Sex differences in neuropeptide staining of song-control nuclei in zebra finch brains

Abstract

This study examined the distribution of the neuropeptides somatostatin (SS) and calcitonin gene-related peptide (CGRP) in forebrain and midbrain song-control nuclei of male versus female brains in adult zebra finches (Poephila guttata) using immunohistochemical techniques. Vocal learning in songbirds is controlled by an interconnected, highly-localized system of brain nuclei. Male zebra finches produce learned vocalizations, and females do not. This behavioral sexual dimorphism is reflected in a substantial difference in the size of these song-control nuclei: males have larger nuclei containing a greater number of neurons than do females. Interestingly, previous studies describing neurochemical aspects of the song-control system in other songbird species have not reported any obvious sex differences. However, the results of this study showed that the level of neuropeptide staining was substantially greater in telencephalic song-control nuclei of male brains compared to female brains in zebra finches. In some brain regions females lacked any apparent staining, whereas male brains were intensely labeled. In other cases the number of neurons and/or intensity of labeling were diminished in females relative to males. The telencephalic pattern of somatostatin labeling revealed a large number of SS-labeled somata in the magnocellular nucleus of the anterior neostriatum species that occupy habitats with different hydrodynamic conditions. (MAN), the high vocal center (HVC) and the robust nucleus of the archistriatum (RA) of males, whereas female song-control nuclei contained many fewer or no labeled cells in these regions. Area X of the striatum appeared to contain a slightly higher level of somatostatin immunoreactivity than surrounding striatum in males, but the region corresponding to Area X in females could not be distinguished from the rest of striatum. The telencephalic pattern of CGRP staining was less extensive than that seen for SS. In male brains the magnocellular core region of lateral MAN contained many darkly labeled neurons, and RA was stained by a dense field of anterograde terminal label. Well labeled somata were also seen in medial MAN of males, but HVC was devoid of immunoreactivity. Area X in male brains contained a light field of terminal immunoreactivity. The pattern of labeling seen in males indicates that CGRP acts selectively as a neuromodulator along the efferent projection from lateral MAN to RA and Area X but not in the HVC-to-RA pathway. No telencephalic song-control nuclei in female brains contained CGRP staining. Although some diencephalic and midbrain nuclei contained well-labeled somata or fibers with either or both neuropeptides, there was little or no evidence of a sex difference in neuropeptide expression in these regions. This latter finding suggests that the greater anatomical specialization seen in cortical regions of songbirds compared to those regions in non-oscine species is accompanied by a greater neurochemical differentiation, whereas thalamic and midbrain regions may be more conserved across sex as well as species. These findings indicate that male neurons produce high levels of somatostatin and calcitonin gene-related peptide in major telencephalic vocal-control regions in zebra finches, whereas female neurons produce less or none at all. These dramatic neurochemical sex differences may be directly related to production of learned vocalizations in males, as well as to other aspects of song behavior and courtship.