Superior collicular projection to intralaminar thalamus in rat

The superior collicular (SC) cells which project to the intralaminar thalamus (IT; nuclei centralis lateralis, CL; paracentralis, PC; parafascicularis, Pf) in the rat were identified by means of retrograde transport of wheatgerm agglutinin conjugated horseradish peroxidase (WGA-HRP). SC-IT cells were located throughout the mediolateral and rostrocaudal exents of the tectum ipsilateral to the thalamic injection. In this SC, they had a primarily bilaminar distribution in the lower one-half of the stratum griseum intermediale (SGI) and upper portion of the stratum griseum profundum (SGP). In these laminae, SC-IT cells were arranged in clusters or patches similar to those which have been described for many inputs to the deep SC laminae. A small number of SC-IT cells were also observed in the deep laminae of the tectum contralateral to the thalamic injection. Double labelling experiments using True Blue (TB) and Diamidino Yellow (DY) demonstrated that < 1% of the contralaterally projecting SC-IT cells also innervated ipsilateral IT. Anterograde tracing with [ 3H]leucine demonstrated further that SC projected heavily to CL, PC and Pf. This projection also extended into the medial protion of the posterior thalamus (PO).