Some antidepressants, although they may take weeks to provide improvements apparent to patients, are capable of altering the architecture of the brain within a matter of hours, a study suggests.

1 in 10 U.S. adults use the drugs, which alter the amount of the chemical transmitter serotonin in the brain by blocking the manner in which it is reabsorbed.

Examples of such selective serotonin reuptake inhibitors, or SSRIs, included Prozac, Paxil, Lexapro, Celexa and Zoloft.

Better knowledge of just exactly how the different areas of the brain are affected by various antidepressant drugs could help doctors decide which drugs might be the best choice appropriate for an individual patient, the researchers say.

In the study, 22 healthy volunteers who had never taken an antidepressant had their brains scanned, after which some of them were administered a dose of Lexapro and then scanned again after three hours.

Connectivity in most areas of the brain was reduced by the single dose, with the exception of the thalamus, the brain area linked to signal recognition, and the cerebellum, the area associated with motor control.

These two areas showed increased connectivity as a result of the antidepressant, the researchers found.

"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," says Julia Sacher of the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany.

"It was interesting to see two patterns that seemed to go in the opposite direction," she says.

Since the majority of people who are put on antidepressants aren't reporting discernable changes in moods for at least 2 weeks after starting a course of the drug, it suggests the rapid changes in brain connectivity may be a precursor to long-term alteration, the researchers suggest.

While it is unknown exactly what the changes in connectivity have to do with the drugs' ultimate effect on the two regions of the brain seeing an increase, knowing that such effects begin almost immediately may offer insights into improved research and treatment of depression, the researchers say.

There may come a day when a simple brain scan may help mental health professionals determine whether a person is likely to respond to antidepressants or not, or exactly which of the many different varieties of SSRIs may be the most effective.

"In a perfect world, you would look not only at SSRIs, but all sorts of medications and non-pharmacological interventions," Sacher says. "That would really help to tailor individual therapy for someone in the midst of a depressive episode."