Neuroscientists at the University of Utah investigate the region of the brain that regulates how sensitive we are to the negative effects of alcohol.
The brain's relationship to recreational drugs, of which alcohol is one, involves a complex system of reward and punishment. Alcohol triggers the brain's reward system, releasing pleasure-inducing neurotransmitters that make us want to consume more.
But the adverse effects of alcohol - hangovers, sickness and impaired motor function, among other things - help us to regulate our intake so that the consumption of intoxicants does not become a problem for us.
To understand more about how this shapes the way we learn to avoid things that are bad for us, the University of Utah researchers investigated how they could inhibit this regulatory mechanism in rats.
Their main area of interest was in a brain region called the lateral habenula.
Other recent studies have suggested that the habenula negatively regulates the motivation to consume nicotine and cocaine, and encourages us to learn from adverse experiences associated with these drugs.
The researchers inactivated the lateral habenula in a group of rats, and both these rats and a group of control rats were given intermittent access to a solution of 20% alcohol over several weeks.
They found that the rats with an inactivated lateral habenula escalated their drinking faster, drinking more alcohol than the control rats.
Study author Sharif Taha, PhD, professor of neurobiology and anatomy at the University of Utah, comments:
"In people, escalation of intake is what eventually separates a social drinker from someone who becomes an alcoholic. These rats drink amounts that are quite substantial. Legally they would be drunk if they were driving."
Did the rats drink excessively because they were failing to learn from bad experiences?
To test whether the inactivated lateral habenula rats were failing to learn from negative experiences associated with the alcohol, Dr. Taha and colleagues devised a second component of the experiment.
They gave the rats a sweet-tasting juice that was desirable to them, but then injected the rats with enough alcohol to cause negative effects - a hangover.
"It's the same kind of learning that mediates your response in food poisoning," says Dr. Taha. "You taste something and then you get sick, and then of course you avoid that food in future meals."
The rats with an inactivated lateral habenula escalated their drinking more rapidly than the control rats.
Image credit: University of Utah
Although the control rats learned to not seek out the juice, the inactivated lateral habenula rats were less likely to learn from the bad experience and continued to seek out the juice.
Co-author Andrew Haack explains:
"The way I look at it is the rewarding effects of drinking alcohol compete with the aversive effects. When you take the aversive effects away, which is what we did when we inactivated the lateral habenula, the rewarding effects gain more purchase, and so it drives up drinking behavior."
The team - who publish their results in PLOS One - believes that more specific research is required to understand exactly how the lateral habenula works. Does it regulate hangover effects, making a person feel the after-effects of drinking more or less strongly? Or does it instead control how the person learns from their experience?
"If we can understand the brain circuits that control sensitivity to alcohol's aversive effects, then we can start to get a handle on who may become a problem drinker," concludes Dr. Taha.