One out of every 10 Americans takes an antidepressant, according to the National Health and Nutrition Examination Survey, while one in every four women in their 40s and 50s do so. Now, a new study finds a single dose of a commonly prescribed SSRI (serotonin reuptake inhibitor) quickly produces dramatic changes in the architecture of the human brain. Specifically, brain scans taken of volunteers before and after one dose show a reduction of connectivity throughout the brain, with an increase of connectivity in two separate regions — all in just three hours.

What are SSRIs?

Worldwide, SSRIs are among the most widely prescribed form of antidepressants, often used to treat depression, anxiety disorders, panic attacks, and personality disorders. Classified as third-generation antidepressants, they are known for having fewer side effects than older pills and work by increasing levels of serotonin, a brain chemical naturally produced by your body. While serotonin serves many roles within your brain, chiefly it balances mood.

For the current study, 22 medication-free participants let their minds wander for about 15 minutes while their brains were scanned with an fMRI, a technology capable of measuring oxygenation of blood flow. Meanwhile, the researchers analyzed the three-dimensional images of each participant’s brain and measured the number of connections between small blocks of neurons known as voxels. After giving each volunteer a single dose of Lexapro (escitalopram), the researchers carefully observed the changes in those connections.

Immediately, the researchers felt surprised to discover the speed with which one dose of the SSRI performed. Within a matter of hours, it had reduced the level of intrinsic connectivity in most parts of the brain, while increasing connectivity within two regions: the cerebellum and thalamus. The cerebellum is responsible for, among other tasks, controling motor skills and balance, while the thalamus regulates consciousness, sleep, and alertness.

"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 Dr. Julia Sacher of the Max Planck Institute for Human Cognitive and Brain Sciences and an author of the study. Sacher believes better understanding of the differences in individual response to SSRIs "could help to better predict who will benefit from this kind of antidepressant versus some other form of therapy.”

Introduced in 2002, Lexapro is approved for the treatment of major depressive disorder and generalized anxiety disorder. Though headaches, nausea, and insomnia are among its most common side effects, the Food and Drug Administration also warns of suicidal thoughts and tendencies brought on by the drug.

Source: Schaefer A, Burmann I, Regenthal R, et al. Serotonergic modulation of intrinisic functional connectivity. Current Biology. 2014.