Transsynaptic retrograde transport of fragment C of tetanus toxin demonstrated by immunohistochemical localization.

Injection of the non-toxic fragment C of tetanus toxin into the superior oblique muscle of the eye results in strong direct retrograde labelling of the motoneurons in the contralateral trochlear nucleus and clear transsynaptic labelling of neurons in the ipsilateral and contralateral vestibular nuclei. Standard immunohistochemical procedures using a monoclonal antibody localize fragment C in the brain with high sensitivity and excellent resolution. An injection of fragment C into the superior oblique muscle labels the same pool of trochlear motoneurons as an HRP injection into the superior oblique muscle. A comparison of the fragment C labelling of trochlear motoneurons with intracellular injections of HRP into trochlear motoneurons suggests that fragment C stains not just the soma, but also the distal dendrites of motoneurons. Moreover, a fragment C injection into the superior oblique muscle labels transsynaptically more neurons in each vestibular nucleus than an injection of HRP into the trochlear nucleus labels directly.