Bird brains display tuneful cell surge.

Bird brains display tuneful cell surge

Adult canaries and zebra finches generate a fresh supply of brain cells to replace those lost with age in forebrain regions that control song learning and production, according to a report in the Sept. 21 SCIENCE. The newly formed cells, known as projection neurons, extend their message-bearing axons over roughly 3 millimeters and link two related structures in the birds' cerebral song-control center.

The findings suggest that adult avian brains possess considerable potential for self-repair as neurons grow old and wither away, says study director Fernando Nottebohm, a neuroscientist at Rockefeller University Field Research Center in Millbrook, N.Y. "But we're not sure if or how these findings apply to humans and other mammals," he adds.

For now, the data indicate a relationship between the development of avian memories for either the perception or production of distinctive songs and the appearance of substantially more neurons in song-control regions of the brain.

Twice daily for two weeks, Nottebohm and his co-workers injected birds with a substance that labels newly emerging brain cells. Four months later, when the labeled population of neurons in the forebrain area concerned with song control had grown and stabilized, they injected the birds with another substance that stained the previously marked neurons. The animals were then killed and neuron counts were conducted.

Four adult male canaries, all 1 year old, received their first injections in May, a time of stable song production. Another six adult makes were injected at 17 months of age in October, when songs undergo seasonal modifications. Two additional males, slightly more than 4 years old, entered the experiment in October.

Canaries injected in October generated significantly more projection neurons in the song-control center than did the May group. Moreover, the 4-year-old October duo displayed considerably more new projecting neurons than did the 1-year-old May quartet. Either a lower rate of cell production or a higher rate of cell death may account for the slower generation of new projection neurons in May, the researchers maintain.

Testing of six adult male zebra finches, which do not modify their songs, revealed projection neuron increases roughly comparable to those observed among canaries injected in May.

Although the October neuron increases in adult canaries apparently stemmed from learning new songs, even zebra finches experienced noticeable, unexplained jumps in neuron genration, the scientists report.

Nottebohm says his team plans to investigate possible ways to boost the natural process of self-repair and neuron generation in the brains of adult birds. Little evidence exists for new cell growth in the forebrains of mammlas, he adds. But if scientists eventually extend the avian observations to mammalian species, they can then seek methods of reversing neuron degeneration in memory-destroying conditions such as Alzheimer's disease, Nottebohm suggests.