Results

(C) Artefactual labeling due to uptake by fibers of passage

After pressure applications in fiber bundles such as the lateral vestibulo-spinal and olivo-cerebellar tracts, the medial lemniscus and medial longitudinal fasciculus, we observed CTb immunostaining of fibers belonging to these tracts not only in the anterograde direction up to the terminals but also in the retrograde direction all the way to their retrogradely labeled parent cell bodies. For example, following pressure injections in the olivocerebellar tract, axons were immunostained on their way back to their retrogradely labeled parent cell bodies in the contralateral inferior olivary complex but also as far as their terminals in the cerebellum. These findings clearly demonstrate that CTb is taken up, and anterogradely but also retrogradely transported by fibers of passage. However, many points suggest that this uptake occurred only in damaged fibers of passage. Indeed, after CTb pressure injections centered on the lateral vestibulospinal tract with its destruction by the needle of the Hamilton Syringe, we detected a number of retrogradely labeled cells in the Deiters nucleus as well as immunostained fibers in the tract rostral and caudal to the injection site. In contrast, when the site involved the tract but without mechanical injury to it, no labeled cells were observed in the Deiters nucleus nor any labeled fibers in the tract. Furthermore, in contrast with pressure injections, 2- uA iontophoretic applications with 20-um micropipettes centered on the olivocerebellar tract gave rise only to occasional retrogradely labeled neurons in the contralateral olivary complex and no or only a few labeled fibers in the tract. Taken together, these results indicate that CTb is taken up and transported by damaged but not by intact fibers of passage, a drawback nearly eliminated by iontophoretic applications. Importantly, in contrast to the specific anterograde labeling (see below), the artefactual labeling of lesioned fibers was observed for long pathways even after the shortest survival (24 h), suggesting that CTb is transported in lesioned axons by a fast axonal transport mechanism. For example, after CTb injections in the fasciculus longitudinalis medialis at the level of the raphe dorsalis and 24-72 h of survival, we detected anterogradely labeled lesioned fibers 13 mm caudally in the inferior olivary complex.

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