Death Triggers Regrowth of Some Neurons

Say what you like about birdbrains, they're actually quite sophisticated. Researchers have known for some time that certain patches of avian neurons grow and shrink with the seasons. Now a team has shown that neuron death can trigger the birth of other neurons--but only for certain cells that have the phoenixlike ability to replace their neighbors. Understanding what makes some neurons grow could guide the search for treatments for stroke or spinal cord damage.

A decade ago, scientists assumed that adult brains were doomed to constant, steady decline. Shortly after birth, it seemed, neurons lost their ability to grow, and cells that died could not be replaced. But in the past several years, scientists have found that certain kinds of neurons can grow in adult brains, including those of humans. Not only has this overturned scientific dogma, it has also provided hope for those suffering from degenerative brain diseases or paralyzing spinal cord injuries. Exactly what allows some brain neurons to grow remains mysterious, however. Now, research on songbirds may provide a clue.

A team led by Jeffrey Macklis of Harvard Medical School in Boston wondered whether the death of some neurons would trigger neighbors to multiply. Working on zebra finches, the team selectively killed neurons in a brain structure that controls singing--a region in which a few new neurons are constantly growing. They then injected the zebra finches with a compound that identifies growing neurons. This abnormal neuronal death did trigger cell growth: About three times as many new neurons sprouted in the treated birds as in control birds, the researchers report in the 24 February Neuron. But the ability to grow new neurons is limited to certain cells. Those that stretch to an area called "RA," which seems to control muscles used while singing, do regrow. But those that stretch to area "X," whose purpose isn't as clear, are not replaced by fresh neurons.

The work reinforces the idea that "the brain can make new neurons, but it can't make all of them," says neuroscientist Allison Doupe of the University of California, San Francisco. Discovering why some neurons are replenished could help scientists find ways to grow particular types of neurons in people, perhaps for treating stroke or other brain damage, says developmental neuroscientist Ron McKay of the National Institute of Neurological Disorders and Stroke in Bethesda, Maryland.