The same amount of alcohol affects the brains of heavy drinkers differently than light drinkers, according to a new study from the University of California, San Francisco.

“The brains of heavy or problem drinkers are changed in a way that makes them more likely to find alcohol pleasant, and may be a clue to how problem drinking develops in the first place,” said Dr. Jennifer Mitchell, lead author of the study published in the journal Science Translational Medicine. “That greater feeling of reward might cause them to drink too much.”

Dr. Mitchell and her colleagues recruited 13 heavy drinkers and 12 control subjects who were light drinkers for the study. Each participant was injected with radioactive carfentanil, an opiate-like drug that binds to opioid receptors in the brain, and then underwent a brain scan using PET technology. The carfentanil caused the receptor sites in the brain to “light up.”

Next participants took a drink of alcohol, were injected with carfentanil second time, and underwent a second PET scan of their brains. This time researchers were able to track changes in the way their brains “lit up” under the new influence of alcohol, and to discover the exact areas where endorphins were released in response to alcohol. Endorphins are natural chemicals with opiate-like effects that cause feelings of pleasure. The heavy drinkers released more endorphins in response to alcohol than other participants, and they felt more intoxicated after just one drink than members of the control group.

“Greater endorphin release was associated with more hazardous drinking,” Dr. Mitchell said. “We believe this is an important step in understanding where and how alcohol acts in the brain.”

Dr. Mitchell’s team hopes to gain a better understanding of how alcohol works in different people so as to develop better, more-individualized treatments for alcoholism. The current drug approved for treatment programs is Naltrexone, but it only works in certain cases because it can cause unwanted side effects.