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Updated 2:00 PM November 8, 2006
 

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  Research
U-M study shows how brain responds to pleasant tastes

We all have tastes we love—and tastes we hate.

And yet our taste for certain flavors and foods can change over time as we get older or get tired of eating the same old thing.

A U-M study gives new evidence about what's going on in the brain when we taste something we like, or develop a liking for something we once hated.

Although the study used rats instead of people, it has direct implications for understanding the way we perceive pleasure—and the reasons some people develop problems such as drug abuse, depression or anorexia that knock their pleasure response off balance.

In a new paper in the November issue of the Journal of Neurophysiology, U-M neuroscientists and psychologists report the findings from direct monitoring of an area of the brain known as the ventral pallidum. Deep inside the brain, it's a kind of traffic center for signals from different areas that process tastes and pleasurable sensations.

Researchers were able to track the activity of brain cells in that area while the rats received water, salt water and sugar water directly into their mouths. They also recorded how the rats behaved while tasting those different solutions, including signs they "liked" or "disliked" the tastes. Tests were repeated when the rats had been treated with drugs that greatly reduced their bodies' salt levels.

At first all rats behaved negatively after tasting a strong salt-water solution, compared to the water or sugar water. Their ventral pallidum brain activity also was much lower in response to the salt water.

But when the researchers put the rats into a salt-deprived state using a combination of diet and hormones that cause the body to get rid of salt, the picture changed. Suddenly, the rats' brain activity rose as high when they received the salt water as it had when they received sugar water. The effect lasted for a while after the rats' bodies returned to normal salt levels, but soon enough it wore off.

"We converted something that wasn't pleasing to something that suddenly became pleasurable, and when we did that, the neurons we were studying switched their response," says senior author J. Wayne Aldridge, a research associate professor in the Department of Neurology at the Medical School. "Pleasure has traditionally been one of the hardest problems for neuroscience to measure, but these results shed light on how it is represented in brain activity."

The study was designed so that the signals from the ventral pallidum were related only to the liking—or disliking—of the taste, and not to a salt-seeking drive or movement. The ventral pallidum is part of the limbic system of the brain, which is involved in motor-muscle control as well as pleasure and reward.

In an accompanying editorial, University of North Carolina researchers Robert Wheeler and Regina Carelli call the study "elegant" and the results a "profound step" toward understanding the nature of pleasure itself, rather than the behaviors and actions triggered by it.

Aldridge collaborated on the study with former U-M psychology graduate student Amy Tindell and with Kent Berridge, a professor in the Department of Psychology in LSA.

"This finding reveals a type of brain Morse code for pleasure," says Berridge. "The faster these neurons fire, the more pleasant the taste seems to become. The hardest test for a pleasure code is whether the brain signal can track the change from nasty to nice. The amazing fact is that these neurons pass that test."

Aldridge notes an analogous effect that occurs in everyday human life, when a formerly favorite food becomes less attractive after we have overindulged in that food.

"Moment by moment, this low-level information processing in the brain helps us react to what we like or don't like," he says. "These neurons respond to a taste as pleasurable, or as not pleasurable."

But research on how these neurons fire to signal pleasure is important for more than just curiosity's sake, he adds. The ventral pallidum is an important brain region for both pleasure and craving. Firing patterns gone wrong in the ventral pallidum possibly could contribute to eating disorders, anorexia and drug addiction.

Eventually, activation in the ventral pallidum in response to pleasurable tastes also could be useful in brain-mapping techniques in humans. All in all, Aldridge says, the new paper is "a really good example of how animal experiments help us understand the human brain. If we can understand how the brain generates normal pleasures, we may have a new focus for effective treatments in people who don't experience normal pleasure."

In addition to Tindell, Berridge and Aldridge, who also holds an adjunct appointment in the Psychology Department, the study's authors include psychology postdoctoral fellow Susana Pecina and graduate student Kyle Smith. The study was funded by the National Institutes of Health and the National Science Foundation.

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