Photoperiod regulation of neuron death in the adult canary.

The avian brain undergoes naturally occurring cell death and neuronal replacement in adulthood. Little is known about how neuron survival in adult birds is regulated. However, previous work suggests that this process is open to environmental control. We now report that a reduction in day length from spring-like to fall-like conditions can dramatically increase cell death in adult male canaries. Many of the dying cells are projection neurons in the motor pathway controlling song learning and production. Circulating levels of gonadal steroids were not correlated with photoperiod-induced changes in the magnitude of cell death. Our results suggest that neuronal death in adult male canaries is regulated by seasonal changes in photoperiod, and that this occurs independent of chronic changes in gonadal steroid hormone levels. Day length may serve as a predictive environmental cue to time cell death in accordance with seasonal reproduction.