There is considerable functional evidence implicating norepinephrine in modulating activity in the vocal control circuit of songbirds. However, our knowledge of noradrenergic inputs to the song system is incomplete. In this study, cholera toxin subunit B (CTB) injections into area X revealed projections from the noradrenergic nuclei locus coeruleus and subcoeruleus, and injections of biotinylated dextran amines into these noradrenergic nuclei labeled fibers in area X. The nonreciprocity of this connection was demonstrated by the absence of retrogradely labeled cells in area X following injections of CTB into the locus coeruleus. Additionally, we found novel inputs to area X from the nidopallium and arcopallium, the mesencephalic central gray, and the dorsolateralis anterior (DLL) and posterior (DLP) lateralis in the thalamus. Area X can be clearly distinguished from the surrounding medial striatum based on cytoarchitectural and chemical neuroanatomical criteria. We show here that neuromodulatory inputs to area X however, exhibit a considerable degree of overlap with the surrounding area. This finding suggests that regional specificity in neuromodulator action is most likely afforded by a specialization in receptor density and enzyme distribution rather than projections from the synthesizing nuclei. Our results extend current knowledge about noradrenergic projections to specialized nuclei of the song control circuit and provide neuroanatomical evidence for the functional action of norepinephrine-modulating context-dependent ZENK expression in area X. Furthermore, the novel projections to area X from telencephalic and thalamic areas could be new and interesting nodes in the striatopallidothalamic loop spanning the songbird brain.
2007. (c) 2007 Wiley-Liss, Inc.