Prototypical Features of the Inhibitory Synapses in the Frog Spinal Cord as Revealed Pharmacologically

Abstract

Inhibitory synapses of the spinal cord modulate excitatory synaptic transmission and control the tones of agonistic and antagonistic muscles to ensure smooth movements of limbs and fingers. Thus terrestrial vertebrates which are destined to move with their limbs under high gravity are expected to have well organized inhibitory synapses as well as excitatory ones. During studies on the functions of inhibitory synapses in the spinal cord of frog, which is the most primitive terrestrial vertebrate, we encountered many prototypical features. For instance bicuculline and strychnine, well-known antagonists for GABA and glycine in mammalian central nervous system, respectively, showed little antagonism of the GABA- and glycine-induced membrane potential changes in amphibians [6]. Interestingly, we found that these drugs block the effect of taurine which exerts biphasic effects on primary afferent terminals [6]. We have also found that diazepam did not influence GABA binding on synaptic membranes prepared from frog spinal cord [7]. Taurine seems to be a major inhibitory neurotransmitter in the spinal cord of this species. These results give the impression that the inhibitory systems in frog spinal cord are prototypical as compared with those in mammalian central nervous system.