Acetylcholine receptor conformational transition on excitation masks disulphide bonds against reduction

KARLIN and Bartels 1 found that dithiothreitol (DTT) inhibits the responses induced by acetylcholine (ACh) in the Electrophorus electricus electroplax preparation and that 5,5′-dithio- bis -(2-nitrobenzoic acid) completely restored the membrane sensitivity to ACh. These results illustrate the importance of disulphide bonds for acetylcholine receptor (AChR) function, and work on other preparations with nicotinic AChRs gave similar results 2–9 . Several facts suggest that DTT affects AChRs specifically; (1), the significance of the quaternary ammonium group in the molecules of alkylating and acylating agents for the rate of their reaction with the AChR reduced by DTT 10 ; (2) changes in pharmacological specificity seen in the modified receptor 5,7,10 ;(3) the decrease in the slope of the dose–response curve to carbamylcholine expressed as the Hillplot 11 ; and (4) the increase in d -tubocurarine affinity to the AChR active site 5 . If we could protect the receptor against chemical modification by an agonist or antagonist, this would be the most reliable proof of the specificity of modifying agent action 12 . We present here a detailed study of the action of DTT on nicotinic AChRs of completely isolated Limnaea stagnalis neurones in order to clarify the ability of various cholinergic ligands to protect the receptor disulphide bonds against reduction by DTT.