Abstract
It has been reported that cholera toxin B subunit (CTb) is a sensitive neuronal tracer with unique features. However, the possible uptake of CTb by non-terminal fibers passing through the injection site has not been examined thoroughly. In the present study, small iontophoretic injections (current = + 2 μA) of CTb were made in the olivocerebellar pathway in the rat ventrolateral medulla. A large number of retrogradely labeled neurons were seen in the contralateral inferior olive. In addition, prominent anterogradely labeled climbing fibers/terminals were found in the cerebellum ipsilateral to the injection site. This study, in contrast to previous report(s), indicates that CTb can be taken up avidly by fibers of passage and transported both anterogradely and retrogradely.