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664 P. N. R. USHERWOOD

coxal adductor muscle fibres, and with even quite low repetition frequencies (e.g. 1 GABA pulse/3 set) there. wasmarked summation of the potentials and as a result the membrane potential of the muscle fibre at the GABA sensitive site slowly drifted towards the GABA equilibrium potential. There was no evidence for desensitization of the GABA receptors under these conditions.

The results of these studies which are, as yet, of a preliminary nature, confirm that GABA mimics the action of the transmitter at locust inhibitory nerve-muscle synapses. It will be of interest to determine whether the GABA receptors pro- liferate over the entire muscle surface following denervation of the coxal adductor muscle. Such a change has‘already been demonstrated for glutamate receptors on insect muscle fibres (Usherwood, 1969).

Acknowledgements-This work was supported by a grant from the S.R.C.

REFERENCES BER.~NEK R. & MILLER P. L. (1968) The action of iontophoretically applied glutamate on

insect muscle fibres. J. exp. Biol. 49, 83-93. HOYLE G. (1966a) An isolated insect ganglion-nerve-muscle preparation. J. exp. Biol. 44,

413-427. HOYLE G. (1966b) Functioning of the inhibitory conditioning axon innervating insect

muscles. -7. exp. Biol. 44, 428-453. KERKUT G. A., SHAPIRA A..& WALKER R. J. (1965) The effect of acetylcholine, glutamic acid

and GABA on contractions of the perfused cockroach leg. Comp. Biochem. Physiol. 16, 37-48.

TAKJIIJCHI A. & TAKEUCHI N. (1965) Localized action of gamma-aminobutyric acid on.the crayfish muscle. 3. Physiol., Lond. 177, 225-238.

USHERW~OD P. N. R. (1968) A critical study of the evidence for peripheral inhibitory axons in insects. J. exp. Biol. 49, 201-222.

USHERWOOD P. N. R. (1969) Glutamate sensitivity of denervated insect muscle fibres. Nature, Lond. 223, 411-413.

USHERWOOD P. N. R. & GRUNDFEST H. (1964) Inhibitory post-synaptic potentials in grass- hopper muscle. Science, N. Y. 143, 817-818.

USHERWOOD P. N. R. & GRUNDFEST H. (1965) Peripheral inhibition in skeletal muscle of insects. J. Neurophysiol. 28, 497-518.

USHERWOOD P. N. R. & MACHILI P. (1968) Pharmacological properties of excitatory neuro- muscular synapses in the locust. r. exp. Biol. 49, 341-361.

Key Word Index-GABA; locust; muscle fibre; glutamate; inhibitory transmitter; reversal potential; inhibitory synapses.

IO-“A

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FIG. l(b-d). Reversal of GABA potentials byhyperpolarizing coxal adductor muscle fibre with brief current pulses. GABA potentials produced by iontophoretic ap- plication of GABA to a GABA sensitive area. Bottom traces are intracellular recordings made close to the sensitive area, the intracellular current electrode also being placed in this region. Upper traces monitor GABA ejection current. The bottom trace in (a) was obtained at the resting potential of the fibre, i.e. - 59 mV.