Transsynaptic transfer of retrogradely transported tetanus protein-peroxidase conjugates

Tetanus toxin and its atoxic binding fragment, C-fragment (CF), are the only known proteins which undergo extensive transfer from motoneurons to presynaptic terminals in the spinal cord. Intramuscular injection of CF conjugated to horseradish peroxidase (HRP) was used to elucidate the ultrastructural basis for this unique property. Motoneurons labeled by retrograde axonal transport contained enzymatic reaction product within cytoplasmic vesicles and were surrounded by presynaptic terminals with label in the synaptic cleft and synaptic vesicles. Membranous structures on both sides of the synapse remained labeled for several days after the injection. Nonsynaptic regions were virtually free of CF-HRP. Transport of CF may reflect the movement of the tetanus toxin receptor in a unique synaptic compartment.