The same brain chemical thought to increase our desire to overeat also appears to increase alcoholic tendencies, according to research released Tuesday.
A new study examined the behavior of drunken rats, but has implications for humans, researchers say. Scientists injected tiny amounts of the neuropeptide galanin, which has been shown to trigger excessive eating, into rats' brains and monitored their eating and drinking behaviors. Their water consumption did not change, but they drank far more alcohol after being injected with the appetite-increasing chemical.
Previously, the researchers found that drinking alcohol increased the amount of galanin in the brain. They described this effect as a "positive feedback loop" between alcohol and the chemical, which also appears to prompt cravings for fatty foods. Foods high in fat have also been shown to trigger a similar feedback loop involving galanin.
"We definitely think there's a linkage there," said Michael Lewis, a Princeton University psychology researcher and one of the study's authors.
While the rats began drinking more after injection with galanin, they showed no increase in eating. The researchers correlated this result with studies showing that alcoholics often stop eating regularly as they drink more.
"In dependent individuals that system sort of gets shifted," Lewis said. "Instead of being motivated by fat, it's motivated by ... alcohol."
Rats are noctural, and while they drank more alcohol at night in this study, they also drank during the day — when they usually sleep — and exhibited similar sleep disturbances to those suffered by alcoholics.
Galanin in the brain appears to increase the presence of dopamine, a neurotransmitter closely tied to the body's sense of pleasure. Many addictive substances, including nicotine and cocaine, are thought to trigger the release of dopamine, which accounts for part of the addictive high.
A caloric effect?
Unlike most addictive substances, both alcohol and food are caloric, which links galanin specifically not only to an addiction but to an addiction focused on calories, especially those from high-fat foods.
Rockefeller University researcher Sarah Leibowitz, who took part in the current research, found in an earlier study that a single high-fat meal — the rodent equivalent of a bacon double cheeseburger — increased levels of galanin in rats' brains.
Dozens of neuropeptides like galanin have been linked to eating, but the latest research helps underscore galanin as one of the best understood elements of behavioral links between overeating and overdrinking. The study was published in the December issue of the journal Alcoholism: Clinical and Experimental Research.
Both fat and alcohol can lead to "nonhomeostatic" behavior, Leibowitz said, meaning that we keep consuming them beyond the point that our bodies would otherwise tell us we've had enough.
But she said it wasn't clear that the same psychological dependencies prompted by alcohol can be seen in fatty foods.
"There's no question that we do overconsume fat," Leibowitz told MSNBC.com. "We like the taste of it. It has positive aspects to it. But that's a far cry from addiction."
'An intriguing possibility'
Todd Thiele, an associate professor at the University of North Carolina at Chapel Hill's Bowles Center for Alcohol Studies, said the latest results indicated "an intriguing possibility" that galanin plays a key role in regulating abuse of both food and alcohol. But he noted that more work is needed to understand whether galanin is triggered by alcohol's inebriating effects or its calories.
"It's hard to know if animals are drinking more alcohol because the galanin is making the alcohol more rewarding," he said. "These are important issues that need to be teased out."
Leibowitz said forthcoming research is expected to show a direct connection between fat and alcohol intake: "We do know that the more fat we eat, the more alcohol we want to drink."
If galanin is eventually shown to play a large role in regulating both eating and drinking, drugs that control its function in the brain could potentially help stem the effects of alcoholism and overeating. When the researchers tried a galanin blocker, they found the rats drank somewhat less during the first hour after injection.
But any such medicine is years away. No effective compounds have emerged to limit galanin's impact over a sustained period of time.
While galanin's fat-craving effects haven't been explicitly tied to obesity, the latest study bolsters theories that overeating has similar triggers and impacts as alcoholism, and that treatments for both may require similar approaches.
"What we're focusing on is trying to make that connection between overeating and overdrinking and what's going on in the brain, and we're delighted that we see some connections there," said Lewis.