Research Could Hold A Key On Human Memory Loss
The song of the zebra finch is a pretty simple one, and not particularly tuneful, but it can tell us a surprising amount about how brains work and preserve memories.
"One of the biggest hurdles in stem cell research now is directing new cells to go only to the site where you want them to go; it's like herding cats almost," said John Kirn, a Wesleyan University neuroscientist who has studied bird brains since the 1980s.
Birds can create new brain cells through most of their brains, while the creation of new neurons, known as neurogenesis, can occur in only a few regions of a mammal's brain. Better understanding of how neurogenesis happens in birds' brains, Kirn said, could lead to medical breakthroughs for humans.
"If we can understand how they manage to do this on the molecular level, it might give us some insights that we can use," he said, adding that stem therapy is one area that could benefit.
"There's something special about the bird brain that might be important in how we can create therapies for human brain damage," he said.
Kirn recently co-authored a study on neurogenesis in the zebra finch. The study, published in the May issue of the Journal of Neuroscience, could influence research into neurodegenerative illnesses in humans,includingAlzheimer's andParkinson's disease.
Typically, the song of a zebra finch will gradually degrade if it loses its hearing. But the researchers found that new neurons that developed in zebra finch brains helped the birds retain their song even after they were surgically deafened.
"It's completely counterintuitive to what everyone thinks neurogenesis is, which is to provide the flexibility for change and to learn new things," Kirn said. "And this is perhaps an example of the opposite of that. New neurons, in this case at least, are designed to preserve function."
To arrive at this insight, the research team studied a group of zebra finches, recorded their songs, injected the birds with a biomarker that would highlight new neurons, and then deafened half the birds. After 30 days, they analyzed the songs of the deafened birds to see which ones best preserved their songs. "We have some really sophisticated software for measuring all sorts of acoustic parameters," Kirn said.
The birds were then killed and their brains examined to see which birds had the most neurons. One of the scientists' predictions was shot down immediately.
"There's a lot of evidence that certain kinds of experience including social enrichment, can augment the number of neurons," Kirn said, adding that the researchers thought the lack of hearing would have the opposite effect. "We thought that the hearing birds would have more new neurons than deaf birds, but there was no difference."
But when the researchers looked at the brains of the deafened birds, he said, "that's when things got interesting."
"We found that the more new neurons a bird had, the longer it preserved the song after it was deafened," he said.
This has some implications for the brains of other species — including humans — and about the possible causes and even treatment of neurodegenerative disorders.
"On the very abstract scale, it suggests the possibility that in some brain regions, it might be possible to preserve information by adding new cells," Kirn said. "If [human patients are] losing cognitive function, if they're losing memories, this may be a way to not just enable you to learn new information, but actually preserve old information."
The link between the brains of birds and humans is indirect, Kirn said, but "not trivial." For instance, it was generally believed that most animals — including humans — could not produce new neurons later in life. By the 1990s, though, the idea was well-accepted that the production of new neurons did occur in certain animals — thanks largely to research on birds. Eventually, scientists accepted that it happened in humans as well, though only in certain parts of the brain.
Fernando Nottebohm, a neuroscientist at Rockefeller University in New York City and a mentor of Kirn's, was among the first to prove that neurogenesis occurs in birds. His study of bird brains grew out of an interest in figuring out how humans learn to vocalize.
"Some birds learn their songs much the way that people learn the sounds of speech," he said. And bird brains are a lot easier to study than the human brain. "We know much less about what goes on in the human than in the bird brain."
Kirn studies the zebra finch almost exclusively, although he did a brief stint concentrating on the canary. Unlike the canary, which learns a new song each year, the zebra finch has a limited repertoire. It learns one song in its first 90 days of life — made of four to eight notes "in very specific order and they don't vary at all" — and then sings it for the rest of its life.
Nottebohm studies both. He said they each have their advantages as study subjects. But, aesthetically, the more tuneful canary wins, hands down.
"Zebra finches have a squawky little song," Nottebohm said. "They sound like a mechanical cat."