Effect of tetanus toxin on the excitatory and the inhibitory post-synaptic potentials in the cat motoneurone.

Tetanus toxin (100 mouse minimal lethal doses per kilogram) was injected into the medial gastrocnemius muscle of the cat. At various times thereafter, homonymous and heteronymous group Ia excitatory post-synaptic potentials (e.p.s.p.s), disynaptic reciprocal Ia inhibitory post-synaptic potentials (i.p.s.p.s) and post-synaptic potentials (p.s.p.s) produced by sural nerve stimulation were recorded in the medial gastrocnemius motoneurones. The duration of the after-hyperpolarization, the input resistance and the axonal conduction velocity of motoneurones were also measured. Homonymous Ia e.p.s.p.s remained normal until 72 h after toxin injection. However, 5 days after toxin injection, the amplitudes of Ia e.p.s.p.s. were significantly smaller than those in control animals (1.5 +/- 1.0 mV versus 5.6 +/- 2.7 mV; t test, P less than 0.001). Heteronymous Ia e.p.s.p.s produced by stimulation of the lateral gastrocnemius-soleus nerve 5 days after toxin injection were also significantly smaller than those in control animals (0.6 +/- 0.6 mV versus 2.5 +/- 1.5 mV; P less than 0.001). However, these heteronymous Ia e.p.s.p.s remained normal when the lateral gastrocnemius-soleus nerve was ligated and sectioned at the entry to those muscles just before the toxin injection. The ascending volleys, which are supposed to represent mainly the action potentials of the dorsal spinocerebellar tract and to be elicited monosynaptically by collaterals of group I afferents, were essentially the same in the left tetanic and right control sides up to 5 days after toxin injection. Ia i.p.s.p.s and the hyperpolarizing component of sural p.s.p.s could not be produced or were very small in motoneurones sampled later than 30 h after toxin injection. The duration of the after-hyperpolarization and the input resistance of motoneurones remained normal. Axonal conduction velocity of motoneurones measured 5 days after toxin injection was 89.4 +/- 12.7 m/s, and was significantly slower than that of control motoneurones (94.1 +/- 15.4 m/s) (P less than 0.005). Differences in the amplitude of group I incoming volleys between tetanic leg and contralateral control leg were not observed. These results suggest that tetanus toxin blocks excitatory synapses in the central nervous system as well as inhibitory synapses.