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Your Get-up-and-Go Got Up and Went
By Heather Mayer

It seems that as people settle into old age, they slow down, mellow out, and generally get less of a kick out of life. Now researchers at the National Institutes of Mental Health (NIMH) have a clue as to why this may be: The pleasure circuits of our brains change dramatically with aging.

New research indicates that as we get older, our brains may stop responding to the neural signals that used to bring us pleasure.

“I think something that many people as they age fear the most is having something wrong with their brains — either a dementia of some type or being depressed or not being able to enjoy life,” says Karen Berman, a neuroscientist at NIMH.

Berman and her team of researchers conducted a first-of-its-kind study, looking at the brain chemical dopamine in healthy young and old people. Dopamine is a chemical that is responsible for the pleasure responses that people feel, and Berman’s team found that dopamine receptors decrease with age, causing a decrease in the brain’s pleasure and reward response.

Measuring Pleasure in the Brain

“Dopamine in the brain is very important for motivating us, for keeping us happy, for letting us experience rewards and for helping us to go about our daily lives and carry out our plans,” Berman explains.

The researchers used two types of brain imaging to compare people’s response to anticipating and receiving rewards. The subjects were young people in their 20’s and older people in their 60’s and 70’s, who had aged successfully in that they were healthy, happy and well-adjusted, Berman explains. While in the brain scanners they were shown a video simulating a slot machine, and any “winners” during the experiment were paid off with real money.

The researchers used functional magnetic resonance imaging (fMRI) to study the subjects’ moment-to-moment brain activity while watching the slot machine during the MRI. This allowed scientists to see how the thinking part of the brain responds to anticipating and then winning a monetary reward. Subjects also viewed the slot machine video while in a positron emission tomography (PET) scanners. The PET scans allowed the researchers to monitor the subjects’ dopamine levels.

What Berman and her research team found was surprising, and the exact opposite of what they expected.

Youth Isn’t Wasted on the Young

In the younger subjects, Berman discovered that the more dopamine there was, the more their brains responded to both anticipating a reward and actually getting it. However, when there was more dopamine in the older subjects’ brains, they had less reward activity.

“We didn’t expect that,” Berman says. “We thought that the relationship might be dampened down. We didn’t expect it to be exactly the opposite.”

They measured this in the prefrontal cortex region of the brain, which is responsible for reward-related activity. The decreased activity in the older subjects’ pleasure center may indicate that older people’s brains are “simply less sensitive to important rewards,” Berman’s research team explains.

The way the pleasure and reward circuit works is that the dopamine neurons, which are deep in the brain where dopamine is made, send wires to the prefrontal cortex. The dopamine is released in the prefrontal cortex, stimulating that part of the brain’s response. Berman and her researchers discovered that this circuit differs among young people and older people.

The docking places for these neurons reduce with age, which may explain the decrease in reward activity in the older subjects, Berman says.

Insight into Successful Aging

While the results show how fragile the reward system in the brain really is and how easy it is for aging brains to veer off course, Berman suggests this may not always be a bad thing.

“It is conceivable that what we found does explain why older folks may be less prone to taking risks and may be less susceptible to becoming addicted to drugs,” Berman says.

And while aging is inevitable, losing the kick you get out of life doesn’t have to be. The NMIH researchers found that healthy-aging brains seem to have adapted mechanisms to modify brain circuits, like the pleasure and reward circuit.

“The important thing we found here is that the brain can adjust to neurobiological changes of aging and that can keep us going,” Berman says.

She says her study and its results open the door for future studies that may help researchers learn about unsuccessful aging, and how to help those who suffer from its effects.

“It is helpful to realize how fragile this system in the brain is,” she explains. “What goes on in successful aging, we’ll be able to gain insights that will help us to study when aging is not so successful — when people become depressed, when they lose their ‘get-up-and-go.’”

Elsewhere:

National Institute on Aging

National Council on Aging

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