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updated 9/18/2014 5:16:03 PM ET 2014-09-18T21:16:03

A single dose of a commonly prescribed antidepressant drug quickly and dramatically changes how "in sync" different parts of the brain are, new research suggests.

In the study, researchers scanned the brains of 22 healthy people who had never taken antidepressants before. Scientists analyzed the brain's so-called "functional connectivity," which is a measure of how synchronized brain activity is in different areas.

The results showed a dramatic decrease in functional connectivity across the whole brain within just three hours of when participants took a type of antidepressant medication known as a selective serotonin reuptake inhibitor (SSRI).

"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 study researcher Dr. Julia Sacher, of the Max Planck Institute for Human Cognitive and Brain Sciences in Germany. "The connectivity changes we report here are much more dramatic and acute than previous reports on SSRI-action in the human brain have indicated," she said. [ 7 Ways Depression Differs in Men and Women ]

Although SSRIs are widely prescribed, researchers still don't know exactly how these medications may work to improve mood. What is known is that the drugs affect levels of the brain chemical serotonin ; specifically, they block the reabsorption of serotonin  brain cells, leading to increased levels of free serotonin outside of cells.

It's thought that changes in serotonin levels may lead to reorganization in the brain, by affecting brain cell proliferation, the efficiency of cell signal transmission and other factors. Researchers have postulated that these changes take several weeks to occur, about the same time it takes for a depressed person to start to respond to the drugs, Sacher told Live Science. The changes the researchers saw in the new study could be "the first step in remodeling the brain," she said.

Sacher stressed that the brain changes seen in the study are not necessarily "good" or "bad," per se.

"It depends on the context" and how well a person's brain can respond to the changes in connectivity that may be needed to adapt to certain situations, Sacher said. Some studies have linked anxiety disorders to patterns of increased function in certain brain networks, she said.

The researchers plan to conduct future studies with people who are recovering from depression, as well as those who have taken SSRIs but haven't benefited from the medicines, to compare those individuals' changes in functional connectivity.

Understanding the differences between the brains of people who respond to SSRIs and those who don't "could help to better predict who will benefit from this kind of antidepressant versus some other form of therapy," Sacher said.

The study is published today (Sept. 18) in the journal Current Biology.

Follow Rachael Rettner@RachaelRettner. FollowLive Science@livescience,Facebook&Google+. Original article on Live Science.

© 2012 LiveScience.com. All rights reserved.

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