Is lectin-coupled horseradish peroxidase taken up and transported by undamaged as well as by damaged fibers in the central nervous system?

Uptake and transport of horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA) in intact and damaged passing fibers were studied by injections of the medulla and pons in 11 cats. Injections with evidence of damage to olivocerebellar fibers and cranial nerve fibers invariably lead to retrograde labeling of neurons in the inferior olive and cranial motor nuclei. With staining around--but apparently no damage of--cranial nerve root fibers, no labeling was found in their motor nuclei. Injections limited to the medullary pyramid with slight fiber damage and limited staining lead to faint retrograde labeling of a small number of cells in the ipsilateral sensorimotor cortex. More extensive staining and fiber damage of the pyramid gave a higher number of labeled cells in the ipsilateral sensorimotor cortex. From these experiments we conclude that HRP-WGA is taken up and transported retrogradely with subsequent significant cell labeling in damaged but not in intact fibers. Anterograde transport of HRP-WGA in fibers passing through the injected area was found to take place only for a very short distance, as judged from cases with injections of either the pons or the medullary pyramid interrupting many corticospinal fibers.