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Can neuronal replacement slow the mind's aging?

American Association For The Advancement Of Science (AAAS) : 11 July, 2005  (Company News)
Pioneering neurobiologist Fernando Nottebohm told a AAAS audience that, after a career of studying the brains of song-birds, he believes humans may someday be able to replace neurons in the human brain to offset the effects of disease, injury or getting old.
Nottebohm cautioned that he was 'crystal-balling' the future, and that such dramatic advances in brain science were not likely to affect anyone alive today. But in delivering the 2005 John P. McGovern Lecture, he said research is building an understanding of how some types of neurons are regenerated in songbirds and mammals.

'The challenge is to extend the courtesy of replacement to those [neurons] that are not normally replaced,' Nottebohm said at the AAAS Annual Meeting in Washington, D.C. If that were possible, the result might be 'a sprightly young mind that is able to learn' even in old age, but also a mind 'that has forgotten much of who you are.'

Nottebohm is the Dorothea L. Leonhardt Distinguished Professor at Rockefeller University in New York and head of the university's Laboratory of Animal Behavior. During an influential, prize-winning career, he has used zebra finches, canaries and other birds to study the complex process by which they learn and produce their songs. His findings have had a strong influence on the understanding of the biological and neural underpinnings of human language.

The human brain contains a network of billions of neurons, each a cell that uses biochemical reactions to receive, process and send information. At the time he began his career in the 1960s, it was assumed that humans had a fixed number of neurons and that though some were lost in aging, they were not replaced.

Nottebohm, however, was the first to observe neuro-genesis in the adult brain In his lecture at AAAS, he described the roots of that research, a project in which he gave a radiological 'label' to songbird neurons, and then was able to track that label being passed on as the cells reproduced.

'So there are hundreds of thousands of neurons being created in the adult canary,' he said, 'presumably to replace old ones.'

Nottebohm also studied neuron generation in chickadees. The birds are very 'conservative,' he said, and in the fall, they scatter their store of winter food into cracks and nooks across a habitat of around 30 acres. Remembering where the food is requires a mental feat. According to Nottebohm, the feat is possible because the chickadee brain steps up neuron creation in the autumn.

And, he noted, the number of new neurons created in free-range chickadees is three times higher than for birds kept in captivity. The dramatic difference suggests that medical researchers may be building a flawed body of knowledge if they base their findings on animals in captivity, where brain processes may be artificially suppressed, Nottebohm said.

In human brains, only about 5 to 8 percent of neurons are capable of regeneration, he said. Still, 'once you realize cells are being replicated in the brain, you begin to see the brain in a slightly different way,' he added. 'Because if you lose cells, you lose a little bit of who you are.'

The neuron losses wouldn't stop if we could regenerate neurons more broadly, Nottebohm explained. Instead, however, the brain might maintain a youthful ability to keep learning despite the losses.

But isn't a canary brain is much different from a human brain?

'I'm not too impressed with the difference between mammals and birds,' he told the audience. 'Most of it is packaging-the circuitry is very similar.'

Nottebohm, a native of Argentina, is a fellow of the American Association for the Advancement of Science and of the American Academy of Arts and Sciences. He also is a member of the U.S. National Academy of Sciences, and of the American Philosophical Society.
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