Is the songbird Area X striatal, pallidal, or both? An anatomical study

Anatomical and neurophysiological studies have established that Area X, a songbird nucleus essential for vocal learning, is a basal ganglia structure, with mammalian striatal properties. However, Area X also sends a gamma-aminobutyric acid (GABA)ergic projection to the medial portion of the dorsolateral thalamus (DLM), a projection characteristic of the pallidum. These findings suggested that Area X contains both striatal and pallidal neurons. To test this hypothesis further, we investigated the neurochemistry and connectivity of Area X and its projections by using neurotransmitter antibodies, in combination with tracing studies. Like the mammalian striatum, Area X contains small enkephalin- and substance P-immunopositive neurons. Choline acetyltransferase-positive cells of Area X do not retrogradely label from DLM and are probably cholinergic interneurons similar to those in mammals. Like pallidal cells, large GABAergic cells project from Area X to the thalamus, but they also contain enkephalin, a characteristic of striatal neurons projecting to indirect pathway pallidal neurons. Moreover, many Area X cells are labeled with the pallidal marker Nkx2.1, but these do not include any thalamus-projecting neurons, suggesting that the projection cells are not of pallidal embryonic origin. Thus, although Area X combines both striatal and pallidal features, it is not a simple recapitulation of the mammalian circuit or of the avian lateral striatopallidal pathway: some individual Area X neurons may function as pallidal-like projection neurons but have striatal characteristics as well. Such heterogeneity of basal ganglia circuitry, both within and across species, may be facilitated by the developmental history of basal ganglia, which involves extensive migration and cellular intermixing.