Neuronal nicotinic alpha 7 receptor expressed in Xenopus oocytes presents five putative binding sites for methyllycaconitine.
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.
Palma
E
E
Department of Physiology, Faculty of Medicine, University of Geneva, Switzerland.
Bertrand
S
S
Binzoni
T
T
Bertrand
D
D
D000157:Aconitine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D005260:Female; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007700:Kinetics; D009474:Neurons / Q000378:metabolism*; D018733:Nicotinic Antagonists / Q000378:metabolism* / Q000494:pharmacology; D009865:Oocytes / Q000378:metabolism; D011978:Receptors, Nicotinic / Q000096:biosynthesis / Q000378:metabolism*; D014982:Xenopus laevis
D018733:Nicotinic Antagonists; D011978:Receptors, Nicotinic; C054634:methyllycaconitine; D000157:Aconitine