Waiting periods versus early innervation: the development of axonal connections in the zebra finch song system
This study examines the development of two neural pathways within the zebra finch forebrain that function respectively in the juvenile acquisition and the adult production of learned song. In the adult male zebra finch forebrain, the song nuclei L-MAN and HVc both innervate nucleus RA; L-MAN plays a crucial role in juvenile song acquisition but, unlike HVc and RA, is not essential for adult song production. Previous studies have shown that HVc axons arrive at the dorsal border of RA as early as posthatch day 15 (day 15), and only enter the male RA after days 25–30, but never enter the female RA. The present study examines the development of axonal projections from L-MAN to RA and finds that, in contrast to HVc axons, L-MAN terminals are present within the male and female RA by day 15, and persist there throughout adult life. Unlike RA-projecting HVc neurons, HVc neurons projecting to area X innervate this target by day 20. Like L-MAN, area X plays a transient role in song acquisition. These results suggest that in the zebra finch forebrain, neural pathways essential to juvenile song learning develop before those needed for adult song production.
© 1994 Society for Neuroscience. Received Dec. 28, 1993; revised Apr. 15, 1994; accepted Apr. 27, 1994. We thank Dr. Mark Konishi, in whose lab this work was performed, Gene Akutagawa for training and technical assistance, Drs. Scott Fraser and Nancy O'Rourke for assistance with confocal microscopy, and Drs. Carla Shatz and Allison Doupe for comments and criticism with respect to the manuscript.
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