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

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.

Ventral Root Responses of the Hemisected Amphibian Spinal Cord to Perfused Amino Acids in the Presence of Procaine

“…For instance bicuculline blocks the inhibitory effects of GABA at mammalian motoneurones (Curtis et al, 1971) and the present results have shown that picrotoxin and bicuculline can antagonize the hyperpolarizing effect of GABA at amphibian motoneurones. The observation of a picrotoxin sensitive component of post-synaptic inhibition at cat motoneurones (Kellerth & Szumski, 1966) supports the possibility of a post-synaptic inhibitory transmitter role for GABA in mammals. However, a picrotoxin or bicuculline-sensitive post-synaptic inhibition of motoneurones in amphibians has yet to be demonstrated.…”

Pre‐ and postsynaptic modulation of monosynaptic reflex by GABAA receptors on turtle spinal cord

“…The modulation of the MSR by GABA A receptors is mediated by two mechanisms: presynaptic inhibition of Ia afferent terminals and postsynaptic inhibition of motoneurones (Kellerth & Szumski, 1966; Schmidt, 1971; Peng & Frank, 1989; Jonas et al 1998; Rudomin & Schmidt, 1999; Kullmann et al 2005). Despite the morphological and pharmacological evidence available on GABA A receptors on the motoneurones, the role of these receptors in the postsynaptic modulation of the MSR has been scarcely studied.…”

The responses of thalamic neurones to iontophoretically applied monoamines

“…It has been suggested that these may be part of a monosynaptic monoaminergic pathway from the reticular formation and medulla oblongata to motoneurones (Dahlstrom & Fuxe, 1965). It is possible that the strychnine resistant, picrotoxin sensitive inhibitions described by Kellerth & Szumski (1966) were adrenergic in nature.…”

“…It has been reported that picrotoxin does not influence the inhibition of Purkinje cells in the cerebellum (Crawford, Curtis, Voorhoeve & Wilson, 1963) or inhibitions in the spinal cord (Curtis, 1963). Kellerth & Szumski (1966), on the other hand, were able to show that picrotoxin abolished strychnine resistant inhibitions of spinal cord motoneurones and its ability to block several types of inhibitory junction in invertebrates is well documented (Robbins & Van der Kloot, 1958;Robbins 1959;Grundfest, Reuben & Rickles, 1959;Usherwood & Grundfest, 1964). J. W. PHILLIS AND A. K. TEBECIS Picrotoxin also blocks the inhibitory actions of y-aminobutyric acid (GABA) at crustacean neuromuscular junctions.…”