New brain pathways found in the vocal control system of a songbird.

Songbirds use a complex network of discrete brain areas and connecting fiber tracts to sing their song, but our knowledge of this circuitry may be incomplete. The forebrain area, "caudal mesopallium" (CM), has received much attention recently for its song-related activities. HVC, a prominent song system nucleus, projects to a restricted area of the CM known as the avalanche nucleus (Av). However, the other connections of Av remain unknown. Here we used tract-tracing methods to examine the connections of Av to other song system nuclei. Injections of biotinylated dextran amine (BDA) into Av labeled both afferent terminals and neurons in HVC and the interfacial nucleus of the nidopallium (NIf), suggesting that there is complex feedforward and feedback communication between these nuclei (HVC<-->Av<-->NIf). Labeled neurons were also found in the uvaeform nucleus (Uva), which was substantiated by BDA injections into Uva that labeled terminals in Av. Double fluorescent tracing experiments confirm that both HVC and Uva project to Av. The present study adds complex new connections that expand the traditional song system circuitry into the caudal mesopallium. These new pathways are likely to have broad implications for deciphering how this intricate system works.