Hear this . . .
By Marina Chicurel WHAT a bird hears, or fails to hear, can determine the fate of new nerve cells in its brain, researchers in Connecticut have found. The same could be true for humans. In birds, certain parts of the brain continue to sprout new cells well into adulthood. Experience can shape the fate of these neurons—chickadees that don’t exercise their memories, for example, end up with brain regions that lack their normal share of cells. To trace how a neuron’s fate is moulded by experience, John Kirn and his colleagues at Wesleyan University in Connecticut tracked the lives of a group of newly formed neurons in hearing and deaf zebra finches. “Most people use one snapshot of time to look at the number of neurons added as a result of a given experience,” says Kirn. Instead, his team looked at the time when the new cells arose and also examined the same cells once they had been integrated into the brain. The researchers deafened adult songbirds by surgically removing their cochleas. Then they injected the birds with a radioactive marker that labels dividing cells so they could trace the insertion of new neurons in a brain region that controls singing, the hyperstriatum ventrale caudalis (HVC). A month after the injections, the deafened birds had incorporated 70 per cent fewer labelled neurons into the HVC than hearing birds. When the researchers checked three months later, only a third of the labelled neurons in hearing birds remained, suggesting that these neurons have very short lifespans. But in the deafened birds, almost all the labelled neurons inserted into the HVC at one month were still there. Kirn argues that fewer new neurons int-egrate into the HVCs of deaf birds, but that those that do live longer. This suggests that the birds suffer a lack of turnover of neurons in their hearing centre as a result of their deafness. He speculates that old neurons in the HVC make way for young ones as they die. If the neurons of deaf birds live longer, their old neurons may bar young ones from occupying positions in the HVC. “This is a very novel and important result,” says Eliot Brenowitz, an expert in the neurobiology of birdsong at the University of Washington. But he believes Kirn has yet to establish that deafening prolongs the time that neurons survive. Kirn is now planning to test his hypothesis by tracking neurons labelled before the birds were deafened. Studies like Kirn’s may help develop new ways to repair brain damage, as Kirn predicts that harnessing the adult human brain’s potential for creating new neurons will greatly depend on understanding how neurons are destined to live or to die. Source:The Journal of Neuroscience(vol 19,