Desperately seeking subunits: are native 5-HT3 receptors really homomeric complexes?

doi:10.1016/S0165-6147(98)01210-3

Trends in Pharmacological Sciences

Volume 19, Issue 6, 1 June 1998, Pages 212-215

https://doi.org/10.1016/S0165-6147(98)01210-3 Get rights and content

Abstract

The 5-HT₃ receptor complex is a ligand-gated ion channel, and is therefore likely to comprise multiple subunits in common with other members of this superfamily. To date, however, only one 5-HT₃ receptor subunit, plus an alternatively spliced variant, have been identified. In this article, Stephanie Fletcher and Nicholas Barnes review some of the extensive data in the literature that suggest the presence of other 5-HT₃ receptor subunits. This is particularly relevant given the recent demonstration that the 5-HT₃ receptor purified from pig brain contains a non-5-HT_3A-like protein(s).

Section snippets

Pharmacological diversity of the 5-HT₃ receptor

It is now well recognized that early attempts to classify subtypes of the 5-HT₃ receptor relied largely on inter-species pharmacological differences[5], for which considerable data exist. For example, the guinea-pig and human 5-HT₃ receptors display a distinct pharmacology relative to a number of other species (e.g. rat, mouse and rabbit6, 7, 8, 9). In addition, 5-HT₃ receptors demonstrate species differences in relative central distribution[9]. Whilst inter-species differences are of interest,

Electrophysiological studies

Electrophysiological studies were amongst the first to suggest the presence of different 5-HT₃ receptors within a species. For instance, while the conductance of the 5-HT₃ receptor ion channel ranges between 9 and 17 pS for the 5-HT₃ receptor expressed in various native tissues, it is sub-pS for the 5-HT₃ receptor expressed in most cell lines (e.g. the N18, NCB20 and N1E-115 cell lines), and for either of the 5-HT₃ receptor spliced variants in heterologous expression systems (for review, see [14]

Concluding remarks

It remains an attractive hypothesis that the native 5-HT₃ receptor complex may contain structurally distinct subunits. Given the established therapeutic actions of 5-HT₃ receptor antagonists, the presence of an additional subunit within the 5-HT₃ receptor complex would be highly significant: it may add to the diversity of 5-HT₃ receptors and hence provide additional targets for drug development, which could also increase the spectrum of therapeutic actions of 5-HT₃ receptor ligands.

Note added in proof

A recent report failed to find evidence that native 5-HT₃ receptors purified from pig brain contain either α1, α3, α4, α5, α7 or β2 subunits of the nicotinic receptor [Fletcher, S., Lindstrom, J. M., McKernan, R. M. and Barnes, N. M. (1998) Neuropharmacology 37, 397–399].

Acknowledgements

The authors are most grateful to Drs Mary Keen and Austen Spruce for comments on the manuscript. The work in the authors' laboratory is funded by the Medical Research Council, the Wellcome Trust and the British Pharmacological Society.

References (32)

A.G. Hope
Eur. J. Pharmacol.
(1993)
D.L. Downie
Neuropharmacology
(1994)
G.J. Kilpatrick
Neurochem. Int.
(1991)
R.M.C. Parker et al.
J. Neurol. Sci.
(1996)
H. Ito
Neuropharmacology
(1995)
S. Akuzawa et al.
Eur. J. Pharmacol.
(1996)
J.A. Peters et al.
Trends Pharmacol. Sci.
(1992)
K.A. Jones et al.
Neurosci. Lett.
(1994)
R.M.C. Parker et al.
Trends Pharmacol. Sci.
(1996)
T. Smart et al.
Prog. Neurobiol.
(1994)

R.M. McKernan

J. Biol. Chem.

(1990)

S.C. Froehner et al.

Neuron

(1990)

J. Kirsch

J. Biol. Chem.

(1991)

A.V. Maricq et al.

Science

(1991)

A.C. Hargreaves et al.

Mol. Pharmacol.

(1994)

B.P. Richardson et al.

Nature

(1985)

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The 5-hydroxytryptamine 3 (5-HT₃) receptor is expressed widely in the central and peripheral nervous systems, where it mediates or modulates a wide range of physiological processes. The receptor is targeted by drugs administered for nausea and/or emesis and irritable bowel syndrome and has been proposed as a potential drug target in various psychiatric disorders. The 5-HT₃ receptor is a pentameric ligand-gated ion channel and belongs to the Cys-loop receptor family. In contrast to the immense heterogeneity characterizing other Cys-loop receptors, native 5-HT₃ receptors historically have been considered a much more homogenous receptor population. However, the recent discovery of additional 5-HT₃ subunits and the dawning realization that central and peripheral 5-HT₃ receptor populations might comprise several subtypes characterized by distinct functional properties has emphasized the complexity of human 5-HT₃ receptor signaling. In this review potential implications of these findings and of the entirely new layer of interindividual diversity introduced to the 5-HT₃ receptor system by genetic variations will be outlined.

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Trends in Pharmacological Sciences

Desperately seeking subunits: are native 5-HT3 receptors really homomeric complexes?

Abstract

Section snippets

Pharmacological diversity of the 5-HT3 receptor

Electrophysiological studies

Concluding remarks

Note added in proof

Acknowledgements

Eur. J. Pharmacol.

Neuropharmacology

Neurochem. Int.

J. Neurol. Sci.

Neuropharmacology

Eur. J. Pharmacol.

Trends Pharmacol. Sci.

Neurosci. Lett.

Trends Pharmacol. Sci.

Prog. Neurobiol.

J. Biol. Chem.

Neuron

J. Biol. Chem.

Science

Mol. Pharmacol.

Nature

Desperately seeking subunits: are native 5-HT₃ receptors really homomeric complexes?

Pharmacological diversity of the 5-HT₃ receptor