Effects of Picrotoxin on Stretch‐Activated Post‐Synaptic Inhibitions in Spinal Motoneurones

Abstract

Abstract 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.