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J. Biol. Chem., Vol. 278, Issue 25, 22482-22491, June 20, 2003

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The M₂ Muscarinic G-protein-coupled Receptor Is Voltage-sensitive^*

Yair Ben-Chaim , Oded Tour  , Nathan Dascal ¶, Itzchak Parnas  || and Hanna Parnas  **

From the The Otto Loewi Minerva Center for Cellular and Molecular Neurobiology, the Hebrew University, Jerusalem 91904 and the ^¶Department of Physiology and Pharmacology, Tel Aviv University, Tel Aviv 69978, Israel

G-protein coupled receptors are not considered to exhibit voltage sensitivity. Here, using Xenopus oocytes, we show that the M₂ muscarinic receptor (m2R) is voltage-sensitive. The m2R-mediated potassium channel (GIRK) currents were used to assay the activity of m2R. We found that the apparent affinity of m2R toward acetylcholine (ACh) was reduced upon depolarization. Binding experiments of [³H]ACh to individual oocytes expressing m2R confirmed the electrophysiological findings. When the GIRK channels were activated either by overexpression of G subunits or by injection of GTPS, the ratio between the currents measured at –60 mV and +40 mV was the same as for the basal activity of the GIRK channel. Thus, the steps downstream to agonist activation of m2R are not voltage-sensitive. We further found that, in contrast to m2R, the apparent affinity of m1R was increased upon depolarization. We also found that the voltage sensitivity of binding of [³H]ACh to oocytes expressing m2R was greatly diminished following pretreatment with pertussis toxin. The cumulative results suggest that m2R is, by itself, voltage-sensitive. Furthermore, the voltage sensitivity does not reside in the ACh binding site, rather, it most likely resides in the receptor region that couples to the G-protein.

Received for publication, February 3, 2003 , and in revised form, April 6, 2003.

^* This work was supported by Grant SFB 391 (to I. P. and H. P.) from the Deutsche Forschungsgemeinschaft, Germany, and by Grant GM 56260 from the National Institutes of Health and a grant from the USA-Israel Binational Science Foundation (to N. D.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Present address: Dept. of Pharmacology, University of California-San Diego, La Jolla, CA 92093-0647.

^|| The Greenfield Professor for Neurobiology.

^** To whom correspondence should be addressed. Tel.: 972-2-658-5900; Fax: 972-2-658-4174; E-mail: hanna{at}vms.huji.ac.il.

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