Characterisation of 5-HT3C, 5-HT3D and 5-HT3E receptor subunits: evolution, distribution and function

The 5‐HT₃ receptor is a member of the ‘Cys‐loop’ family of ligand‐gated ion channels that mediate fast excitatory and inhibitory transmission in the nervous system. Current evidence points towards native 5‐HT₃ receptors originating from homomeric assemblies of 5‐HT_3A or heteromeric assembly of 5‐HT_3A and 5‐HT_3B. Novel genes encoding 5‐HT_3C, 5‐HT_3D, and 5‐HT_3E have recently been described but the functional importance of these proteins is unknown. In the present study, in silico analysis (confirmed by partial cloning) indicated that 5‐HT_3C, 5‐HT_3D, and 5‐HT_3E are not human–specific as previously reported: they are conserved in multiple mammalian species but are absent in rodents. Expression profiles of the novel human genes indicated high levels in the gastrointestinal tract but also in the brain, Dorsal Root Ganglion (DRG) and other tissues. Following the demonstration that these subunits are expressed at the cell membrane, the functional properties of the recombinant human subunits were investigated using patch clamp electrophysiology. 5‐HT_3C, 5‐HT_3D, and 5‐HT_3E were all non‐functional when expressed alone. Co‐transfection studies to determine potential novel heteromeric receptor interactions with 5‐HT_3A demonstrated that the expression or function of the receptor was modified by 5‐HT_3C and 5‐HT_3E, but not 5‐HT_3D. The lack of distinct effects on current rectification, kinetics or pharmacology of 5‐HT_3A receptors does not however provide unequivocal evidence to support a direct contribution of 5‐HT_3C or 5‐HT_3E to the lining of the ion channel pore of novel heteromeric receptors. The functional and pharmacological contributions of these novel subunits to human biology and diseases such as irritable bowel syndrome for which 5‐HT₃ receptor antagonists have major clinical usage, therefore remains to be fully determined.