Horseradish Peroxidase Study of Tectal Afferents in Xenopus Laevis with Special Emphasis on Their Relationship to the Lateral-line System

Afferent projections to the tectum opticum of the clawed toad Xenopus laevis were studied by injections of horseradish peroxidase (HRP) into the tectum. Cells were labelled in the following nuclei, listed from rostral to caudal: nucleus entopeduncularis anterior, nucleus anterior thalami, nucleus posterior thalami, nucleus ventromedialis thalami, nucleus ventrolateralis thalami pars dorsalis, nucleus lateralis thalami pars posterodorsalis, nucleus neuropilis postthalamici, nucleus lentiformis mesencephali, nucleus praetectalis, nucleus laminaris tori semicircularis, nucleus principalis tori semicircularis, nucleus magnocellularis tori semicircularis, nucleus profundus mesencephali, nucleus anterodorsalis tegmenti, nucleus posterodorsalis tegmenti, nucleus posteroventralis tegmenti, nucleus isthmi, nucleus lineae lateralis pars rostralis, nucleus lineae lateralis pars caudalis, nucleus intermedius, nucleus lateralis nervi octavi, nucleus descendens nervi trigemini, nucleus reticularis superior, nucleus reticularis medius, nucleus reticularis inferior, nucleus reticularis lateralis, nucleus cuneatus and area dorsalis medullae spinalis. Four of these nuclei can be associated with lateral-line processing: the nuclei lineae lateralis rostralis and caudalis of the medulla and the centrolateral nuclei magnocellularis and principalis of the torus semicircularis. The toric input is particularly prominent; it is topologically organized in that central parts of the torus project to the medial tectum, and lateral parts of the torus project to the rostrolateral tectum. For comparison, the torotectal connection was also examined in several anuran species that lose their lateral line at metamorphosis. In these animals, this projection is less well developed than in Xenopus. Therefore, it is argued that the torotectal connection primarily conveys lateral-line information.