AXONAL REGROWTH AFTER SPINAL-CORD TRANSECTION IN ADULT ZEBRAFISH

Using axonal tracers, we characterized the neurons projecting from the brain to the spinal cord as well as the terminal fields of ascending spinal projections in the brain of adult zebrafish with unlesioned or transected spinal cords. Twenty distinct brain nuclei were found to pr oject to the spinal cord. These nuclei were similar to those found in the closely related goldfish, except that additionally the parvocellul ar preoptic nucleus, the medial octavolateralis nucleus, and the nucle us tangentialis, but not the facial lobe, projected to the spinal cord in zebrafish. Terminal fields of axons, visualized by anterograde tra cing, were seen in the telencephalon, the diencephalon, the torus semi circularis, the optic tectum, the eminentia granularis, and throughout the ventral brainstem in unlesioned animals. Following spinal cord tr ansection at a level approximately 3.5 mm caudal to the brainstem/spin al cord transition zone, neurons in most brain nuclei grew axons beyon d the transection site into the distal spinal cord to the level of ret rograde tracer application within 6 weeks. However, the individually i dentifiable Mauthner cells were never seen to do so up to 15 weeks aft er spinal cord transection. Nearly all neurons survived axotomy, and t he vast majority of axons that had grown beyond the transection site b elonged to previously axotomized neurons as shown by double tracing. T erminal fields were not re-established in the torus semicircularis and the eminentia granularis following spinal cord transection. (C) 1997 Wiley-Liss, Inc.