New research suggests that tobacco smokers have a greater tendency toward alcohol consumption.

Researchers at the Scripps Research Institute found that, in rat models, nicotine exposure actually promotes alcohol dependence.

"It's a vicious cycle," Olivier George, a senior author of the new study, said in a statement. "Nicotine makes individuals crave alcohol to 'reward' the brain and reduce stress."

For the study, the team first tested whether nicotine exposure could affect alcohol-drinking behavior in rat models. They started with two groups of male rats. Both groups were given access to alcohol to establish the baseline of how much they would drink. The rats drank a little bit, perhaps the equivalent of one or two beers for a human, but they stopped before showing signs of drunkenness.

After this baseline test, the researchers used alcohol vapor to induce alcohol dependence in one group of rats. Dependence developed in about two months. In subsequent tests where alcohol was freely available, these rats consumed the equivalent of a six-pack of beer and had blood alcohol levels close to three times the legal limit for humans.

The second group of rats was exposed to both nicotine and alcohol vapor. These rats developed alcohol dependence much faster -- and they began drinking the equivalent of a six-pack in just three weeks. "We had never seen such a rapid escalation of alcohol drinking before," said George.

The researchers then offered the rats alcohol with the bitter compound quinine added to see if they could stop the rats from drinking. Most rats decreased their alcohol consumption to avoid the bitter taste, but the nicotine-exposed rats just kept drinking. This indicated that their behavior was compulsive, much like alcoholism in humans.

Using further neurological studies, George and his colleagues tracked this compulsive behavior to the activation of "stress" and "reward" pathways in the brain.

Previous studies from George's lab had shown that nicotine activates certain "reward" neurons in the brain -- giving positive reinforcement to keep smoking. At the same time, nicotine activates "stress" neurons in the brain, giving negative reinforcement. This stress can lead individuals to crave alcohol to both activate the reward system and calm the stress system.

The compulsive alcohol consumption and neurological pathways seen in the new study suggest that alcohol works with nicotine to further activate the brain's reward system and dampen the stress of nicotine exposure.

Researchers said this interaction may explain why it is difficult for smokers to quit drinking, and vice versa. Interestingly, the combination of neurons activated by nicotine and alcohol together is different from the neurons activated by each substance on its own.

The findings are detailed in The Journal of Neuroscience.