This sparrow massively enlarges part of its brain in preparation for mating.

Gambel's White-crowned Sparrow (Zonotrichia leucophrys gambelii), is found on stony ground in the Azores.

Gambel’s White-crowned Sparrow (Zonotrichia leucophrys gambelii) is native to North America.Credit: blickwinkel/Alamy

Researchers have long known that the areas of songbird brains responsible for singing expand during the mating season, then shrink once the season is over. But one species, the white-crowned sparrow (Zonotrichia leucophrys gambelii), does so on a scale that scientists struggle to understand. A part of the male sparrow’s brain, called the HVC, grows from about 100,000 neurons to about 170,000, almost doubling in size, during the bird’s mating season.

Although how the bird achieves this feat remains a mystery, scientists who data presented at the annual meeting of the Society for Neuroscience in Washington DC November 11-15, we’re about to get some answers. They hope their findings could one day point to ways to treat abnormalities in the human brain.

In most animals, when a region of the brain grows and shrinks, “often it has quite detrimental consequences for behavior and brain function,” says Tracy Larson, a neuroscientist at the University of Virginia in Charlottesville who directed the work. In particular, growth on this scale in mammals would cause inflammation and increase pressure inside their skulls. But when it comes to sparrows, “there’s something really fascinating about these birds: They manage to do this without having any harmful impacts,” Larson adds.

The skull as a “rigid suitcase”

Larson’s research so far has suggested that the sparrow’s brain uses a range of tactics to form and then quickly kill large numbers of neurons. One question Larson wanted to answer is how the sparrow’s brain shrinks significantly at the end of the mating season. So, she and her colleagues labeled cells in and around the HCVs of male sparrows with a molecule called bromodeoxyuridine (BrdU), which can incorporate itself into the DNA of dividing cells. They also used hormonal supplements to simulate the breeding season in birds.

Toward the end of the simulated breeding season, they found that BrdU-labeled stem cells near the HVC began to divide and develop into astrocytes, brain cells that support neurons in various ways. The labeled stem cells were located between the HVC and the hippocampus, a deep brain region involved in learning and memory. In sparrows, the seahorse also generates new neurons at certain times, particularly before mating season, when birds are migrating and need to increase the amount of brain power dedicated to navigation.

Birds appear to allocate space in their brains based on which region needs it most at different times of the year. The skull is like “a hard suitcase,” Larson says. “If you put more neurons in the HCV, something has to give somewhere else.”

The extra HVC neurons die at the end of the mating season. Astrocytes located in the same brain region clean up the damage and die in the process. Stem cells located outside the HCV then appear to replace these lost support cells. According to Larson’s observations, almost all astrocytes in the sparrow’s HVC are replaced during this period.

The work is still preliminary, but it is innovative, explains Jacques Balthazart, biologist at the University of Liège in Belgium. People have quantified quite extensively the neurons that appear in CVH, but not the astrocytes, he adds.

Larson hopes the cycle of neurogenesis and degeneration will provide a model for studying depression in other animals, including humans. When the sparrow loses neurons, it shows signs of loss of motivation to sing, which could be comparable to a form of depression. So how the bird exits this phase — the chemical or physical mechanisms that return HCV toward growth — could offer clues about how to treat depression, Larson says. “How do these birds go from this depressed state to a naturally super excited state? »


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