Since autoradiographic studies demonstrating the retrograde axonal transport of 125I-tetanus toxin have been questioned, the present study was designed to determine whether locally injected, unlabeled toxin followed an intraaxonal pathway toward the CNS. Rats were injected intramuscularly with purified tetanus toxin, double ligatures were placed on both sciatic nerves, and, 24 h later, the nerves were removed and fixed. The indirect immunoperoxidase method showed reaction product within axons on the distal but not on the proximal side of the distal ligature. No label was detected in other parts of the nerve or in other tissue compartments of the sciatic nerve. These results demonstrate that immunoreactive tetanus toxin is carried toward the central nervous system by retrograde axonal transport.
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