Sex differences in the developmental changes of GABAergic neurons in zebra finch song control nuclei

Small neurons in a sexually dimorphic nucleus of the zebra finch, the robust nucleus of the archistriatum (RA), were immunoreactive for gamma-aminobutyric acid (GABA). This dimorphism arose from the sex differences in the development of the RA neurons. We examined whether neurons with GABA-like immunoreactivity (GABA-LI) undergo the same developmental changes as the cresyl-violet-stained neurons in the RA. Somata with GABA-LI from male finches became larger during development, while somata with GABA-LI from female finches became smaller, at a rate similar to that in the total population of RA neurons. However, there were marked sex differences in the development of the number of RA neurons with GABA-LI. In the female, neurons with GABA-LI were lost at a rate which was similar to the rate of loss of the total population of RA neurons. On the other hand, in the male, the number of neurons with GABA-LI increased transiently during the sensitive period of song learning, while the total number of RA neurons did not change. This transient increase may be related to the fact that only male birds learn their song during a sensitive period.