Cell death during development of a forebrain nucleus involved with vocal learning in zebra finches.

Lateral MAN (magnocellular nucleus of the anterior neostriatum) is a forebrain nucleus that is known to be importantly involved with vocal learning in juvenile male zebra finches only during a restricted period of the learning process: lesions of lMAN completely disrupt song behavior in zebra finches prior to 50 days of age but have little or no effect in older juvenile or adult birds. The development of lMAN, as of other song-control regions, is delayed until the time that song behavior is being learned. Lateral MAN undergoes a substantial loss of neurons between 25 and 55 days of age, a time that encompasses initial stages of vocal production as well as the interval during which lMAN lesions become ineffective. In this study, we measured both the time course of neuronal loss and the incidence of pyknotic cells within lMAN during the period of cell loss. There is a pronounced loss of neurons from lMAN between 20 and 35 days, after which the adult number of neurons is established. The incidence of pyknosis is greatest at 20 days, around the time when the loss of live cells is also most pronounced, suggesting that the loss of neurons from lMAN is attributable to cell death. The loss of neurons occurs well before lesions of lMAN become ineffective in disrupting vocal behavior. Thus the neurons remaining in lMAN after the period of cell loss apparently undergo a substantial change in function at the time lesions lose effectiveness (about 55-60 days).