High affinity binding of epibatidine to serotonin type 3 receptors.
Epibatidine and mecamylamine are ligands used widely in the study of nicotinic acetylcholine receptors (nAChRs) in the central and peripheral nervous systems. In the present study, we find that nicotine blocks only 75% of (125)I-epibatidine binding to rat brain membranes, whereas ligands specific for serotonin type 3 receptors (5-HT(3)Rs) block the remaining 25%. (125)I-Epibatidine binds with a high affinity to native 5-HT(3)Rs of N1E-115 cells and to receptors composed of only 5-HT(3A) subunits expressed in HEK cells. In these cells, serotonin, the 5-HT(3)R-specific antagonist MDL72222, and the 5-HT(3)R agonist chlorophenylbiguanide readily competed with (125)I-epibatidine binding to 5-HT(3)Rs. Nicotine was a poor competitor for (125)I-epibatidine binding to 5-HT(3)Rs. However, the noncompetitive nAChR antagonist mecamylamine acted as a potent competitive inhibitor of (125)I-epibatidine binding to 5-HT(3)Rs. Epibatidine inhibited serotonin-induced currents mediated by endogenous 5-HT(3)Rs in neuroblastoma cell lines and 5-HT(3A)Rs expressed in HEK cells in a competitive manner. Our results demonstrate that 5-HT(3)Rs are previously uncharacterized high affinity epibatidine binding sites in the brain and indicate that epibatidine and mecamylamine act as 5-HT(3)R antagonists. Previous studies that depended on epibatidine and mecamylamine as nAChR-specific ligands, in particular studies of analgesic properties of epibatidine, may need to be reinterpreted with respect to the potential role of 5-HT(3)Rs.
Drisdel
Renaldo C
RC
Department of Neurobiology, University of Chicago, Chicago, IL 60637, USA.
Sharp
Douglas
D
Henderson
Tricia
T
Hales
Tim G
TG
Green
William N
WN
D000818:Animals; D001921:Brain / Q000378:metabolism*; D001923:Brain Chemistry / Q000187:drug effects; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000494:pharmacology*; D045744:Cell Line, Tumor; D006801:Humans; D008464:Mecamylamine / Q000494:pharmacology; D051379:Mice; D009538:Nicotine / Q000378:metabolism / Q000494:pharmacology; D018722:Nicotinic Agonists / Q000494:pharmacology*; D018733:Nicotinic Antagonists / Q000494:pharmacology; D011485:Protein Binding; D011725:Pyridines / Q000494:pharmacology*; D051381:Rats; D011978:Receptors, Nicotinic / Q000235:genetics / Q000378:metabolism; D044406:Receptors, Serotonin, 5-HT3 / Q000235:genetics / Q000378:metabolism; D058831:Serotonin 5-HT3 Receptor Antagonists
D019086:Bridged Bicyclo Compounds, Heterocyclic; D018722:Nicotinic Agonists; D018733:Nicotinic Antagonists; D011725:Pyridines; D011978:Receptors, Nicotinic; D044406:Receptors, Serotonin, 5-HT3; D058831:Serotonin 5-HT3 Receptor Antagonists; D008464:Mecamylamine; D009538:Nicotine; C082748:epibatidine
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