Anticancer therapy-induced emesis: existence of an intestinal subtype of the 5-HT3 receptor?

Pharmacological and electrophysiological properties of the naturally occurring Pro391Arg variant of the human 5-HT3A receptor

The 5-HT3A receptor, a ligand-gated ion channel, is involved in pain pathways, nausea and emesis, and irritable bowel syndrome, and may play a role in the pathogenesis of psychiatric diseases such as schizophrenia and depression. Recently, a naturally occurring variation (ProArg) in the second intracellular loop of the human (h) 5-HT3A receptor was identified in a schizophrenic patient. Because the substitution of proline, an alpha-imino acid, by arginine may affect the conformation of the whole receptor, the aim of the present study was to determine the pharmacological and functional properties of this variant compared to the wild-type receptor in stably transfected HEK293 cells. Studies of binding of [H]GR65630, a 5-HT3 receptor antagonist, to membranes (saturation and competition experiments with 5-HT3 receptor ligands) and patch-clamp studies of agonist-induced currents in outside-out patches were carried out. In comparison to the wild-type, the variant receptor exhibited no changes in the receptor density and the affinities for nine representative ligands (five agonists and four antagonists). The potencies and efficacies of three 5-HT3 receptor agonists in inducing currents through the ion channel and the potencies of two 5-HT3 receptor antagonists in blocking 5-HT-evoked currents did not differ between wild-type and variant receptors. In addition, there were no differences in the desensitization kinetics of both receptor isoforms. In conclusion, the ArgPro variation of the h5-HT3A receptor does not change ligand binding to the h5-HT3A receptor, nor does it modify current through the receptor channel.

Exon-intron organization of the human 5-HT3A receptor gene

The gene structure of the human 5-HT3A receptor gene was analyzed by exon to exon polymerase chain reaction and subsequent sequencing. The results were confirmed by restriction analysis and genomic Southern blotting. The coding region of the human gene was found to be split by eight introns at identical positions as in the murine 5-HT3A receptor gene. All exon-intron boundaries exhibited fully conserved splice donor and acceptor consensus sequences. The alternative splice acceptor in intron eight of the murine gene was not found in the human counterpart. The length of particular introns differs markedly from the murine gene. With the exception of intron 5, all human introns are longer than their murine counterparts. From the start to the stop codon the human gene stretches over about 14.5 kb. The human exon sequences confirm one of three published human 5-HT3A receptor cDNA sequences. Knowledge of the gene structure, including 1.9 kb of the 5' noncoding region, all introns and the exon-intron boundaries of the human 5-HT3A receptor gene should facilitate investigation of its potential role in psychiatric disorders.