Deafening alters neuron turnover within the telencephalic motor pathway for song control in adult zebra finches.

In the telencephalon of adult songbirds, projection neurons are lost and replaced within the efferent pathway controlling learned vocal behavior. We examined the potential role of auditory experience in regulating the addition and long-term survival of vocal control neurons in adult male zebra finches. Deafened and control birds were injected with the cell birth marker [(3)H]thymidine and then killed 1 or 4 months later. At the 1 month survival time, the number of [(3)H]-labeled neurons present in the high vocal center (HVC) was 70% lower in deafened birds compared with controls. This was true for all [(3)H]-labeled HVC neurons, as well as the subset that projected to the robust nucleus of the archistriatum. Over the next 3 months, two-thirds of the [(3)H]-labeled HVC neurons in control birds were lost, presumably through cell death. Surprisingly, deafened birds showed no loss over this interval. The total number of HVC neurons did not differ between control and deafened birds at either survival time. Nuclear diameters of [(3)H]-labeled HVC neurons decreased with cell age in both control and deafened birds, a process that may relate to the eventual death and replacement of these cells. These results suggest that experience influences the addition and also the longer-term fate of neurons formed in adulthood. We propose that auditory deprivation decreases the incorporation of new neurons and prolongs their life span. Alterations in the neuronal replacement cycle may relate to the gradual deterioration in song that occurs after deafening in adult zebra finches.