Seasonal changes in neuronal turnover in a forebrain nucleus in adult songbirds.

Neuronal death and replacement, or neuronal turnover, in the adult brain is one of many fundamental processes of neural plasticity. The adult avian song control circuit provides an excellent model for exploring mature neuronal death and replacement by new neurons. In the song control nucleus HVC of adult male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelli) nearly 68,000 neurons are added each breeding season and die during the subsequent nonbreeding season. To accommodate large seasonal differences in HVC neuron number, the balance between neuronal addition and death in HVC must differ between seasons. To determine whether maintenance of new HVC neurons changes within and between breeding and nonbreeding conditions, we pulse-labeled two different cohorts of new HVC neurons under both conditions and quantified their maintenance. We show that the maintenance of new HVC neurons, as well as new non-neuronal cells, was higher at the onset of breeding conditions than at the onset of nonbreeding conditions. Once a steady-state HVC volume and neuronal number was attained in either breeding or nonbreeding conditions, neuronal and non-neuronal maintenance were similarly low. We found that new neuronal number correlated with new non-neuronal number within each cohort of new neurons. Together, these data suggest that sex steroids promote the survival of an initial population of new neurons and non-neuronal cells entering HVC. However, once HVC is fully grown or regressed, neuronal and non-neuronal cell turnover is regulated by a common mechanism likely independent of direct sex steroid signaling. This article is protected by copyright. All rights reserved.