Serotonin reuptake inhibitors or SSRIs are some of the most prescribed medication for depression treatment. A new study reveals that a single dose of SSRIs such as Prozac or Celexa results in changes in the brain structure within a few hours.

Depression affects as many as 6.7 percent of U.S adults each year. SSRIs are, by far, the most studied and most prescribed medication for depression. According to researchers at Max Planck Institute for Human Cognitive and Brain Sciences, these drugs work almost instantly to reconnect brain areas.

SSRIs work by blocking a key receptor in the brain for serotonin. The neurotransmitter serotonin is known to affect mood.

"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.

For the study, the researchers obtained brain scans of people with depression. The participants had to lie for 15 minutes in a brain scanner, which measured the oxygenation of blood flow in the brain. The researchers used the data to create 3D structure of the brain. They then compared the number of connections before and after depression treatment. In the study, the participants were given a single dose of escitalopram (trade name Lexapro).

The team found that connectivity between different parts of the brain changed almost immediately, but two regions - the cerebellum and thalamus - showed greatest changes.

The researchers hope that further studies will help scientists understand why some people fail to respond to SSRIs even weeks after the treatment.

"The hope that we have is that ultimately our work will help to guide better treatment decisions and tailor individualized therapy for patients suffering from depression," said Sacher in a news release.

The study is published in the journal Current Biology.