WEDNESDAY, Jan. 13 (HealthDay News) -- New research is giving scientists a greater understanding of how a brain becomes susceptible to depression and anxiety on a molecular level and why some people are resistant to antidepressants.

Scientists think two things -- biological factors and stressful life events -- cause the mental disorders. Antidepressants (such as Prozac) are available to treat them, often by increasing serotonin levels, but they don't always work.

"Unfortunately, more than half of all depressed patients fail to respond to their first drug treatment," senior study author Rene Hen, of Columbia University, said in a news release. "The reasons for this treatment resistance remain enigmatic. Elucidating the exact nature of both the factors predisposing to depression and the mechanisms underlying treatment resistance remains an important and unmet need."

In the study, researchers used mice to investigate the way the brain deals with serotonin. The brain's mechanism for handling serotonin appears to be related to levels of so-called "autoreceptors," the study authors found. Mice with higher levels didn't respond to treatment with antidepressants, but they did better when the levels went down, the study showed.

The research could lead to treatments to make people more responsive to antidepressants before they take them, Hen said.

The findings are published in the Jan. 14 issue of the journal Neuron.

More information

The U.S. National Library of Medicine has more on antidepressants.