New Brain Cells Grow in Songbirds Each Year: Neuron Growth Investigated

You may think that dying brain cells are a bad thing but for birds, it's a natural part of their yearly cycle. Brain cells the multiply to help birds sing their best during breeding season end up dying back naturally later in the year. Now, scientists have found out how neuron growth in these birds begins anew each spring.

Neurons can be generated in the adult brains of many animals. Yet the birth of new neurons, called neurogenesis, is limited in mammals and humans, especially where new neurons are generated after there's been a blow to the head or other physical loss of brain cells. That's why scientists decided to examine the brain's ability to replace cells that have been lost naturally.

"Many neurodegenerative disorders are not injury-induced, so it is critical to determine if and how reactive neurogenesis occurs under non-injury-induced neurodegenerative conditions," wrote the co-authors in a news release.

In this case, the researchers examined Gamble's white-crowned sparrows. These birds, like most songbirds, experience growth in the area of the brain that controls song output during the breeding season when a superior song helps them attract mates and define their territories. At the end of the season, though, the cells begin dying naturally and the bird's song degrades.

"As the hormones level decrease, the cell in the part of the brain controlling song no longer have the signal to 'stay alive,'" said Tracy Larson, lead author of the new paper, in a news release. "Those cells undergo programmed cell death-or cell suicide as some call it. As those cells die it is likely they are releasing some kind of signal that somehow gets transmitted to the stem cells that reside in the brain. Whatever that signal is then triggers those cells to divide and replace the loss of the cell that sent the signal to begin with."

In this case, the researchers blocked cell death at the end of the breeding season by using chemicals to turn off the cell "suicide" pathway. This caused far fewer stem cells to divide and there wasn't the large uptick in new neurons being born.

"There's no reason to think what goes on in a bird brain doesn't also go on in mammal brains, in human brains," said Eliot Brenowitz, co-author of the new paper. "As far as we know, the molecules are the same, the pathways are the same, the hormones are the same. That's the ultimate purpose of all this, to identify these molecular mechanisms that will be of use in repairing human brains."

The findings reveal  a bit more about how these cells die and then grow again each year. This could help with future research into human cell regeneration.

The findings are published in the Journal of Neuroscience.