Identification of the zebrafish red nucleus using Wheat Germ Agglutinin transneuronal tracing

The red nucleus is located in the rostral midbrain of the vertebrate brain and controls motor coordination during locomotion. It receives input from the cerebellum and sends its output to the spinal cord. The presence of the red nucleus is well established in tetrapods, and its existence has also been suggested in teleosts but its presence and position has still been under discussion. By using wheat germ agglutinin (WGA) as a genetically encoded anterograde tracer, we recently identified contralateral projections from the cerebellum to a putative red nucleus in the zebrafish midbrain tegmentum. In this report we further revealed red nucleus derived from this contralateral afferent from the cerebellum using WGA and contralateral projections to the hindbrain-spinal cord junction site using DiI-mediated retrograde tracing. Thus the structure that we have identified by anterograde and retrograde tracing fulfills the anatomical demands for the red nucleus: the location in the midbrain tegmentum, contralateral afferent from the cerebellum (cerebello-ruber projection) and contralateral efferent to the spinal cord (rubro-spinal projection).