Inhibiton and gamma-aminobutyric acid-induced conductance on locust ( Schistocerca gregaria) extensor tibiae muscle fibres

Abstract

1. 1. Increases in membrane conductance ( g m) were induced by GABA in distal bundles 32, 33 and 34 of extensor tibiae muscles of the locust ( Schistocerca gregaria). 2. 2. Bath application of GABA (10 −5−5 × 10 −3 M) induced reductions in muscle fibre space constant (λ). 3. 3. GABA (5 × 10 −3 M) induced additional membrane conductance of 2.21 ± 0.03 × 10 −6 S/mm, 0.38 ± 0.03 × 10 −6 S/mm and 0.29 ± 0.06 × 10 −6 S/mm on muscle bundles 34, 33 and 32 respectively. The greater sensitivity of muscle fibres in bundle 34 to GABA is due at least in part to a larger number of GABA receptors on bundle 34 muscle fibres. 4. 4. The decrement of electrotonic potentials in the presence of GABA were measured over distances of both half fibre length and whole fibre length. Good agreement was obtained between changes in space constant produced by GABA using half fibre length and whole fibre length data. 5. 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 excitatory postsynaptic potentials by GABA. 6. 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-fibre GABA sensitivity and could not be completely accounted for by GABA-induced changes in the cable properties of the muscle fibres.