Neuron death and replacement are fundamental components of brain plasticity. Much remains unknown, however, about the mechanistic interaction between neuron death and neurogenesis in adult vertebrates. In seasonally breeding adult male white-crowned sparrows, the song system nucleus HVC loses ∼26% of its neurons via caspase-dependent apoptosis within 4 d after a transition to nonbreeding physiological conditions. To determine whether neuronal death is necessary for the recruitment of new neurons, we infused caspase inhibitors into HVC in vivo and suppressed neurodegeneration for at least 20 d after the transition to nonbreeding conditions. The blockade of HVC neuron death reduced the number and density of new neurons recruited to the ipsilateral HVC by 48 and 29%, respectively, compared with contralateral HVC. Our results are the first to show that reducing neuronal death in the adult brain decreases the recruitment of new neurons.
