Tiny strands of genetic material called microRNA can interfere with a brain chemical that normally protects against excessive alcohol drinking, a study suggests.

Why is it that half of U.S. college students binge drink, but just a fraction develop long-term drinking problems? Dorit Ron, a neurologist at the University of California, San Francisco, points to a regulatory system in the brain that can break down when people binge drink, leading some into unhealthy alcohol habits.

When people drink moderately, the body boosts the supply of brain-derived neurotrophic factor, or BDNF. BDNF allows the brain to change over time. It has also been recognized as a player in drug and alcohol addiction.

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Ron has shown that the presence of BDNF in the brain tends to ward off excessive drinking. But binge drinking pushes down BDNF levels in a mouse model of human alcohol abuse, Ron and her colleagues report in a paper published this week in the journal Molecular Psychiatry.

The blame lies with a bit of microRNA, a genetic substance that helps regulate gene expression, or how genes influence a person’s biology at a given time.

Ron and her colleagues saw a clear pattern in a part of mouse brains called the medial prefrontal cortex, which plays an important role in decision-making. When the microRNA, called 30a-5p, increased, BDNF levels dropped off. The mice then couldn’t control their alcohol cravings.

“They really binge drink — and the solution is bitter. It doesn’t taste good, but still they escalate and they drink large amounts of alcohol. It’s pretty amazing,” Ron said.

When the scientists limited the microRNA, BDNF levels went back up and the mice went back to drinking alcohol in moderation. The chemical switch may explain why some people get into bad patterns of binge drinking even after it’s no longer fun, and why they find their habits hard to break even if they aren’t physically dependent on alcohol.