High vocal center growth and its relation to neurogenesis, neuronal replacement and song acquisition in juvenile canaries.

It is generally thought that most circuits of the adult central nervous system (CNS) are sculpted, in part at least, by selective elimination of some of the neurons present in an initial overabundant set. In this scenario, the birth of neurons precedes the period when brain functions, such as learning, first occur. In contrast to this form of brain assembly, we describe here the delayed development of the high vocal center (HVC) and one of its efferent pathways in canaries. The retrograde tracer Fluoro-Gold (FG) was injected into one of HVC's two efferent targets, the nucleus robustus archistriatalis (RA), to define the boundaries of HVC. The HVC grows markedly between 1 and 4 months, invading neighboring territories of the caudal telencephalon. During this same period, 0.43%-0.64% of the HVC neurons present at 1 year of age are labeled per day of [3H]-thymidine injection. [3H]-Thymidine labeling is a marker of cell birth, and during the first 4 months HVC neuron number increases, probably accounting for part of the HVC growth observed. Thereafter, the number of HVC neurons remains constant, but neuronal birth persists. We infer from this that neuronal replacement starts as early as 4 months after hatching and perhaps before then. About half of the neurons born after posthatching day 10 grow an axon to RA to form the main efferent pathway exiting from HVC. HVC growth, neurogenesis, axogenesis, and the observed replacement of neurons happen during the period of juvenile vocal learning. However, the recruitment of neurons that are still present at 1 year shows no particular inflections corresponding to the various stages in song learning, and continues at essentially the same rate after the more stereotyped adult song has been acquired. We suggest that a combination of neurogenesis and neuronal replacement provides unique advantages for learning.