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J. Biol. Chem., Vol. 277, Issue 12, 10367-10373, March 22, 2002

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Determinants of the Anesthetic Sensitivity of Neuronal Nicotinic Acetylcholine Receptors^*

David Lindsay Downie, Francisco Vicente-Agullo^§, Antonio Campos-Caro^¶, Trevor John Bushell, William Robert Lieb, and Nicholas Peter Franks

From the Biophysics Group, The Blackett Laboratory, Imperial College of Science, Technology & Medicine, London SW7 2BW, United Kingdom and the  Instituto de Neurociencias, Universidad Miguel Hernandez, E-03550 San Juan de Alicante, Spain

Some neurotransmitter-gated ion channels are very much more sensitive to general anesthetics than others, even when they are genetically and structurally related. The most striking example of this is the extreme sensitivity of heteromeric neuronal nicotinic acetylcholine receptors to inhalational general anesthetics compared with the marked insensitivity of the closely related homomeric neuronal nicotinic receptors. Here we investigate the role of the  subunit in determining the anesthetic sensitivity of these receptors by using ₃/₇ chimeric subunits that are able to form functional homomeric receptors. By comparing the sensitivities of a number of chimeras to the inhalational agent halothane we show that the short (13 amino acids) putative extracellular loop connecting the second and third transmembrane segments is a critical determinant of anesthetic sensitivity. In addition, using site-directed mutagenesis, we show that two particular amino acids in this loop play a dominant role. When mutations are made in this loop, there is a good correlation between increasing anesthetic sensitivity and decreasing acetylcholine sensitivity. We conclude that this extracellular loop probably does not participate directly in anesthetic binding, but rather determines receptor sensitivity indirectly by playing a critical role in transducing anesthetic binding into an effect on channel gating.

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