Date of Award
neurogenesis, zebra finch, HVC neurons, birdsong
The study of neurogenesis in adult songbirds focused initially on the canary, a species that learns new song elements each year. Many neurons in the canary song control nucleus, HVC, are discarded every year, with cell loss peaking at the end of the breeding season. New neurons numerically replace those that have died and this replacement process occurs at a time when vocal output is most plastic, suggesting adult neurogenesis may have a role in the learning process. It is not known, however, whether spontaneous neuronal replacement accompanies recruitment of new HVC neurons in the adult zebra finch, a species that learns its song only once prior to sexual maturity. To test for numerical replacement in the adult zebra finch, we conducted three experiments. First, we retrogradely labeled and counted the two populations of projection neurons in HVC in birds ranging in age from 90 days, when they are first sexually mature, to 11 years. We found that the neurons projecting from HVC to the robust nucleus of the arcopallium (HVC-RA neurons), which are known to be recruited after hatching and into adult life, substantially increased in number with age. In contrast, the number of neurons that project to Area X (proper name, HVC-X neurons) and are only produced during early development, remained constant throughout adulthood. Second, we retrogradely labeled the HVC-RA neurons present at 95 days of age and found no loss in their numbers over the following 9 months, suggesting that recruitment of the HVC-RA neurons in adulthood is not accompanied by loss of the existing HVC-RA neurons. Third, we labeled neurons born at sexual maturity with a birthdate marker and tracked their survival from 1 month to 4 years. We found no loss of labeled neurons in HVC over this time period, showing that adult-born recruited to the zebra finch HVC can survive for the longterm. Thus, the consequences of neuronal recruitment in the HVC of adult zebra finches are markedly different from those observed in adult canaries. New HVC-RA neurons are recruited without any appreciable loss in the existing neuron population and the recruited neurons survive for many years in HVC. Overall, these events result in a substantial net increase in the total number of HVC-RA neurons across adulthood. We speculated that a net addition of neurons to the vocal motor pathway in adulthood would have implications for adult song, specifically in song stereotypy which has been shown to improve with age after sexual maturity. However, we found that neuronal addition occurred as normal in deafened birds and that HVC-RA numbers were not correlated with song stereotypy or song stability after deafening. Thus, the functional role of neuron addition to the zebra finch song system is still unknown. Perhaps adult-born HVC-RA neurons have a perceptual rather than a motor function in this species. This research highlights that animal models for understanding the mechanism and function of adult neurogenesis must take into account the different lifestyles and needs of the species under investigation.
Walton, Clare, "Net Addition and Long-Term Survival of Adult-Born Neurons in the Zebra Finch HVC: Why Replace When You Can Keep Them All?" (2011). Student Theses and Dissertations. 133.