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Günter Gisselmann, Desiree Alisch, Brigitte Welbers-Joop, Hanns Hatt

• The genus \(\textit {Cinchona}\) is known for a range of alkaloids, such as quinine, quinidine, cinchonine, and cinchonidine. \(\textit {Cinchona}\) bark has been used as an antimalarial agent for more than 400 years. Quinine was first isolated in 1820 and is still acknowledged in the therapy of chloroquine-resistant falciparum malaria; in lower dosage quinine has been used as treatment for leg cramps since the 1940s. Here we report the effects of the quinoline derivatives quinine, quinidine, and chloroquine on human adult and fetal muscle nicotinic acetylcholine receptors (nAChRs). It could be demonstrated that the compounds blocked acetylcholine (ACh)-evoked responses in \(\textit {Xenopus laevis}\) oocytes expressing the adult nAChR composed of \(\alpha\)\(\beta\)\(\epsilon\)\(\delta\) subunits in a concentrationdependent manner, with a ranked potency of quinine (IC\(_{50}\) = 1.70 \(\mu\)M), chloroquine (IC\(_50\) = 2.22 \(\mu\)M) and quinidine (IC\(_50\) = 3.96 \(\mu\)M). At the fetal nAChR composed of abgd subunits, the IC\(_50\) for quinine was found to be 2.30 \(\mu\)M. The efficacy of the block by quinine was independent of the ACh concentration. Therefore, quinine is proposed to inhibit ACh-evoked currents in a non-competitive manner. The present results add to the pharmacological characterization of muscle nAChRs and indicate that quinine is effective at the muscular nAChRs close to therapeutic blood concentrations required for the therapy and prophylaxis of nocturnal leg cramps, suggesting that the clinically proven efficacy of quinine could be based on targeting nAChRs.