Effects of picrotoxin on stretch-activated post-synaptic inhibitions in spinal motoneurones.

The effects of picrotoxin on post-synaptic inhibitions were intracellularly investigated in cat popliteal, common peroneal and hamstring motoneurones which were activated by stretch of the triceps surae, tibialis anterior or semitendinosus muscles. In some experiments a single electrical shock to the nerve was also used as a stimulus. Inhibitions from muscle stretch were studied by the following criteria: i) hyperpolarization of the post-synaptic membrane, ii) decrease in size of the monosynaptic excitatory post-synaptic potential, iii) reduction of firing rate of a motoneurone stimulated by injected depolarizing current, iv) ‘synaptic activation noise’ in a hyperpolarizing direction, and v) sensitivity of the post-synaptic inhibition to an increased intracellular concentration of chloride ions. Post-synaptic inhibitions remaining after picrotoxin (picrotoxin-resistant) and those abolished by picrotoxin (picrotoxin-sensitive) were identified. Both types of inhibition were shown definitely to act on the post-synaptic membrane. As with strychnine-resistant post-synaptic inhibition (Kellerth and Szumski 1966), picrotoxin-sensitive post-synaptic inhibition was found to have characteristics similar to those described for presynaptic inhibition. In no case was a post-synaptic inhibition found to be resistant to both strychnine and picrotoxin. and in strychninized cats, the remaining post-synaptic inhibitions were always abolished by picrotoxin.