A study by Italian researchers may offer the answer to sudden infant death syndrome, the third leading cause of death among infants aged between a week and a year, killing 2,500 infants annually in the U.S. and thousands more globally.

Some previous studies have linked SIDS to smoking around infants and during pregnancy; some others have stressed the importance of putting babies to sleep on their backs, thus the SIDS dropping almost three quarters.

According to the new findings, a malfunction in the regulation of the brain chemical serotonin may be at the root of SIDS. Serotonin is a signaling chemical that has far-reaching effects in the brain and other organs, helping in the heart rate regulation, breathing, temperature regulation and more.

For the study, Cornelius Gross, PhD, and colleagues at the European Molecular Biology Laboratory near Rome analyzed mice susceptible to sudden death that had been engineered to slow the production of serotonin, a process that affected the brain stem. The brain stem is that part of the brain that is linked to the spinal cord and helps control the lungs and the heart.

Although “at first sight the mice were normal,” the researchers surprisingly found that “they suffered sporadic and unpredictable drops in heart rate and body temperature,” which caused more than half of the mice die, Dr. Gross said.

What is more surprising is the fact that “having a dysfunction in the serotonin system is somehow worse than having no serotonin at all,” as a complete block of serotonin doesn’t cause death, the study found.

This is not the first time a study links serotonin to SIDS. Two years ago, scientists at Children’s Hospital Boston and Harvard Medical School led by Dr. Hannah Kinney linked serotonin to SIDS by performing autopsies on 31 SIDS babies. Their autopsies revealed abnormalities in the SIDS infants’ brainstem, which uses serotonin to tell the body how to react to environmental changes.

The findings of the Italian study were welcomed by the Foundation for the Study of Infants Deaths. A spokesperson of the institute said the study “reinforced” the findings from 2006 but he further added that that more study needs to be done in order to find what exactly “causes the serotonin imbalance – genetic factors or environmental?”

The findings of the Italian study were published in the July 4 issue of Science.