Presynaptic inhibition of the monosynaptic reflex during local tetanus in the cat

Tetanus toxin at doses of 2-2000 mouse MLD/kg was injected into the gastrocnemius muscle of the left hind limb of the cat. Acute experiments were performed at various times thereafter, when the intoxicated hind leg was strongly extended. Presynaptic inhibition of the monosynaptic reflex (MSR) of gastrocnemius motoneurones was tested by applying conditioning single electric stimuli to the antagonistic deep peroneal nerve. In most intoxicated animals the delayed inhibition of the MSR could still be observed at time intervals typical for presynaptic inhibition. However, the amplitude of the MSR often showed a strong toxin-induced facilitation at about 30 msec after the conditioning stimulus which could mask the presynaptic inhibition and sometimes made it difficult to observe it at all. After spinal transection at the Th1 level the inhibition could be better observed in such cases. Further evidence for the resistance of the presynaptic inhibitory system against the tetanus toxin in the given dose range was given by recordings of distinct dorsal root potentials which were abolished, together with the MSR inhibition by i.v. injection of picrotoxin. It is concluded that the mechanism of presynaptic inhibition remains intact or is even lengthened during local tetanus after i.m. injection of moderate, clinically relevant, toxin doses.