Two types of corticostriatal projection neurons have been identified: 1) one whose intrastriatal arborization arises as a collateral of a projection to the ipsilateral brainstem via the pyramidal tract (PT-type); and 2) one that projects intratelencephalically to the cortex and striatum, in many cases bilaterally, but not extratelencephalically (IT-type). To assess possible functional differences between these two neuron types, we characterized their laminar location in the cortex, their perikaryal size, and the morphology of their intrastriatal terminals. IT-type neurons were retrogradely labeled by tetramethylrhodamine-dextran amine (RDA)3k injection into the contralateral striatum, whereas their intrastriatal terminals were labeled anterogradely by biotinylated dextran amine (BDA)10k injection into the contralateral motor or primary somatosensory cortex. To label PT-type neurons and their ipsilateral intrastriatal terminals retrogradely, BDA3k was injected into the pontine pyramidal tract. We found that IT-type neuronal perikarya are medium-sized (12-13 microm) and located in layer III and upper layer V, whereas PT-type perikarya are larger (18-19 microm) and most commonly located in lower layer V. At the electron microscopic level, the intrastriatal terminals of both corticostriatal neuron types made asymmetric synaptic contact with spine heads and less frequently with dendrites. IT-type axospinous terminals were characteristically small (0.4-0.5 microm) and regular in shape, whereas PT-type terminals were typically large (0.8-0.9 microm) and often irregular in shape. Perforated postsynaptic densities were common for PT-type terminals, but not IT-type. The clear differences between these two corticostriatal neuron types in perikaryal size and laminar location in the cortex, and in the size and shape of their intrastriatal terminals, suggest that they may differ in the nature of their influence on the striatum.

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.

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