Abstract
The 5-hydroxytryptamine type 3 (5-HT3) receptor is a transmitter-gated ion channel mediating neuronal excitation. The receptor native to neurons, or as a homopentameric assembly of 5-HT3A receptor subunits, displays a species-dependent pharmacology exemplified by a 1800-fold difference in the potency of (+)-tubocurarine [(+)-Tc] as an antagonist of the current response mediated by mouse and human receptor orthologs. Here, we attempt to identify amino acid residues involved in binding (+)-Tc by use of chimeric and mutant 5-HT3A subunits of mouse and human expressed in Xenopus laevis oocytes. Replacement of the entire extracellular N-terminal domain of the mouse 5-HT3A(m5-HT3A) subunit by that of the human ortholog and vice versa exchanged the differential potency of (+)-Tc, demonstrating the ligand binding site to be contained wholly within this region. Mutagenesis of multiple amino acid residues within a putative binding domain that exchanged nonconserved residues between mouse and human receptors shifted the apparent affinity of (+)-Tc in a reciprocal manner. The magnitude of the shift increased with the number of residues (3, 5, or 7) exchanged, with septuple mutations of m5-HT3A and human 5-HT3A subunits producing a 161-fold decrease and 53-fold increase in the apparent affinity of (+)-Tc, respectively. The effect of point mutations was generally modest, the exception being m5-HT3A D206E, which produced a 9-fold decrease in apparent affinity. We conclude that multiple amino acids within a binding loop of human and mouse 5-HT3Asubunits influence the potency of (+)-Tc.
Footnotes
- Received November 2, 1998.
- Accepted March 17, 1999.
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Send reprint requests to: Dr. John A. Peters, Department of Pharmacology and Neuroscience, Ninewells Hospital and Medical School, The University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom. E-mail: j.a.peters{at}dundee.ac.uk
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↵1 Present address: Department of Pharmacology, The Medical School, The University of Birmingham, Edgbaston, Birmingham B15 2TT.
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This work was supported by grants from the Wellcome Trust to J.A.P. and J.J.L.
- The American Society for Pharmacology and Experimental Therapeutics
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