Growth and regression of thalamic efferents in the song-control system of male zebra finches.

A serial forebrain pathway in the songbird brain plays a critical role in vocal learning; Area X of the parolfactory lobe (X) projects to the medial portion of the dorsolateral nucleus of the anterior thalamus (DLM), which in turn projects to the lateral magnocellular nucleus of the anterior neostriatum (IMAN). Lesions of this pathway in juvenile birds disrupt vocal development, whereas identical lesions in adult birds do not influence the production of already learned song. During the course of vocal learning, IMAN undergoes a phase of massive neuronal loss, whereas the neuronal population of X more than doubles. In the present study, the development of neuron number in DLM was analyzed and found not to change during the course of vocal learning. Anterograde DiI labeling of DLM efferent fibers was then used to analyze the morphological development of this projection in relation to both the loss of neurons from lMAN and the loss of the ability of X-DLM-lMAN lesions to influence vocal production. We found that DLM axons arrive within lMAN by 15 days of age, prior to both the loss of neurons from lMAN and the onset of vocal production. The volume of anterograde DiI label over lMAN did not change between 15 and 20 days of age, but this volume more than doubled between 20 and 35 days of age. During this phase of exuberant growth, anterograde label matched the dorsal border of lMAN but extended beyond all other borders of lMAN into a surrounding "shell" of parvicellular neurons.(ABSTRACT TRUNCATED AT 250 WORDS)