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Neuronal nicotinic alpha 7 receptor expressed in Xenopus oocytes presents five putative binding sites for methyllycaconitine

Published in The Journal of Physiology. 1996, vol. 491.1, p. 151-161
Abstract 1. The recently isolated compound methyllycaconitine (MLA) is a plant toxin which is a competitive inhibitor of nicotinic acetylcholine receptors (nAChRs). We found that homomeric alpha 7 receptors display a very high sensitivity to MLA with an IC50 in the picomolar range. 2. The competitive nature of the alpha 7 MLA blockade was reinforced by the observation that this compound has no action on wild-type serotoninergic receptors (5-HT3), whereas it is a powerful antagonist of chimaeric receptors alpha 7-5-HT3. 3. The time course of MLA inhibition of the wild-type (WT) alpha 7 follows a monotonic exponential decay whose time constant is proportional to the MLA concentration and could be described by a bimolecular mechanism with a forward rate constant (k+) of 2.7 x 10(7) S-1 M-1. In contrast, recovery from MLA inhibition displays an S-shaped time course that is incompatible with a simple bimolecular reaction. 4. Given the pentameric nature of the neuronal nicotinic receptors, a linear chain model, including five putative MLA binding sites corresponding to the homomeric nature of alpha 7, is proposed. 5. Both onset and recovery data obtained on the alpha 7 wild-type receptor are adequately described by this model assuming that a single MLA molecule is sufficient to block receptor function. 6. Analysis of MLA blockade and recovery of reconstituted heteromeric alpha 4 beta 2 receptors reveals, as expected, a time course compatible with only two binding sites for the toxin and, thus, further supports the validity of our model.
Keywords Aconitine/analogs & derivatives/metabolism/pharmacologyAnimalsBinding Sites/drug effectsFemaleIon Channel Gating/drug effects/physiologyKineticsNeurons/metabolismNicotinic Antagonists/metabolism/pharmacologyOocytes/metabolismReceptors, Nicotinic/biosynthesis/metabolismXenopus laevis
PMID: 9011607
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Research group Groupe Binzoni Tiziano (radiologie) (234)
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PALMA, Eleonora et al. Neuronal nicotinic alpha 7 receptor expressed in Xenopus oocytes presents five putative binding sites for methyllycaconitine. In: The Journal of Physiology, 1996, vol. 491.1, p. 151-161. https://archive-ouverte.unige.ch/unige:91038

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