A collection of drunken rats is helping University of North Carolina researchers understand how brains repair themselves after chronic drinking — and possibly find new ways to help alcoholics recover full mental capacity.

Fulton T. Crews and Kimberly Nixon have discovered that heavy drinking slows the creation of new brain nerve cells, or neurons, in animals.

Rats that have been intoxicated and then sobered up produce more than normal amounts of neurons. That might explain why the brains of rats with simulated alcoholism shrink during chronic drinking but grow after the abuse stops — just like the brains of alcoholic people.

That growth is often accompanied in people by life-saving behavioral changes, such as the ability to understand how drinking harms them and their families. If that insight is tied to replenishing neurons, medicine may get a new way to speed healing among alcoholics.

"More experiments are needed, but this is a first tantalizing step," said Antonio Noronha, director of the National Institute of Alcohol Abuse and Alcoholism's division of neuroscience and behavior.

An estimated 8 percent of Americans are thought to be alcoholics.

For decades, scientists thought that adult humans and other animals had a finite number of neurons. But brain stem cells producing neurons even in adults were discovered in 1997.

Counting new cells made in living human brains is impossible. So Crews, who runs the Bowles Center for Alcohol Studies at UNC, asked Nixon in 2001 to find out how alcohol affects the production of new neurons in rats.

Nixon, a research scientist working for Bowles, found a way to chemically tag the new nerve cells and then tally them at different stages.

In each experiment, Nixon and lab workers regularly dose the rats over four days at eight-hour intervals — mimicking drinking binges — a mix of ethanol and a nutritional beverage. They aim to bring the animals' blood-alcohol levels up to 0.3, much higher than the 0.08 tolerated in drivers.

The scientists stay awake one full night to watch over the animals while they withdraw from the alcohol's effects, then sober up the rodents. Each rat is killed at a different point in recovery, depending on the experiment.

The bodies are injected with a form of formaldehyde that hardens its brain, and the researchers take slices from the hippocampus, a region of the brain linked to learning and memory in animals and people.

The slivers are then soaked in substances that stain new neurons dark enough to be seen with microscopes using 1,000-fold magnification.

"We get obsessive," Nixon said. "We need the numbers of how many cells there are. That is worth counting."

Crews' lab now hopes to try to track the new neurons to see where in the brain they become functional. That will deliver more clues to their function.

Dennis Parnell runs The Healing Place of Wake County, a residential treatment center for recovering alcoholics and drug addicts in Raleigh. He expects no simple cure for alcoholism, but the results produced by Crews, who has received $10.9 million in federal research grants over the past five years, make sense to him.

"We used to think that when people got wet brains they damaged themselves and couldn't come back," he said. "Now we see if you can keep a person sober, they make improvements over 18 months."

  And, he said, the research gives alcoholics something that surpasses science — hope.

"It helps people understand," Parnell said. "It helps them hang in there."