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Journal of Experimental Biology 151,21-39 (1990)
Published by Company of Biologists 1990

Ionic Basis of Membrane Potential and of Acetylcholine-evduced Currents in the Cell Body of the Cockroach Fast Coxal Depressor Motor Neurone

JONATHAN A. DAVID ¹ and DAVID B. SATTELLE ²

¹ AFRC Unit of Insect Neurophysiology and Pharmacology, Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK; Department of Biology and Preclinical Medicine, Gatty Marine Laboratory, University of St Andrews, St Andrews, Fife KY16 8LB, UK
² AFRC Unit of Insect Neurophysiology and Pharmacology, Department of Zoology, University of Cambridge Downing Street, Cambridge, CB2 3EJ, UK

The ionic basis of the resting potential and of the response to acetylcholine (ACh) has been investigated in the cell body membrane of the fast coxal depressor motor neurone in the metathoracic ganglion of the cockroach Periplaneta americana. By means of ion-sensitive microelectrodes, intracellular concentrations of three ion species were estimated (mmoll^-1): [K⁺]_i, 1443; [Na⁺]_i, 9±1; [Cl^-], 7±1. The resting potential of continuously superfused cells was -75.6±1.9mV at 22° C. A change in resting potential of 42.0±2.5mV accompanied a decade change in [K⁺]_o. Experiments with (10^-4moll^-1) ouabain, Na⁺ injection, low temperature (10°C) and non-superfused cells indicated the presence of an electrogenic sodium pump.

Under current-clamp, the cell body membrane was depolarized by sequentially applied, ionophoretic pulses (500ms duration) of ACh. Under voltage-clamp, such doses of ACh resulted in an inward current which was abolished in low-Na+ saline. Ion-sensitive electrodes revealed an increase in [Na⁺]_i but no change in [Cl^-1]j in response to externally applied ACh. The ACh-induced current-voltage relationship was shifted in a negative direction by low-K⁺ saline. The AChinduced inward current was usually followed by a delayed outward current which reversed at E_k. Low-K⁺ saline had the same effect on this outward component as depolarizing the membrane. This suggests that the outward current component is carried by K⁺.

The ACh-induced inward current and the delayed outward current were potentiated either when [Ca²⁺]_i was lowered by injecting the calcium chelator BAPTA or by exposure of the cell to low-Ca²⁺ saline. High-Ca²⁺ saline reduced the inward component of the response and produced a negative shift in the AChinduced current-voltage relationship. The amplitude of the delayed outward

Key words: acetylcholine receptor, identified insect neurone, voltage-clamp, ion channel, resting potential

Accepted on February 1, 1990

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