It is almost forty years since Fernando Nottebohm first began to describe some of the dynamic changes that occur in the brains of songbirds as the seasons change. Every year some regions of the brain grow in response to changes in ambient light levels and others regress. There are marked seasonal changes in the brains of fish, reptiles, amphibians, birds and even some mammals such as gerbils, mice and perhaps even in humans. But the magnitude of the changes in birds far outweighs any other species. It is hoped that that understanding the mechanism controlling that change may help us to develop treatments for age-related degenerative diseases of the brain such as Parkinson's and dementia.
Researchers from the University of Washington and the University of California, Berkeley, have published some interesting new data in the Proceedings of the National Academy of Sciences. They report a striking shrinkage in the size of the brain regions that control singing behavior of Gambel's white-crowned sparrows. This transformation is triggered by the withdrawal of testosterone and can be seen within 12 hours. The study is the first to report such rapid regression of brain nuclei caused by the withdrawal of a hormone and a change in daylight conditions in adult animals.
The research protocol was designed to mimic the natural seasonal changes that occur in the brains of the sparrows. Their song-control regions expand in the spring and summer leading up to the breeding season, as they use songs to establish territories and attract mates in Alaska. Later in the summer, as the birds get ready to migrate back to California, the same brain regions shrink.
To better understand what happens in the sparrows' brain, the researchers received federal and state permits to capture 25 of the migrating male birds in Eastern Washington. They then housed the birds for 12 weeks before exposing them to 20 days of long-day conditions comparable to the natural lighting the sparrows would experience in Alaska during the breeding season. The birds were also implanted with testosterone.
At the end of 20 days, six of the birds were euthanized and the remaining 19 were castrated and testosterone implants were removed so there would not be any circulating testosterone in their systems. After 12 hours five more birds were euthanized and the remainder were euthanized at 2, 4, 7 and 20 days.
The researchers found that the size of the high vocal center (HVC) region decreased 22 percent within 12 hours after the withdrawal of testosterone and that the number of neurons in this song-control region fell by 26 percent after four days. In addition, the size of two other song-control regions called Area X and the RA significantly regressed after 7 and 20 days.
Much as I dislike animal experiments of any kind, it is invaluable to have an animal model system in which we can observe predictable neurodegeneration. As men age, circulating levels of testosterone decrease, and other research has shown that this decline may contribute to cognitive impairment.
This is an important new approach to understanding the interplay between nerve cell degeneration and hormones.