Ontogeny of brain nuclei controlling song learning and behavior in zebra finches

An anatomically discrete system of interconnected brain nuclei controls production of learned song patterns in adult male zebra finches (Poephila guttata). The corresponding nuclei in females, who do not sing, are greatly reduced in volume. Cells in some telencephalic song- control nuclei accumulate androgens, and male nuclei contain a much higher proportion of hormone-concentrating cells than do female nuclei. The main purpose of this experiment was to examine the normal ontogeny of the total volume of various telencephalic nuclei that have been directly or indirectly implicated in song learning and behavior in male zebra finches. In addition, the corresponding nuclei of age-matched females were examined. The major findings were as follows: the volumes of the caudal nucleus of the ventral hyperstriatum (HVc), the robust nucleus of the archistriatum (RA), and area X increased sharply in males between 12 and 53 days of age, whereas brain regions not involved with song control increased much less. Increases in the size of HVc occurred prior to those in RA and area X. Previous studies have shown that males are learning the auditory and motor characteristics of song during this time. In contrast, the volume of the magnocellular nucleus of the anterior neostriatum (MAN) decreased precipitously in males between 25 and 53 days. Measurements of neuron density and soma size demonstrated that this decreased volume is attributable to a loss of greater than 50% of the neurons in MAN. We have previously shown that lesions of MAN during this time completely disrupt song development. The volumes of HVc and RA were smaller in females than in males at all ages studied. Between 12 and 25 days the volumes of female HVc and RA increased less than brain regions not involved with song control. Female HVc and RA decreased slightly in volume between 25 and 53 days.