INHIBITION AND GAMMA-AMINOBUTYRIC ACID-INDUCED CONDUCTANCE ON LOCUST (SCHISTOCERCA-GREGARIA) EXTENSOR TIBIAE MUSCLE-FIBERS

1. Increases in membrane conductance (gm) were induced by GABA in distal bundles 32, 33 and 34 of extensor tibiae muscles of the locust (Schistocerca gregaria). 2. Bath application of GABA (10-5 .times. 10-3 M) induced reductions in muscle fiber space constant (.lambda.). 3. GABA (5 .times. 10-3 M) induced additional membrane conductance of 2.21 .+-. 0.03 .times. 10-6 S/mm, 0.38 .+-. 0.03 .times. 10-6 S/mm and 0.29 .+-. 0.06 S/mm on muscle bundles 34, 33 and 32 respectively. The greater sensitivity of muscle fibers in bundle 34 to GABA is due at least in part to a larger number of GABA receptors on bundle 34 muscle fibers. 4. The decrement of electrotonic potentials in the presence of GABA were measured over distances of both half fiber length and whole fiber length. Good agreement was obtained between changes in space constant produced by GABA using half fibre length and whole fiber length data. 5. By taking into account changes in space constant induced by GABA it was possible to demonstrate that presynaptic GABA receptors were involved in the inhibition of slow postsynaptic potentials by GABA. 6. "Slow" excitatory postsynaptic potentials recorded under current clamp were inhibited in a dose-dependent manner by GABA. This inhibition was not dependent on muscle-fiber GABA sensitivity and could not be completely accounted for by GABA-induced changes in the cable properties of the muscle fibers.