Developmental neurobiology of vocal learning in song birds.
Neurogenesis, neuronal death, and pathway formation are largely restricted to embryogenesis in most warm- blooded vertebrates. However, neurogenesis in birds continues throughout adulthood in the walls of the lateral ventricles, and the neuroblasts produced there infiltrate most of the telencephalon. In effect, much of the adult avian brain is constantly being “rewired.” My lab is interested in understanding the kinds of neurons produced in adulthood, how long they live, the way neuronal replacement is regulated, and the impact of neuronal turnover on adult brain function and behavior.
These questions are being investigated with a variety of techniques in the vocal control system of oscine songbirds. These birds posses a discrete network of brain areas that control song learning and production. We have shown that one cell type produced in adult canaries is a projection neuron that synapses on target neurons 2-3 millimeters away to become part of the motor pathway for vocal control. Cells of this type can live for eight months or longer and replace older cells that have died. Over the six months from spring to fall, between one-third and one-half of all projection neurons of this type are lost and replaced. Interestingly, the period from spring to fall encompasses the seasonal transition from stable song-to-song learning in adult male canaries. Thus, there may be a relationship between the seasonal renewal of brain circuits and song learning.
Adult neurogenesis is regulated, in part, by cell death. In a collaborative effort, we have shown that selective laser photolysis of vocal control neurons results in an increase in new neuron replacement. Interestingly, when neurons are killed in this way, there is a transient loss of song acoustic structure. Partial-to-complete song recovery then follows as the dying cells are replaced.
We have also shown that experiential factors influence cell turnover in adult songbirds. Moreover, there is an age-related decrease in neuron addition that parallels increased stability in the song motor program. This age effect may reflect the amount of vocal practice a bird has had. Current work attempts to isolate the contributions of auditory and motor activity in the regulation of neuron turnover. We are also using retroviruses to chart morphological change in new neurons as they age. An intriguing hypothesis is that singing regulates cell replacement and cell replacement is necessary for song structure.
Funding: National Institutes of Health and National Science Foundation.