A study of the inhibitory action of gamma-amino-butyric acid on neuromuscular transmission in the crayfish.

1. The effect of gamma-aminobutyric acid (GABA) on the neuromuscular junction of the crayfish (Cambarus clarkii) was investigated. The drug was applied iontophoretically to a junction and the potential changes were recorded intracellularly as well as extracellularly with micro-electrodes.2. The amplitude of the potential produced by iontophoretically applied L-glutamate was decreased by simultaneous application of GABA to the same junctional area. When the membrane potential was clamped at the resting level the clamping current for the L-glutamate injection was not changed by GABA application, indicating that there is no appreciable competition between L-glutamate and GABA for the glutamate-sensitive receptor.3. The excitatory nerve was stimulated repetitively and the excitatory junctional potentials (e.j.p.s) were recorded extracellularly with a micro-electrode placed on the surface of the muscle. The average size of the extracellular e.j.p.s was reduced by the application of GABA.4. The number of quanta released per impulse calculated from the extracellular e.j.p.s was decreased during the application of GABA, while the quantum size remained unchanged.5. The size of the excitatory presynaptic nerve spike was decreased by the application of GABA. However, no appreciable change was observed in the inhibitory nerve spike.6. These results indicate that in addition to its post-synaptic action GABA acts on the excitatory presynaptic nerve terminal and reduces the release of the transmitter.