Differential morphology of pyramidal tract‐type and intratelencephalically projecting‐type corticostriatal neurons and their intrastriatal terminals in rats

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.