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CHICAGO (Reuters) - The middle-aged brain is not as nimble as it used to be because of all the stress it has endured, a study released on Tuesday showed.
U.S. researchers said stress causes nerve cells in a part of the brain needed for learning to shrink and lose plasticity -- the ability to quickly form connections called synapses.
Younger animals can recover from this but older animals start losing this ability beginning in middle age.
The findings add new understanding to the process of aging, and may help explain why some people decline more quickly than others.
"We suspected that these nerve cells would be altered by age but the loss of synaptic plasticity in the context of life experience has profound implications for age-related cognitive decline," John Morrison of the Mount Sinai School of Medicine, whose study appears in the Journal of Neuroscience, said in a statement.
For the study, Morrison and colleagues studied young, middle-aged and elderly rats who were placed in a confined area for several hours, causing the release of stress hormones that bring about nerve cell changes in the prefrontal cortex -- a part of the brain used in learning.
The team then studied changes in a part of the nerve cells called spines that are used to form synapses. When they looked under a microscope, they saw changes in the spines of the young rats, showing they were able to adapt to the stressful experience. There were few changes in the spines in middle-aged rats and none in the oldest rats.
The findings suggest aging causes a significant loss in the brain's ability to respond to stress, something that is crucial to learning, Morrison said.
"The prefrontal cortex is constantly 'rewiring' in response to life experiences," Morrison said.
But he said the aged brain has already suffered significant loss of these nerve spines and the ones that remain are less able to respond in situations that require rewiring.
"The aged animal essentially loses its capacity for experience-induced plasticity," Morrison, who is 58, said in a telephone interview.
He was surprised by the findings at first and now finds them a bit sobering.
"I wouldn't want to try to learn a language," Morrison said. "People can do it in a partial way but nothing like a child can do it."
But you do not lose everything while aging, he said.
"One of the great stories about aging is you don't lose expertise. You are not losing the very stable synapses and circuits," Morrison said.
He said the findings point to a new approach in the search for treatments that protect the brain from age-related declines, such as in Alzheimer's disease.
"If we really want to understand Alzheimer's disease and deal with it effectively, we have to prevent it. And preventing it will require very early intervention," he said.