Lateral asymmetries and testosterone-induced changes in the gross morphology of the hypoglossal nucleus in adult canaries.

The caudal portion of the hypoglossal nucleus (tracheosyringeal, nXIIts) contains the motor neurons that innervate the syrinx in songbirds. It receives projections from telencephalic and midbrain nuclei that are necessary for song production. Its neurons concentrate androgens. The present study assesses the gross morphology of the hypoglossal nucleus in canaries. In this species song is more frequent, elaborate, and stereotyped in males than in females. Adult females respond to testosterone by developing a stereotyped song that is sung frequently. Song in male canaries is much more disrupted by damage on the left side of the song system than by damage on the right. We find anatomical correlates for each of these attributes in the nXIIts. This nucleus is 83% larger in males than in females. This is caused primarily by a sex difference in neuropil volume as there is no significant sex difference in the number of neurons in nXIIts. nXIIts grows by 34% in females given testosterone as adults. It is about 8% larger on the left than on the right in males, females, and females treated with testosterone. Sex differences are also found in the rostral (lingualis) portion of nXII, which controls muscles of the tongue, but there is no effect here of adult treatment with testosterone. Comparisons of these data with earlier measures of synaptic density and morphology in nXIIts suggest that the testosterone acts on this nucleus by inducing a modest increase in synapse numbers and by altering the efficacy of synapses in nXIIts. This contrasts with the effects of testosterone on n. robustus archistriatalis, a telencephalic component of the song system in which testosterone induces massive amounts of synaptogenesis.