In a study on genetically-engineered rats, researchers in France and from Columbia University and Yale University found that two types of antidepressants appeared to stimulate growth of brain cells in the hippocampus. The theory that antidepressants fight depression by creating new brain cells may explain why the drugs often take a few weeks to work, researchers said.

The study involved mice with signs of depression -- decreased grooming -- who were given one of two antidepressants: fluoxertine, a.k.a. Prozac, or tricyclic imipramine. The mice given the drugs exhibited an increase in brain-cell growth.

To ensure that the cell growth, or neurogenesis, was the reason for the depression lifting, researchers used X-rays on some of the mice to stop new cell growth in the hippocampus and then gave the mice the antidepressants. These mice remained depressed.

"This is an important new insight into how antidepressants work," said Dr. Thomas Insel, director of the National Institute of Mental Health. "We have known that antidepressants influence the birth of neurons in the hippocampus. Now it appears that this effect may be important for the clinical response."

The study's findings could lead to development of more effective drugs to treat depression.

"The proof in humans is going to come when we extend the work into finding drugs that stimulate [growth of brain cells]. If these drugs have antidepressant effects in humans, this is going to be proof that the process is critical in humans," said Rene Hen, who led the study. "There is a push already in the pharmaceutical industry to find such compounds."

The study is published in the Aug. 8 issue of the journal Science.

Santarelli, L., et al. (2003) Requirement of Hippocampal Neurogenesis for the Behavioral Effects of Antidepressants. Science, 301(5634): 805-809.

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