Vocal control neuron incorporation decreases with age in the adult zebra finch.

In adult male zebra finches, high vocal center (HVC) neurons continuously die and are replaced. Many of these cells are projection neurons that form part of the efferent pathway controlling learned song production. Although it is known that HVC receives new neurons well into adulthood, it is unknown whether this occurs at a constant rate or declines with adult age. We used [3H]thymidine to label new HVC neurons in male zebra finches that were 3-36 months of age. Birds were killed 4 months after 3H injections to measure the long-term incorporation of new HVC neurons. HVC neurons projecting to the robust nucleus of the archistriatum (HVC-RA) were retrogradely labeled with Fluoro-Gold 4 d before death. We found a dramatic age-related decline in the number of 3H-labeled HVC-RA neurons present 4 months after cell birth dating. A similar decline in new HVC neurons was found as soon as 1 month after their formation. These results indicate that the production or early survival of adult-formed neurons decreases with age. HVC volume and total neuron number did not change with bird age, suggesting that the age-related decrease in new neuron addition was balanced by increased survivorship of neurons incorporated previously. Reliance of song structure on auditory feedback also wanes with age. We propose that with aging, fewer new cells are added as the numbers of functionally appropriate cells increase, a process that may be linked to age-related increases in motor program stability.