Assessment of volumetric sex differences in the song control nuclei HVC and RA in zebra finches by immunocytochemistry for methionine enkephalin and vasoactive intestinal polypeptide.

In this study we assessed sex differences in the volume of two vocal control nuclei, the high vocal center (HVC) and the robust nucleus of the archistriatum (RA) in zebra finches (Taeniopygia guttata) exposed to various endocrine treatments (neonatal treatment with an aromatase inhibitor and/or an adult treatment with testosterone). The boundaries of these nuclei were defined in sections stained for Nissl substance and compared to alternate sections stained with an immunocytochemical procedure for two neuropeptides, methionine enkephalin and vasoactive intestinal polypeptide. A high density of immunoreactive fibers for methionine enkephalin and vasoactive intestinal polypeptide completely covered the high vocal center in male and female zebra finches and these fibers were not observed in the surrounding neostriatum. With the use of these markers to define the boundary of the high vocal center, it became possible to reconstruct its volume. Positively staining perikarya were not apparent within the boundaries of this nucleus. Immunoreactive fibers for vasoactive intestinal polypeptide and enkephalin also allowed one to define the boundary of the robust nucleus of the archistriatum but they did not fill the entire area of the nucleus as is the case for the high vocal center. When the volumes of both nuclei were reconstructed based on enkephalin or vasoactive intestinal polypeptide immunoreactivity, the presence of a marked sex difference in the volume of these nuclei was confirmed. Neonatal and adult endocrine manipulations did not affect the volumes of these two nuclei measured in Nissl-stained material nor did they affect the volumes as defined based on the peptidergic innervations of the nuclei. Also, the volumetric estimates of these two nuclei derived from sections stained by immunocytochemistry were in good agreement in all groups of birds with the values obtained based on an analysis of the Nissl-stained material. These results illustrate the usefulness of employing a variety of histochemical markers to define a brain area when investigating brain variation and plasticity.