Distribution of 5-HT(3), 5-HT(4), and 5-HT(7) Receptors Along the Human Colon

BACKGROUND/AIMS: Several disorders of the gastrointestinal tract are associated with abnormal serotonin (5-HT) signaling or metabolism where the 5-HT(3) and 5-HT(4) receptors are clinically relevant. The aim was to examine the distribution of 5-HT(3), 5-HT(4), and 5-HT(7) receptors in the normal human colon and how this is associated with receptor interacting chaperone 3, G protein coupled receptor kinases, and protein LIN-7 homologs to extend previous observations limited to the sigmoid colon or the upper intestine. METHODS: Samples from ascending, transverse, descending, and sigmoid human colon were dissected into 3 separate layers (mucosa, longitudinal, and circular muscles) and ileum samples were dissected into mucosa and muscle layers (n = 20). Complementary DNA was synthesized by reverse transcription from extracted RNA and expression was determined by quantitative or end point polymerase chain reaction. RESULTS: The 5-HT(3) receptor subunits were found in all tissues throughout the colon and ileum. The A subunit was detected in all samples and the C subunit was expressed at similar levels while the B subunit was expressed at lower levels and less frequently. The 5-HT(3) receptor E subunit was mainly found in the mucosa layers. All splice variants of the 5-HT(4) and 5-HT(7) receptors were expressed throughout the colon although the 5-HT(4) receptor d, g, and i variants were expressed less often. CONCLUSIONS: The major differences in 5-HT receptor distribution within the human colon are in relation to the mucosa and muscular tissue layers where the 5-HT(3) receptor E subunit is predominantly found in the mucosal layer which may be of therapeutic relevance.