Site of action of the general anesthetic propofol in muscarinic M1 receptor-mediated signal transduction

Although a potential target site of general anesthetics is primarily the GABA (A) receptor, a chloride ion channel, a previous study suggested that the intravenous general anesthetic propofol attenuates the M1 muscarinic acetylcholine receptor ( M1 receptor)-mediated signal transduction. In the present study, we examined the target site of propofol in M1 receptor-mediated signal transduction. Two-electrode voltage-clamp method was used in Xenopus oocytes expressing both M1 receptors and associated G protein alpha subunits (Gqalpha). Propofol inhibited M1 receptor-mediated signal transduction in a dose-dependent manner (IC50 = 50 nM). Injection of guanosine 5'-3-O( thio) triphosphate (GTPgammaS) into oocytes overexpressing Gqalpha was used to investigate direct effects of propofol on G protein coupled with the M1 receptor. Propofol did not affect activation of Gqalpha-mediated signal transduction with the intracellular injection of GTPgammaS. We also studied effects of propofol on l-[N-methyl-H-3] scopolamine methyl chloride ([H-3] NMS) binding and M1 receptor-mediated signal transduction in mammalian cells expressing M1 receptor. Propofol inhibited the M1 receptor-mediated signal transduction but did not inhibit binding of [ H-3] NMS. Effects of propofol on Gs- and Gi/o-coupled signal transduction were investigated, using oocytes expressing the beta2 adrenoceptor (beta2 receptor)/cystic fibrosis transmembrane conductance regulator or oocytes expressing the M2 muscarinic acetylcholine receptor ( M2 receptor)/Kir3.1 ( a member of G protein-gated inwardly rectifying K+ channels). Neither beta2 receptor-mediated nor M2 receptor-mediated signal transduction was inhibited by a relatively high concentration of propofol ( 50 muM). These results indicate that propofol inhibits M1 receptor-mediated signal transduction by selectively disrupting interaction between the receptor and associated G protein.