Induced cell death in a thalamic nucleus during a restricted period of zebra finch vocal development.

A discrete network of forebrain nuclei underlies vocal learning and production in male zebra finches. Three nuclei within this network form a neural pathway that is particularly important for vocal learning in juveniles: area X of the avian striatum projects to the medial dorsolateral nucleus of the anterior thalamus (DLM), which in turn projects to the lateral magnocellular nucleus of the anterior neostriatum (IMAN). Lesions of any of these nuclei in juvenile birds disrupt normal vocal development, whereas the same lesions in adult birds have no effect on already-learned song. Because numerous studies have shown that neuronal survival in the developing nervous system depends on access to efferent targets, we have investigated the possibility that the survival of DLM neurons is similarly regulated over the course of vocal learning. Thus, the efferent target of DLM (IMAN) was lesioned electrolytically in male birds at various stages of vocal development (20, 40, 60 d of age and adult) and birds were killed either 2, 4, or 6 d postlesion. Electrolytic lesions of IMAN removed the single identified efferent target of DLM projection neurons and axotomized the terminal arborizations of these neurons. Although DLM does not normally lose neurons during vocal development, IMAN lesions in 20-d-old birds yielded numerous pyknotic cells throughout DLM by 4 d postlesion and a two-thirds reduction in DLM neuron density by 6 d postlesion. In contrast, IMAN lesions in adult birds had little or no effect on neuronal survival in DLM. Analysis of 40-d-old birds revealed significant but less substantial cell loss than in 20-d-old birds, whereas 60-d-old birds were not different from adults. The age-related decline in the vulnerability of DLM cells to IMAN lesion-induced death suggests that factors that regulate DLM neuron survival may also be involved in the acquisition of learned vocal behavior in songbirds.