LONG-DISTANCE AXONAL REGENERATION OF IDENTIFIED LAMPREY RETICULOSPINAL NEURONS

Retrograde labeling with horseradish peroxidase was used to examine th e time course and extent of axonal regeneration of 12 pairs of individ ually identifiable reticulospinal Muller cells and 2 pairs of Mauthner cells in larval lamprey that received transections of the rostral spi nal cord in the gill region. With increasing recovery times (3-32 week s post-transection) the descending axons of many of these neurons rege nerated to progressively more caudal levels of the spinal cord. These results confirm that some reticulospinal neurons are capable of true r egeneration. However, the regenerative capacity of these neurons was n ot uniform, even for neurons in the same brain stem nucleus in close p roximity. For example, at 32 weeks post-transection some identifiable reticulospinal neurons could regenerate their axons to 60% body length or as much as 57 mm below the transection site. In contrast, previous studies indicated regeneration dis tances of 5-6 mm. Other neurons sh owed modest axonal regeneration, while one cell type showed very limit ed regeneration. The factors which may be responsible for the variable extent of regeneration among these neurons are considered. (C) 1994 A cademic Press, Inc.