Acetylcholine activates a chloride channel as well as glutamate and GABA

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Abstract
In conventional two microelectrode experiments, acetylcholine had qualitatively the same effect as GABA and glutamate on membrane potential and input resistance of muscle fibres of the opener and intrinsic stomach muscles of crayfish (Austropotamobius torrentium). In patch-clamp experiments, acetylcholine occasionally elicited single channel openings in cell-attached patches on these muscles. If outside-out patches were excised and the Cl− concentration was high on both sides of the membrane, acetylcholine at concentrations of 1 nM regularly elicited single channel currents. The amplitude of single channel currents depended strongly on the intracellular concentration of Cl−. The reversal potential of the channel, determined after replacing intracellular K+ with Cs+, corresponded to the Nernst potential for Cl−. The voltage dependence and the reversal potential of single channel current amplitudes elicited by ACh, glutamate and GABA were identical. The distribution of life times of openings (>1 ms) elicited by ACh and glutamate could be fitted by a single exponential with a time constant of about 2.5 ms, corresponding to the mean open time. ACh and glutamate applied to the same outside-out patch showed cross-desensitization, and thus ACh and glutamate activate the same channels. An excitatory, cationic ACh-activated channel could not be identified. Permeabilities of the chloride channel were calculated according to the Goldman-Hodgkin-Katz equation at different membrane potentials. Negative single channel current amplitudes (inward currents) could be fitted with a permeability ofπ 2= 3.9×10−14 cm3s−1. For positive currents (outward) the channel had a permeability ofπ 1= 1.4× 10−14 cm3s−1. The permeability of the channel declined from 16×10−14 cm3s−1 to 2.3×10−14 cm3s−1 if the intracellular Cl−-concentration was raised from 6 to 257 mM. The activation elicited by acetylcholine was inhibited by extracellular Ca++. The mean current activated by ACh was reduced by a factor of 50 if the extracellular concentration of Ca++ was raised from 0.1 mM to the physiological concentration of 13.5 mM.