Received for publication October 24, 2006.
Revised March 27, 2007.
Accepted for publication March 28, 2007.

Characterization of the Novel Human Serotonin Receptor Subunits 5-HT_3C, 5- HT_3D and 5-HT_3E

Abstract

Within the family of serotonin receptors, the 5-HT₃ receptor is the only ligand-gated ion channel. It is composed of five subunits, of which the 5-HT_3A and 5-HT_3B subunits are best characterized. Several studies, however, have reported on the functional diversity of native 5-HT₃ receptors which cannot solely be explained on the basis of the 5-HT_3A and 5-HT_3B subunits. Following our discovery of further putative 5-HT₃ serotonin receptor encoding genes, HTR3C, HTR3D and HTR3E, we investigated wether these novel candidates and the isoform 5-HT_3Ea are able to form functional 5-HT₃ receptor complexes. Using immunofluorescence and immunoprecipitation studies of heterologous expressed proteins, we found that each of the respective candidates co-assembles with 5-HT_3A. To investigate if the novel subunits modulate 5-HT₃ receptor function, we performed radioligand-binding assays and calcium-influx studies in HEK293 cells. Our experiments revealed that the 5-HT_{3C, D, E} and _Ea subunit alone cannot form functional receptors. Co-expression with 5-HT_3A, however, result in the formation of functional heteromeric complexes with different serotonin efficacies. Potencies of two agonists and antagonists were nearly identical with respect to homomeric 5-HT_3A and heteromeric complexes. However, 5-HT showed increased efficacy with respect to 5-HT_3A/D and 5-HT_3A/E receptors, which is consistent with the increased surface expression compared to 5-HT_3A receptors. In contrast, 5-HT_3A/C and 5-HT_3A/Ea receptors exhibited decreased 5-HT efficacy. This data shows for the first time that the novel 5-HT₃ subunits are able to form heteromeric 5-HT₃ receptors which exhibit quantitatively different functional properties compared to homomeric 5-HT_3A receptors.

Key words: Serotonin, Serotonin, Func. analysis receptor/ion channel mutants, Immunocytochemistry, Receptor binding studies

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