Organization of the intercollicular pathway in rat.

The intercollicular pathway of the rat was studied using autoradiographic (ARG) and horseradish peroxidase (HRP) tracing techniques. The HRP experiments demonstrated that the cells of origin of the intertectal pathway were located primarily in the rostral stratum griseum intermediale ( SGI ), stratum album intermedium (SAI) and stratum griseum profundum (SGP). Intertectal neurons were in most cases multipolar and had average somal diameters which ranged between 8 and 33 micron. Only a small number of superficial layer neurons contributed axons to the intercollicular pathway. ARG tracing showed that the intertectal pathway terminated in the deep layers of the rostral one half of the colliculus. The primary terminal zone was SGP. In addition, labeled axons left this region and coursed dorsally to terminate in a series of patches in the lower SGI and upper SAI. A small number of labeled fibers also reached the stratum opticum (SO) and lower stratum griseum superficiale (SGS).