Axonal connections of a forebrain nucleus involved with vocal learning in zebra finches.

Connections of a telencephalic vocal-control nucleus, the lateral magnocellular nucleus of the anterior neostriatum (lMAN), were studied in adult male zebra finches. Anterograde transport of horseradish peroxidase (alone or conjugated to wheat germ agglutinin) revealed that neurons in lMAN project to another forebrain song-control nucleus, the robust nucleus of the archistriatum (RA). RA is known to project onto the hypoglossal motor neurons that innervate the vocal organ. Retrograde transport of HRP from lMAN labeled a large thalamic nucleus, the medial portion of the dorsolateral nucleus of the thalamus (DLM). DLM in turn receives input from another nucleus of the song-control system, area X of the parolfactory lobe. We confirmed results of previous studies showing that area X receives a projection from the ventral area of Tsai (AVT) in the midbrain. In addition, we replicated results of previous experiments with canaries showing that the song-control nucleus HVc (caudal nucleus of the ventral hyperstriatum) receives input from three sources: the medial magnocellular nucleus of the anterior neostriatum (mMAN), the interfacial nucleus (NIf), and the uvae-form nucleus (Uva) of the thalamus. HVc neurons project to area X and to RA. In summary, there is a path from AVT in the midbrain, to area X, to DLM, and then to lMAN; HVc projects to X and hence indirectly to lMAN. We do not yet know the afferent connections of AVT. Thus, lMAN receives indirect input from a variety of other sources, including other regions known to be involved with vocal control.