Neurogenesis in adult canary telencephalon is independent of gonadal hormone levels.

Neurons generated in adulthood are found throughout the canary telencephalon. We are interested in the factors that control the rate of proliferation of stem cells that give rise to these new neurons. The rate of incorporation of newly generated neurons into vocal-control regions varies seasonally. This difference could reflect a higher rate of neurogenesis, a lower rate of cell death, or an altered migration. We examined the incidence of thymidine-labeled cells in the telencephalic ventricular zone of adult canaries as a function of variations in gonadal hormone levels. Adult female canaries maintained on a short-day photoperiod were anesthetized and gonadectomized. Four separate groups of birds received systemic exposure to either testosterone, estradiol, a combination of an anti-androgen and an inhibitor of estrogen synthesis, or nothing. All birds were also implanted with an osmotic minipump that released 3H-thymidine for 3 d and were killed 4 or 7 d following the onset of treatment. Analysis of autoradiograms revealed no differences between groups in the incidence of labeling within the ventricular zone either at the level of the anterior commissure or directly adjacent to the vocal-control nucleus HVC (higher vocal center). These results suggest that sex steroids do not regulate the rate of cell division in the ventricular zone. Seasonal differences in the incorporation of labeled cells into HVC may therefore be due to regulation of neurogenesis by photoperiodic factors other than gonadal steroids or to some other cellular mechanism, such as differential migration or survival of neurons.