Then they had the participants have a drink of alcohol and injected them with the tagged drug again. The researchers expected to see much less "lighting up" in the brain this time around, since the natural endorphins in the participants' brains should bind to the opioid receptors before the radioactively labeled drug could get there.

This was exactly what they found. One type of opioid receptor in particular, known as Mu, was bound by the endorphins.

"This is something that we've speculated about for 30 years, based on animal studies, but haven't observed in humans until now," said lead author Jennifer Mitchell in a press release. "It provides the first direct evidence of how alcohol makes people feel good."

All of the participants, whether heavy drinkers or controls, reported more feelings of pleasure from the alcohol when more endorphins were released in a brain region called the nucleus accumbens, involved in pleasure and reward. When there was more activity in another area, the orbitofrontal cortex, the heavy drinkers -- but not the controls -- reported more feelings of intoxication.

"This indicates that 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 Mitchell. "That greater feeling of reward might cause them to drink too much."

The results of the study could also help researchers design a better drug to treat alcohol addiction. Senior author Howard L. Fields pointed out that there are drawbacks to naltrexone, the FDA-approved medication to treat alcohol and opioid dependence, by binding to opioid receptors. It's not been a terribly effective method for treating alcoholism "not because it isn't effective at reducing drinking," Fields said, "but because some people stop taking it because they don't like the way it makes them feel."

Instead of blocking a range of opioid receptors, a more specialized drug could target only the ones that play a role in alcohol use, and perhaps come with fewer side effects. "If we better understand how endorphins control drinking," said Fields, "we will have a better chance of creating more targeted therapies for substance addiction. This paper is a significant step in that direction because it specifically implicates the Mu opioid receptor in alcohol reward in humans."

The research was carried out at the Ernest Gallo Clinic and Research Center at the University of California, San Francisco, and published in Science Translational Medicine.

This article originally appeared on TheDoctorWillSeeYouNow.com, an Atlantic partner site.