Electron microscopic evidence for a transsynaptic migration of tetanus toxin in spinal cord motoneurons: an autoradiographic and morphometric study.

Adult rats were injected intramuscularly (musculus deltoideus) with 125I-labeled tetanus toxin. First signs of tetanic rigidity became visible 12-13 h after injection: death occurred 1-2 h later. Electron microscopic autoradiography of the ventrolateral spinal cord gray matter showed a large part of the radioactivity to be located in motoneurons (perikaryon, axon and dendrites) at 7 and 14 h after injection. A great majority of the rest of the label was found over synaptic terminals, most of them were afferent to motoneurons. The percentage of labeled nerve terminals increased significantly (P less than 0.05) between 7 h and 14 h after injection. Glial elements surrounding the motoneurons were mostly unlabeled. These observations strongly favor the assumption of a transsynaptic migration of tetanus toxin. Dorsal root, dorsal horn and cell bodies of spinal cord interneurons were free of label.