Using FoxP2 to Distinguish Direct and Indirect Basal Ganglia Pathways for Vocal Learning in Songbirds

The cortico-basal ganglia pathways that mediate vocal learning in zebra finches (Taeniopygia guttata) are localized in parallel circuits formed by CORE and SHELL subregions. These circuits traverse a specialized region of the basal ganglia essential for vocal learning (Area X), which includes intermixed striatal and pallidal neurons. The pallidal neurons within Area X exhibit analogs of mammalian direct and indirect pathways that may have opposing effects and thereby increase or inhibit thalamic activity, respectively. Direct pallidal neurons of Area X send projections to the medial portion of the dorsolateral anterior thalamic nucleus (DLM), whereas indirect pallidal neurons form intrinsic connections onto DLM-projecting neurons. Expression of the transcription factor FoxP2 in the basal ganglia is necessary for normal vocal learning and production in both humans and songbirds. We used tract-tracing techniques to label direct pallidal Area X→DLM projection neurons and immunohistochemical techniques to label neurons expressing the transcription factor FoxP2 in adult and juvenile male zebra finches. Our results showed that DLM-projecting neurons did not express FoxP2 in either adults or juveniles. Measurements of nuclear sizes revealed a population of large neurons that expressed FoxP2 but were not retrogradely labeled from DLM. A putative marker of striatal neurons (DARPP-32) did not co-localize with FoxP2 in many of these large neurons, suggesting that they form a class of indirect pallidal neurons. These findings offer FoxP2 as a possible marker for indirect pallidal neurons and support the existence of different subpopulations of neurons that correspond to direct and indirect pathways within Area X.