels in an image) and the change in those connections with a single dose of escitalopram (trade name Lexapro).
However, Sacher and her colleagues observed an increase in connectivity within two brain regions, specifically the cerebellum and thalamus.
The researchers said the new findings represent an essential first step toward clinical studies in patients suffering from depression.
The study is published in the Cell Press journal Current Biology.