Developmental changes in the distribution of NADPH-diaphorase-containing neurons in telencephalic nuclei of the zebra finch song system.

Extensive recent research has focused on the potential role of nitric oxide (NO) in synaptic plasticity. Could the capacity to synthesize NO be associated with neural and behavioral plasticity in the song system? The timing of song learning and of major developmental changes in song system anatomy are known. We searched for an association between NO and these developmental events by observing the distribution of neurons staining for NADPH-diaphorase, an enzyme used in the synthesis of NO, in the brains of zebra finches. Both male and female brains were taken at different developmental ages from day 21 to adulthood. We found that the incidence of stained neurons in the song system nuclei is lower than in surrounding areas. The incidence of staining decreases with development, with most of the decrease occurring prior to the auditory learning phase of song learning. The developmental changes were quantified for area X and found to be highly significant, with a 56% decrease in staining frequency from day 21 to adulthood in males and a 23% decrease in females for the equivalent region. We also found a sexual dimorphism in the song system of adult birds, consisting of a reduced incidence of stained neurons in song system nuclei area X, high vocal center (HVC), and nucleus robustus (RA) archistrialis in males compared with females. These findings suggest that NO is less involved in the plasticity underlying song acquisition than in the earlier formation of the song system.