Barbiturate Action Is Dependent on the Conformational State of the Acetylcholine Receptor

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Abstract
Barbiturates act on many neuronal ion channels by poorly understood mechanisms. The authors investigated the hypothesis that barbiturates inhibit the transient open-channel conformation of the nicotinic acetylcholine receptor (nAchR) by binding to a discrete site. Inhibition curves of the agonist-stimulated efflux of 8bRb+ from nAchR-rich membrane vesicles prepared from the electric tissue of Torpedo nobiliana were obtained for 14 barbiturates using a filter assay. When added simultaneously with agonist, all agents inhibited the ion efflux with half-inhibitory concentrations (IC50), varying from 23 microM for pentobarbital to 880 microM for barbital, and with Hill coefficients of one. The effect of several barbiturates on the agonist concentration-response curve for carbachol-stimulated efflux indicated that this inhibitory action was not competitive. The IC50s of these agents did not correlate with their octanol/water partition coefficients, nor with general anesthetic potency, although a degree of channel inhibition occurred with many agents at general anesthetic concentrations. The existence of a barbiturate-inhibitory site of action was indicated by the structural specificity. This conclusion was supported by the Hill coefficient of one, and by the high inhibitory potencies, which ruled out membrane perturbations as a mechanism. This site on the transient open-channel conformation exhibits different structure-activity relationships than an allosteric site established by equilibrium barbiturate binding on the resting conformation of the AchR. Thus, barbiturate action depends on the nAchR's conformational state.

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