Presynaptic inhibition in the phallic neuromuscular system of the cockroach Periplaneta americana

1. GABA 10^-4 W/V applied to the phallic neuromuscular system of the cockroach markedly reduced the amplitude of evoked epsp's without appreciable change in membrane resistance. 2. In most of the cells studied GABA did not cause any change in membrane-conductance. In two cases, only, a transient increase in membrane conductance was observed after application of 10^-4 W/V GABA. 3. In one cell only, was there a sustained effect of GABA, to reduce the time constant of decay of epsp's. 4. It is concluded that, if at all, post-synaptic inhibitory receptors are found in the phallic muscle cells, they show rapid desensitization. 5. The effect of GABA to reduce the amplitude of evoked epsp's, the lack of change in amplitude histogram of mepp's and the reduction of mepp frequency all point that GABA acts presynaptically on the excitatory nerve terminals. 6. Picrotoxin 10^-3 W/V or removal of chloride ions abolished the effects of GABA. 7. The presence of inhibitory axons which act presynaptically to reduce the release of excitatory transmitter, was demonstrated by "timing experiments". 8. The method described enables identification of inhibitory axons, neither with the physical isolation of an inhibitory axon, nor with the presence of ipsp's which usually serve as a "marker" in "timing experiments".