The overall goal of this project was to further define the connectional patterns and terminal morphologies of ascending axons from each subnucleus of the trigeminal nuclear complex (PrV, SpVo, SpVi, and SpVc) to zona incerta (ZI). Tract tracing techniques and immunocytochemistry for the calcium binding proteins (CBPs) parvalbumin (PV), calbindin (CB), and calretinin (CR) were utilized.; Our data revealed that: (1) PrV and SpVi provided dense and overlapping afferents to ZI; SpVc only send sparse inputs to ZI; no terminals were observed originating from SpVo; (2) axonal morphologies and branching patterns of trigeminal terminals in ZI varied in a subnuclear specific pattern; and the morphologies were quite different from those in ventroposteriomedial nucleus (VPM) at both light and electron microscopic level; (3) PrV, SpVi and SpVc terminals all formed direct synaptic contacts and exhibited asymmetric characteristics with the incerto-tectal neurons; (4) PrV and SpVi terminals were larger and tended to make synaptic contacts with larger dendrites than SpVc; (5) two separate cell populations of small neurons in PrV projected to VPM and ZI independently; these neurons selectively express PV; (6) a few large cells were observed in PrV to project to both VPM and ZI, and these neurons were more likely to express CB; (7) a moderate percentage of all three CBPs were detected in SpVi-incertal pathway; (8) only a few incertal projection neurons in SpVc were positive for CBPs, but all three CBPs were detected in the SpVc-incertal pathway.; Taken together, these findings indicate that the trigemino-incertal pathway converges neuroanatomically and neurochemically upon ZI and differs from the trigemino-dorsal thalamic circuitry. In particular, our data suggest that ZIv may serve to integrate different sensory information arising from PrV and SpVi and then project to SC in order to generate orienting movement. In contrast, ZId receives only sparse input from TGC, but has extensive connection with the neocortex as well as brainstem centers. As such, ZId may function as a modulator for feedforward and feedback information processing to the cerebral hemisphere.
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