A chemical important for brain development may play a role in explaining why some people are genetically predisposed to anxiety and could lead to new treatments, U.S. researchers said on Tuesday.

They said rats bred to be highly anxious had very low levels of a brain chemical called fibroblast growth factor 2 or FGF2 compared with rats that were more laid back.

But when they improved the anxious rats' living conditions, — giving them new toys to explore, an obstacle course and a bigger cage to live in — levels of this brain chemical increased and they became less anxious.

"The levels of this molecule increased in response to the experiences that the rats were exposed to. It also decreased their anxiety," Javier Perez of the University of Michigan, whose study appears in the Journal of Neuroscience, said in a telephone interview.

"It made them behave the same way as the rats that were laid back and had low anxiety to begin with," he said.

Injecting the rats with the chemical also made them less anxious, he said.

In a prior study of people who were severely depressed before they died, the team found the gene that makes FGF2 was producing very low levels of the growth factor, which is known primarily for organizing the brain during development and repairing it after injury.

Perez thinks the brain chemical may be a marker for genetic vulnerability to anxiety and depression. But it can also respond to changes in the environment in a positive way, possibly by preserving new brain cells.

While both the calm and anxious rats produced the same number of new brain cells, these cells were less likely to survive in the high-anxiety rats, the team found.

Giving the rats better living conditions or injecting them with FGF2 helped improve cell survival.

"This discovery may pave the way for new, more specific treatments for anxiety that will not be based on sedation, like currently prescribed drugs, but will instead fight the real cause of the disease," Dr. Pier Vincenzo Piazza of the University of Bordeaux in France, who had seen the study, said in a statement.

Perez said the study was funded in part by the Pritzker Neuropsychiatric Disorders Research Fund, which is seeking to patent the molecule.