On the permeability of the presynaptic terminal of the crayfish neuromuscular junction during synaptic inhibition and the action of gamma-aminobutyric acid.

1. The contribution of chloride to presynaptic inhibition of the crayfish neuromuscular junction during the action of gamma-aminobutyric acid (GABA) and the inhibitory transmitter was investigated. Chloride in van Harreveld's solution was replaced with propionate, acetate, methylsulphate or glycerophosphate and electrical changes were recorded intracellularly and extracellularly with micro-electrodes.2. When the preparation was soaked in Cl-deficient solutions, the quantum content of the excitatory junctional potentials (e.j.p.s), calculated from the number of failures, was increased.3. The presynaptic inhibitory action of GABA was reduced or almost absent when the preparation was soaked in Cl-deficient solution. In Cl-deficient solution the stimulation of the inhibitory axon showed little or no inhibitory action on e.j.p.s.4. If GABA was applied shortly after the outside solution was changed to the Cl-deficient one, the frequency of the spontaneous e.j.p.s was increased transiently.5. It is suggested that GABA or the inhibitory transmitter act on the presynaptic excitatory terminal and predominantly increase its permeability to chloride.