Rhesus monkeys with a particular version of a gene involved in the brain's reward system tended to be more stimulated by alcohol and drink more than other monkeys, according to new research from the National Institute on Alcohol Abuse and Alcoholism (NIAAA).

The study focused on monkeys with a specific version of the "mu" subtype of opioid receptors in the brain, which past studies indicated is most likely responsible for transmitting the positive effects of alcohol use. In humans, the 118G variant of the mu receptor is suspected of causing increased euphoria from alcohol consumption because it is better able to bind opioid peptides, which activate the reward systems of the brain.

"A mu-opioid receptor variant that is functionally similar to the human 118G variant occurs in these animals," said Christina Barr, M.D., Ph.D., of NIAAA's Laboratory of Clinical and Translational Studies and Laboratory of Neurogenetics. "... We hypothesized that monkeys that had the gene for this receptor variant would experience enhanced alcohol stimulation and, therefore, consumption."

All of the monkeys with the gene variant exhibited increased activity after alcohol consumption compared to other monkeys, and males with the gene variant drank twice as much alcohol and were intoxicated more often, as well.

"The male-restricted effect of this gene is interesting, and parallels other recent evidence that opioid transmission may play a greater role in alcohol problems among some males than among females," said Markus Heilig, M.D., Ph.D., NIAAA clinical director and the study's senior author.

"Although the pathway to alcoholism is influenced by many factors, our findings affirm that individuals who possess this receptor variant may experience enhanced pleasurable effects from alcohol that could increase their risk for developing alcohol abuse and dependence," added Heilig.

The study was published in the March 2007 issue of the Archives of General Psychiatry.