Transsynaptic retrograde labeling in the oculomotor system of the monkey with [125I]tetanus toxin BIIb fragment.

The injection of radioactive [125I]tetanus toxin BIIb fragment into the extraocular eye muscles of monkeys led to strong retrograde labeling of motoneurons. In addition, two patterns of weaker labeling were found: (1) discrete deposits of silver grains associated with cell soma, and (2) diffuse deposits unrelated to cell bodies. The discrete cell soma labeling was found in all areas known to make synaptic contact with the retrogradely filled motoneurons, and is indicative of transsynaptic retrograde transport. The use of tetanus toxin as a transsynaptic retrograde tracer substance is shown here for the first time at the light microscopic level.