Recruitment of FoxP2-expressing neurons to area X varies during song development.

In adult songbirds, neural progenitors proliferate along the lateral ventricles. After migration, many of the subsequently formed neuroblasts integrate into the song nuclei HVC and Area X that participate in auditory-guided vocal motor learning and singing. Recruitment of postembryonically generated neurons into HVC, rodent hippocampus, and olfactory bulb has been linked to learning and memory. The cellular identity and the role of postembryonically generated neurons in Area X are unknown. Here we describe that the majority of new neurons in postembryonic Area X of male zebra finches expressed DARPP32 but not choline acetyltransferase or parvalbumin. This suggests that they are spiny neurons. Retrogradely labeled neurons projecting to thalamic nucleus DLM were not renewed. The spiny neurons in Area X were recently shown to express FoxP2, a transcription factor critical for normal speech and language development in humans. Since increased FoxP2 mRNA expression was previously observed during periods of vocal plasticity we investigated whether this increase might be associated with neuronal recruitment. Consistent with their spiny phenotype, new neurons in Area X did express FoxP2 and recruitment increased transiently during the juvenile song learning period. Moreover we found that FoxP2 was expressed in the ventricular zone of adult songbirds but was absent from the germinal zones in adult mouse brains, the hippocampus, and the subventricular zone. Together these results raise the possibility that neuronal recruitment and FoxP2 expression in Area X are associated with vocal learning.