Distribution of serotonin receptors and interacting proteins in the human sigmoid colon

This study aimed to examine the distribution of 5-HT receptors in the human colon. 5-HT induces desensitization of the circular muscle and as this is facilitated by G-protein coupled receptor kinases (GRKs) and other proteins, we also examined their distribution. Human sigmoid colon samples were dissected into three separate layers (mucosa, taeniae coli and intertaenial strips) and RNA was amplified by RT-PCR. The 5-HT2B receptor and all 5-HT7 receptor splice variants were expressed in all tissues. 5-HT4 a,b,c and n splice variants were also expressed in all tissues and 5-HT4d, 5-HT4g and 5-HT4i were only detected in some samples. The 5-HT2A receptor was seen predominantly in the intertaenial strips of the colon. Only one transcript of the serotonin transporter (SERT) was detected in the muscle layers. Variation was seen in GRK expression with GRK2 and 3 predominantly expressed in the mucosa, while GRK5 and 6 were found more commonly in the taeniae coli. PDZ (named after postsynaptic density protein, Drosophila disc large tumour suppressor and tight junction protein ZO-1) domain containing proteins, which may be involved in 5-HT receptor trafficking, were also detected throughout the sigmoid colon. The 5-HT3A subunit was expressed in all tissues, whereas the 5-HT3E subunit was mainly found in the mucosa layer while the 5-HT3B subunit was more common in the muscle layers. Receptor interacting chaperone (RIC-3), which is involved in transporting 5-HT3 receptor subunits, is expressed less in mucosa compared to muscle layers. In conclusion, these results show that there is variation in distribution of 5-HT receptors and interacting proteins within the sigmoid colon that may contribute to colonic function.