Local anesthetic inhibition of m1 muscarinic acetylcholine signaling

Background: Local anesthetics inhibit lipid mediator signaling (lysophospha tidate, thromboxane) by acting on intracellular domains of the receptor or on the G protein. On receptors for polar agonists, the ligand-binding pocke t could form an additional site of interaction, possibly resulting in super additive inhibition The authors therefore investigated the effects of local anesthetics on m1 muscarinic receptor functioning. Methods: The authors expressed receptors in isolation using Xenopus oocytes , Using a two-electrode voltage clamp, the authors measured the effects of lidocaine, QX314 (permanently charged), and benzocaine (permanently uncharg ed) on Ca2+ - activated Cl- currents elicited by methylcholine. The authors also characterized the interaction of lidocaine with [H-3] quinuclydinyl b enzylate ([H-3]QNB) binding to mi receptors. Results: Lidocaine Inhibited muscarinic signaling with a half-maximal inhib itory concentration (IC50 18 nM) 140-fold less than that of extracellularly administered QX314 (IC50 2.4 mu M), Intracellularly injected QX314 (IC50 0 .96 mM) and extracellularly applied benzocaine (IC50 1.2 mM) inhibited at h igh concentrations only. Inhibition of muscarinic signaling by extracellula rly applied QX314 and lidocaine was the result of noncompetitive antagonism . Intracellularly injected QX314 and benzocaine inhibited muscarinic and ly sophosphatidate signaling at similar concentrations, suggesting an action o n the common G-protein pathway. Combined administration of intracellularly injected (IC50 19 mu M) and extracellularly applied QX314 (IC50 49 nM) exer ted superadditive inhibition. Lidocaine did not displace specific [H-3]QNB binding to m1 receptors, Conclusions: m1 Muscarinic signaling is inhibited by clinically relevant co ncentrations of lidocaine and by extracellularly administered QX314, sugges ting that the major site of action is a extracellular domain of the muscari nic receptor. An additional less potent but superadditive inhibitory effect on the G-protein is suggested.