An immunohistochemical and pathway tracing study of the striatopallidal organization of area X in the male zebra finch

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Abstract
Area X is a nucleus within songbird basal ganglia that is part of the anterior forebrain song learning circuit. It receives cortical song‐related input and projects to the dorsolateral medial nucleus of thalamus (DLM). We carried out single‐ and double‐labeled immunohistochemical and pathway tracing studies in male zebra finch to characterize the cellular organization and circuitry of area X. We found that 5.4% of area X neuronal perikarya are relatively large, possess aspiny dendrites, and are rich in the pallidal neuron/striatal interneuron marker Lys8‐Asn9‐neurotensin8–13 (LANT6). Many of these perikarya were found to project to the DLM, and their traits suggest that they are pallidal. Area X also contained several neuron types characteristic of the striatum, including interneurons co‐containing LANT6 and the striatal interneuron marker parvalbumin (2% of area X neurons), interneurons containing parvalbumin but not LANT6 (4.8%), cholinergic interneurons (1.4%), and neurons containing the striatal spiny projection neuron marker dopamine‐ and adenosine 3′,5′‐monophosphate‐regulated phosphoprotein (DARPP‐32) (30%). Area X was rich in substance P (SP)‐containing terminals, and many ended on area X neurons projecting to the DLM with the woolly fiber morphology characteristic of striatopallidal terminals. Although SP+ perikarya were not detected in area X, prior studies suggest it is likely that SP‐synthesizing neurons are present and the source of the SP+ input to area X neurons projecting to the DLM. Area X was poor in enkephalinergic fibers and perikarya. The present data support the premise that area X contains both striatal and pallidal neurons, with the striatal neurons likely to include SP+ neurons that project to the pallidal neurons. J. Comp. Neurol. 469:239–261, 2004.