Circuit Plasticity

Natural neuronal death prompts reactive adult neurogenesis

Neuronal birth and death are tightly coordinated to establish and maintain properly functioning neural circuits. Disruption of the equilibrium between neuronal birth and death following brain injury or insult often induces reactive neurogenesis. We used a model of naturally occurring neural degradation in a neural circuit that controls song behavior in Gambel’s white-crowned sparrows and examined the temporal dynamics between neuronal birth and death. We found that during seasonal regression of the song control nucleus HVC, caspase-mediated apoptosis increased rapidly following transition from breeding to nonbreeding conditions and neural stem cell proliferation in the nearby ventricular zone increased shortly thereafter. We also found that as apoptosis increased and the number of both recently born and pre-existing neurons in HVC decreased, the structure of song, a learned sensorimotor behavior, degraded. Our data illustrate that reactive neurogenesis is not limited to injury-induced neuronal death, but also can result from normally occurring degradation of a telencephalic neural circuit.