July 3 (Bloomberg) -- Sudden Infant Death Syndrome may be caused by imbalances in the brain chemical serotonin, suggests a study in mice that could lead to ways to identify babies at risk for the condition.

Mice genetically modified to produce low levels of the brain signaling protein suffered drops in heart rate and other symptoms of SIDS, and many of the animals died at an early age, the study by Italian scientists found. Depleted levels of serotonin in the animals' brainstems, which control heartbeat and breathing, may have caused sudden death, researchers said.

SIDS took the lives of about 2,200 babies in the U.S. in 2005, the last year National Center for Health Statistics data is available. The experiment may give researchers an animal model to test the effect of drugs on serotonin system dysfunctions in the brain and help pinpoint biological risks for crib death in humans, according to the report today in the journal Science.

``This is one more significant source of information that shows that serotonin deficiency may be the key in SIDS,'' said William Fifer, professor of psychiatry and pediatrics at Columbia who advised the study's authors. ``Research in this area has been painstakingly slow.''

Public awareness campaigns and physicians have warned parents about possible SIDS triggers, such as exposure to smoke, cluttered cribs or tummy sleeping. While risk-avoiding tactics have reduced the number of SIDS deaths from annual highs of 6000 during the 1980s, doctors have not been able to explain the disease.

Previous Evidence

The mouse study builds on evidence of serotonin's role in SIDS, researchers said. Two years ago scientists at Children's Hospital Boston and Harvard Medical School linked serotonin to SIDS by performing autopsies on 31 SIDS babies. Their analysis showed abnormalities in the SIDS infants' brainstem, which uses serotonin to tell the body how to react to environmental changes.

Serotonin acts as a chemical messenger in the brain that transmits signals between nerve cells. Among its many effects, the chemical helps regulate sleep, appetite and mood, as well as body temperature and breathing rates during periods of excitement.

In the latest study, investigators genetically modified mice to create a serotonin system imbalance. After being exposed to slight external temperature changes, the rodents suddenly died because their bodies could not adjust, researchers said.

``The serotonin system can be lethal if it's not regulated in some way,'' said Cornelius Gross, a researcher at the Monterotondo, Italy-based European Molecular Biology Laboratory and co-author of the study.

Impaired serotonin response prevents infants that sleep on their stomachs from reacting when they breathe exhaled carbon dioxide, researchers said. Non-SIDS babies will automatically respond by turning their heads to the side so that they could breathe more oxygen.

To contact the reporter on this story: Cristina Alesci in New York at calesci@bloomberg.net

Last Updated: July 3, 2008 14:00 EDT