Sex differences in the densities of α₂-adrenergic receptors in the song control system, but not the medial preoptic nucleus in zebra finches

In songbirds, song is regulated by a specialized group of brain nuclei known as the song system. Other aspects of courtship, such as male sexual interest in a female, are likely regulated by the medial preoptic nucleus (POM). The song control system and the POM are rich in norepinephrine, which appears to regulate courtship behaviors, including song. Zebra finches (Taeniopygia guttata) exhibit an extreme sexual dimorphism in song behavior; males sing, primarily to attract or maintain mates, and females do not. We explored possible sex differences in the distribution and density of the α₂-adrenergic receptors in the song system and POM of zebra finches. Receptors were labeled with the selective ligand, [³H] RX821002, via autoradiographic procedures. In males, dense α₂-receptors were observed in the song system (Area X, the high vocal center (HVc), the lateral portion of the magnocellular nucleus of the anterior neostriatum, and the robust nucleus of the archistriatum). In contrast, in females neither the lateral portion of the magnocellular nucleus of the anterior neostriatum nor the HVc could be identified based on α₂-receptor binding. Females lack Area X and indeed differential α₂-binding was not observed within the female lobus parolfactorius. The robust nucleus of the archistriatum contained less dense α₂-binding in females compared to males. Alpha₂-binding in the POM was similar in males and females. The dimorphism in α₂-binding in nuclei of the song system likely relates to the dimorphism in song behavior observed in male and female zebra finches.