|BRAIN CELL GROWTH AFFECTED BY STRESS
For years, neurobiologists clung to a fundamental truth: as animals
and people reach adulthood, they lose brain cells and they never grow
new ones. There were a couple of exceptions such as birds and rats,
but the thought was that these were peculiarities of nature and not
evidence of a general principle.
Experts say they fully expect that humans are no different and that they, too, make new brain cells in adult life. That raises the glimmer of a possibility of eventually treating degenerative disorders like Alzheimer's or Parkinsons disease and injuries such as those resulting from stroke or trauma — by prompting the brain to grow replacement cells.
It also means that neurobiologists must rethink basic notions of the way the brain changes with learning or life experiences.
Dr. Elizabeth Gould of Princeton University, Dr. Bruce S. McEwen of Rockefeller University in Ncw York and their colleagues investigated using marmoset monkeys, adding two tracer chemicals to the animals' brains: one that labeled cells that were dividing — the process that gives rise to new cells and one that labeled mature nerve cells. Cells that were born during adult life and that grew into mature brain cells would be marked by both chemicals.
With this method, the researchers looked for, and found, new cells in the animals' hippocampuses.
Dr. Gould estimated that thousands of such cells were being made each day. She said she suspected other cells were dying to make room for new ones, but her study did not count numbers of dying cells.
The hippocampus was particularly intriguing for another reason, Dr. Gould said. Earlier research had shown that when people are under stress, the hippocampus shrinks in size. For example, people with tumors that pour out the stress hormone cortisol have a diminished hippocampus. So do people with recurrent depression and people with posttraumatic stress disorder, Dr. Gould said.
It might be possible, she reasoned, that monkeys under stress might decrease their production of new brain cells in the hippocampus, making that area of the brain shrink.
To test the hypothesis, Dr. Gould and her colleagues stressed monkeys by putting a male monkey who had always lived alone into a small cage where another male was living. The intruder was terrified and cowered in the cage, with a rapidly beating heart. When Dr. Gould and her colleagues examined the brains of the frightened monkeys, they found that after just one hour of this stress, the monkeys were making substantially fewer new I brain cells.
The study is being published in The Proceedings of the National Academy of Sciences.
As so often happens in science, the seeds for the new view of brain regeneration were sown decades ago, but were largely ignored.
In the 1960s, Dr. Joseph Altman, a Purdue University scientist who is now retired, reported that rats make new brain cells throughout their lives. The cells were in the hippocampus and in the olfactory bulb, an area used to sense smells, he noted. "No one paid attention,"Dr. Gould said.
Twenty years later, Dr. Fernando Nottebohm, who is head of the laboratory of animal behavior at Rockefeller University, asked whether brain cells were being born in adult birds. Bird brains, he noticed, grow and shrink with the seasons, swelling when the animals need to learn new songs to attract mates and shrinking after they had bred. He wondered whether the swelling brains during breeding seasons could represent the actual growth of new brain cells. At the time, Dr. Nottebohm said, he knew nothing of Dr. Altman's work.
In a series of painstaking experiments, Dr. Nottebohm showed that birds constantly make new brain cells and that the new cells replace old ones that die. "There was a program of constant brain rejuvenation,"Dr. Nottebohm said.
In 1984, Dr. Nottebohm organized a meeting in New York that he called Hope for a New Neurology A colleague at Rockefeller, Dr. Arturo Alvarez-Buylla recalled that Dr. Nottebohm "was pushing the idea that in the adult brain, there is no impediment to the formation of new neurons." But, Dr. Alvarez-Buylla added, "people thought that was bordering on fantasy."Nonetheless, some researchers persisted, showing in rats and mice and in tree shrews that new brain cells are born throughout life, at least in the hippocampus and olfactory bulb.
Dr. Alvarez-Buylla, for example, recently found that adult mice make 5,000 to 10,000 new brain cells each hour. The brain cells that end up in the olfactory bulb are born on the walls of the ventricles, cavities in the brain that are called with cerebrospinal fluid. They travel in "little trains of cells" to their destination, he said. Those that end up in the hippocampus are born there. But many scientists believed that monkeys and humans could not be growing new brain cells -- that in order to store memories for a lifetime, you need a stable brain.
Dr. Gould said. "If cells are constantly dying and new ones being produced, how would that be possible?" Dr. Gould, however, was persuaded by the findings on other species. ''Why not monkeys?" she asked. Others also began seeking and finding brain regeneration in monkeys, but Dr. Gould is the first to publish her findings.
Can anything be done about this? Are we destined to lose our faculties as we age? There is some hope on several fronts. Recent research on Alzheimer's Disease suggests that there might some day be a vaccine that protects us from Alzheimer's build-up of plaques and tangles in our brains. Some studies suggest that physical exercise keeps brains healthy. There is also research that suggests that we can keep our brains working well by using them regularly. This is the principle behind a recent book by Lawrence Katz, Ph.D. and Manning Rubin. Katz is a neuroscientist at Duke University who studies brain aging. Their book, Keep your Brain Alive, describes practical exercises that anyone can do to keep the brain working at top efficiency. Eating or brushing your teeth with your non-dominant hand, for example, allows your brain to use pathways that are not frequently accessed.
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