Neurogenesis in juvenile and adult ring doves.
Studies in songbirds suggest that neurogenesis during the first few years of life is related to song learning. In this study, we examined whether postnatal neurogenesis occurs in a nonsongbird, the ring dove (Streptoplia risoria), and whether it persists to old age. Twenty-four hours after a single intramuscular injection of [3H]thymidine, labeled cells were present in the brains, particularly in the lateral wall of the lateral ventricle of juvenile (3-month and 8-month) and adult (1-year to 8-year) doves. Two months after multiple [3H]thymidine injections, there were fewer labeled cells in the ventricular zone (VZ), but many labeled cells with neuronal morphology in the parenchyma of the forebrain; labeled cells were confirmed as neurons by using neuron-specific markers, microtubule-associated protein-2 (MAP-2) and anti-neuronal nucleus (NeuN). In general, new neurons were distributed in the forebrain without clustering in any particular nucleus. During the first year of life, however, neostriatum caudale and hyperstriatum, the regions known to be essential for proper integration of sensory cues and reproductive behavior, contained more new neurons than any other brain regions. These neuronal additions showed an age-related decline; the first reduction coincided with the dove's attainment of adult physical size (about 3 months old) and the second occurred when the dove would normally attain reproductive fitness (about 1 year old). A low level of forebrain neurogenesis persisted up to 8 years of age (the oldest animals studied). These observations suggest that neurogenesis in adulthood is widespread among birds but that the biological significance of adult neurogenesis in the ring dove remains to be determined.