New Clues to Why We Become More Forgetful as We Age.
Neurobiologists have discovered why the aging brain produces progressively fewer new nerve cells in its learning and memory center. The scientists said the finding, made in lab animals, refutes current ideas on how long crucial "progenitor" stem cells persist in the aging brain. Results of the study appear online in the journal Neurobiology of Aging. The research was funded by the National Institutes of Health and the U.S. Department of Veterans Affairs.
Previous studies by Shetty and others had demonstrated that as the brain ages, fewer new nerve cells, or neurons, are born in the hippocampus, the brain's learning and memory center. In one study, Shetty and colleagues showed that the production of new neurons in rats slows down dramatically by middle age -- the equivalent of 50 years in humans. But scientists did not know what causes this decline.
“The common assumption had been that the brain drain was due to a decreasing supply of neural stem cells in the aging hippocampus,” said lead study investigator Bharathi Hattiangady, Ph.D., research associate in neurosurgery. Neural stem cells are immature cells that have the ability to give rise to all types of nerve cells in the brain.
In the current study, however, the researchers found that the stem cells in aging brains are not reduced in number, but instead, as the rodents aged, they divided less frequently, resulting in dramatic reductions in the addition of new neurons in the hippocampus. They found that in young rats, the hippocampus contained 50,000 stem cells -- and, significantly, this number did not diminish with aging. This finding, the researchers said, suggested that the decreased production of new neurons in the aged brain was not due to a lack of starting material but rather a decrease in the division of stem cells.
"This discovery provides a new avenue to pursue in trying to combat the cognitive decline associated with conditions such as Alzheimer's disease and with aging in general," Hattiangady said.
The team now is searching for ways to stimulate the brain to replace its own cells in order to improve learning and memory function in the elderly.
One approach being explored is to treat older rats with drugs designed to mimic the action of compounds called neurogenic factors, which encourage stem cells in the brain to divide, Shetty said. The researchers also are grafting neural stem cells grown in culture dishes into the hippocampus, to stimulate those already present. Additional approaches include using behavioral modification techniques, such as physical exercise and exposure to an enriching environment, that are known to stimulate proliferation of stem cells.