Blocking effects of tetanus toxin and its fragment [A-B] on the excitatory and inhibitory synapses of the spinal motoneurone of the cat.

A highly purified fragment [A-B] preparation (5.5 micrograms of protein in 5 or 10 microliters which is far less than the minimal lethal dose), free from contamination with the whole toxin, was injected intraspinally into the neighbouring area of the gastrocnemius motoneurone pool (eight cats). Tetanus toxin (27 micrograms protein, about 100 LD50) in 5 or 10 microliters (four cats) and 10 microliters of Ringer's solution (four cats) were injected in the same way. Fragment [A-B] depressed both the monosynaptic reflex (MSR) of the gastrocnemius motoneurone pool and its inhibition by conditioning stimulus on the antagonistic peroneal nerve almost simultaneously in the individual cats, in contrast to the whole toxin which blocked first the inhibition of the MSR and later the MSR itself. The effects of fragment [A-B] appeared late, at about the time when the whole toxin blocked the MSR. The cats injected with fragment [A-B] did not die by intoxication. Fragment [A-B] at high doses (up to 120 micrograms) injected intramuscularly into the rat had no effect on neuromuscular transmission. The results clearly showed a blocking action of fragment [A-B] on inhibitory and excitatory synapses of the spinal motoneurones.