November 20 (HealthDay News) -- Variations in the genetic makeup of alcoholics may affect how much they drink, a new study suggests.

And the key might be the brain's control of serotonin, a mood-influencing neurological chemical.

The research could potentially help doctors understand who might be at highest risk of becoming an alcoholic, and then treat that person, said study co-author Ming D. Li, head of neurobiology at the University of Virginia.

Li added that the research is unique, because it shows that a single gene variation is connected to a kind of behavior -- alcoholism.

The genetic blueprint that people inherit from their parents accounts for an estimated 40 percent to 50 percent of a person's risk of becoming alcoholic, said Dr. Robert Philibert, director of the Laboratory of Psychiatric Genetics at the University of Iowa.

The interplay between genetic makeup and environmental factors is responsible for the rest of the risk, said Philibert, who's familiar with the new study's findings.

"This study really takes the next step down the line," he said, in understanding the role that genes play in alcoholism.

For the study, the researchers looked at the DNA of 275 alcoholics who had sought treatment. Almost 80 percent were men, and all were of European descent. The researchers found that differences in the genes that affect serotonin levels in the brain coincided with the amount of alcohol consumed by the drinkers.

The findings were published online Nov. 20 and were expected to be in the February 2009 issue of Alcoholism: Clinical & Experimental Research.

Scientists think serotonin, a neurotransmitter, is crucial to human moods and emotions as well as things like sleep. Low levels of serotonin can lead to depression; some antidepressants aim to help the brain do a better job of processing serotonin.

"We know that serotonin is critical to maintaining a positive sense of self and for controlling our anxiety," Philibert said. That could explain a possible connection between serotonin levels and alcoholism, he added.

Li cautioned, however, that it's unlikely that a single genetic trait by itself would make someone more susceptible to alcoholism. It's more likely that a genetic variation works with other genes to raise the risk, he said.

Philibert said research might lead to a day when doctors could look at an alcoholic's genetic traits and discover whether antidepressants could help that person.

Doctors, he said, might say, "You have this genotype and you drink a lot, so you may benefit from a drug like Prozac."

SOURCES: Ming D. Li, Ph.D., professor and head of neurobiology, University of Virginia, Charlottesville; Robert Philibert, M.D., Ph.D., director of the Laboratory of Psychiatric Genetics, University of Iowa, Iowa City; February 2009, Alcoholism: Clinical & Experimental Research