The labelling of motor end-plates in skeletal muscle of mice with 125I tetanus toxin.

Twelve hours after injection of 125I labelled tetanus toxin into the shank of one hindlimb of mice radioactivity was found in the end-plate region of soleus muscles. The ratio between the radioactivity of the end-plate and the end-plate-free region was 2.5 +/- 0.4 S.D. Autoradiographs showed intense labelling of end-plates and a slight but clear labelling of axons. When 125I tetanus toxin was injected 3 days after denervation of the soleus muscle the former end-plate region still accumulated a higher radioactivity (ratio 2.0 +/- 0.5 S.D.), however, autoradiographs showed a diffuse distribution of labelled tetanus toxin. It can be concluded that tetanus toxin binds to the presynaptic nerve terminal. This binding is not dependent on activity of the nerve terminal or transmitter release.