Abstract
Synaptic events in a chloride-deficient condition were studied to elucidate functional aspects of presynaptic inhibitory synapses. The extracellular junctional potentials and nerve terminal potentials were concurrently recorded from a synaptic region. Inhibitory stimulation produced repetitive spikes on the inhibitory nerve terminal and then the excitatory nerve terminal, which resulted in the extracellular excitatory junctional potentials. Excitatory stimulation did not produce repetitive spikes on the inhibitory nerve terminal, indicating one-way signal transmission in this axo-axonal synapse from inhibitory to excitatory axon. The interval required for an inhibitory stimulation to produce the first response in the postsynaptic muscle membrane ranged widely from 10 to 800msec. When γ-aminobutyric acid (GABA, 1×10-4M) was added in these experimental conditions, the muscle membrane was transiently depolarized by about 10 mV. The action of GABA mimics that of the neurotransmitter at presynaptic inhibitory synapses. The experimental observations may be well explained by the concept of synapses on synapses, i. e., presynaptic inhibition, where the neurotransmitter may be GABA and chloride ions may be playing essential roles as in the case of postsynaptic inhibition.
