Muscle-dependent and hormone-dependent differentiation of the vocal control premotor nucleus robustus archistriatalis and the motornucleus hypoglossus pars tracheosyringealis of the zebra finch.

Sex differences in the vertebrate brain (brain sex) are thought to develop owing to the tissue specific action of gonadal hormones similar to the development of secondary sex characteristics of the body. Small sex differences in body anatomy could, however, retrogradely control the sexual differentiation of the central nervous system. This possibility has so far been verified only for motorneuron pools, since the connectivity of sex-specific higher brain areas to the sexual dimorphic periphery is frequently not well known. Here, we tested whether somatic sex differences feed back on higher brain areas by bilateral denervation of the syringeal musculature of zebra finches before, during, and after onset of estrogen-sensitive sexual differentiation of forebrain vocal nuclei such as RA (nucleus robustus archistriatalis). In the zebra finch, the sound-producing musculature (the syrinx), the syrinx motornucleus hypoglossus pars tracheosyringealis (nXIIts), and the RA are much larger in males compared to females. Tract tracing studies revealed that the volume and neuron size distribution of the nXIIts was sexually dimorphic in intact but not in animals denervated as juveniles. In contrast, the volume of RA and size of RA neurons of denervated animals were highly sexually dimorphic. Furthermore, estrogen masculinized the RA of denervated females. Thus, sexual differentiation of the RA but not of the nXIIts appears independent of somatic sex differences. The syrinx muscles are, however, important for the soma size of those RA neurons that project to the nXIIts.