Relative number of cells projecting from contralateral and ipsilateral nucleus isthmi to loci in the optic tectum is dependent on visuotopic location: Horseradish peroxidase study in the leopard frog

The leopard frog optic tectum is the principal target of the contralateral retina. The retinal terminals form a topographic map of the visual field. The tectum also receives bilateral topographic input from a midbrain structure called nucleus isthmi. In this study we determined the relative strength of n. isthmi projections to different loci in the tectum. Horseradish peroxidase (HRP) was applied at single superficial tectal locations in a series of leopard frogs. The application sites were distributed across the tectum. Retrogradely filled cells were counted in ipsilateral and contralateral nucleus isthmi. Although all regions of the tectum receive input from both n. isthmi, the relative number of labeled cells in the two n. isthmi is dependent on visuotopic location. Input to the rostromedial tectum representing the visual field ipsilateral to the labeled tectum comes primarily from the contralateral n. isthmi. Input to the caudolateral tectum representing the visual field contralateral to the labeled tectum originates mostly from the ipsilateral n. isthmi. Tectal application sites representing the visual midline had approximately equal numbers of labeled cells in the two n. isthmi. The results are similar at postapplication survival times ranging from 2 to 14 days. Using application of HRP to rostral tectum and application of nuclear yellow to caudal tectum, we show that the anisotropy in isthmi labeling is not due to take up of these labels by isthmotectal fibers passing through the application sites that terminate elsewhere. J. Comp. Neurol. 414:212–216, 1999. © 1999 Wiley‐Liss, Inc.